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... Leaves are determined as such when the primordia are extremely small (Sachs, 1969). indeed at the time of initiation (Poethig & Sussex, 1985) or even up to half a plastochron before tbis (Snow & Snow, 1933). ...
... Sepals, petals, stamens and carpels also seem to become determined at, or very soon after initiation (Hicks, 1972, J973, J9S2;Hicks & BeJJ, 1975). However the parts of a leaf and its constituent cells may be determined gradually as the organ develops (Sachs, 1969;Feldman & Cutter, 1970). As it develops the leaf behaves as a collection of cell clones (Poethig, 1984). ...
The processes involved in the formation of primordia on the shoor apex are those controlling (1) growth rate, (2) division plane. (3) surface microstructure, and (4) extensibility of the surface. Changes in growth rate and division planes may accompany primordium formation but are considered as probably not in themselves being causal. Changes in surface microstructure may be necessary to delimit the position and area occupied by an incipient primordium. However, attention is directed to changes in surface extensibility as perhaps being the overriding factor in primordium formation. Nevertheless, the position and form of the primordia will also depend on growth rate, division plane, and surface microstructurc being permissive. The relative importance of these four sets of processes may differ from species to species and from one stage of development to another. Chemical and metabolic changes within the apex may first be necessary to determine whether the surface can extend sufficiently for any primordia to form at all, but their positions and time of initiation may depend more on the other factors. The surface microstructure may become more important when patterning is detailed and precise as it is in the developing flower, whereas a less precise mechanism dependent on localized induction of synthesis of a morphogen (auxin?) may provide sufficient information to determine the general position and liming of primordium initiation in vegetative apices. In determining the pattern of primordia on the apex, primordial area at initiation is important and reasons for believing that auxin may be involved in determining this are summarised. The different developmental pathways of primordia seem to diverge from the moment of initiation. Developmental fate of primordia is determined by the hamcotic genes which may in fact be heterochronic genes. How these regulatory genes control the processes involved in differentiation of different types of primordia is so far unknown.
Contents
Summary
1
I.
Introduction
2
II.
The mechanism of primordium formation: what causes an outgrowth of the apical surface?
2
III.
The positions of successive primordia: what determines their size and localization, so giving rise to pattern?
11
IV.
What determines the developmental pathways of the primordia once initiated?
13
V.
Conclusions
15
VI.
Acknowledgements
16
VII.
References
16
... Although decapitation of cotyledon-stage dicotyledonous seedlings outside plant tissue culture generally ends in seedling death (Adams 1924;Mendel 1938), there are reports of hypocotyledonary bud and shoot regeneration in a few unrelated species following decapitation; e.g., tomato (Solanum lycopersicum) (Adams 1924), flax (Linum usitatissimum) (Adams 1924), custard apple (Annona muricata) (Mendel 1938), and cranberry (Vaccinium oxycoccus) (Bain 1940). Hypogeal seedlings (e.g., Pisum sativum) may produce axillary buds around the cotyledonary node on decapitation (Sachs 1969), a well-known Fabaceae tissue culture response (Amutha et al. 2006). We examined the seedling developmental response of the great majority of dicotyledonous species that cannot survive decapitation. ...
... Normal leaves were only produced by the new SAM. Abnormal regenerated leaves similar to those observed here were noted following cuts separating leaf primordia from the SAM of P. sativum seedlings (Sachs 1969). Some distorted leaves (Fig. 2) resembled the Arabidopsis mutants blade-on-petiole (Norberg et al. 2005) and Asymmetric leaves 1 (Byrne et al. 2000), and Arabidopsis plants expressing the jaw-D leaf phenotype (Palatnik et al. 2003). ...
Regeneration of new shoots in plant tissue culture is often associated with appearance of abnormally shaped leaves. We used
the adventitious shoot regeneration response induced by decapitation (removal of all preformed shoot apical meristems, leaving
a single cotyledon) of greenhouse-grown cotyledon-stage seedlings to test the hypothesis that such abnormal leaf formation
is a normal regeneration progression following wounding and is not conditioned by tissue culture. To understand why shoot
regeneration starts with defective organogenesis, the regeneration response was characterized by morphology and scanning electron
and light microscopy in decapitated cotyledon-stage Cucurbita pepo seedlings. Several leaf primordia were observed to regenerate prior to differentiation of a de novo shoot apical meristem from dividing cells on the wound surface. Early regenerating primordia have a greatly distorted structure
with dramatically altered dorsoventrality. Aberrant leaf morphogenesis in C. pepo gradually disappears as leaves eventually originate from a de novo adventitious shoot apical meristem, recovering normal phyllotaxis. Similarly, following comparable decapitation of seedlings
from a number of families (Chenopodiaceae, Compositae, Convolvulaceae, Cucurbitaceae, Cruciferae, Fabaceae, Malvaceae, Papaveraceae,
and Solanaceae) of several dicotyledonous clades (Ranunculales, Caryophyllales, Asterids, and Rosids), stems are regenerated
bearing abnormal leaves; the normal leaf shape is gradually recovered. Some of the transient leaf developmental defects observed
are similar to responses to mutations in leaf shape or shoot apical meristem function. Many species temporarily express this
leaf development pathway, which is manifest in exceptional circumstances such as during recovery from excision of all preformed
shoot meristems of a seedling.
... The developmental origin of early leaf polarity was proposed by Sussex (1955) to be an instructive signal from the SAM because surgically separated leaf primordia of potato (Solanum tuberosum) produced unifacial radial organs. Sachs (1969), however, showed that wounded flat leaflet primordia of pea (Pisum sativum) can give rise to tendrils and suggested that wounding per se may induce the formation of radial centric organs. A more encompassing model by Hagemann and Gleissberg (1996), effectively a plant version of the prepattern paradigm (Stern and Tokunaga, 1967), suggests that the seemingly radial leaf primordium is intrinsically dorsiventral from the outset. ...
... The maturation in many leaves progresses from the distal tip basally ( Figure 1C) as indicated by the sequential appearance of morphogenetic markers: trichomes (Avery, 1933;Poethig and Sussex, 1985b), provascular strands (Aloni, 1987), enlarged epidermal cells (Hagemann and Gleissberg, 1996), modified cellular morphology, and differentiating guard cells (Bergmann and Sack, 2007). The gradual changes are also apparent in the extent of developmental potential in the developing leaves, determined, for example, by regeneration capacity of the primordium following injury (Sachs, 1969;Sena et al., 2009). These maturation processes are also reflected molecularly in temporally dynamic changes in >50% of the transcriptome of the growing blades (Schmid et al., 2005), thereby serving as a molecular signature that predicts the maturation state of leaf samples (Efroni et al., 2008). ...
