Figure 2 - uploaded by Reidunn B Aalen
Content may be subject to copyright.
Source publication
In Arabidopsis thaliana, the final step of floral organ abscission is regulated by INFLORESCENCE DEFICIENT IN ABSCISSION (IDA): ida mutants fail to abscise floral organs, and plants overexpressing IDA display earlier abscission. We show that five IDA-LIKE (IDL) genes are expressed in different tissues, but plants overexpressing these genes have phe...
Contexts in source publication
Context 1
... A significant partial rescue of the ida phenotype was also observed ( Figure 6B). The IDL1 EPIP peptide gave similar results (see Supplemental Figure 8 online), and a small percentage of the flowers (5 to 10%) exposed to IDA EPIP-C and IDL1 EPIP displayed an excessive rounding of AZ cells and secretion of white substance from this region (Figure 6C). Both the preco- cious floral abscission, the expansion of the AZ cells, and the secretion is similar to what is seen in 35S:IDA and 35S:IDL flowers (Figure 2). We also exposed other organs, such as cauline leaves and pedicels, to IDA EPIP-C to see whether this would mimic the 35S: IDA response in these tissues as well. Although none of the cauline leaves abscised, after 1 week, some of the pedicels portrayed an enlargement compared with the wild type and the epidermis ruptured as cell separation occurred (Figure 6C). The effect of the synthetic peptides on the hae hsl2 mutant was investigated. Although the floral abscission defect of ida could be partially rescued when exposed to all peptides tested, the hae hsl2 flowers remained totally unaffected. Irrespective of the presence of IDA EPIP-C, IDA EPIP, or IDL1 EPIP peptides, 100% of the hae hsl2 flowers tested ( n = 90) retained their floral organs, substantiating that these RLKs are needed for IDA/IDL peptide signaling. Studies of CLAVATA3 (CLV3) and related proteins have shown both sequence conservation and functional sufficiency of the C-terminally located CLE domain (Jun et al., 2008); moreover, strong evidence both in vitro and in vivo of proteolytic processing to release the active CLE peptide has been observed (Kondo et al., 2006; Ni and Clark, 2006). Given the parallel evidence of both conservation and functional importance of the IDA and IDL EPIP domain, we hypothesized that a peptide might be proteolytically released from the IDA protein. As a first approach to investigate whether IDA can be processed, a glutathione S -transferase (GST)-tagged version of the protein lacking the signal peptide (Stenvik et al., 2006), GST-IDA D SP, was expressed in Escherichia coli . The fusion protein was incubated with cauliflower extracts in in vitro reactions previously shown to process the CLE domain from CLV3 (Ni and Clark, 2006). Upon detection by protein gel blots, mobility shifts consistent with a distinct C-terminal processing of GST-IDA D SP and GST-mCLV3 (Ni and Clark, 2006) were observed (Figure 7A). To determine if IDA processing was catalyzed by the same activity as CLV3, competition experiments were performed. Excess His-mCLV3, included in GST-IDA D SP processing reactions, partially inhibited processing of GST-IDA D SP, while excess BSA had no effect (Figure 7B). This suggests the presence of a single or related processing activity capable of generating peptides from the C terminus of both CLV3 and IDA. Interestingly, there is some similarity between the PIP motif and the active mature CLV3 peptide, MCLV3 (Kondo et al., 2006), as well as the residues preceding these peptides (Figure 7C). MCLV3 and PIP have a common core, PS[G,A]P, surrounded by small residues, and two amphoteric residues at the end, while hydrophobic residues flank each side of the charged E or K preceding the CLE/PIP motifs. One of the aims of this work has been to elucidate the function of the IDL genes, which, in contrast with IDA , were identified using bioinformatics tools (Butenko et al., 2003). The promoter-GUS transgenic plants showed that IDL 2, IDL3 , and IDL4 are expressed in floral AZs (Figure 1B; see Supplemental Figure 1 online), suggesting that these genes are the results of gene duplication events of an ancient abscission-related gene. The most closely related genes, IDL2 (At5g64667) and IDL3 (At5g09805) (Figure 4D), are in fact found in two duplicated regions on chromosome 5, encompassing the genes At5g63600- 65640 and At5g08570-10570 (cf. dup). The expression of IDL genes in the AZ is apparently ...
