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FIMBRIATA controls flower development by mediating between meristem and organ identity genes
Abstract
Two major classes of genes directing flower development have so far been described: early activated genes regulating meristem identity and later acting genes controlling organ identity. Here, we show that the fimbriata (fim) gene acts between these two classes in a sequence of gene activation. The fim gene, originally described in 1930, was cloned by transposon tagging from Antirrhinum majus and encodes a product with no detectable homology to other proteins. Mutations in fim result in partial homeotic transformations of floral organs and in reduced determinacy of the meristem. Expression and function of fim depends on the activity of meristem identity genes, and fim in turn controls the spatial and temporal expression of organ identity genes. The pattern of fim expression defines a new domain of the floral meristem that changes with time in a complementary manner to those of the meristem identity gene floricaula and the organ identity gene plena.
... CsaTEN can bind the GGTCCC motif by its TCP and R domain; additionally, CsaTEN can bind a new CDCCRCC motif by its C terminus (Yang et al., 2020). UNUSUAL FLORAL ORGANS (UFO), encoding an F-box protein, plays a conserved role in establishing the petal and stamen whorls during the flower morphogenesis (Simon et al., 1994;Ingram et al., 1995;Levin & Meyerowitz, 1995;. Interestingly, the unusual flower and tendril (uft) mutant, which acts as a loss-of-function mutation in CsaUFO (premature translation termination), produces defective flowers and abnormal tendrils in cucumber (Chen et al., 2021). ...
... After multiple whole-genome duplication event, the burst of cucurbit diversification is driven by morphological innovations and molecular innovations, and the tendril is one of key morphological innovation in cucurbits (Guo et al., 2020). UNUSUAL FLORAL ORGAN is a conserved flower morphogenesis regulator in plants, which is expressed in floral meristem and plays a role in establishing the petal and stamen whorls (Simon et al., 1994;Ingram et al., 1995;Levin & Meyerowitz, 1995;. Interestingly, the previous study (Chen et al., 2021) and the results in this study (Figs 1, 3) indicated that CsaUFO was also expressed in tendril and played an additional role in regulating tendril morphogenesis in cucumber. ...
... UNUSUAL FLORAL ORGAN encodes an F-box protein and plays a conserved role in establishing the petal and stamen whorls during the flower morphogenesis (Simon et al., 1994;Ingram et al., 1995;Levin & Meyerowitz, 1995;. Interestingly, CsaUFO obtains an additional role in regulating tendril morphogenesis in cucumber. ...
Tendril is a morphological innovation during plant evolution, which provides the plants to obtain climbing ability. However, the tendril morphogenesis is poorly understood.
A novel tendril morphogenesis defective mutant (tmd1) was identified in cucumber. The apical part of tendril was replaced by a leaf blade in tmd1 mutant, and it lost the climbing ability. Map‐based cloning, qPCR detection, bioinformatic analysis, yeast one‐hybrid assay, electrophoretic mobility shift assay, and luciferase assay were used to explore the molecular mechanism of CsaTMD1 in regulating tendril morphogenesis.
CsaUFO was the candidate causal gene, and a fragment deletion within promoter impaired CsaUFO expression in tmd1 mutant. A conserved motif 1, which harbored two putative TCP transcription factor binding sites, was located within this deleted fragment. CsaTEN directly bound the motif 1 and positively regulated CsaUFO, and mutation in motif 1 removed this regulation.
Our work shows a CsaTEN–CsaUFO module in regulating tendril morphogenesis, indicating that evolution of tendril in cucumber due to simply drive of CsaUFO by CsaTEN in tendril. Additionally, the conserved motif 1 provides a strategy for engineering tendril‐less Cucurbitaceae crops.
... Curiously, the expression pattern of UFO, FIM and DOT within the flowers also diverged (supplementary material Fig. S1), even though they are thought to have a similar role in the activation of organ-identity genes Schultz et al., 2001;Souer et al., 2008). In young FMs, UFO and FIM mRNA are expressed throughout the meristem dome and in later stages become confined to the petal/sepal boundary (Simon et al., 1994;Lee et al., 1997). DOT and AN mRNA, however, are never expressed in the FM center (Lippman et al., 2008;Souer et al., 2008). ...
... In Antirrhinum, FIM is already expressed during the vegetative phase (supplementary material Fig. S11), and during reproductive growth its expression is restricted to the (lateral) FM and excluded from the apical IM (Simon et al., 1994). In petunia seedlings we never observed expression of pFIM:GUS (Fig. 6C), whereas in Arabidopsis seedlings pFIM:GUS was expressed at the base of newly formed leaves (Fig. 6D). ...
... The divergent expression patterns of DOT, UFO and FIM within the FM are intriguing because they seem to have similar functions within the flower. UFO and FIM are initially expressed throughout the FM, overlapping with the expression of subordinate organidentity genes that specify petal and stamen fate (Simon et al., 1994;Lee et al., 1997;Schultz et al., 2001), whereas in petunia and tomato FMs, the DOT and AN mRNA expression patterns have little or no overlap with those of the downstream B and C-type genes (Schultz et al., 2001;Souer et al., 2008). Hence, we hypothesized that DOT protein moves between cells in the FM , which might also explain why (small) changes in their expression pattern in the flower have limited consequences for development. ...
Higher plant species diverged extensively with regard to the moment (flowering time) and the position (inflorescence architecture) where flowers are formed. This seems largely caused by variation in the expression patterns of conserved genes that specify floral meristem identity (FMI), rather than changes in the encoded proteins. Here we report a functional comparison of the promoters of homologous FMI genes from Arabidopsis, petunia, tomato and Antirrhinum. Analysis of promoter-reporter constructs in petunia and Arabidopsis and complementation experiments showed that the divergent expression of LEAFY (LFY) and the petunia homolog ABERRANT LEAF AND FLOWER (ALF) results from alterations in the upstream regulatory network rather than cis-regulatory changes. The divergent expression of UNUSUAL FLORAL ORGANS (UFO) from Arabidopsis and the petunia homolog DOUBLE TOP (DOT), on the other hand, is caused by the loss or gain of cis-regulatory promoter elements, which respond to trans-acting factors that are expressed in similar patterns in both species. Introduction of pUFO:UFO causes no obvious defects in Arabidopsis, but in petunia it causes the precocious and ectopic formation of flowers. This provides an example of how a change in a cis-regulatory region can account for a change in the plant body plan.
© 2015. Published by The Company of Biologists Ltd.
... Identification of Fimbriata ( Fim ) from Antirrhinum and UN-USUAL FLORAL ORGANS ( UFO ) from Arabidopsis suggested a mechanism by which Flo and LFY may regulate activity of B-and C-class floral homeotic genes (Simon et al., 1994;Levin and Meyerowitz, 1995;Wilkinson and Haughn, 1995;Ingram et al., 1997;Lee et al., 1997). Mutations in Fim and UFO disrupt floral meristem identity and normal expression of B-and C-class genes. ...
... Overexpression of the class B gene APETALA3 ( AP3 ) in a ufo mutant background is able to rescue floral organ defects in ufo mutants, demonstrating that AP3 acts downstream of UFO (Krizek and Meyerowitz, 1996). In Antirrhinum, Fim expression is dependent on early Flo expression (Simon et al., 1994), and regulation of class B activity by Flo may occur through Fim . However, in Arabidopsis, UFO does not act downstream of LFY because overexpression of UFO is unable to rescue floral defects in lfy mutants (Lee et al., 1997). ...
... Phenotypic analysis of these two mutants indicated that Stp , in addition to its role in the flower, is involved in leaf and inflorescence development. Many of the effects of stp on flower development in pea are also seen in fim mutants from Antirrhinum and ufo mutants from Arabidopsis (see Simon et al., 1994;Ingram et al., 1995Ingram et al., , 1997Levin and Meyerowitz, 1995;Wilkinson and Haughn, 1995). Molecular analysis reveals that Stp is the pea ortholog of Fim / UFO . ...
Isolation and characterization of two severe alleles at the Stamina pistilloida (Stp) locus reveals that Stp is involved in a wide range of developmental processes in the garden pea. The most severe allele, stp-4, results in flowers consisting almost entirely of sepals and carpels. Production of ectopic secondary flowers in stp-4 plants suggests that Stp is involved in specifying floral meristem identity in pea. The stp mutations also reduce the complexity of the compound pea leaf, and primary inflorescences often terminate prematurely in an aberrant sepaloid flower. In addition, stp mutants were shorter than their wild-type siblings due to a reduction in cell number in their internodes. Fewer cells were also found in the epidermis of the leaf rachis of stp mutants. Examination of the effects of stp-4 in double mutant combinations with af, tl, det, and veg2-2—mutations known to influence leaf, inflorescence, and flower development in pea—suggests that Stp function is independent of these genes. A synergistic interaction between weak mutant alleles at Stp and Uni indicated that these two genes act together, possibly to regulate primordial growth. Molecular analysis revealed that Stp is the pea homolog of the Antirrhinum gene Fimbriata (Fim) and of UNUSUAL FLORAL ORGANS (UFO) from Arabidopsis. Differences between Fim/UFO and Stp mutant phenotypes and expression patterns suggest that expansion of Stp activity into the leaf was an important step during evolution of the compound leaf in the garden pea.
... However, the mechanisms responsible for this alignment between morphology and gene expression boundaries are unclear. A candidate gene involved in this process is the fimbriata (fim) gene of Antirrhinum, and its Arabidopsis orthologue, UFO, which affect both the identity and arrangement of organs within the flower (Simon et al., 1994; Ingram et al., 1995; Levin and Meyerowitz, 1995; Wilkinson and Haughn, 1995). The FIM protein contains a novel structural motif, the F-box, which is present in a range of proteins, including cyclin A, cyclin F and CDC4, that are involved in cell cycle control in yeast (Yochem and Byers, 1987; Zhang et al., 1993, 1995; Bai et al., 1994, 1996). ...