The leaves of seed plants evolved from a primitive shoot system and are generated as determinate dorsiventral appendages at the flanks of radial indeterminate shoots. The remarkable variation of leaves has remained a constant source of fascination, and their developmental versatility has provided an advantageous platform to study genetic regulation of subtle, and sometimes transient, morphological changes. Here, we describe how eudicot plants recruited conserved shoot meristematic factors to regulate growth of the basic simple leaf blade and how subsets of these factors are subsequently re-employed to promote and maintain further organogenic potential. By comparing tractable genetic programs of species with different leaf types and evaluating the pros and cons of phylogenetic experimental procedures, we suggest that simple and compound leaves, and, by the same token, leaflets and serrations, are regulated by distinct ontogenetic programs. Finally, florigen, in its capacity as a general growth regulator, is presented as a new upper-tier systemic modulator in the patterning of compound leaves.
... Current concepts about the way in which leaves are specified are based almost entirely on microsurgical studies of compound leaves, such as those of ferns (Steeves, 1966; Steeves and Sussex, 1989), potato (Sussex, 1955b), or pea (Sachs, 1969). This conceptual framework is often applied to simple leaves, although there is relatively little evidence that this generalization is valid. ...
... This phenotype demonstrates that PHAN is required for the specification of adaxial identity both in the midrib and in the lamina and suggests that the differentiation of the lamina depends on the juxtaposition of cells with adaxial and abaxial identity (Waites and Hudson, 1995). Microsurgical analysis of pea leaf development (Sachs, 1969) of the leaf. The character of the marginal structures produced by the leaf primordium is determined at a later stage of development; shallow cuts along the margins of primordia that are -100 pm in length do not prevent the formation of pinnae but usually transform pinnae primordia into tendrils. ...
... Surgical experiments on isolated pea leaf primordia have shown that the characteristic shape of a leaf is resolved gradually and acropetally, over four plastochrons, from the time a primordium is initiated on the shoot apical meristem [21,22]. Likewise, the four distinctive whorls of a pea flower are developed within four plastochrons [11]. ...
... The novel function of UNI in regulating leaf morphogenesis is supported by the presence of PEAFLO mRNA during the first four plastochrons after leaf primordium initiation ( Figure 5), at the time when pea leaf pattern is established [21,22]. Strong PEAFLO expression was detected in petiole-rachis progenitor cells of the P1 primordium, at a stage prior to the differentiation of leaf lateral organs [25,26]. ...
The vegetative phenotype of the pea mutant unifoliata (uni) is a simplification of the wild-type compound leaf to a single leaflet. Mutant uni plants are also self-sterile and the flowers resemble known floral meristem and organ identity mutants. In Antirrhinum and Arabidopsis, mutations in the floral meristem identity gene FLORICAULA/LEAFY (FLO/LFY) affect flower development alone, whereas the tobacco FLO/LFY homologue, NFL, is expressed in vegetative tissues, suggesting that NFL specifies determinacy in the progenitor cells for both flowers and leaves. In this paper, we characterised the pea homologue of FLO/LFY.
The pea cDNA homologue of FLO/LFY, PEAFLO, mapped to the uni locus in recombinant-inbred mapping populations and markers based on PEAFLO cosegregated with uni in segregating sibling populations. The characterisation of two spontaneous uni mutant alleles, one containing a deletion and the other a point mutation in the PEAFLO coding sequences, predicted that PEAFLO corresponds to UNI and that the mutant vegetative phenotype was conferred by the defective PEAFLO gene.
The uni mutant demonstrates that there are shared regulatory processes in the morphogenesis of leaves and flowers and that floral meristem identity genes have an extended role in plant development. Pleiotropic regulatory genes such as UNI support the hypothesis that leaves and flowers derive from a common ancestral sporophyll-like structure. The regulation of indeterminancy during leaf and flower morphogenesis by UNI may reflect a primitive function for the gene in the pre-angiosperm era.
... A normal pea leaf is composed of a pair of basal stipules and a rachis bearing two or more pairs of lateral leaflets, two or more pairs of tendrils, and a terminal tendril (see Figure 5A below). A surgical study (Sachs, 1969) suggests that the morphology of the pea leaf is determined progressively during early stages of leaf development. As illustrated in Figures 4A and 48, bisection or surgical removal of portions of the leaf primordium early in the first plastochron of pea leaf development (before the 30-pm stage) leads to the regeneration of leaves with normal morphology, but by the end of plastochron 1 (the 70-pm stage), leaf primordia have lost this ability to regenerate. ...
... Thus, the wild-type AF gene must begin to function no later than plastochron 3, and the TL gene no later than plastochron 6, to influence leaflet and tendril identity; the surgical study described earlier suggests that their functions are probably critical even earlier, during plastochron 2. Young (1983) has proposed that the identity of lateral appendages in the pea leaf is determined by primordial size at a critical stage of development; primordia below the size threshold would develop as tendrils and those above it as leaflets. Consistent with this model, surgical cuts bisecting or removing portions of lateral leaflet primordia during plastochron 2 sometimes cause a tendril to form instead of a leaflet ( Figure 4C; Sachs, 1969). Young (1983) hypothesizes that the afand tl mutations alter the critical thresholds rather than altering primordial size at the critical time, a proposal consistent with the lack of morphological differences between mutant and wildtype leaf primordia during the first two plastochrons of pea leaf development (Meicenheimer et al., 1983;Gould et al., 1986). ...
... Our current understanding of the leaf margin is based on foundational work that defined the margin by explicitly tracking developmental landmarks (Avery, 1933;Poethig and Sussex, 1985a;Wolf et al., 1986;Dolan and Poethig, 1998). Early literature defined the leaf primordium as broadly meristematic during early development, with this meristematic potential becoming restricted and gradually lost as the leaf develops (Foster, 1936;Sachs, 1969;Hagemann and Gleissberg, 1996). Although such studies provide a roadmap for describing growth patterns in the margin, a major challenge is to understand how these patterns are specified at the genetic level Whitewoods and Coen, 2017) and how this fits with our interpretation of the recruitment of regulatory mechanisms suppressing the morphogenetic potential of the margin during the evolution of leaves in seed plants. ...
Leaf morphogenesis involves cell division, expansion, and differentiation in the developing leaf, which take place at different rates and at different positions along the medio-lateral and proximal–distal leaf axes. The gene expression changes that control cell fate along these axes remain elusive due to difficulties in precisely isolating tissues. Here, we combined rigorous early leaf characterization, laser capture microdissection, and transcriptomic sequencing to ask how gene expression patterns regulate early leaf morphogenesis in wild-type tomato (Solanum lycopersicum) and the leaf morphogenesis mutant trifoliate. We observed transcriptional regulation of cell differentiation along the proximal–distal axis and identified molecular signatures delineating the classically defined marginal meristem/blastozone region during early leaf development. We describe the role of endoreduplication during leaf development, when and where leaf cells first achieve photosynthetic competency, and the regulation of auxin transport and signaling along the leaf axes. Knockout mutants of BLADE-ON-PETIOLE2 exhibited ectopic shoot apical meristem formation on leaves, highlighting the role of this gene in regulating margin tissue identity. We mapped gene expression signatures in specific leaf domains and evaluated the role of each domain in conferring indeterminacy and permitting blade outgrowth. Finally, we generated a global gene expression atlas of the early developing compound leaf.