Context 2
... to induce cell separation of floral organs when ida is mutated. This could be due to their different expression patterns (e.g., in contrast with IDA , the peak of IDL expression is not seen until after the floral organs have been shed) (see Supplemental Figure 1 online). In addition, the gene swap experiments show that the IDL2, IDL3, and IDL4 proteins have limited ability to substitute for IDA even when expressed under control of the IDA promoter (Figure 4B). This suggests that the active parts of these IDL proteins, most likely found within the EPIPs, are not suffi- ciently similar to IDA to fully substitute for IDA function (cf. Figure 4C). Involvement in floral organ abscission may not be the normal and/or major function of the IDL genes, as the IDL:GUS expression was seen in a range of tissues (Figure 1). The expression of IDL2 and IDL3 at the base of the pedicel (Figure 1B) could be a remainder of an ancient expression pattern. Pedicel abscission occurs naturally in a number of plant species, although not Arabidopsis , which has a vestigial AZ at the pedicel junction (Stenvik et al., 2006). In plant species displaying such abscission, it can be hypothesized that an IDA or IDL ortholog induces the pedicel separation event. Sloughing (i.e., detachment of the root- cap from the remaining root), release of the seed body from the funiculus, and disruption of adhesion at the junction between two adjacent cells during the formation of stomata are also cell–cell separation processes (Stevens and Martin, 1978; Zhao and Sack, 1999; Roberts et al., 2002; del Campillo et al., 2004). During the maturation of continuous tubes of tracheary elements, the middle lamella between two neighboring tracheary elements is digested completely by cell wall–degrading enzymes (Fukuda, 2004). Interestingly, IDL genes are also expressed at these sites (Figures 1A and 1D to 1F; see Supplemental Figure 2 online). The investigation of future IDL knockout mutants may reveal whether the IDL proteins, similar to IDA, represent signals that trigger the cell separation processes at these sites. The overexpression of the IDL genes resulted in phenotypes similar to the overexpression of IDA itself (Figure 2; see Supplemental Figure 3 online). Therefore, one could have expected that all IDL proteins would be capable of complementing the ida mutant when expressed under the control of the IDA promoter. However, this was only the case for IDL1, which has the highest overall sequence similarity to IDA (Figures 4C and 4D). The failure of the other IDL proteins to rescue the mutant phenotype was not due to the low sequence similarity in the N terminus and variable regions, as all the IDA:IDL-IDA constructs were functional. The gene deletion analysis substantiated that the variable region of IDA was not required for IDA function (Figure 5). All constructs containing the EPIP motif rescued ...
Context 3
... to induce cell separation of floral organs when ida is mutated. This could be due to their different expression patterns (e.g., in contrast with IDA , the peak of IDL expression is not seen until after the floral organs have been shed) (see Supplemental Figure 1 online). In addition, the gene swap experiments show that the IDL2, IDL3, and IDL4 proteins have limited ability to substitute for IDA even when expressed under control of the IDA promoter (Figure 4B). This suggests that the active parts of these IDL proteins, most likely found within the EPIPs, are not suffi- ciently similar to IDA to fully substitute for IDA function (cf. Figure 4C). Involvement in floral organ abscission may not be the normal and/or major function of the IDL genes, as the IDL:GUS expression was seen in a range of tissues (Figure 1). The expression of IDL2 and IDL3 at the base of the pedicel (Figure 1B) could be a remainder of an ancient expression pattern. Pedicel abscission occurs naturally in a number of plant species, although not Arabidopsis , which has a vestigial AZ at the pedicel junction (Stenvik et al., 2006). In plant species displaying such abscission, it can be hypothesized that an IDA or IDL ortholog induces the pedicel separation event. Sloughing (i.e., detachment of the root- cap from the remaining root), release of the seed body from the funiculus, and disruption of adhesion at the junction between two adjacent cells during the formation of stomata are also cell–cell separation processes (Stevens and Martin, 1978; Zhao and Sack, 1999; Roberts et al., 2002; del Campillo et al., 2004). During the maturation of continuous tubes of tracheary elements, the middle lamella between two neighboring tracheary elements is digested completely by cell wall–degrading enzymes (Fukuda, 2004). Interestingly, IDL genes are also expressed at these sites (Figures 1A and 1D to 1F; see Supplemental Figure 2 online). The investigation of future IDL knockout mutants may reveal whether the IDL proteins, similar to IDA, represent signals that trigger the cell separation processes at these sites. The overexpression of the IDL genes resulted in phenotypes similar to the overexpression of IDA itself (Figure 2; see Supplemental Figure 3 online). Therefore, one could have expected that all IDL proteins would be capable of complementing the ida mutant when expressed under the control of the IDA promoter. However, this was only the case for IDL1, which has the highest overall sequence similarity to IDA (Figures 4C and 4D). The failure of the other IDL proteins to rescue the mutant phenotype was not due to the low sequence similarity in the N terminus and variable regions, as all the IDA:IDL-IDA constructs were functional. The gene deletion analysis substantiated that the variable region of IDA was not required for IDA function (Figure 5). All constructs containing the EPIP motif rescued ...