... In addition, mutant flowers sometimes fail to produce carpels and instead produce large numbers of mosaic organs (apical indeterminacy). These phenotypic effects can be accounted for by reduced expression of the b function gene deficiens (def) and c function gene plena (ple), suggesting that one role of fim is to promote transcription of organ identity genes (Simon et al., 1994). Consistent with this, expression of fim is detected in meristems before any organ primordia are visible, shortly before the onset of b and c organ identity gene expression. ...
... In wild-type, fim is first expressed in the ventral region of floral meristems and then in their centre before any floral organ primordia are visible (stage 2). The expression of fim then appears to spread outwards and, by the floritypic stage (stage 4), becomes localized as a ring around the centre of the floral meristem, adjacent to the sepal primordia (Simon et al., 1994). By stage 5, when petal primordia become visible, fim transcripts are present in domains around the base of each developing petal (Figure 5A). ...
The fimbriata (fim) gene of Antirrhinum affects both the identity and arrangement of organs within the flower, and encodes a protein with an F-box motif. We show that FIM associates with a family of proteins, termed FAPs (FIM-associated proteins), that are closely related to human and yeast Skp1 proteins. These proteins form complexes with F-box-containing partners to promote protein degradation and cell cycle progression. The fap genes are expressed in inflores-cence and floral meristems in a pattern that incorporates the domain of fim expression, supporting an in vivo role for a FIM–FAP complex. Analysis of a series of novel fim alleles shows that fim plays a key role in the activation of organ identity genes. In addition, fim acts in the regions between floral organs to specify the correct positioning and maintenance of morphological boundaries. Taking these results together, we propose that FIM–FAP complexes affect both gene expression and cell division, perhaps by promoting selective degradation of regulatory proteins. This may provide a mechanism by which morphological boundaries can be aligned with domains of gene expression during floral development.
... At the molecular level, little is known on the nature of LFY-UFO synergy. Unlike most floral regulators, UFO encodes not for a TF but for an F-box protein, one of the first to be described in plants [14][15][16] . UFO is part of a SKP1-Cullin1-F-box (SCF) E3 ubiquitin ligase complex through the interaction of its F-box domain with ARABIDOPSIS SKP1-LIKE (ASK) proteins 15,17 . ...
... The F-box domain is represented 76 . In selected species, presented proteins were identified as UFO homologs and their role was confirmed genetically 7,11,12,16,[77][78][79][80][81][82][83][84] . Source data are available in Supplementary Data 4. Fig. 2 | pAP3 DEE LFYBS is not required for LFY-UFOdependent pAP3 activation. ...
In angiosperms, flower development requires the combined action of the transcription factor LEAFY (LFY) and the ubiquitin ligase adaptor F-box protein, UNUSUAL FLORAL ORGANS (UFO), but the molecular mechanism underlying this synergy has remained unknown. Here we show in transient assays and stable transgenic plants that the connection to ubiquitination pathways suggested by the UFO F-box domain is mostly dispensable. On the basis of biochemical and genome-wide studies, we establish that UFO instead acts by forming an active transcriptional complex with LFY at newly discovered regulatory elements. Structural characterization of the LFY–UFO–DNA complex by cryo-electron microscopy further demonstrates that UFO performs this function by directly interacting with both LFY and DNA. Finally, we propose that this complex might have a deep evolutionary origin, largely predating flowering plants. This work reveals a unique mechanism of an F-box protein directly modulating the DNA binding specificity of a master transcription factor.
... Several genes involved in organ identity and flower symmetry have been isolated using this technology, mostly in A. majus, A. thaliana and maize. For example, AINTEGUMENTA [74], CENTRORADIALIS [75], CYCLOIDEA [76], CURLY LEAF [77], DEFICIENS [78,79], FIMBRIATA [80], FLORICAULA [81], GLOBOSA [82], GLOSSY15 [83], MIXTA [84], PLENA [85,86], SEPALLATA2 and 3 [53,87], SILKY1 [88] and ZAG1 [89] have been cloned using the transposon tagging tool. Although this method has made it possible to clarify many aspects of the flower architecture, this review will emphasize the discovery and analysis of spontaneous flower's mutants generated by the activity of TEs. ...
... In A. thaliana, several key regulators of floral meristem identity have been reported [102]. Among them, LEAFY (LFY) and UNUSUAL FLORAL ORGANS (UFO) are positive regulators of floral meristem identity and work in the same genetic pathway [80,[103][104][105][106]. In rice, the ABERRANT PANICLE ORGANIZATION1 (APO1) gene, encoding an F-box-containing protein orthologous to A. thaliana UFO, suppresses precocious conversion of rachis branch meristems to spikelets to ensure generation of certain number of spikelets [107]. ...
Transposable elements (TEs) are ubiquitous in the plant kingdom and can be a major component of plant genomes. TEs are DNA sequences that can change their position within genomes. Transposition of TEs can influence plant genes and genomes in many ways. TEs can restructure genomes through element-mediated chromosomal rearrangements and alter the genome size thus acting as agents of genome evolution. They also cause mutations by insertion into genes and affect the regulation of genes by inserting near promoters. There are some examples of mutations and other types of genetic variations associated with the activity of mobile elements and involved in flower development. The origin and extremely rapid diversification of flowering plants, which Darwin famously referred to as an “abominable mystery,” is one of the most extraordinary phenomena in evolutionary history. The most extensive manifestation of this morphological variability is found in the innumerable and surprising flower structures in plants adapted to the most contrasting environments. The wide floral diversification is a consequence of the arrangement of the organs (sepals, petals, stamens and carpels) in the four whorls of flowers and the shape that the various organs take, in particular the petal symmetry both in the individual flowers and in the organization of the inflorescences. This review will focus on how the activity of TEs has altered the activity of some genes controlling floral shape and inflorescence architecture in angiosperms.
... We showed that this activation was mediated, at least in part, by another gene,fimbriata (fim). The phenotype offim mutants had features of both meristem and organ identity mutants, displaying transformations of floral organ identity and reduced determinacy of the floral meristem Simon et al., 1994). Riudiger Simon and Sandra Doyle identified the transposon inserted atfim and were able to isolate the locus. ...
... Riudiger Simon and Sandra Doyle identified the transposon inserted atfim and were able to isolate the locus. Riidiger showed that fim occupies an intermediate position in a sequence of gene activation that starts with the meristem identity genes and leads to the expression of organ identity genes in specific whorls of the floral meristem (Simon et al., 1994). Analysis of cell lineages in the flower suggests that organ identity gene expression may be allocated to cells at a very early stage in floral development . ...
... In Arabidopsis, LFY and WUS can act either together or redundantly to activate AG (Lenhard et al. 2001;Lohmann et al. 2001). Moreover, the UFO homologs are required for the normal expression of AG homologs in Antirrhinum (Simon et al. 1994) and rice (Ikeda et al. 2005). Generating additional mutants, such as WUS or UFO homologs mutants, may help to understand AG regulation in F. vesca. ...
The plant-specific transcription factor LEAFY (LFY), generally maintained as a single copy gene in most angiosperm species, plays critical roles in flower development. The woodland strawberry (Fragaria vesca) possesses four LFY homologs in the genome; however, their respective functions and evolution remain unknown. Here, we identified and validated that mutations in one of the four LFY homologs, FveLFYa, cause homeotic conversion of floral organs and reiterative outgrowth of ectopic flowers. In contrast to FveLFYa, FveLFYb/c/d appear dispensable under normal growth conditions, as fvelfyc mutants are indistinguishable from wild type and FveLFYb and FveLFYd are barely expressed. Transgenic analysis and yeast one-hybrid assay showed that FveLFYa and FveLFYb, but not FveLFYc and FveLFYd, are functionally conserved with AtLFY in Arabidopsis (Arabidopsis thaliana). Unexpectedly, LFY binding site prediction and yeast one-hybrid assay revealed that the transcriptional links between LFY and the APETALA1 (AP1) promoter/the large AGAMOUS (AG) intron are missing in F. vesca, which is due to the loss of LFY binding sites. The data indicate that mutations in cis-regulatory elements could contribute to LFY evolution. Moreover, we showed that FveLFYa is involved in leaf development, as approximately 30% of mature leaves have smaller or fewer leaflets in fvelfya. Phylogenetic analysis indicated that LFY homologs in Fragaria species may arise from recent duplication events in their common ancestor and are undergoing convergent gene loss. Together, these results provide insight into the role of LFY in flower and leaf development in strawberry and have important implications for the evolution of LFY.
... The LFY-UFO regulatory mechanism appears to be widely conserved because orthologues have been identified in numerous plant species. For example, a similar interaction for regulation of B-class genes (DEFICIENS and GLOBOSA) has been proposed in Antirrhinum, involving FLO and FIMBRIATA (FIM), the UFO orthologue (Ingram et al., 1995;Simon et al., 1994). In rice protoplasts, APO2/RFL has been shown to regulate and physically interact with ABERRANT PANI-CLE ORGANIZATION1 (APO1), the rice homologue of UFO (Kawakatsu et al., 2012). ...