... 11 est possible qu'un pareil cours des marges du rachis, avec unifacialisation de celui-ci au ni\'eau des noeuds, soit également présent dans les feuilles normales de Gleditsia, oii il serait souligné par l'existence de sortes de poils massifs qui, près de l'insertion des folioles ou foliolules, semblent évoquer ce trajet. Ce dernier ne conduit pas toutefois à une unifa- jeune lors de rintervention, ou même si celle-ci est intervenue dans la région apicale où son apparition est simplement prévisible, ses deux moitiés subissent une régulation, de sorte que se forment deux primordiunis entiers, ou bien un primordium bifide si seule la partie supérieure du primordium originel avait été fendue (Neville, 1968;Sachs, 1969;Amer et al.^ 1977). ...
... If the two sides of this primordium are removed in the 70-μm-long primordium, the rachis alone is formed with the terminal tendril also formed from the leftover lower part. Through other surgical manipulations, it was possible to change a future leafl et into a tendril, or vice versa (Sachs 1969 ). From all these surgical experiments, it is clear that the main elements of leaf architecture are laid down early in leaf ontogeny, but the morphology of leafl ets is determined later. ...
Leaves are the most important organs of plants and carry out very vital physiological activities such as photosynthesis, respiration, transpiration, photoreception and synthesis and supply of signal compounds, including growth regulators. They are always associated with shoot apical meristems from which they arise. Leaves are arranged on the stem with very characteristic and non-mutagenic phyllotactic pattern characteristic of each plant. The various theories to explain this pattern are briefly described. The leaf primordia are initially with a leaf axis from which the lamina, petiole and phyllopodium regions of the mature leaf arise. This chapter deals with the genetic network that operates during various phases of leaf ontogeny. The genetic basis for shoot apical meristem (with indeterminate growth)-leaf primordium (with determinate growth) boundary is also discussed. This article also discusses the ontogenetic and genetic bases of the differences between simple and compound leaves. A short account each on heteroblasty, heterophylly, senescence and evolution of leaf is also provided.
... Although there are no consensus HAG member sets found among these diverse tissue types, ear tissue is promising for identifying yield and heterosis-associated genes because it is a reproductive organ, and therefore straightforwardly linked to yield. Previous studies demonstrated that early primordium stages are critical in determining the organ developmental fate or pattern (Sachs 1969;Cho et al. 2002). Therefore, analysis of immature ear will likely reveal YAGs. ...
Heterosis has contributed greatly to yield in maize, but the nature of its contribution is not completely clear. In this study, two strategies using whole-genome oligonucleotide microarrays were employed to identify differentially expressed genes (DEGs) associated with heterosis and yield. The analysis revealed 1 838 heterosis-associated genes (HAGs), 265 yield-associated genes (YAGs), and 85 yield heterosis-associated genes (YHAGs). 37.1% of HAGs and 22.4% of YHAGs expressed additively. The remaining genes expressed non-additively, including those with high/low-parent dominance and over/under dominance, which were prevalent in this research. Pathway enrichment analysis and quantitative trait locus (QTL) co-mapping demonstrated that the metabolic pathways for energy and carbohydrates were the two main enriched pathways influencing heterosis and yield. Therefore, the DEGs participating in energy and carbohydrate metabolism were considered to contribute to heterosis and yield significantly. The investigation of potential groups of HAGs, YAGs, and YHAGs might provide valuable information for exploiting heterosis to improve yield in maize breeding. In addition, our results support the view that heterosis is contributed by multiple, complex molecular mechanisms.
... These become concentrated at the distal end of the elongating primordium by plastochrons 3 and 4, and are down-regulated by plastochron 5. Gourlay et al. (2000) did not observe uni expression in stipule, leaflet and tendril primordia ( Fig. 4A and B). Earlier surgical experiments had already established the first four plastochrons of pea leaf development as the window of time in which lateral organs initiate before the rachis becomes determined (Sachs, 1969;Gould et al., 1994). It appeared then, that the timing of uni gene expression was coincident with the potential of the leaf-rachis primordium to initiate lateral organs, but expression was not observed in the lateral organs themselves, which have no potential for organogenesis. ...
The partial-shoot theory of the leaf was a controversial hypothesis revived by Arber and supported by her morphological and
anatomical studies. This theory highlighted the parallels between leaves and shoots and contrasted with an alternative view
that leaves, with their limited growth potential, are completely distinct from shoots. Pea morphological mutants with altered
growth potential in their compound leaves are described. The unifoliata mutant has a limited growth potential relative to wild-type;cochleata, afila and insecatus have extended potentials. Characterization of theunifoliata mutation and gene expression patterns indicate that unifoliata is a common factor in pea compound leaf and floral shoot development, and so provides rudimentary support for the idea that
some leaves have shoot-like characteristics. Tomato leaves are also considered to lend tentative support. The afila and insecatus leaf forms are described as bipinnate and weakly bipinnate, respectively. These and the tendril-less mutant are potential phenocopies of legume relatives, an idea based on Vavilov's law of homologous series of variation. Arber
illustrated, but did not articulate in genetical terms, that morphological variation in structures within an individual plant
can be interpreted as reiteration of design. Analogous with Vavilov's view, this can be considered a consequence of the same
genetic programme in a different location.Copyright 2001 Annals of Botany Company
... During this period, form is determined but little growth occurs . At a later stage, the primordium grows at a relatively fast rate, but only in accordance with the organization determined earlier (Sachs 1969). KNOX gene activity was not detected in leaf primordia of pea (Hofer et al. 2001), in contrast to other compound-leafed species, such as tomato (Hareven et al. 1996). ...