Context 4
... Figure 2 online). IDL4:GUS was also expressed in guard cells of young seedlings (Figure 1F), whereas both IDL4: GUS and IDL5:GUS showed expression in hydathodes (Figure 1G). To elucidate IDL gene function, constructs for overexpression of all the IDL genes were made, using the strong constitutive cauliflower mosaic virus 35S promoter (see Methods), and primary transformants were investigated for abnormal phenotypes. The AZ of 35S:IDA plants, characterized by premature floral organ abscission, shows an increased number of rounded AZ cells secreting a white substance shown to be rich in arabino- galactan (AG) (Stenvik et al., 2006). Overexpression of all IDL genes resulted in a similar abscission phenotype. However, the severity of the phenotype varied; 35S:IDL1 plants usually had the strongest resemblance to 35S:IDA and 35S:IDL5 the weakest (Figure 2; see Supplemental Figures 3A to 3C online). Compared with the wild type (Figure 2A) and similar to 35S:IDA plants (Figure 2B), the 35S:IDL lines showed premature abscission of floral organs and had shed their floral organs by position 7 (Figures 2C and 2D). After abscission, AZs with an increase in the number of rounded AZs cells and secretion of AG were found in all lines (Figures 2F to 2H; see Supplemental Figures 3A to 3C) but not found in wild-type AZs (Figure 2E). As for plants overexpressing IDA (Stenvik et al., 2006), siliques, flowers, and cauline leaves were in some cases abscised, and AZ-like cells covered with AG were also seen at these abscission sites (see Supplemental Figures 3D to 3F online). These results indicate some degree of functional redundancy. To further investigate the role of HAE in abscission, a line (SALK 021905) (Alonso et al., 2003) with a T-DNA inserted in the sequence encoding the 10th LRR (see Supplemental Figure 4 online) was analyzed. Unlike the HAE antisense lines, which were reported to have delayed floral organ abscission (Jinn et al., 2000), all homozygous plants for the T-DNA insertion ( hae mutants) were indistinguishable from wild-type plants (data not shown). HAE is closely related to the two genes HSL1 ( At1g28440 ) and HSL2 ( At5g65710 ) (Shiu and Bleecker, 2001b). HSL1 and HSL2 share 58 and 45% overall identity, respectively, to HAE (see Supplemental Figure 4 online). The gene expression pattern of HSL2 in flower development and during stamen abscission (Schmid et al., 2005; Cai and Lashbrook, 2008) is similar to HAE , with low transcript levels prior to anthesis followed by an increase shortly before the onset of abscission (see Supplemental Figure 5 online). HSL1 , on the other hand, shows a different profile, with levels decreasing shortly before the onset of abscission. The congruent expression profile of HAE and HSL2 suggests that these two genes may be functionally redundant. A homozygous T-DNA insertion line for HSL2 (SALK 030520), with an insertion in the predicted transmembrane domain coding sequence (see Supplemental Figure 4 online) depicting normal abscission (data not shown) was crossed to the hae T-DNA mutant. Similar to the ida mutant, hae hsl2 double mutant plants were completely deficient in floral organ abscission, retaining their petals, sepals, and filaments indefinitely (Figure 3A). RT- PCR performed on cDNA from AZ tissue of flowers from positions 4 to 8 failed to detect any HAE or HSL2 transcripts (Figure 3B). This combined with the position of the T-DNA insertions suggests that the insertions have resulted in null alleles for both genes. A quantitative determination of hae hsl2 floral organ abscission was obtained by measuring the force needed to remove the petals from the receptacle of the plant (i.e., petal break strength [pBS]) (Fernandez et al., 2000; Lease et al., 2006). The double mutant had high pBS at positions 2 and 4, exceeding 2-gram equivalents, following a substantial decrease by position 8, which compared with wild-type flowers is a delay by two positions (Figure 3C). This pBS profile was also recorded for ida (Figure 3C) and is indicative of cell wall loosening (Butenko et al., 2003). For both hae hsl2 and ida but not the wild type, the drop in pBS is followed by an increase, where flowers at position 20 have pBS similar to those at position 6 (Figure 3C). To explore the possibility that the IDA , HAE , and HSL2 genes could be acting in the same genetic pathway, a single locus 35S: IDA line overexpressing IDA (Figure 3A) was crossed to hae hsl2 . An F2 population of plants was genotyped for the presence of homozygote T-DNA insertions in HAE and HSL2 and for the inclusion of the 35S:IDA sequence. All homozygous hae ...