The transition to the reproductive phase, inflorescence formation and flower development are crucial elements that ensure maximum reproductive success in a plant’s life cycle. To understand the regulatory mechanisms underlying correct flower development in barley (Hordeum vulgare) we characterised the multiovary 5 (mov5.o) mutant. This mutant develops abnormal flowers that exhibit mosaic floral organs typified by multiple carpels at the total or partial expense of stamens. Genetic mapping positioned mov5 on the long arm of chromosome 2H, incorporating a region that encodes HvLFY, the barley orthologue of LEAFY from Arabidopsis. Sequencing revealed that in mov5.o plants, HvLFY contains a single amino acid substitution in a highly conserved proline residue. CRISPR‐mediated knockout of HvLFY replicated the mov5.o phenotype, suggesting that HvLFYmov5 represents a loss of function allele. In heterologous assays, the HvLFYmov5 polymorphism influenced protein‐protein interactions and affinity for a putative binding site in the promoter of HvMADS58, a C‐class MADS‐box gene. Moreover, molecular analysis indicated that HvLFY interacts with HvUFO and regulates the expression of floral homeotic genes including HvMADS2, HvMADS4 and HvMADS16. Other distinct changes in expression differ from those reported in the rice LFY mutants apo2/rfl, suggesting LFY function in the grasses is modulated in a species‐specific manner. This pathway provides a key entry point for the study of LFY function and multiple ovary formation in barley, as well as cereal species in general.
... Whereas some conserved regulatory mechanisms still exist between LFY and RFL/APO2, for example, LFY interacts with its co-regulator UFO (UNUSUAL FLORAL ORGANS) in Arabidopsis 14 , while the rice RFL/APO2 can interact with APO1 10 , an ortholog of UFO. The interaction between LFY and UFO homologs were reported in several other eudicots, such as petunia 15 , A. majus 16 and pea 17,18 . These studies revealed both partial functional divergence and conservation of LFY between rice and Arabidopsis. ...
The LFY transcription factor gene family are important in the promotion of cell proliferation and floral development. Understanding their evolution offers an insight into floral development in plant evolution. Though a promiscuous transition intermediate and a gene duplication event within the LFY family had been identified previously, the early evolutionary path of this family remained elusive. Here, we reconstructed the LFY family phylogeny using maximum likelihood and Bayesian inference methods incorporating LFY genes from all major lineages of streptophytes. The well-resolved phylogeny unveiled a high-confidence duplication event before the functional divergence of types I and II LFY genes in the ancestry of liverworts, mosses and tracheophytes, supporting sub-functionalization of an ancestral promiscuous gene. The identification of promiscuous genes in Osmunda suggested promiscuous LFY genes experienced an ancient transient duplication. Genomic synteny comparisons demonstrated a deep genomic positional conservation of LFY genes and an ancestral lineage-specific transposition activity in grasses.
... For example, the floral switch ALF has a similar role in petunia as its orthologues in tomato (FALSIFLORA; FA), Arabidopsis (LEAFY; LFY), and Antirrhinum (FLORICAULA; FLO), since they all trigger the acquisition of FM identity in these divergent angiosperm species 26 (Table S3). Similarly, the function of the orthologous group represented by the petunia DOUBLE TOP (DOT), 18 the tomato ANANTHA (AN), 27 the Antirrhinum FIMBRIATA (FIM), 28 and the Arabidopsis UNUSAL FLORAL ORGANS (UFO) genes 29,30 is conserved since they all promote FM identity, although their mutant phenotypes are weaker in Arabidopsis and Antirrhinum 28-30 than in the other species (Table S3). Furthermore, the three orthologous groups containing the Arabidopsis genes SHOOT MERISTEMLESS, WUSCHEL, and FLOWERING LOCUS T, play functionally well conserved regulatory roles in meristem identity and activity (Table S3). ...
Angiosperm inflorescences develop in two fundamentally different ways. In monopodial plants, for example in Arabidopsis thaliana, the flowers are initiated as lateral appendages of a central indeterminate inflorescence meristem. In sympodial plants, flowers arise by terminal differentiation of the inflorescence meristem, while further inflorescence development proceeds from new sympodial meristems that are generated at the flank of the terminal flower. We have used the sympodial model species Petunia hybrida to investigate inflorescence development. Here, we describe a mutant, bonsai (bns), which is defective in flower formation, inflorescence branching, and control of meristem size. Detailed microscopic analysis revealed that bns meristems retain vegetative charateristics including spiral phyllotaxis. Consistent with a block in flower formation, bns mutants exhibit a deregulated expression of various MADS-box genes. Molecular analysis revealed that the bns mutant carries a transposon insertion in the previously described EVERGREEN (EVG) gene, which belongs to the WUSCHEL-LIKE HOMEOBOX (WOX) transcription factor gene family. EVG falls in the WOX9 subfamily, which has diverse developmental functions in angiosperms. The comparison of WOX9 orthologues in five model species for flowering shows that these genes play functionally divergent roles in monopodial and sympodial plants, indicating that the WOX9 regulatory node may have played an important role in the evolution of shoot architecture.
... For example, A. thaliana and A. majus have a raceme architecture with lateral flowers, and LFY/FLO expression is the limiting factor for acquisition of floral fate in these flowers (Coen et al., 1990;Bl azquez et al., 1997). UFO is expressed in both vegetative and reproductive tissues, and neither UFO nor its A. majus orthologue FIMBRIATA is sufficient to specify floral meristem identity (Simon et al., 1994;Lee et al., 1997). By contrast, Solanaceae species such as petunia and tomato have a cyme architecture with terminal flowers, and rather than LFY, it is the UFO orthologues DOUBLE TOP and ANANTHA that are specifically expressed in these floral meristems and are necessary and sufficient to specify floral identity (Souer et al., 1998;Lippman et al., 2008). ...
A conserved genetic toolkit underlies the development of diverse floral forms among angiosperms. However, the degree of conservation vs divergence in the configuration of these gene regulatory networks is less clear.
We addressed this question in a parallel genetic study between the closely related species Arabidopsis thaliana and Cardamine hirsuta .
We identified leafy ( lfy ) and apetala1 ( ap1 ) alleles in a mutant screen for floral regulators in C. hirsuta . C. hirsuta lfy mutants showed a complete homeotic conversion of flowers to leafy shoots, mimicking lfy ap1 double mutants in A. thaliana . Through genetic and molecular experiments, we showed that AP 1 activation is fully dependent on LFY in C. hirsuta , by contrast to A. thaliana . Additionally, we found that LFY influences heteroblasty in C. hirsuta , such that loss or gain of LFY function affects its progression. Overexpression of UNUSUAL FLORAL ORGANS also alters C. hirsuta leaf shape in an LFY ‐dependent manner.
We found that LFY and AP 1 are conserved floral regulators that act nonredundantly in C. hirsuta , such that LFY has more obvious roles in floral and leaf development in C. hirsuta than in A. thaliana .
... Chez les mutants lfy, les pétales et les étamines sont les organes les plus affectés, voire totalement absents chez les allèles forts. Par hybridation in situ, ce phénotype se traduit par une réduction du niveau et du domaine d'expression d'AP3 et PI Weigel and Meyerowitz, 1993a (Ingram et al., 1995;Lee et al., 1997a;Simon et al., 1994). Par ailleurs, l'expression de UFO est détectée à partir du stade floral 2, au moment où l'identité des pétales et étamines se détermine (Laufs et al., 2003). ...
LEAFY (LFY) protein is a key regulator of floral development. Its gradually increased expression governs the sharp floral transition and LFY subsequently controls the patterning of flower meristems by inducing the expression of the floral homeotic genes APETALA1 (AP1), APETALA3 (AP3) et AGAMOUS (AG). According to some theories of evolution, LFY should have also played a central role in the appearance of flowering plants (Angiosperms). Despite a wealth of genetic data, how LFY functions at the molecular level is poorly understood. LFY is the only member of a transcription factor family specific of plant kingdom and its primary sequence looks like no other. The most important breakthrough of my PhD work, the obtention of the 3-D structure of LFY C-terminal domain (LFY-C) from Arabidopsis thaliana bound to two of its target genes, AP1 and AG, now helps us to better understand how this original protein works. LFY-C adopts a novel seven-helix fold and forms basespecific contacts in both the major and the minor grooves of DNA. LFY-C binds DNA as a cooperative dimer mediated by two basic residues. This cooperativity could partly explain LFY's effectiveness in triggering sharp flowering transition. The structure reveals also an unexpected similarity between LFY and HTH proteins such as homeodomain transcription factors or paired protein involve in animal development. Finally, allowing us to study LFY orthologues from all terrestrial plants in a novel way, these data provide a unique framework to elucidate the molecular mechanisms underlying the evolutionary history of flowering plants.
... However, as for homologues of the ABC and SEP genes, lineage specific gene duplications have led to different patterns of functional redundancy, and are possibly linked to interspecific differences in organ fusion (Weir et al. 2004). In A. majus, the UFO orthologue FIM regulates organ fusion and phyllotaxy by negatively regulating CHORIPETALA (CHO) and DES-PENTEADO (DESP), themselves negative regulators of Bclass genes in the first whorl (Simon et al. 1994;Egea-Cortines et al. 1999;Wilkinson et al. 2000). Similarly, P. hybrida PhSUP can partly complement the A. thaliana sup organ boundary mutant (Nakagawa et al. 2004). ...
The genetic basis of flowering is best understood in the model core eudicot species Arabidopsis thaliana (Brassicaceae), and involves the genetic reprogramming of shoot apical meristems, ending in the production of flowers. Although inflorescences and flowers of core eudicots share a common ground plan, variation in architecture, shape and ornamentation suggests repeated modifications to this ancestral plan. Comparative studies, primarily in Brassicaceae and Leguminoseae (rosids), and Asteraceae, Plantaginaceae and Solanaceae (asterids), have revealed a common developmental framework for flowering across core eudicots. This serves as a basis for understanding genetic changes that underlie the diversification of inflorescence and floral form. Recent work is starting to reveal the relative importance of regulatory versus protein coding changes in genes involved in diversification of inflorescence and flower development across core eudicots. Furthermore, these studies highlight the importance of phylogenetic history for understanding functional conservation of duplicated genes.