Leaf shapes and sizes vary naturally from simple with smooth, serrated or lobed margins to compound with a few or a lot of leaflets. Simple leaves develop through gradients of cell division and cell expansion from tip to base, resulting in a fully differentiated mature leaf without meristematic activity, referred to as a determinate structure. Cell numbers and cell expansion influence leaf size and shape as observed by manipulation of the core cell cycle or cell wall extensibility. However, mechanisms exist that compensate changes in leaf growth by affecting cell expansion or number, which indicates that leaf size is also under supracellular control. Foliar morphology is used for classification in botany, demonstrating its genetic and evolutionary basis. A developmental biology approach is taken to identify the molecular control of leaf size and shape by using a limited number of model species. Mutational and transgene analysis in Arabidopsis thaliana has uncovered more than 100 loci important for simple leaf development. Regulatory genes, such as transcription factors, have been shown to regulate leaf growth and development, of which some might act upstream of the hormonal responses and core cell cycle. Chromatin modification complexes are involved in the control of leaf growth and might form the interface with developmental and environmental cues to influence leaf formation – a phenomenon known as leaf plasticity. In contrast to simple leaves, compound leaves develop discernable meristems that will form leaflets along the rachis. Molecular-genetic work in snapdragon (Antirrhinum majus), tomato (Solanum lycopersicum), and pea (Pisum sativum) identified independent molecular pathways for compound leaf development. These regulatory pathways have previously been shown to be important for meristem identity and suggest that compound leaves can be considered as transitional forms from determinate simple leaves to indeterminate shoots. The use of developmental genes for applications in agriculture, horticulture and ornamentals will be discussed.
... Leaves are characterized by complex networks of vascular tissues (Sachs 1975Sachs , 1981Sachs , 1989; Nelson and Dengler 1997; Candela and others 1999), fast vascular development and early determination (Mattsson and others 1999). Regeneration of both leaf shape (Sachs 1969 ) and leaf vascular differentiation is limited to early developmental stages (Sachs 1975Sachs , 1981; Mattsson and others 1999; Sieburth 1999). In complex networks of leaves, there are strands in which the individual cells do not have an obvious shoot-to-root polarity and therefore can be deemed not polar (Sachs 1975). ...
A comparison is made between foliar and axial vascular differentiation. Current thoughts and new evidence are presented on the role of hormones in controlling the differentiation of vascular tissues in organized and tumorous tissues, focusing on the role of auxin and cytokinin in controlling phloem and xylem relationships, vessel size and density, cambium sensitivity, vascular adaptation and xylem evolution in deciduous hardwood trees. The possible role of wounding is also considered. A new hypothesis, namely, the leaf-venation hypothesis, is proposed to explain the hormonal control of vascular differentiation in leaves of dicotyledonous plants. Experimental evidence in support of the hypothesis is presented showing that hydathodes, the water-secreting glands, are the primary sites of auxin synthesis during leaf morphogenesis. Vessel element patterns similar to those found in hydathodes were experimentally induced by exogenous auxin application.
... Morphological and genetic evidence point to striking similarities between leaf initiation from the flanks of the SAM and leaflet initiation from the leaf margin (Barkoulas et al., 2008;Brand et al., 2007;Hagemann and Gleissberg, 1996;Mathan and Jenkins, 1962;Ori et al., 2007;Sachs, 1969). Here we show that NAM-like genes, shown to be involved in boundary specification during lateral organ formation from the SAM, are also utilized for boundary specification at the site of leaflet initiation. ...
Leaves are formed at the flanks of the shoot apical meristem (SAM) and develop into a variety of forms. In tomato, prolonged leaf patterning enables the elaboration of compound leaves by reiterative initiation of leaflets with lobed margins. In goblet (gob) loss-of-function mutants, primary leaflets are often fused, secondary leaflets and marginal serrations are absent, and SAMs often terminate precociously. We show that GOB encodes a NAC-domain transcription factor expressed in narrow stripes at the leaf margins, flanking the distal side of future leaflet primordia, and at the boundaries between the SAM and leaf primordia. Leaf-specific overexpression of the microRNA miR164, a negative regulator of GOB-like genes, also leads to loss of secondary-leaflet initiation and to smooth leaflet margins. Plants carrying a dominant gob allele with an intact ORF but disrupted miR164 binding site produce more cotyledons and floral organs, have split SAMs and, surprisingly, simpler leaves. Overexpression of a form of GOB with an altered miR164 binding site in leaf primordia leads to delayed leaflet maturation, frequent, improperly timed and spaced initiation events, and a simple mature leaflet form owing to secondary-leaflet fusion. miR164 also affects leaflet separation in Cardamine hirsuta, a Brassicaceae species with complex leaves. Genetic and molecular analyses suggest that GOB expression is intact in the simplified leaves of entire tomato mutants, which have a defect in a putative repressor of auxin responses. Our results show that GOB marks leaflet boundaries and that its accurate spatial, temporal and quantitative activity affects leaf elaboration in a context-dependent manner.
... Depending on their length, ectopic laminae and bifurcations alter leaf patterning along the proximodistal axis or result in complete duplication of the organ. These observations are reminiscent of the observations made by Sachs (1969) following surgical experiments of leaf primordia in pea. Depending on the developmental stage of the primordium, incisions resulted either in duplication of the entire organ or in altered patterning of regions within the organ, suggesting that leaf primordia become gradually more patterned and determined. ...
The maize leafbladeless1 (lbl1) mutant displays a variety of leaf and plant phenotypes. The most extreme manifestation in the leaf is the formation of radially symmetric, abaxialized leaves due to a complete loss of adaxial cell types. Less severe phenotypes, resulting from a partial loss of adaxial cell identity, include the formation of ectopic laminae at the boundary between abaxialized, mutant sectors on the adaxial leaf surface and the bifurcation of leaves. Ectopic laminae and bifurcations arise early in leaf development and result in an altered patterning of the leaf along the proximodistal axis, or in complete duplication of the developing organ. Leaf-like lateral organs of the inflorescences and flowers show similar phenotypes. These observations suggest that Lbl1 is required for the specification of adaxial cell identity within leaves and leaf-like lateral organs. Lbl1 is also required for the lateral propagation of leaf founder cell recruitment, and plays a direct or indirect role in the downregulation of the homeobox gene, knotted1, during leaf development. Our results suggest that adaxial/abaxial asymmetry of lateral organs is specified in the shoot apical meristem, and that formation of this axis is essential for marginal, lateral growth and for the specification of points of proximodistal growth. Parallels between early patterning events during lateral organ development in plants and animals are discussed.
... Howell (1998) suggested that`Becausethat`Because leaf primordia develop sequentially rather than synchronously, they must be able to control much of their own development.' Experiments show that the number of lobes that a leaf will grow is determined very early in its development (Sussex 1955; Sachs 1969; Fuchs 1975). Indeed, the gross pattern of a leaf seems to be determined typically when the leaf primordium is much less than 1mm across (Poethig & Sussex 1985; Alberts et al. 1994), when the primordium has of the order of 10 2 cells (Howell 1998), although there are changes in detail later. ...