Context 5
... Figure 2 online). IDL4:GUS was also expressed in guard cells of young seedlings (Figure 1F), whereas both IDL4: GUS and IDL5:GUS showed expression in hydathodes (Figure 1G). To elucidate IDL gene function, constructs for overexpression of all the IDL genes were made, using the strong constitutive cauliflower mosaic virus 35S promoter (see Methods), and primary transformants were investigated for abnormal phenotypes. The AZ of 35S:IDA plants, characterized by premature floral organ abscission, shows an increased number of rounded AZ cells secreting a white substance shown to be rich in arabino- galactan (AG) (Stenvik et al., 2006). Overexpression of all IDL genes resulted in a similar abscission phenotype. However, the severity of the phenotype varied; 35S:IDL1 plants usually had the strongest resemblance to 35S:IDA and 35S:IDL5 the weakest (Figure 2; see Supplemental Figures 3A to 3C online). Compared with the wild type (Figure 2A) and similar to 35S:IDA plants (Figure 2B), the 35S:IDL lines showed premature abscission of floral organs and had shed their floral organs by position 7 (Figures 2C and 2D). After abscission, AZs with an increase in the number of rounded AZs cells and secretion of AG were found in all lines (Figures 2F to 2H; see Supplemental Figures 3A to 3C) but not found in wild-type AZs (Figure 2E). As for plants overexpressing IDA (Stenvik et al., 2006), siliques, flowers, and cauline leaves were in some cases abscised, and AZ-like cells covered with AG were also seen at these abscission sites (see Supplemental Figures 3D to 3F online). These results indicate some degree of functional redundancy. To further investigate the role of HAE in abscission, a line (SALK 021905) (Alonso et al., 2003) with a T-DNA inserted in the sequence encoding the 10th LRR (see Supplemental Figure 4 online) was analyzed. Unlike the HAE antisense lines, which were reported to have delayed floral organ abscission (Jinn et al., 2000), all homozygous plants for the T-DNA insertion ( hae mutants) were indistinguishable from wild-type plants (data not shown). HAE is closely related to the two genes HSL1 ( At1g28440 ) and HSL2 ( At5g65710 ) (Shiu and Bleecker, 2001b). HSL1 and HSL2 share 58 and 45% overall identity, respectively, to HAE (see Supplemental Figure 4 online). The gene expression pattern of HSL2 in flower development and during stamen abscission (Schmid et al., 2005; Cai and Lashbrook, 2008) is similar to HAE , with low transcript levels prior to anthesis followed by an increase shortly before the onset of abscission (see Supplemental Figure 5 online). HSL1 , on the other hand, shows a different profile, with levels decreasing shortly before the onset of abscission. The congruent expression profile of HAE and HSL2 suggests that these two genes may be functionally redundant. A homozygous T-DNA insertion line for HSL2 (SALK 030520), with an insertion in the predicted transmembrane domain coding sequence (see Supplemental Figure 4 online) depicting normal abscission (data not shown) was crossed to the hae T-DNA mutant. Similar to the ida mutant, hae hsl2 double mutant plants were completely deficient in floral organ abscission, retaining their petals, sepals, and filaments indefinitely (Figure 3A). RT- PCR performed on cDNA from AZ tissue of flowers from positions 4 to 8 failed to detect any HAE or HSL2 transcripts (Figure 3B). This combined with the position of the T-DNA insertions suggests that the insertions have resulted in null alleles for both genes. A quantitative determination of hae hsl2 floral organ abscission was obtained by measuring the force needed to remove the petals from the receptacle of the plant (i.e., petal break strength [pBS]) (Fernandez et al., 2000; Lease et al., 2006). The double mutant had high pBS at positions 2 and 4, exceeding 2-gram equivalents, following a substantial decrease by position 8, which compared with wild-type flowers is a delay by two positions (Figure 3C). This pBS profile was also recorded for ida (Figure 3C) and is indicative of cell wall loosening (Butenko et al., 2003). For both hae hsl2 and ida but not the wild type, the drop in pBS is followed by an increase, where flowers at position 20 have pBS similar to those at position 6 (Figure 3C). To explore the possibility that the IDA , HAE , and HSL2 genes could be acting in the same genetic pathway, a single locus 35S: IDA line overexpressing IDA (Figure 3A) was crossed to hae hsl2 . An F2 population of plants was genotyped for the presence of homozygote T-DNA insertions in HAE and HSL2 and for the inclusion of the 35S:IDA sequence. All homozygous hae ...