... In a very general sense, however, genes ®rst expressed as chemical patterns, such as annuli, in any branching process in plants, are of potential interest. These include a few¯ower development genes intermediate between meristem identity and organ identity genes: FIMBRIATA (FIM) in Antirrhinum (Simon et al., 1994); UNUSUAL FLORAL ORGANS (UFO) in Arabidopsis (Ingram et al., 1995); and BLH2, a homeodomain gene in Arabidopsis, expressed in various tissues at different times in development, including expression in the¯oral apex in an annular pattern where the sepal whorl is about to form (M. Pidcowich, D. Godt, K. Kushalappa and G. Haughn, pers. comm.). ...
d Background and Aims Many conifer embryos, both in natural seeds and in clonal populations of somatic embryos, display variability in the number of cotyledons. In hybrid larch, Larix 3 leptoeuropaea (synonymous with L. 3 marschlinsii Coaz), such variability has previously been reported in somatic embryos, together with a decrease in the average cotyledon number when benzyladenine (BA) is applied exogenously. Described here is a spatially quantitative study with the aim of throwing some light on the way cotyledon number is determined, and hence the mechanism of cotyledon formation. d Methods Stock cultures of embryogenic tissue were maintained and later made embryogenically active by standard methods. Development through cotyledon formation was followed by optical microscopy with quantita- tive measurement of embryo diameter and number of cotyledons. SEMs of representative stages and cotyledon numbers were done for purposes of illustration in this account. Existing mathematics of waveforms on a disc were cast into a form suitable to compare with the quantitative data. d Key Results The number of cotyledons is linearly related to the diameter of the apical surface of the embryo (which approximates a circular disc) at the time of first appearance of the cotyledon primordia. This linearity is a constant-spacing phenomenon between adjacent primordia. Addition of BA to the medium restricts the range of apical diameters without changing inter-cotyledon spacing. Slope/intercept ratio of the linear plot matches expectation for initiation of cotyledon pattern as a harmonic waveform on a circular disc. d Conclusions The entire pattern of cotyledon primordia arises as a single entity coordinated by a mechanism with wave-forming properties. This is explicable by diverse mechanisms, especially either mechanical buckling ('biophysical') or reaction-diffusion kinetics ('physicochemical'). ª 2004 Annals of Botany Company
... This is a refinement of the conclusion reached by that DOT is expressed in a normal intensity and pattern in an alf mutant background. Our data are in line with studies in tomato and Antirrhinum, which showed by quantitative RT-PCR that a tomato DOT-ortholog, ANANTHA, is not expressed if the ALF-ortholog FALSIFLORA is mutated , and that in Antirrhinum normal expression of the DOT-ortholog FIMBRIATA requires the ALF-ortholog FLORICAULA (Simon et al., 1994). ...
... -2 phenotype might be the consequence of a malfunction of a gene regulating the expression of B genes in pea. Such activities have been characterized for both Arabidopsis and Antirrhinum , as two homologous genes, UNUSUAL FLORAL ORGANS (UFO) and FIMBRIATA (Fim), respectively (Simon et al. 1994; Levin and Meyerowitz 1995; Lee et al. 1997 ...
This work provides new evidence of the complex genetic regulation necessary to accomplish flower development in legumes. Using scanning electron microscopy (SEM) analysis, we have characterized the early developmental events of the wild type Medicago truncatula flower and selected morphological characters as markers to break it down into eight different developmental stages. The order of floral organ initiation in M. truncatula and pea (Pisum sativum L.), in contrast to Arabidopsis and Antirrhinum, is unidirectional in all whorls starting from the abaxial position of the flower with a high degree of overlap. Another main difference is the existence of four common primordia from which petals and stamens differentiate. The formation of common primordia, as opposed to discrete petal and stamen primordia, has been described in many legume and non-legume plants. The main differences between pea and M. truncatula floral ontogeny are in carpel and fruit development. We also used these morphological markers as tools to characterize early alterations in the flower development of a male-sterile M. truncatula floral homeotic mutant named mtapetala. This mutant displays a phenotype resembling those of weak class B mutants with homeotic conversions of floral organ whorls 2 and 3 into sepaloid and carpelloid structures, respectively. Ontogeny studies of the mtapetala mutant flowers showed similarities with the effects of previously described loss-of-B-function mutations. Differences between ontogeny of wild type and mtapetala flowers could not be detected during the first stages (1-5) of flower development. In late stage 5, abnormal-shaped petals with acute lobes and trichomes as well as abnormal-shaped stamens were visible in whorls 2 and 3. At stage 6, the morphology of petals began to change, developing enlarged sepaloid structures bearing trichomes and first the antesepalous stamens and then the antepetalous stamens began to differentiate carpelloid anthers from filaments. Third whorl organs presented different degrees of carpelloidy. The present study should provide tools for the characterization and comparative analyses of new Medicago floral homeotic mutants and could be useful in elucidating how floral organ identity functions work in legumes.
... In the young FM, DOT mRNA is initially expressed as a stripe at the base of the first sepal primordium (Figure 3B). At this stage, FIM and UFO are expressed in the center of the FM (Simon et al., 1994; Lee et al., 1997). Slightly later, when all sepal primordia are formed, DOT mRNA has expanded into a ring overlapping the presumptive whorls 1 and 2 (Figures 3C) and subsequently moves outward to the sepal/petal boundary (Fig- ures 3D, 3F, and 3H) to cease by the time the carpels fuse. ...
Angiosperms display a wide variety of inflorescence architectures differing in the positions where flowers or branches arise. The expression of floral meristem identity (FMI) genes determines when and where flowers are formed. In Arabidopsis thaliana, this is regulated via transcription of LEAFY (LFY), which encodes a transcription factor that promotes FMI. We found that this is regulated in petunia (Petunia hybrida) via transcription of a distinct gene, DOUBLE TOP (DOT), a homolog of UNUSUAL FLORAL ORGANS (UFO) from Arabidopsis. Mutation of DOT or its tomato (Solanum lycopersicum) homolog ANANTHA abolishes FMI. Ubiquitous expression of DOT or UFO in petunia causes very early flowering and transforms the inflorescence into a solitary flower and leaves into petals. Ectopic expression of DOT or UFO together with LFY or its homolog ABERRANT LEAF AND FLOWER (ALF) in petunia seedlings activates genes required for identity or outgrowth of organ primordia. DOT interacts physically with ALF, suggesting that it activates ALF by a posttranslational mechanism. Our findings suggest a wider role than previously thought for DOT and UFO in the patterning of flowers and indicate that the different roles of LFY and UFO homologs in the spatiotemporal control of floral identity in distinct species result from their divergent expression patterns.
... UFO and its homologs, e.g. FIMBRIATA (FIM) in Antirrhinum, encode F-box proteins (Simon et al., 1994; Ingram et al., 1995; Samach et al., 1999). F-box proteins act as components of the SKP1-cullin-F-box (SCF) complex, which target substrates for ubiquitin-mediated protein degradation. ...
... Methods for tissue preparation, digoxigenin labelling of RNA probes and in situ hybridization were as described by Ingram et al. (1997). A sense and an antisense RNA probe corresponding to the entire Bo-PIP1b2 RNA (nucleotides ±36 to 1082) were prepared as described by Simon et al. (1994). ...
The pollen grains of angiosperms are usually desiccated at maturity. Following pollination, pollen hydrates on the stigma surface before germination takes place. Rehydration is an essential step for the success of pollination and depends on the movement of water from the stigmatic cells. This water flow has been shown to be biologically regulated, and components of both pollen and stigma surfaces have been demonstrated to play a role in the control of pollen hydration. Regulation of water transport between animal or plant cells involves membrane proteins, designated aquaporins, which possess water-channel activity. Such molecules may be candidates for controlling pollen hydration, and consequently we investigated whether aquaporins are present in the pollen and stigma cells in Brassica oleracea. Here, we report the identification of two new aquaporin genes, Bo-PIP1b1 and Bo-PIP1b2, which are highly homologous to PIP1b from Arabidopsis thaliana. Both Bo-PIP1b1 and Bo-PIP1b2 proteins are active water channels when expressed in Xenopus oocytes. Expression of Bo-PIP1b1 and Bo-PIP1b2 was observed in reproductive organs as well as in vegetative tissues. Interestingly, the use of a Bo-PIP1b2 cDNA probe revealed that PIP1-like transcripts were not present in the pollen grains or in the stigma papillae, but were present in the stigma cell layers underlying the papillar cells. This observation suggests that water flow between the pollen and stigma papillae may be dependent on aquaporins expressed in cells that are not directly in contact with the pollen grain.
... The delay in requirement for DEF function in second whorl petal development, compared to that of stamens in the third whorl, was revealed during analysis of a temperaturesensitive DEF mutant (Zachgo et al., 1995). Furthermore, an outward`movement' of the expression pattern of FIMBRIATA (FIM), an F-box protein required for full DEF expression during¯ower development, has been reported (Ingram et al., 1997;Simon et al., 1994). Section analysis revealed that early FIM mRNA expression precedes DEF expression by one to two developmental stages. ...
A three-dimensional whole-mount technique for detection of mRNA and protein expression patterns of floral regulatory genes in inflorescences from Antirrhinum majus is reported. This technique allows the observation of complex expression patterns in situ in developing flowers at different developmental stages initiated sequentially on the same inflorescence and labelled under the same conditions. Thereby, reconstruction from serial two-dimensional sections can be circumvented. The technique was used to study early changes in the expression of DEFICIENS (DEF), a class B floral homeotic transcription factor. Whole-mount analysis revealed that the order of appearance of DEF mRNA and protein expression in the floral primordium is opposite to the order of initiation of organ primordia. As a consequence, stamen primordia express the DEF gene prior to their initiation in whorl three, while petal primordia in the second whorl are morphologically distinct structures when second whorl DEF expression becomes established. This interesting feature was not readily detectable by previous analysis of serial sections. The particular usefulness of in situ analyses in whole mounts is further demonstrated in floral mutants with variable phenotypes and unpredictable sites of aberrant organ development.