We consider mechanisms that may determine certain simple leaf shapes. Compared with other aspects of plant morphogenesis, such as phyllotaxis or spiral leaf arrangement, rather little is known about leaf-shape-determining mechanisms. We develop mathematical models for the gross pattern of leaf shape based on reaction diffusion systems. These models are consistent with what is known about factors that might determine leaf shape. They show that diverse leaf shapes may be obtained from a single reaction diffusion system. This has implications in terms of both convergent and divergent evolution. The models make predictions that can be tested experimentally. We predict the form of pre-patterns of growth promoters in leaf primordia of different sizes when the morphogens either diffuse into the primordia or are produced locally. We also predict the effects on leaf shape of removing parts of primordia at different times. The models can also predict the effects on leaf shape of the topical application of activators and inhibitors to leaf primordia.
... Leaves are characterized by complex networks of vascular tissues (Sachs 1975(Sachs , 1989Nelson and Dengler 1997;Candela et al. 1999;Aloni 2001), rapid vascular development and early determination (Mattsson et al. 1999;Sieburth 1999). Regeneration of both leaf shape (Sachs 1969) and leaf vascular differentiation is limited to early developmental stages (Sachs 1975(Sachs , 1981Mattsson et al. 1999;Sieburth 1999). However, the molecular and physiological mechanisms that control vascular differentiation and determine venation pattern formation in leaves are poorly understood (Telfer and Poething 1994;Nelson and Dengler 1997;Candela et al. 1999;Carland et al. 1999;Mattsson et al. 1999;Sieburth 1999;Deyholos et al. 2000;Koizumi et al. 2000;Semiarti et al. 2001). ...
The major regulatory shoot signal is auxin, whose synthesis in young leaves has been a mystery. To test the leaf-venation hypothesis [R. Aloni (2001) J Plant Growth Regul 20: 22-34], the patterns of free-auxin production, movement and accumulation in developing leaf primordia of DR5::GUS-transformed Arabidopsis thaliana (L.) Heynh. were visualized. DR5::GUS expression was regarded to reflect sites of free auxin, while immunolocalization with specific monoclonal antibodies indicated total auxin distribution. The mRNA expression of key enzymes involved in the synthesis, conjugate hydrolysis, accumulation and basipetal transport of auxin, namely indole-3-glycerol-phosphate-synthase, nitrilase, IAA-amino acid hydrolase, chalcone synthase and PIN1 as an essential component of the basipetal IAA carrier, was investigated by reverse transcription-polymerase chain reaction. Near the shoot apex, stipules were the earliest sites of high free-auxin production. During early stages of primordium development, leaf apical dominance was evident from strong beta-glucuronidase activity in the elongating tip, possibly suppressing the production of free auxin in the leaf tissues below it. Hydathodes, which develop in the tip and later in the lobes, were apparently primary sites of high free-auxin production, the latter supported by auxin-conjugate hydrolysis, auxin retention by the chalcone synthase-dependent action of flavonoids and also by the PIN1-component of the carrier-mediated basipetal transport. Trichomes and mesophyll cells were secondary sites of free-auxin production. During primordium development there are gradual shifts in sites and concentrations of free-auxin production occurring first in the tip of a leaf primordium, then progressing basipetally along the margins, and finally appearing also in the central regions of the lamina. This developmental pattern of free-auxin production is suggested to control the basipetal maturation sequence of leaf development and vascular differentiation in Arabidopsis leaves.
... These events include the three-dimensional axes determination (dorsal-ventral, basal-distal and medio-lateral; for review see Poethig 1997), tissue differentiation (including epidermal, ground and vascular tissues) and cellular differentiation (such as sporogenous cells in anthers or ovules, and photosynthetic cells in leaves). The developmental fate or pattern is determined at an early primordium stage (Sussex 1955a, b;Sachs 1969). Thus, analysis of the molecular events governing early primordial development will likely reveal regulatory mechanisms underlying organ formation. ...
Elucidating the regulatory mechanisms of plant organ formation is an important component of plant developmental biology and will be useful for crop improvement applications. Plant organ formation, or organogenesis, occurs when a group of primordial cells differentiates into an organ, through a well-orchestrated series of events, with a given shape, structure and function. Research over the past two decades has elucidated the molecular mechanisms of organ identity and dorsalventral axis determinations. However, little is known about the molecular mechanisms underlying the successive processes. To develop an effective approach for studying organ formation at the molecular level, we generated organ-specific gene expression profiles (GEPs) reflecting early development in rice stamen. In this study, we demonstrated that the GEPs are highly correlated with early stamen development, suggesting that this analysis is useful for dissecting stamen development regulation. Based on the molecular and morphological correlation, we found that over 26 genes, that were preferentially up-regulated during early stamen development, may participate in stamen development regulation. In addition, we found that differentially expressed genes during early stamen development are clustered into two clades, suggesting that stamen development may comprise of two distinct phases of pattern formation and cellular differentiation. Moreover, the organ-specific quantitative changes in gene expression levels may play a critical role for regulating plant organ formation.
Development of plants and animals depends on the formation of complex vascular systems for the delivery of water, nutrients, and hormonal signals. This review clarifies major controlling mechanisms that regulate vascular differentiation, regeneration, adaptation, and evolution of plants, which were discovered during the past 50 years. Hypotheses and evidence on the hormonal mechanisms that regulate vascular differentiation are discussed, focusing on phloem and xylem relationships, control of vessel width, fiber differentiation, leaf and flower development, root initiation, evolution of ring-porous wood, parasitism, gall formation, cancer development and prevention.
Life evolved in the presence of reactive oxygen species (ROS) which resulted in an early integration of ROS signals and properties for cellular physiology. Originally ROS were viewed merely as metabolic by-products, but now they are viewed as conserved regulators of physiological and developmental processes throughout all kingdoms of life. Plants monitor constantly the fluctuating environmental conditions in which they grow, for which ROS signaling is essential. However, in recent years it has become clear that plants also use ROS homeostasis to guide their development. Here, an overview is presented for the role of ROS gradients and homeostasis in the development of leaves from the shoot apical meristem based mainly on the model plant species Arabidopsis thaliana. Furthermore, this chapter summarizes current knowledge on the involvement of different ROS species and molecular components in the regulation of leaf development. Although our current understanding of how plants employ ROS to guide their development is far from complete, it is clear that ROS is utilized during many developmental phases of leaf growth.
Leaf initiation depends on dynamic gradients of upward polar auxin flows toward the shoot apex, where the development of auxin maxima induces the initiation of new leaf primordia in an orderly pattern, namely, phyllotaxis. Young growing leaves induce their own vascular tissues by polar auxin flows that they produce. The question dealt in this chapter is how leaves produce their vascular tissues in orderly patterns characterized by plasticity. Polar auxin flows induce asymmetric patterns of vascular veins in the same leaf, demonstrating the auxin-independent genetic control of vascular tissues. The hydathodes of the midvein and the secondary veins are induced by dynamic downward developmental patterns of auxin maxima at the leaf periphery; whereas tertiary veins and freely-ending veinlets are induced in the lamina at a later developmental stage by minor low-auxin synthesis sites, after the cessation of the inhibiting auxin maxima along the primordium periphery. The controlling mechanisms of these patterns are discussed below.