Similar publications
Polyamine oxidases (PAOs) are FAD-dependent enzymes involved in polyamine catabolism. In Arabidopsis thaliana, five PAOs (AtPAO1-5) are present with cytosolic or peroxisomal localization. Here, we present a detailed study of the expression pattern of AtPAO1, AtPAO2, AtPAO3 and AtPAO5 during seedling and flower growth and development through analysi...
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 preco...
Appropriate cell division and differentiation ensure normal anther development in angiosperms. BARELY ANY MERISTEM ½ (BAM½) and RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2), two groups of leucine-rich repeat receptor-like protein kinase (LRR-RLK), are required for early anther cell specification. However, little is known about the molecular mechanisms und...
TERMINAL FLOWER1 (TFL1) is a key regulator of flowering time and the development of the inflorescence meristem in Arabidopsis thaliana. TFL1 and FLOWERING LOCUS T (FT) have highly conserved amino acid sequences but opposite functions. For example, FT promotes flowering and TFL1 represses it; FT-overexpressing plants and TFL1 loss-of-function mutant...
Arabidopsis NSN1 encodes a nucleolar GTP-binding protein and is required for flower development. Defective flowers were formed in heterozygous nsn1/+ plants. Homozygous nsn1 plants were dwarf and exhibited severe defects in reproduction. Arrests in embryo development in nsn1 could occur at any stage of embryogenesis. Cotyledon initiation and develo...
Citations
... IDA binding to HAE and HSL2 promotes receptor association with members of the co-receptor somatic embryogenesis receptor kinase (SERK) family and further downstream signaling leading to cell separation events. (Cho et al., 2008;Meng et al., 2016;Santiago et al., 2016;Stenvik et al., 2008). Recently, it has been shown that IDA and IDL family members also bind and activate HSL1 in the regulation of leaf epidermal pattering, indicating subfunctionalisation within this clade of receptors (Roman et al., 2022) and opens for the possibility that HAE or HSL2 could have additional functions than regulating cell separation events. ...
... A deficiency in the IDA signaling pathway prevents the expression of genes encoding secreted cell wall remodeling and hydrolase enzymes thus hindering floral organs to abscise (Butenko et al., 2003;Cho et al., 2008;Kumpf et al., 2013;Meng et al., 2016;Niederhuth et al., 2013;Stenvik et al., 2008;Taylor and Walker, 2018). Interestingly, components of the IDA signaling pathway control different cell separation events during Arabidopsis development. ...
... These results are in stark contrast to what has previously been reported, where cell separation during floral abscission was shown to be dependent on RBOHD and RBOHF (Lee et al., 2018). We used a stress transducer to quantify the force needed to remove petals, the petal breakstrength (pBS; Stenvik et al., 2008), from the receptacle of gradually older flowers along the inflorescence of WT and rbohd rbohf flowers. Measurements showed a significant lower pBS value for the rbohd rbohf mutant compared to WT at the developmental stage where cell loosening normally occurs, indicating premature cell wall loosening (Figure 2-figure supplement 4e). ...
The abscission of floral organs and emergence of lateral roots in Arabidopsis is regulated by the peptide ligand inflorescence deficient in abscission (IDA) and the receptor protein kinases HAESA (HAE) and HAESA-like 2 (HSL2). During these cell separation processes, the plant induces defense-associated genes to protect against pathogen invasion. However, the molecular coordination between abscission and immunity has not been thoroughly explored. Here, we show that IDA induces a release of cytosolic calcium ions (Ca ²⁺ ) and apoplastic production of reactive oxygen species, which are signatures of early defense responses. In addition, we find that IDA promotes late defense responses by the transcriptional upregulation of genes known to be involved in immunity. When comparing the IDA induced early immune responses to known immune responses, such as those elicited by flagellin22 treatment, we observe both similarities and differences. We propose a molecular mechanism by which IDA promotes signatures of an immune response in cells destined for separation to guard them from pathogen attack.
... IDA Peptides IDA peptides, a class of small signaling proteins, are generated via the proteolytic cleavage of their precursor protein catalyzed by subtilases (Katharina et al., 2016;Stin i and Schaller, 2022). They possess an N-terminal secretory signal peptide, guiding the protein to the extracellular milieu, while the C-terminal proline-rich extended motifs, known as the EPIP motif, comprised of 20 amino acids, play a pivotal role in inducing the abscission of floral organs post-pollination (Stenvik et al., 2008;Vie et al., 2017). Reportedly, IDA peptides regulate the shedding of cauline leaves in response to dehydration stress (Taylor et al., 2019). ...