Keywords: whole mount, in situ hybridization, immunolocalization, Antirrhinum majus, flower development.
... Although the UFO gene has been cloned ( Ingram et al., 1995;Lee et al., 1997), unlike LFY there have been few clues as to its molecular mode of action. The deduced aminoacid sequence of UFO has high similarity only to FIMBRIATA (FIM; Simon et al., 1994), the product of an orthologous gene from Antirrhinum majus. Both UFO and FIM proteins contain a newly characterized motif named the F-box ( Bai et al., 1996;Ingram et al., 1997;Samach et al., 1996), a domain that has been shown to mediate interaction with the SKP1 gene product in yeast ( Bai et al., 1996;Connelly and Hieter, 1996). ...
Genetic and molecular studies have suggested that the UNUSUAL FLORAL ORGANS (UFO) gene, from Arabidopsis thaliana, is expressed in all shoot apical meristems, and is involved in the regulation of a complex set of developmental events during floral development, including floral meristem and floral organ identity. Results from in situ hybridization using genes expressed early in floral development as probes indicate that UFO controls growth of young floral primordia. Transgenic constructs were used to provide evidence that UFO regulates floral organ identity by activating or maintaining transcription of the class B organ-identity gene APETALA 3, but not PISTILLATA. In an attempt to understand the biochemical mode of action of the UFO gene product, we show here that UFO is an F-box protein that interacts with Arabidopsis SKP1-like proteins, both in the yeast two-hybrid system and in vitro. In yeast and other organisms both F-box proteins and SKP1 homologues are subunits of specific ubiquitin E3 enzyme complexes that target specific proteins for degradation. The protein selected for degradation by the complex is specified by the F-box proteins. It is therefore possible that the role of UFO is to target for degradation specific proteins controlling normal growth patterns in the floral primordia, as well as proteins that negatively regulate APETALA 3 transcription.
... This must therefore involve other genes that link the early acting meristem identity genes and the step involving the activation of organ identity genes. One such intermediate step in Antirrhinum is controlled by the gene FIMBRIATA (FIM) (Simon et al 1994). This gene is activated soon after the activation of the meristem identity genes FLO and SQUA but before the expression of the organ identity genes DEF and PLE. ...
Flower development provides a model system to study mechanisms that govern pattern formation in plants. Most flowers consist
of four organ types that are present in a specific order from the periphery to the centre of the flower. Reviewed here are
studies on flower development in two model species:Arabidopsis thaliana andAntirrhinum majus that focus on the molecular genetic analysis of homeotic mutations affecting pattern formation in the flower. Based on these
studies a model was proposed that explains how three classes of regulatory genes can together control the development of the
correct pattern of organs in the flower. The universality of the basic tenets of the model is apparent from the analysis of
the homologues of theArabidopsis genes from other plant species
... A combination of expression patterns and mutant phenotypes has suggested that certain floral genes may play a role in limiting cell-cell communication in flower development, fimbriata (tim) is expressed in the junction between whorls 1 and 2 and whorls 2 and 3. fire mutants have homeotic conversions and chimeric organs in whorls 2 and 3 (Simon et al. 1994). unusual floral organs (ufo) is considered to be the cognate ortholog of fire in Arabidopsis (Ingram et al. 1995). ...
... In petunia, the LFY ortholog ABERRANT LEAF AND FLOWER (ALF) is required for the establishment of floral meristem identity and interacts physically with the UFO ortholog DOUBLE TOP (DOT) (Souer et al., 2008), suggesting that the genetic and physical interactions are conserved between Arabidopsis and petunia. Genetic interactions between LFY and UFO orthologs have been reported in Antirrhinum majus and pea (Simon et al., 1994;Hofer et al., 1997;Taylor et al., 2001). ...
The temporal and spatial control of meristem identity is a key element in plant development. To better understand the molecular mechanisms that regulate inflorescence and flower architecture, we characterized the rice aberrant panicle organization 2 (apo2) mutant which exhibits small panicles with reduced number of primary branches due to the precocious formation of spikelet meristems. The apo2 mutants also display a shortened plastochron in the vegetative phase, late flowering, aberrant floral organ identities and loss of floral meristem determinacy. Map-based cloning revealed that APO2 is identical to previously reported RFL gene, the rice ortholog of the Arabidopsis LEAFY (LFY) gene. Further analysis indicated that APO2/RFL and APO1, the rice ortholog of Arabidopsis UNUSUAL FLORAL ORGANS, act cooperatively to control inflorescence and flower development. The present study revealed functional differences between APO2/RFL and LFY. In particular, APO2/RFL and LFY act oppositely on inflorescence development. Therefore, the genetic mechanisms for controlling inflorescence architecture have evolutionarily diverged between rice (monocots) and Arabidopsis (eudicots).
... Genetic stocks and plant material Antirrhinum plants were grown in the greenhouse at 18-25°C with additional light during winter. The wild-type lines JI98 (the progenitor of line 165E), flo-640 (Carpenter et al., 1995) andflo-662 (McSteen et al., 1998;Simon et al., 1994) were kindly provided by Rosemary Carpenter (John Innes Centre, Norwich, UK). The wildtype line Sippe 50 and the mutants, def-gli , inco-pannosa and inco-deformis (referred to as def, inco-1 and inco-2, respectively) were obtained from the collection at the IPK, Gatersleben (Accession numbers MAM88, MAM162 and MAM161, respectively). ...
INCOMPOSITA (INCO) is a MADS-box transcription factor and member of the functionally diverse StMADS11 clade of the MADS-box family. The most conspicuous feature of inco mutant flowers are prophylls initiated prior to first whorl sepals at lateral positions of the flower primordium. The developing prophylls physically interfere with subsequent floral organ development that results in aberrant floral architecture. INCO, which is controlled by SQUAMOSA, prevents prophyll formation in the wild type, a role that is novel among MADS-box proteins, and we discuss evolutionary implications of this function. Overexpression of INCO or SVP, a structurally related Arabidopsis MADS-box gene involved in the negative control of Arabidopsis flowering time, conditions delayed flowering in transgenic plants, suggesting that SVP and INCO have functions in common. Enhanced flowering of squamosa mutants in the inco mutant background corroborates this potential role of INCO as a floral repressor in Antirrhinum. One further, hitherto hidden, role of INCO is the positive control of Antirrhinum floral meristem identity. This is revealed by genetic interactions between inco and mutants of FLORICAULA, a gene that controls the inflorescence to floral transition, together with SQUAMOSA. The complex regulatory and combinatorial relations between INCO, FLORICAULA and SQUAMOSA are summarised in a model that integrates observations from molecular studies as well as analyses of expression patterns and genetic interactions.
The formation of inflorescences and flowers is essential for the successful reproduction of angiosperms. In the past few decades, genetic studies have identified the LEAFY transcription factor and the UNUSUAL FLORAL ORGANS (UFO) F‐box protein as two major regulators of flower development in a broad range of angiosperm species. Recent research has revealed that UFO acts as a transcriptional cofactor, redirecting the LEAFY floral regulator to novel cis‐elements. In this review, we summarize the various roles of UFO across species, analyze past results in light of new discoveries and highlight the key questions that remain to be solved.
The sections in this article are
Introduction
Shoot Development
Organogenesis of the Leaf
Organogenesis of the Root
Conclusions
Acknowledgements
The sections in this article are
Introduction
Flower Primordium Outgrowth
Regulating Flower Meristem Size
Early Control of Organogenesis in the Flower
Generating Organ Boundaries
Floral Organ Size
Flower Shape and Symmetry
Dorsoventral Symmetry
Outlook: to Boldly go Where no One has Gone Before …
Acknowledgements
The sections in this article are
Introduction
What is a Flower?
Phylogenetic and Paleontological Context
Evolutionary Novelties of the Flower
Ordering the Key Steps in Floral Evolution
Developmental Genetic Background
Models for the Origin of Bisexuality
Apical Megasporophyll Production on a Microsporangiate Axis?
The Compression of the Floral Axis
The Evolution of the Perianth
The Origin of a Dimorphic Perianth
Conclusion
Acknowledgements
The many deep-seated changes that characterize the vegetative growth of plants described in previous chapters are terminated by the production of flowers. This entails the transformation of the shoot apical meristem into a single flower or into a cluster of flowers known as the inflorescence. What prompts, at the physiological and molecular levels, the making of flowers is still a central and unanswered question in plant developmental biology. Despite more than a century of experimentation, we are still in the dark about the intracellular agents and gene products that start the cells of the shoot apical meristem along the developmental pathway of a floral primordium. However, these experiments have greatly advanced our understanding of the links between identifiable types of extracellular factors and early changes in the shoot apical meristem that lead to the formation of flowers. Morphological and anatomical studies aimed at following in the cells of the shoot apical meristem those known features that identify them as related to flower initiation have been an integral part of the early investigations on flowering.
In animals most organogenesis takes place during embryogenesis and further development consists mostly of growth and maturation of the embryonic organs. This is in contrast to plant embryo development, where only a very basic structure is set up [1]. Mature plant embryos are composed of a single shoot-root axis. The primary shoot consists of a small stem, and, depending on the species, one or several cotyledons (specialized embryonic leaf-like structures) and a limited number of leaves can be formed. Almost all the organs found in an adult plant are formed after germination by small groups of dividing cells called meristems. This implies that multiple ramifications that characterize the architecture of many adult plants take place during postembryonic plant development. The continuous mode of development in plants makes them very flexible, as they can adapt their growth and architecture (development) to environmental conditions. This adaptability counterbalances their inability to move and to flee local unpropitious conditions.