The sections in this article are
Introduction
Shoot Development
Organogenesis of the Leaf
Organogenesis of the Root
Conclusions
Acknowledgements
Pea leaf determination was examined by culturing excised leaf, leaflet, and tendril primordia of different ages on a nutrient medium. Pinna primordia were designated as 1) determined, if they grew normally in culture; 2) undetermined, if they grew into differentiated structures that were morphologically and anatomically different from either leaflet or tendril; or 3) partially determined, if the two pinnae of an opposite pair developed unequally in isolation, or for leaflet pinnae only, if laminae were initiated but did not develop completely. The compound pea leaf as a whole is determined over four plastochrons of development. Proximal pinnae are determined during the second leaf plastochron, approximately 0.8 plastochron after their initiation. The second most proximal pair of pinnae is determined during the third plastochron, and the terminal portion of the rachis is determined last, during the fourth plastochron. Determination of leaflet dorsiventrality is gradual, requiring a critical minimum period with the leaf in physiological contact with the shoot system. The rachis primordium, when isolated from the shoot, does not affect determination of its pinnae as leaflets or tendrils. Afila and tendril-less homeotic mutations do not alter the timing of pinna determination.
Leaves are among the most specialized of all plant organs, devoting most of their activity to the production of ribulose-l,5-bisphosphate carboxylase-oxygenase (RuBISCO). This key protein is mentioned here to introduce the chapter on leaves not only because of its importance as a primary enzyme for carbon fixation in the initial reaction of photosynthesis, but also to emphasize that all of the, structural and developmental adaptations of leaves are geared to maximize the production of RuBISCO.
Polarity can be defined as the bipolar, axiate character of organisms (Bloch 1965). It is considered the first, visible morphological indication of an internal asymmetrical state within a living system and is a fundamental component of differentiation and spatial organization. Polarity appears to be present in all organisms at some stage of their life cycle and is currently assumed to be an outcome of a biochemical gradient or gradients within a cell or group of cells due to asymmetric environmental stimuli (Fig. 1).
… our ignorance of the pathways from genes to characters is one of the chief obstacles to our understanding of evolutionary trends (Stebbins, 1974, p. 102).
Experimental evidence has shown that all forms of peas previously described as species have a diploid chromosome number of 14, that no sterility barriers exist, and that gene exchange is complete. The genus Pisum is therefore best regarded as monospecific in accordance with Lamprecht’s (1966) view. He classified the different forms as ecotypes included under Pisum arvense Linné, the wild-growing form of the two described by Linné. The ecotypes abyssinicum Braun; arvense (Linné) Lamprecht (including elatius Steven, jomardi Schrank, and transcaucasicum Stankov); fulvum Sibthorp and Smith; and humile Boissier (including syriacum/Berger/Lehmann) occur as wild-growing populations. All man-made genetic variations were collected together under the name sativum, the domesticated race. This system of classification is practical and workable, though perhaps not taxonomically orthodox. Pisum formosum Steven, which is a tuber-forming perennial, was separated to form the genus Alophotropsis (Boissier) Lamprecht.
To the best of our current knowledge, every living plant cell contains all the genetic information for programming a whole mature and reproductive adult individual. This implies that at no time during growth and development is this information lost or added to. Assuming this is so, then the basic question of morphogenesis concerns the nature and regulation of the expression of this information and the means by which it is directed to the development of an organized form and behaviour. Morphogenesis is thus the succession by which the genetic dictionary of the plant is revealed in an ordered sequence of differentiation and developmental events, all of which are constrained within a format that is the specific dictate of the species.
The paper is meant to explain and, where possible, briefly to substantiate the following central principles:
In compound leaves, leaflet primordia are initiated directionally along the lateral sides. Our understanding of the molecular basis of leaflet initiation has improved, but the regulatory mechanisms underlying spatio-temporal patterns remain unclear. In this study, we investigated the mechanisms of acropetal (from the base to the tip) progression of leaflet initiation in Eschscholzia californica. We established an ultraviolet-laser ablation system to manipulate compound-leaf development. Local ablation at the leaflet incipient site generated leaves with asymmetric morphology. In the majority of cases, leaflets that were initiated on the ablated sides shifted apically. Finite time-course observation revealed that the timing of leaflet initiation was delayed, but the distance from the leaf tip did not decrease. These results were suggestive of the local spacing mechanism in leaflet initiation, whereby the distance from the leaf tip and adjacent pre-existing leaflet determines the position of leaflet initiation. To understand how such a local patterning mechanism generates a global pattern of successive leaflet initiation, we assessed the growth rate gradient along the apical-basal axis. Our time-course analysis revealed differential growth rates along the apical-basal axis of the leaf, which can explain the acropetal progression of leaflet initiation. We propose that a leaflet is initiated at a site where the distances from pre-existing leaflets and the leaf tip are sufficient. Furthermore, the differential growth rate may be a developmental factor underlying the directionality of leaflet initiation.
The cellular parameters of leaf development in tobacco (Nicotiana tabacum L.) have been characterized using clonal analysis, an approach that provides unequivocal evidence of cell lineage. Our results indicate that the tobacco leaf arises from a group of around 100 cells in the shoot apical meristem. Each of these cells contributes to a unique longitudinal section of the axis and transverse section of the lamina. This pattern of cell lincage indicates that primordial cells contribute more or less equally to the growth of the axis, in contrast to the more traditional view of leaf development in which the leaf is pictured as arising from a group of apical initials. Clones induced prior to the initiation of the lamina demonstrate that the subepidermal layer of the lamina arises from at least six files of cells. Submarginal cells usually divide with their spindles parallel to the margin, and therefore contribute relatively little to the transverse expansion of the lamina. During the expansion of the lamina the orientation and frequency of cell division are highly regulated, as is the duration of meristematic growth. Initially, cell division is polarized so as to produce lineages that are at an oblique angle to the midrib; later cell division is in alternating perpendicular planes. The distribution of clones generated by irradiation at various stages of development indicates that cell division ceases at the tip of the leaf when the leaf is about one tenth its final size, and then ceases in progressively more basal regions of the lamina. Variation in the mutation frequency within the lamina reflects variation in the frequency of mitosis. Prior to the mergence of the leaf the frequency of mutation is maximal near the tip of the leaf and extremely low at its base; after emergence, the frequency of mutation increases at the base of the leaf. In any given region of the lamina the frequency of mutation is highest in interveinal regions, and is relatively low near the margin. Thus, both the orientation and frequency of cell division at the leaf margin indicate that this region plays a minor role in the growth of the lamina.