... Reportedly, IDA peptides regulate the shedding of cauline leaves in response to dehydration stress (Taylor et al., 2019). Under water deficit conditions, when leaves show signs of wilting, the bioactive IDA peptide ligand is perceived by a receptor complex composed of either the receptor kinases HAESA or HAESA-like 2 (HSL2) along with SERK coreceptors ( Figure 5E; Stenvik et al., 2008;Taylor et al., 2019). This recognition event initiates a downstream signaling cascade involving MAPKs, ultimately governing the expression of cell-wall modifying and hydrolytic enzymes, particularly polygalacturonases and xyloglucan endotransglucosylase/hydrolases ( Figure 5E; Stenvik et al., 2008;Taylor et al., 2019). ...
... Under water deficit conditions, when leaves show signs of wilting, the bioactive IDA peptide ligand is perceived by a receptor complex composed of either the receptor kinases HAESA or HAESA-like 2 (HSL2) along with SERK coreceptors ( Figure 5E; Stenvik et al., 2008;Taylor et al., 2019). This recognition event initiates a downstream signaling cascade involving MAPKs, ultimately governing the expression of cell-wall modifying and hydrolytic enzymes, particularly polygalacturonases and xyloglucan endotransglucosylase/hydrolases ( Figure 5E; Stenvik et al., 2008;Taylor et al., 2019). These enzymes degrade the middle lamella, thereby inducing cell separation. ...
Small peptides (SPs), ranging from 5 to 100 amino acids, play integral roles in plants due to their diverse functions. Despite their low abundance and small molecular weight, SPs intricately regulate critical aspects of plant life, including cell division, growth, differentiation, flowering, fruiting, maturation, and stress responses. As vital mediators of intercellular signaling, SPs have garnered significant attention in plant biology research. This comprehensive review delves into SPs' structure, classification, and identification, providing a detailed understanding of their significance. Additionally, we summarize recent findings on the biological functions and signaling pathways of prominent SPs that regulate plant growth and development. The review also offers a forward-looking perspective on future research directions in peptide signaling pathways.
... Genetic studies have demonstrated that once IDA binds to HAE/HSL2, HAE/HSL2 can modulate KNOTTED1-LIKE HOME-OBOX (KNOX) transcription factors through a mitogen-activated protein (MAP) kinase cascade [34,60,61] . Although the knat2/knat6 mutant only exhibits a weak phenotype, it is suggested that KNAT2 and KNAT6, together with other transcription factors, induce the transcription of cell wall remodeling and degrading enzymes responsible for cell separation and subsequent organ abscission.Thus, IDA-HAE/HSL2 signaling is To date, IDA-like and HAE-like genes have been identified in crop species and highly expressed in the AZ, including tomato [65−67] , soybean [68] , yellow lupine [69,70] , rose [71] , and woody fruit crops oil palm [72] , citrus [73] , and litchi [74,75] . Research has demonstrated that synthetic IDA peptides have the ability to induce premature abscission of floral organs in Arabidopsis [76] , flower separation in yellow lupine [70,77] , leaf abscission in poplar and mature fruit abscission in oil palm [78] . ...
... In addition, when IDA homologous genes from citrus (CitIDA3), litchi (LcIDL1), and rose (RbIDL1 and RbIDL4) are ectopically expressed in Arabidopsis, they can cause early flower abscission and rescue the abscission defects in ida [71,73,74] . Furthermore, the transformation of the litchi LcHSL2 into Arabidopsis hae hsl2 mutants can reactivate the abscission of floral organs [75] . Together, these evidences suggest that the function of the IDA-HAE/HSL2 signaling module is conserved in many species, including woody fruit crops. ...
... Furthermore, the expression of these genes can be postponed by employing an ethylene inhibitor [65,69,[71][72][73]81] . During fruit abscission in litchi, it has been observed that LcIDL1 and LcHSL2, which are located in the AZ, are also induced by ethylene [74,75] . Recent studies further demonstrated that low light intensity associated stress induces ethylene synthesis, which in turn increases the expression of the SlIDL6, ultimately promoting the abscission of tomato flowers [66] . ...
From an evolutionary perspective, fruit abscission is an intelligent regulatory mechanism by which fruit trees adapt to their environment and ensure offspring. However, from an agricultural production standpoint, unwanted fruit abscission can cause significant loss in fruit yield and economic value. Therefore, investigating the mechanisms of fruit abscission has always been an important focus in the field of plant research. Acquiring a thorough comprehension of the underlying mechanisms responsible for fruit abscission is highly valuable for enhancing fruit crop breeding and optimizing harvesting practices. In this review, we focus on fruit abscission, particularly discussing the nature of abscission cues within the abscising fruit, how these signals are generated and transmitted, and how the abscission zone cells perceive and respond to these signals in woody fruit crops.