Flowers are the beautiful and complex reproductive structures of the angiosperms, one of the most diverse and successful groups of living organisms. The underlying thesis of this book is that to understand fully plant development (and why flowers differ in shape, structure, and colour), it is necessary to understand why it is advantageous for them to look like they do. Conversely, in order to fully understand plant ecology, it is necessary to appreciate how floral structures have adapted and evolved. Uniquely, this book addresses flowers and flowering from both a molecular genetic perspective (considering flower induction, development, and self-incompatibility) and an ecological perspective (looking at the selective pressures placed on plants by pollinators, and the consequences for animal-plant co-evolution). This book first considers the evolution of flowers and the history of research into their development. This is followed by a detailed description of the processes which lead to flower production in model plants. The book then examines how flowers differ in shape, structure, and colour, and how these differences are generated. Finally, it assesses the role of these various aspects of floral biology in attracting pollinators and ensuring successful reproduction. In so doing, it provides the first truly integrated study of the topic - one that discusses both the how and why of flowering plant reproductive biology.
Seabuckthorn is an economically important dioecious plant in which mechanism of sex determination is unknown. The study was conducted to identify seabuckthorn homologous genes involved in floral development which may have role in sex determination. Forty four putative Genes involved in sex determination (GISD) reported in model plants were shortlisted from literature survey, and twenty nine seabuckthorn homologous sequences were identified from available seabuckthorn genomic resources. Of these, 21 genes were found to differentially express in either male or female flower bud stages. HrCRY2 was significantly expressed in female flower buds only while HrCO had significant expression in male flowers only. Among the three male and female floral development stages (FDS), male stage II had significant expression of most of the GISD. Information on these sex-specific expressed genes will help in elucidating sex determination mechanism in seabuckthorn.
Plants have many differences, like protandry, protogyny, etc. However, amidst these differences, all angiosperms have a common mechanism of flowering, i.e. concentric pattern of flowering (sepal, petal, stamen and carpel). The mechanism or the genes thorough which plants maintain boundaries between the four, sepal-petal-stamen-carpel, are also given due importance. Once a plant attains competence, flowers may be produced through the reorganization of SAM directly to floral meristem, or through the inflorescence or co-inflorescence meristem, in response of exogenous and endogenous signals. Initiation, determination and differentiation are classed into four stages and this is the region which is studied here in detail. Some organ specificity genes, like MALE STERILITY (MS) and BICAUDAL (BIC), move us towards a better understanding of the mechanism like male sterility and self-incompatibility in plants. Models like ABC, biophysical, MCDK, etc., help in explaining the mechanism of flowering on a molecular basis. However, in general, the path towards flowering is laid when the floral repressor genes are down regulated. Recently identified miRNAs in plants authenticate them and also give out the mechanism by which they down regulate. It has also been found out that MADS-box gene family and CArG-box genes (where those MADS domain protein binds) are highly conserved. Their role in flower development is also touched upon.
A complete understanding of the genetic control of flower development requires a comparative approach, involving species from across the angiosperm lineage. Using the accessible model plant Arabidopsis thaliana many of the genetic pathways that control development of the reproductive growth phase have been delineated. Research in other species has added to this knowledge base, revealing that, despite the myriad of floral forms found in nature, the genetic blueprint of flower development is largely conserved. However, these same studies have also highlighted differences in the way flowering is controlled in evolutionarily diverse species. Here, we review flower development in the eudicot asterid lineage, a group of plants that diverged from the rosid family, which includes Arabidopsis, 120 million years ago. Work on model species such as Antirrhinum majus, Petunia hybrida, and Gerbera hybrida has prompted a reexamination of textbook models of flower development; revealed novel mechanisms controlling floral gene expression; provided a means to trace evolution of key regulatory genes; and stimulated discussion about genetic redundancy and the fate of duplicated genes.
Transposon mutagenesis has provided one of the first and most important routes to gene identification and characterization. In the 17 years since the bz1 gene was first tagged with Activator (Ac), more than 60 genes involved in plant development have been cloned using elements such as Supressor-mutator (Spm) and Mutator (Mu) from maize and Tag1 from Arabidopsis. The advantages of transposon-induced alleles in the study of developmental processes go beyond cloning to include sector analysis, generation of new alleles, and conditional expression based on suppression. The laborious technique of directed tagging that led to many of these successes is now being supplanted by systematic projects to produce large collections of transposon insertions that are precharacterized using PCR-based methods and publicly accessible for both forward and reverse genetics. Of the tens of thousands of new genes postulated to exist in Arabidopsis and other species, most are turning out to have no obvious phenotypic effect. The challenge for functional genomics is now to elucidate the apparently subtle actions of genes at a rate commensurate with their discovery. Referee: Dr. Paul Chomet, Monsanto Co., 62 Maritime Dr., Mystic, CT 06355
Micro-Electro-Mechanical Systems (MEMS) is a big name for tiny devices that will soon make big changes in everyday life and the workplace. These and other types of Microsystems range in size from a few millimeters to a few microns, much smaller than a human hair. These Microsystems have the capability to enable new ways to solve problems in commercial applications ranging from automotive, aerospace, telecommunications, manufacturing equipment, medical diagnostics to robotics, and in national security applications such as nuclear weapons safety and security, battlefield intelligence, and protection against chemical and biological weapons. This broad range of applications of Microsystems reflects the broad capabilities of future Microsystems to provide the ability to sense, think, act, and communicate, all in a single integrated package. Microsystems have been called the next silicon revolution, but like many revolutions, they incorporate more elements than their predecessors. Microsystems do include MEMS components fabricated from polycrystalline silicon processed using techniques similar to those used in the manufacture of integrated electrical circuits. They also include optoelectronic components made from gallium arsenide and other semiconducting compounds from the III-V groups of the periodic table. Microsystems components are also being made from pure metals and metal alloys using the LIGA process, which utilizes lithography, etching, and casting at the micron scale. Generically, Microsystems are micron scale, integrated systems that have the potential to combine the ability to sense light, heat, pressure, acceleration, vibration, and chemicals with the ability to process the collected data using CMOS circuitry, execute an electrical, mechanical, or photonic response, and communicate either optically or with microwaves.
In the last decade, the study of mutants defective in floral development has contributed significantly to our understanding of floral evocation and morphogenesis. Genes in Arabidopsis thaliana and Antirrhinum majus that play key roles in (i) the transition from the vegetative to reproductive phase, (ii) the activation of floral development in specific shoots, and (iii) the unique arrangement of floral organs have been identified genetically and in many cases cloned. Many of the genes appear to encode transcription factors that act to select specific developmental programs of division and differentiation for groups of primordial cells. Other genes may be involved in detecting environmental conditions and transducing the signal to the developing meristems. Key questions remaining include how the regulatory proteins are produced in specific temporal and spatial patterns, interact with each other and initiate specific morphological programs. Although current research on floral morphogenesis has been limited to only a few species there is growing evidence that the basic processes are common to all flowering plants.Thus the information and tools currently being generated should be useful for studying a wide variety of flowering species. It seems reasonable to predict that within the next decade, we should have a fairly complete understanding of the basic mechanisms underlying floral morphogenesis and its evolution among the angiosperms. Key words: Arabidopsis thaliana, floral morphogenesis, molecular genetics.
IntroductionFlower primordium outgrowthRegulating flower meristem sizeEarly control of organogenesis in the flowerGenerating organ boundariesFloral organ sizeFlower shape and symmetryDorsoventral symmetryOutlook: to boldly go where no one has gone before…
Abstract Recent developments in plant molecular genetics have revealed a direct relationship between gene structure and its function in plant structure, development, response to stimuli, and metabolic pathways. The rapid progress in this field depends mainly on intensive efforts at isolation of a series of mutants using selected model plants, such as Arabidopsis thaliana, snapdragon (Antirrhinum majus), rice (Oryza sativa), and maize (Zea mays). Arabidopsis thaliana is a small crucifer, called “botanical Drosophila” because it has some remarkable features: small genome size, short life-cycle, small size, and ease of propagation. More than 200 research groups in many countries have isolated mutants defective in the development of embryo, shoot, flower, and root, as well as in response reactions to physical and chemical stimuli such as gravity, light, nutrients, and phytohormones. Some of the Arabidopsis mutants show aberrant structures that may be identified as non-crucifer characters. For example, a flower mutant, SAKURA. often bears five petals; another mutant, LEUNIG, has no ovarian septum. TERMINAL FLOWER mutant changes indeterminate inflorescence to determinate. Some root mutants do not form lateral roots. Several mutants form one cotyledon. These morphology mutants will provide hints for considering critical genetic changes that may have caused the past evolutionary events.
Genes isolated from the morphology mutants are classified into three groups: coding transcription factors, kinases, and other protein motifs. Although the detailed molecular mechanism in the mutants is not known, hierarchical regulatory networks of the genes are being investigated. We will provide examples of the genetic networks at work in organ development, and discuss possible genetic changes that result in drastic morphological variation.
This chapter discusses plant transposable elements. Eukaryotic transposable elements are divided into two groups, according to their transposition mechanism and mode of propagation—retrotransposons transpose move by RNA intermediate, whereas the DNA transposable elements move by excision and reintegration. Both element classes are subdivided into distinct element superfamilies, which share structural and sequence similarities at the DNA and protein levels. The list of mobile elements is constantly growing. It has become apparent that mobile elements occur ubiquitously in prokaryotes and eukaryotes alike. However, it seems that certain classes or superfamilies of mobile elements are lacking in some genomes. Retrotransposons occur ubiquitously in the lower eukaryotes, in fungi, and in the plant and animal kingdoms, but are apparently not represented in prokaryotes. All retroelements propagate through an RNA intermediate, and consequently they depend on the presence of a reverse transcriptase activity. All together, retroelements comprise a substantial fraction of eukaryotic genomic DNA. Based on the structures of their DNA copies, three classes of retroelements are distinguished: long terminal repeat (LTR) retrotransposons; non-LTR retrotransposons or long interspersed element (LINE)-like retrotransposons; and short interspersed element (SINE)-like Retrogenes. The LTR retrotransposons strikingly resemble retroviral proviruses, except that they are lacking the env genes, which encode the envelope proteins of retroviruses. Thus, the distinctive difference between retrotransposons and retroviruses is the absence of infectious extracellular particles of the former.