The characteristic morphology and anatomy of the maize leaf reflects the outcome of developmental patterning along three axes,
proximodistal, mediolateral, and adaxial-abaxial, which are specified relative to the main axis of the plant. The past decade
has seen a dramatic increase in our understanding of the genetic control of leaf development. Gene regulatory networks involved
in the specification of organ polarity are beginning to emerge. These are distinguished by contributions of highly conserved,
often redundant transcription factor families whose expression or activity are modulated to give rise to the distinctive maize
leaf. Small regulatory RNAs, hormones, as well as proteins that selectively trafficking between cells have emerged as candidate
signals conveying positional information within the shoot to the newly initiated leaf. This chapter outlines findings of both
classical genetic and recent molecular studies that have led to a framework for axial patterning of the maize leaf.
Leaf anatomy, ontogeny, and morphology were described and compared in a pea line (Pisum sativum L.) with conventional leaves and in isogenic lines carrying the mutations af (afila) or tl (tendril-less or acacia). The anatomy of stem, petiole, and rachis is not modified by these mutations. The tendrils, which in af replace leaflets, have normal tendril anatomy, and the terminal leaflets of the tl form have normal leaflet anatomy. The shoot apical dome has the same size and shape in the three genotypes, as does the leaf primordium up to the stage of initiation of the first laterals. The mature morphology of leaves varies with node of insertion. Some leaves, especially at nodes 3 and 4, have structures that are not typical of their genotype. An in vitro culture system is described for axillary shoots. Such shoots recapitulate most of the foliar features of seedling plants, but leaf morphology is on average more complex, and aberrant structures are more frequent. All these observations are discussed in relation to Young's algebraic model for compound leaf development.
A number of single-gene, recessive mutations have been described in Pisum sativum L. that alter the form of the normal pinnately compound leaf and that show promise in elucidating genetic mechanisms of leaf development. Two recessive mutant alleles are known for the Unifoliata gene (the putative Lfy/Flo orthologue): uni and uni-tac (tendrilled acacia). To better understand the role of Uni in pea, we made observations on shoot development, leaf development, and in situ expression of Uni mRNA in these two mutants in comparison to wild-type plants. Although uni plants have abnormal, sterile flowers, those of uni-tac are usually normal and fertile. The uni and uni-tac plants produce more leaves and flower later than wild type, especially under long days. Some shoot features that are altered under long days are unaffected in uni plants, indicating that Uni may play a role in some photoperiodic responses. Adult uni leaves exceed one lateral leaflet pair only under short days, whereas uni-tac leaves typically possess two to three lateral leaflet pairs and one lateral tendril pair. Fusions between the ultimate lateral pinnae and the terminal leaflet are common in both mutants. Pinnae are initiated in an acropetal sequence over five plastochrons (P) for wild type, four for uni-tac, and three for uni. Lateral leaflet initiation and expansion occur earlier on wild-type leaves than on the mutants. Uni mRNA is expressed in the tips of juvenile leaf primordia through P4 in wild type, through P3 in uni-tac, and through P2 in uni. Ectopic expression also occurs in the shoot apical meristem of the mutants. We conclude that the Uni gene affects leaf development in pea by prolonging leaf tip growth and the period of pinna initiation and by delaying leaf tip differentiation. Therefore, it allows larger and more complex leaves to be produced by altering the timing of developmental events.
Three well-defined genes affect the morphological and anatomical features of the pea (Pisum sativum) compound leaf. Either singly or in combination, they specify five distinct pinna types. Using simple genetics, classical criteria for establishing homology, SEM of leaf development, and pinna histology, the phenotypes of the afila (af), tendril-less (tl), and tendrilled acacia (uni-tac)/unifoliata (uni) mutants are compared with that of wild-type plants, and the roles of the Af, Tl, and Uni genes are deduced. Marx's concept of inherent regions within the pea leaf is upheld. The leaf blade consists of three genetically/developmentally determined regions: proximal, distal, and terminal. All three genes modify leaf blade form by altering the timing of events during leaf development. In addition, these genes affect most aspects of leaf morphology (pinna pair number, pinna, petiole and leaf lengths, pinna branching) and histology (cell arrangement and size) as well as characteristics of shoot ontogeny (number of leaves, first node to flower, leaf heteroblasty).
Cette etude de la croissance et du developpement inflorescentiel de Brassica napus L. var. oleifera, en conditions controlees du phytotron de Gif-sur-Yvette, a mis en evidence la presence de differents phenomenes lies a la floraison. Nous avons constate une modification de la morphologie foliaire, une diminution des plastochrones reels et apparents, de la longueur finale des feuilles ainsi que de leur vitesse d'allongement. L'ensemble de ces phenomenes apparait comme partie integrante d'un processus plus complexe mais parfaitement synchronise. Nos resultats mettent en evidence la presence d'une feuille (appelee feuille α) caracteristique du passage a la floraison et a laquelle nous nous sommes particulierement interesses. Les donnees peuvent constituer un premier pas vers la mise au point d'un modele permettant de predire le nombre final de feuilles base sur le numero d'ordre de la feuille α et tenant compte de l'effet des conditions culturales du colza
A modified mathematical model based on the concept of generative centers is proposed to describe organogenesis in young leaf primordia of Murraya paniculata. Measurements of specific parameters on leaf primordia at different stages of development support the basic assumptions of the model. These assumptions are exponential elongation and widening of primordia in the organogenetic phase and rhythmic production of lateral elements at a fixed distance from the apex of the developing primordium. In general, the model provides good estimates for growth parameters such as elemental growth rates. It also provides a relatively accurate description of the shape of the primordium during the organogenesis of lateral elements or leaflet primordia. Key words: leaf development, mathematical model, organogenesis, compound leaf, Murraya paniculata.
Unlabelled:
•
Premise of the study:
Processes of leaf morphogenesis provide the basis for the great diversity of leaf form among higher plants. The common garden pea (Pisum sativum) offers a developmental model system for understanding how gene and hormone interactions impart a large array of mutant leaf phenotypes. •
Methods:
To understand the role of auxin in AF and UNI gene function and their interaction, we compared the range of leaf phenotypes on afila (af) and unifoliata (uni) double mutants, examined the effects of these mutations on auxin levels, auxin transport, auxin response via DR5::GUS, and expression of auxin-regulated genes. •
Key results:
The adult leaves of af uni double mutants have leaflets and tendrils and typically possess two lateral pinna pairs and a terminal leaflet. The af mutants have higher auxin content, stronger auxin response, and higher expression of auxin responsive genes than wildtype. The uni mutant has reduced auxin content and transport, whereas the uni-tac mutant has higher auxin content and transport and reduced auxin response compared to wildtype. •
Conclusions:
Auxin concentration and response differences characterize the antagonistic relationship between AF and UNI in pea leaf development. The mechanism involves modulation of auxin mediated by one or both genes; UNI is expressed in and promotes high auxin levels, and AF suppresses auxin levels.