... The ER signaling pathways result from forming complexes with signaling proteins, such as SERK coreceptors and too many mouths (TMM) transmembrane proteins (Lee et al., 2012;Meng et al., 2015). Similarly, another RLK, meristematic receptor-like kinase (MRLK) expressed in root and (Tsuwamoto et al., 2008;Pfister et al., 2014;Racolta et al., 2014) CLAVATA1 AT1G75820 LRR Floral meristem size, shoot meristems (Clark et al., 1997;Fletcher et al., 1999;Han et al., 2020;Shang et al., 2021) HAESA/ HSL AT4G28490 AT1G28440 AT5G65710 LRR Floral organ abscission (Jinn et al., 2000;Stenvik et al., 2008;Meng et al., 2016;Santiago et al., 2016) ZAR1 AT2G01210 LRR Zygotic division ER/ERL1/2 AT2G26330 AT5G62230 AT5G07180 LRR Root and shoot apical meristem, ovule, and seed density (Yokoyama et al., 1998;Shpak et al., 2003;Kawamoto et al., 2020;Herrmann and Torii, 2021 RLKs involved in plant growth and development. The plant hormone BR binds BRI1, and signaling peptides are perceived by the other membranelocalized RLKs. ...
All living organisms must develop mechanisms to cope with and adapt to new environments. The transition of plants from aquatic to terrestrial environment provided new opportunities for them to exploit additional resources but made them vulnerable to harsh and ever-changing conditions. As such, the transmembrane receptor-like kinases (RLKs) have been extensively duplicated and expanded in land plants, increasing the number of RLKs in the advanced angiosperms, thus becoming one of the largest protein families in eukaryotes. The basic structure of the RLKs consists of a variable extracellular domain (ECD), a transmembrane domain (TM), and a conserved kinase domain (KD). Their variable ECDs can perceive various kinds of ligands that activate the conserved KD through a series of auto- and trans-phosphorylation events, allowing the KDs to keep the conserved kinase activities as a molecular switch that stabilizes their intracellular signaling cascades, possibly maintaining cellular homeostasis as their advantages in different environmental conditions. The RLK signaling mechanisms may require a coreceptor and other interactors, which ultimately leads to the control of various functions of growth and development, fertilization, and immunity. Therefore, the identification of new signaling mechanisms might offer a unique insight into the regulatory mechanism of RLKs in plant development and adaptations. Here, we give an overview update of recent advances in RLKs and their signaling mechanisms.
... When abscission zones have developed and floral organs are no longer required, abscission zones secrete the peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) to trigger the cell separation (Butenko et al., 2003). IDA is perceived in the AZ cells by leucinerich repeat (LRR) receptor kinases (RKs) HAESA (HAE) and HAESA-LIKE 2 (HSL2) and their coreceptors, members of the family of somatic embryogenesis receptor kinases (SERKs; (Jinn et al., 2000;Cho et al., 2008;Stenvik et al., 2008;Meng et al., 2016;Santiago et al., 2016)). When the HAE-A c c e p t e d M a n u s c r i p t 4 SERK/HSL2-SERK receptor complexes activate, they trigger an intracellular signaling cascade of MITOGEN-ACTIVATED PROTEIN KINASES (MAPKs;(Cho et al., 2008;Zhu et al., 2019)). ...
... Other regulators influence abscission indirectly by modulating the IDA-induced signaling pathway (Liljegren et al., 2009;Leslie et al., 2010;Burr et al., 2011;Liu et al., 2013;Gubert and Liljegren, 2014;Baer et al., 2016;Taylor et al., 2019) The signaling cascade induced by IDA and HAE/HSL2 in AZs is a requisite for floral organ abscission. Double mutants hae hsl2 retain all floral organs across floral positions in the inflorescence (Cho et al., 2008;Stenvik et al., 2008). Meanwhile, ida knockouts display a weaker abscission phenotype, with floral organs being loosely attached in floral positions in which siliques are elongating Alling and Galindo-Trigo, 2023). ...
Plants shed organs like leaves, petals or fruits through the process of abscission. Monitoring cues like age, resource availability, biotic and abiotic stresses allows plants to abscise organs in a timely manner. How these signals are integrated in the molecular pathways that drive abscission is largely unknown. The INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) gene is one of the main drivers of floral organ abscission in Arabidopsis and is known to transcriptionally respond to most abscission-regulating cues. Interrogating the IDA promoter in silico and in vitro we identified transcription factors that can potentially modulate IDA expression. We probed the importance of ERF and WRKY binding sites for IDA expression during floral organ abscission, with WRKYs being of special relevance to mediate IDA upregulation in response to biotic stress in tissues destined for separation. We further characterized WRKY57 as a positive regulator of IDA and IDA-like gene expression in abscission zones. Our findings highlight the promise of promoter element-targeted approaches to modulate the responsiveness of the IDA signaling pathway to harness controlled abscission timing for improved crop productivity.