In Antirrhinum, development of petals and stamens in the second and third whorl is controlled by the homeotic B-function genes DEFICIENS and GLOBOSA, which belong to the MADS-box family of transcription factors. The encoded proteins form heterodimers and control petal and stamen organogenesis. A first step for a better understanding of the molecular control mechanisms was the isolation of ROSINA (RSI), a putative regulator of DEF, in a yeast one-hybrid screen using a DEF promoter fragment. RSI is a member of the b-ZIP family of transcription. Since such factors often need partners for exerting their regulatory function as heterodimers or multidimers, RSI was used as ‘bait’ in a yeast two-hybrid system to search for such potential partners. The most interesting candidate, named AmGAI-like, showed strong similarity to members of the GRAS family (GAI and RGA) of Arabidopsis. Expression of AmGAI-like revealed great similarity to the expression patterns of GAI and RGA of Arabidopsis. The protein–protein interaction between AmGAI-like and RSI was confirmed also biochemically by GST-pull down experiments.
Research with the snapdragon, Antirrhinum majus, has a long history with many highlights, making this species a significant model system for comparative genetic, molecular, and ecological studies. In this chapter, we focus interest on flower development, in particular the genetic control of floral organ identity, floral asymmetry, and petal cell‐type specification, where results obtained with Antirrhinum provided the first insights into the underlying molecular mechanisms, leading to advances in the field. In addition to reviewing past and recent scientific achievements, we propose simple models to aid the understanding of complex genetic observations and also to resolve the inconsistencies and contradictions, which inhibit the general application of existing models to other species. In particular, we propose a revision of the ABC‐model to reflect better the experimental results obtained in a variety of model species including Arabidopsis. For this we revive the (A)BC‐scheme in which sepal identity follows from floral meristem identity and represents the ground state for floral organs. In addition to controlling sepal identity, the complex (A)‐function performs several roles that are necessary for the initiation, maintenance, spatial restriction, and functionality of the B‐ and C‐organ identity functions. By providing information on current resources for molecular research and newly arising research areas we intend to encourage the scientific community to utilize Antirrhinum for research in the future.
Recently, several genes have been cloned that affect plant architecture: CLAVATA1, which controls the balance between maintenance and organogenesis in the meristem; CUC2, which separates organ primordia in the meristem; and teosinte branched 1 and cycloidea, which use growth suppression to cause morphological change.
Phylogenetic analysis of DNA sequences of the chloroplast genes rcbL and ndhf revealed a highly supported clade composed of the families Plantaginaceae, Callitrichaceae, and Hippuridaceae in close association with the model organism Antirrhinum majus and other members of family Scrophulariaceae. Plantago has miniature actinomorphic wind-pollinated flowers that have evolved from zygomorphic animal-pollinated precursors. The aquatic Hippuridaceae have reduced windpollinated flowers with one reproductive organ per whorl, and three, rather than four, whorls. In monoecious aquatic Callitrichaceae, further reduction has occurred such that there is only one whorl per flower containing a single stamen or carpel. Optimization of character states showed that these families descended from an ancestor similar to Antirrhinum majus. Recent studies of plant developmental genetics have focused on distantly related species. Differences in the molecular mechanisms controlling floral development between model organisms are difficult to interpret due to phylogenetic distance. In order to understand evolutionary changes in floral morphology in terms of their underlying genetic processes, closely related species exhibiting morphological Variation should be examined. Studies of genes that regulate morphogenesis in the clade described here could aid in the elucidation of a general model tot such fundamental issues as how changes in floral symmetry, organ number, and whorl number are achieved, as well as providing insight on the evolution of dicliny and associated changes in pollination syndrome.
We describe allelic series for three loci, mutations in which result in homeotic conversions in two adjacent whorls in the Arabidopsis thaliana flower. Both the structure of the mature flower and its development from the initial primordium are described by scanning electron microscopy. New mutations at the APETALA2 locus, ap2-2, ap2-8 and ap2-9, cause homeotic conversions in the outer two whorls: sepals to carpels (or leaves) and petals to stamens. Two new mutations of PISTILLATA, pi-2 and pi-3, cause second and third whorl organs to differentiate incorrectly. Homeotic conversions are petals to sepals and stamens to carpels, a pattern similar to that previously described for the apetala3-1 mutation. The AGAMOUS mutations, ag-2 and ag-3, affect the third and fourth whorls and cause petals to develop instead of stamens and another flower to arise in place of the gynoecium. In addition to homeotic changes, mutations at the APETALA2, APETALA3 and PISTILLATA loci may lead to reduced numbers of organs, or even their absence, in specific whorls. The bud and flower phenotypes of doubly and triply mutant strains, constructed with these and previously described alleles, are also described. Based on these results, a model is proposed that suggests that the products of these homeotic genes are each active in fields occupying two adjacent whorls, AP2 in the two outer whorls, PI and AP3 in whorls two and three, and AG in the two inner whorls. In combination, therefore, the gene products in these three concentric, overlapping fields specify the four types of organs in the wild-type flower. Further, the phenotypes of multiple mutant lines indicate that the wild-type products of the AGAMOUS and APETALA2 genes interact antagonistically. AP2 seems to keep the AG gene inactive in the two outer whorls while the converse is likely in the two inner whorls. This field model successfully predicts the phenotypes of all the singly, doubly and triply mutant flowers described.
The analysis of mutations affecting flower structure has led to the identification of some of the genes that direct flower development. Cloning of these genes has allowed the formulation of molecular models of how floral meristem and organ identity may be specified, and has shown that the distantly related flowering plants Arabidopsis thaliana and Antirrhinum majus use homologous mechanisms in floral pattern formation.
Deficiens (defA+) is a homeotic gene involved in the genetic control of Antirrhinum majus flower development. Mutation of this gene (defA-1) causes homeotic transformation of petals into sepals and of stamina into carpels in flowers displaying the 'globifera' phenotype, as shown by cross sections and scanning electronmicroscopy of developing flowers. A cDNA derived from the wild type defA+ gene has been cloned by differential screening of a subtracted 'flower specific' cDNA library. The identity of this cDNA with the defA+ gene product has been confirmed by utilizing the somatic and germinal instability of defA-1 mutants. According to Northern blot analyses the defA+ gene is expressed in flowers but not in leaves, and its expression is nearly constant during all stages of flower development. The 1.1 kb long mRNA has a 681 bp long open reading frame that can code for a putative protein of 227 amino acids (mol. wt 26.2 kd). At its N-terminus the DEF A protein reveals homology to a conserved domain of the regulatory proteins SRF (activating c-fos) in mammals and GRM/PRTF (regulating mating type) in yeast. We discuss the structure and the possible function of the DEF A protein in the control of floral organogenesis.
We describe a novel mutant of Arabidopsis, Flo10, which is the result of a recessive allele, flo10, in the nuclear gene FLO10. The first three organ whorls (sepals, petals, and stamens) of Flo10 flowers are normal, but the fourth, gynoecial whorl is replaced by two to eight stamens or stamen-carpel intermediate organs. Studies of ontogeny suggest that the position of the first six of these fourth-whorl organs often resembles that of the wild-type third-whorl organs. To determine the interaction of the FLO10 gene with the floral organ homeotic genes APETALA2 (AP2), PISTILLATA (PI), AP3, and AGAMOUS (AG), we generated lines homozygous for flo10 and heterozygous or homozygous for a recessive allele of the homeotic genes. On the basis of our data, we suggest that FLO10 functions to prevent the expression of the AP3/PI developmental pathway in the gynoecial (fourth) whorl.
We have determined the structure of the floral homeotic deficiens (defA) gene whose mutants display sepaloid petals and carpelloid stamens, and have analysed its spatial and temporal expression pattern. In addition, several mutant alleles (morphoalleles) were studied. The results of these analyses define three functional domains of the DEF A protein and identify in the deficiens promoter a possible cis-acting binding site for a transcription factor which specifically upregulates expression of deficiens in petals and stamens. In vitro DNA binding studies show that DEF A binds to specific DNA motifs as a heterodimer, together with the protein product of the floral homeotic globosa gene, thus demonstrating that the protein encoded by deficiens is a DNA binding protein. Furthermore, Northern analysis of a temperature sensitive allele at permissive and non-permissive temperatures provides evidence for autoregulation of the persistent expression of deficiens throughout flower development. A possible mechanism of autoregulation is discussed.
We used a yeast genetic screen to isolate cDNAs that encode a protein, SRF accessory protein-1 (SAP-1), that is recruited to the c-fos serum response element (SRE) as part of a ternary complex that includes serum response factor (SRF). SAP-1 requires DNA-bound SRF for ternary complex formation and makes extensive DNA contacts to the 5′ side of SRF, but does not bind DNA autonomously. Ternary complex formation by SAP-1 requires only the DNA-binding domain of SRF, which can be replaced by that of the related yeast protein MCM1. We isolated cDNAs encoding two forms of SAP-1 protein, SAP-1a and SAP-1b, which differ at their C termini. Both SAP-1 proteins contain three regions of striking homology with the elk-1 protein, including an N-terminal ets domain. Ternary complex formation by SAP-1 requires both the ets domain and a second conserved region 50 amino acids to its C-terminal side. SAP-1 has similar DNA binding properties to the previously characterized HeLa cell protein .