Reversion from floral to vegetative growth is under environmental control in many plant species. However the factors regulating floral reversion, and the events at the shoot apex that take place when it occurs, have received less attention than those associated with the transition to flowering.
Reversions may be categorized as flower reversion, in which the flower meristem resumes leaf production, or inflorescence reversions, in which the meristem ceases to initiate bracts with flowers in their axils and begins instead to make leaves with vegetative branches in their axils. Related to these two types of reversion, but distinct from them, are examples of partial flowering, when non-floral meristems grow out so that the plant begins to grow vegetatively again. Anomalous or proliferous flowers may form as a result of unfavourable growth conditions or viral infection, but these do not necessarily involve flower reversions.
There are many examples of inflorescence reversion but fewer clearly defined cases of flower reversion. In flower reversion the meristem of the flower itself reverts to vegetative growth so that flowers with basal floral organs and distal leaves on the same axis are formed successively by the apical meristem. InPharbitis nil, Anagallis arvensis, andImpatiens balsamina flower reversions have been caused by defined environmental conditions. However, only inImpatiens has detailed study of the changes in growth and development at the shoot apex during reversion been carried out. These studies show that changes in apical growth and phyllotaxis that typically accompany flowering can be separated from the development of floral organs and suggest that the floral stimulus plays a role throughout flower morphogenesis. The occurrence of reverting organs intermediate between leaves and petals is of particular interest in allowing experiments to be done on the progress of determination at the cell, tissue and organ levels. Reversion indicates that the flowering process must be regarded as a continuum, with physiological stages such as commitment to flower, and even morphological stages such as different floral organ types, being to varying extents artificial. Further study of the regulation of floral morphogenesis, and of the events associated with reversion, may provide important information on the nature of the factors that bring about the onset of flowering itself.
The leaf is the major site of photosynthesis and the prototypical organ of terrestrial plants. The past five years have seen a dramatic increase in our understanding of the genetic control of leaf development. The establishment of determinacy, the role of the meristem, the establishment of polarity, the control of cell division, vascularization and epidermal patterning have all been the subject of genetic screens and many mutants have been isolated and characterized. Many of these mutants affect floral and embryonic structures, demonstrating the inherent similarities in all lateral organs. In this review we focus on the interactions between these genes and the networks that establish pattern in lateral organs.
In this review, we summarize recent research on leaf development in Arabidopsis. Topics include leaf initiation, ontogeny, the acquisition of leaf shape, heteroblasty, and the differentiation of the tissue types that make up a leaf. New data obtained by a molecular genetic approach and advanced microscopy are discussed in relation to existing hypotheses derived from earlier surgical experiments, morphological and anatomical descriptions, and mutational analyses. Molecular markers are providing new tools with which to investigate cell identity and growth patterns. An example is shown in which cell division is monitored during leaf growth. The cloning of the first set of genes critical for leaf development has shown that transcription factors control a number of important switches in developmental programs, such as the transition from indeterminate to determinate growth during leaf initiation, dorsiventral patterning and cell determination during trichome formation. In short, the study of leaf development seems to be about to come of age.
Shoot-like compound leaves are ancient and predate the evolutionary origin of whorled flowers. By studying leaves of this type it may be possible to gain further insight into similar fundamental processes in plant development. Here, we describe the pea compound leaf from the viewpoint that considers it as a determinate lateral shoot. By integrating research on developmental mutants, we present a model of the genetic control of patterning in the pea leaf.
During leaf development, the formation of dorsal-ventral and proximal-distal axes is central to leaf morphogenesis. To investigate the genetic basis of dorsoventrality and proximodistality in the leaf, we screened for mutants of Arabidopsis thaliana (L.) Heynh. with defects in leaf morphogenesis. We describe here the phenotypic analysis of three mutant alleles that we have isolated. These mutants show varying degrees of abnormality including dwarfism, broad leaf lamina, and aberrant floral organs and fruits. Genetic analysis revealed that these mutations are alleles of the previously isolated mutant asymmetric leaves1 (as1). In addition to the leaf phenotypes described previously, these alleles display other phenotypes that were not observed. These include: (i) some rosette leaves with petiole growth underneath the leaf lamina; (ii) leaf vein branching in the petiole; and (iii) a leaf lamina with an epidermis similar to that on the petiole. The mutant phenotypes suggest that the ASYMMETRIC LEAVES1 (AS1) gene is involved in the control of cell differentiation in leaves. As the first step in determining a molecular function for AS1, we have identified the AS1 gene using map-based cloning. The AS1 gene encodes a MYB-domain protein that is homologous to the Antirrhinum
PHANTASTICA (PHAN) and maize ROUGH SHEATH2 (RS2) genes. AS1 is expressed nearly ubiquitously, consistent with the pleiotropic mutant phenotypes. High levels of AS1 expression were found in tissues with highly proliferative cells, which further suggests a role in cell division and early cell differentiation.
Balancing shoot apical meristem (SAM) maintenance and organ formation from its flanks is essential for proper plant growth and development and for the flexibility of organ production in response to internal and external cues. Leaves are formed at the SAM flanks and display a wide variability in size and form. Tomato (Solanum lycopersicum) leaves are compound with lobed margins. We exploited 18 recessive tomato mutants, representing four distinct phenotypic classes and six complementation groups, to track the genetic mechanisms involved in meristem function and compound-leaf patterning in tomato. In goblet (gob) mutants, the SAM terminates following cotyledon production, but occasionally partially recovers and produces simple leaves. expelled shoot (exp) meristems terminate after the production of several leaves, and these leaves show a reduced level of compoundness. short pedicel (spd) mutants are bushy, with impaired meristem structure, compact inflorescences, short pedicels and less compound leaves. In multi drop (mud) mutants, the leaves are more compound and the SAM tends to divide into two active meristems after the production of a few leaves. The range of leaf-compoundness phenotypes observed in these mutants suggests that compound-leaf patterning involves an array of genetic factors, which act successively to elaborate leaf shape. Furthermore, the results indicate that similar mechanisms underlie SAM activity and compound-leaf patterning in tomato.
The results of studies of genetic regulation of the early leaf morphogenesis, demarcation of the future primordium and transition of cells to determination, have been reviewed. The genetic systems of control of these developmental stages were shown to be conservative and hypotheses of possible mechanisms underlying the evolution of leaf morphology on their basis have been considered.
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