We describe a locus, SUPERMAN, mutations in which result in extra stamens developing at the expense of the central carpels in the Arabidopsis thaliana flower. The development of superman flowers, from initial primordium to mature flower, is described by scanning electron microscopy. The development of doubly and triply mutant strains, constructed with superman alleles and previously identified homeotic mutations that cause alterations in floral organ identity, is also described. Essentially additive phenotypes are observed in superman agamous and superman apetala2 double mutants. The epistatic relationships observed between either apetala3 or pistillata and superman alleles suggest that the SUPERMAN gene product could be a regulator of these floral homeotic genes. To test this, the expression patterns of AGAMOUS and APETALA3 were examined in superman flowers. In wild-type flowers, APETALA3 expression is restricted to the second and third whorls where it is required for the specification of petals and stamens. In contrast, in superman flowers, APETALA3 expression expands to include most of the cells that would normally constitute the fourth whorl. This ectopic APETALA3 expression is proposed to be one of the causes of the development of the extra stamens in superman flowers. The spatial pattern of AGAMOUS expression remains unaltered in superman flowers as compared to wild-type flowers. Taken together these data indicate that one of the functions of the wild-type SUPERMAN gene product is to negatively regulate APETALA3 in the fourth whorl of the flower. In addition, superman mutants exhibit a loss of determinacy of the floral meristem, an effect that appears to be mediated by the APETALA3 and PISTILLATA gene products.
We have determined the structure of the floral homeotic deficiens (defA) gene whose mutants display sepaloid petals and carpelloid stamens, and have analysed its spatial and temporal expression pattern. In addition, several mutant alleles (morphoalleles) were studied. The results of these analyses define three functional domains of the DEF A protein and identify in the deficiens promoter a possible cis-acting binding site for a transcription factor which specifically upregulates expression of deficiens in petals and stamens. In vitro DNA binding studies show that DEF A binds to specific DNA motifs as a heterodimer, together with the protein product of the floral homeotic globosa gene, thus demonstrating that the protein encoded by deficiens is a DNA binding protein. Furthermore, Northern analysis of a temperature sensitive allele at permissive and non-permissive temperatures provides evidence for autoregulation of the persistent expression of deficiens throughout flower development. A possible mechanism of autoregulation is discussed.
Anomalous flowering of the Antirrhinum majus mutant squamosa (squa) is characterized by excessive formation of bracts and the production of relatively few and often malformed or incomplete flowers. To study the function of squamosa in the commitment of an inflorescence lateral meristem to floral development, the gene was cloned and its genomic structure, a well as that of four mutant alleles, was determined. SQUA is a member of a family of transcription factors which contain the MADS-box, a conserved DNA binding domain. In addition, we analysed the temporal and spatial expression pattern of the squa gene. Low transcriptional activity of squa is detectable in bracts and in the leaves immediately below the inflorescence. High squa transcript levels are seen in the inflorescence lateral meristems as soon as they are formed in the axils of bracts. Squa transcriptional activity persists through later stages of floral morphogenesis, with the exception of stamen differentiation. Although necessary for shaping a normal racemose inflorescence, the squa function is not absolutely essential for flower development. We discuss the function of the gene during flowering, its likely functional redundancy and its possible interaction with other genes participating in the genetic control of flower formation in Antirrhinum.
Plants carrying the floricaula (flo) mutation cannot make the transition from inflorescence to floral meristems and have indeterminate shoots in place of flowers. The flo-613 allele carries a Tam3 transposon insertion, which allowed the isolation of the flo locus. The flo gene encodes a putative protein (FLO) containing a proline-rich N-terminus and a highly acidic region. In situ hybridization shows that the flo gene is transiently expressed in the very early stages of flower development. The earliest expression seen is in bract primordia, followed by sepal, petal, and carpel primordia, but no expression is detected in stamen primordia. This pattern of expression has implications for how flo affects phyllotaxis, organ identity, and determinacy. We propose that flo interacts in a sequential manner with other homeotic genes affecting floral organ identity.
In the yeast Saccharomyces cerevisiae cell cycle-regulated SW15 transcription is essential for ensuring that mother and not daughter cells switch mating type. We have identified a 55-bp promoter sequence that appears to be responsible for restricting transcription to the late S, G2, and M phases of the cell cycle. Two proteins, MCM1, a transcription factor described previously, and SFF (SWI five factor, a newly identified factor) bind this sequence in vitro. MCM1 binds the DNA tightly on its own, but SFF will only bind as part of a ternary complex with MCM1. We observe a strong correlation between the ability of mutated SWI5 promoter sequences to form a ternary MCM1-SFF-containing complex in vitro and to activate transcription in vivo, which suggests that efficient transcription requires that both proteins bind DNA. Through its interactions with cell type-specific coactivators and corepressors, MCM1 controls cell type-specific expression of pheromone and receptor genes. By analogy, we propose that SFF enables MCM1 to function as a part of a cell cycle-regulated transcription complex.
To isolate and study genes controlling floral development, we have carried out a large-scale transposon-mutagenesis experiment in Antirrhinum majus. Ten independent floral homeotic mutations were obtained that could be divided into three classes, depending on whether they affect (1) the identity of organs within the same whorl; (2) the identity and sometimes also the number of whorls; and (3) the fate of the axillary meristem that normally gives rise to the flower. The classes of floral phenotypes suggest a model for the genetic control of primordium fate in which class 2 genes are proposed to act in overlapping pairs of adjacent whorls so that their combinations at different positions along the radius of the flower can specify the fate and number of whorls. These could interact with class 1 genes, which vary in their action along the vertical axis of the flower to generate bilateral symmetry. Both of these classes may be ultimately regulated by class 3 genes required for flower initiation. The similarity between some of the homeotic phenotypes with those of other species suggests that the mechanisms controlling whorl identity and number have been highly conserved in plant evolution. Many of the mutations obtained show somatic and germinal instability characteristic of transposon insertions, allowing the cell-autonomy of floral homeotic genes to be tested for the first time. In addition, we show that the deficiens (def) gene (class 2) acts throughout organ development, but its action may be different at various developmental stages, accounting for the intermediate phenotypes conferred by certain def alleles. Expression of def early in development is not necessary for its later expression, indicating that other genes act throughout the development of specific organs to maintain def expression. Direct evidence that the mutations obtained were caused by transposons came from molecular analysis of leaf or flower pigmentation mutants, indicating that isolation of the homeotic genes should now be possible.
Recessive mutations at the plena (ple) locus result in a homeotic conversion of sex organs to sterile perianth organs in flowers of Antirrhinum majus. A complementary phenotype, in which sex organs replace sterile organs, is conferred by semidominant ovulata mutations. The ple locus was identified and isolated using a homologous gene, agamous from Arabidopsis, as a probe. The expression of ple is normally restricted to the inner two whorls of the flower, where sex organs develop. However, in ovulata mutants, ple is expressed ectopically in the outer two whorls of the flower and in vegetative organs. These mutants correspond to gain-of-function alleles of ple, suggesting that ple is sufficient for promoting sex organ development within the context of the flower. The plena and ovulata phenotypes result from opposite orientations of the transposon Tam3 inserted in the large intron of ple.
We used a yeast genetic screen to isolate cDNAs that encode a protein, SRF accessory protein-1 (SAP-1), that is recruited to the c-fos serum response element (SRE) as part of a ternary complex that includes serum response factor (SRF). SAP-1 requires DNA-bound SRF for ternary complex formation and makes extensive DNA contacts to the 5' side of SRF, but does not bind DNA autonomously. Ternary complex formation by SAP-1 requires only the DNA-binding domain of SRF, which can be replaced by that of the related yeast protein MCM1. We isolated cDNAs encoding two forms of SAP-1 protein, SAP-1a and SAP-1b, which differ at their C termini. Both SAP-1 proteins contain three regions of striking homology with the elk-1 protein, including an N-terminal ets domain. Ternary complex formation by SAP-1 requires both the ets domain and a second conserved region 50 amino acids to its C-terminal side. SAP-1 has similar DNA binding properties to the previously characterized HeLa cell protein p62/TCF.
One of the unifying theories of plant biology is that the variety of plant forms are simply different modifications of a common growth plan. Different permutations of a few key features of plant growth can generate a bewildering array of seemingly distinct forms. There is perhaps no better illustration of this than the comparison of a flower and a shoot. The idea that these two apparently different structures might be fundamentally equivalent goes back to Goethe's treatise on metamorphosis, published in 1790. He concluded, "Flowers which develop from lateral buds are to be regarded as entire plants, which are set in the mother plant, as the mother plant is set in the earth" (Goethe, 1790). In equating flowers and shoots, four key assertions need to be made. First, the different parts of the flower (sepals, petals, stamens, and carpels) are equivalent to the leaves of a shoot. Second, the organs of both shoot and flower are separated by internodes, but in the case of the flower these are so short as to be barely visible. Third, the organs of shoot and flower usually have a distinct phyllotaxy, or arrangement around the central axis. Finally, the indeterminate growth that so characterizes a shoot is suppressed in the case of a flower, both apically, because it eventually stops producing organs around the central axis, and laterally, because branches do not normally arise in the axils of floral organs. The comparison of flower and shoot therefore highlights four key variables: organ identity, internode length, phyllotaxy, and determinacy. The numerous forms and habits of plants simply reflect different variations and permutations of these four fundamental aspects of growth. What is their developmental basis?
Homeotic mutants have been useful for the study of animal development. Such mutants are also known in plants. The isolation and molecular analysis of several homeotic genes in Antirrhinum majus provide insights into the underlying molecular regulatory mechanisms of flower development. A model is presented of how the characteristic sequential pattern of developing organs, comprising the flower, is established in the process of morphogenesis.
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