... The families of putative peptides, such as PSK, PSY, RALF, CLE, IDA, CEP, and Pep span across different taxonomic groups of plants. Putative systemin [13,57], PSK [58,59], PSY [60], CLE40 [61][62][63][64][65], PpCLE1-7 [66], IDA [67][68][69][70][71], CEP [72], and Pep [73][74][75] were discovered in different plant families and studied using precursor overexpression, knockouts, and solid-phase peptide synthesis combined with the bioassays. ...
... The search for peptide receptors in plants is a highly challenging task [79]. To date, receptors have been identified for only a couple of dozen isolated peptides, including tomato systemin [37], PSK [38], IDA [67], CLV3 [41], TDIF/CLE41/44 [42], RALF [43], AtPep1 [80], RGF [47], PSY [9,39], CEP1 [45] and CIF [48] ( Table 1). Plant peptide receptors have been identified using sophisticated photoaffinity and radioligand methods, but these methods can be time-consuming and labor-intensive. ...
Plant peptides are a new frontier in plant biology, owing to their key regulatory roles in plant growth, development, and stress responses. Synthetic peptides are promising biological agents that can be used to improve crop growth and protection in an environmentally sustainable manner. Plant regulatory peptides identified in pioneering research, including systemin, PSK, HypSys, RALPH, AtPep1, CLV3, TDIF, CLE, and RGF/GLV/CLEL, hold promise for crop improvement as potent regulators of plant growth and defense. Mass spectrometry and bioinformatics are greatly facilitating the discovery and identification of new plant peptides. The biological functions of most novel plant peptides remain to be elucidated. Bioassays are an essential part in studying the biological activity of identified and putative plant peptides. Root growth assays and cultivated plant cell cultures are widely used to evaluate the regulatory potential of plant peptides during growth, differentiation, and stress reactions. These bioassays can be used as universal approaches for screening peptides from different plant species. Development of high-throughput bioassays can facilitate the screening of large numbers of identified and putative plant peptides, which have recently been discovered but remain uncharacterized for biological activity.
... Despite this divergence in HSL1 function, and similar to our result for BAM1 and CLV1, the full HSL1 coding sequence driven under the HAE promoter can complement hae;hsl2 double mutants (Fig. 3c), consistent with what others have found (Roman et al. 2022). These data indicate that the failure of the endogenous HSL1 locus to trigger abscission in hae;hsl2 mutants is likely due to HSL1's divergent expression pattern, not the functional evolution of the HSL1 protein (Cho et al. 2008;Stenvik et al. 2008;Roman et al. 2022). ...
The coding sequences of developmental genes are expected to be deeply conserved, with cis-regulatory change driving the modulation of gene function. In contrast, proteins with roles in defense are expected to evolve rapidly, in molecular arms-races with pathogens. However, some gene families include both developmental and defense genes. In these families, does the tempo and mode of evolution differ between genes with divergent functions, despite shared ancestry and structure? The leucine-rich repeat receptor-like kinase (LRR-RLKs) protein family includes members with roles in plant development and defense, thus providing an ideal system for answering this question. LRR-RLKs are receptors that traverse plasma membranes. LRR domains bind extracellular ligands, RLK domains initiate intracellular signaling cascades in response to ligand binding. In LRR-RLKs with roles in defense, LRR domains evolve faster than RLK domains. To determine whether this asymmetry extends to LRR-RLKs that function primarily in development, we assessed evolutionary rates and tested for selection acting on eleven sub-families of LRR-RLKs, using deeply sampled protein trees. To assess functional evolution, we performed heterologous complementation assays in Arabidopsis thaliana (arabidopsis). We found that the LRR domains of all tested LRR-RLK proteins evolved faster than their cognate RLK domains. All tested sub-families of LRR-RLKs had strikingly similar patterns of molecular evolution, despite divergent functions. Heterologous transformation experiments revealed that multiple mechanisms likely contribute to the evolution of LRR-RLK function, including escape from adaptive conflict. Our results indicate specific and distinct evolutionary pressures acting on LRR vs. RLK domains, despite diverse organismal roles for LRR-RLK proteins.
... Double mutants hae hsl2 retain all floral organs across floral positions in the inflorescence (Cho et al., 54 2008;Stenvik et al., 2008). Meanwhile, ida knockouts display a weaker abscission phenotype, with 55 floral organs being loosely attached in floral positions in which siliques are elongating (Stenvik et al.,56 2008; Alling and Galindo-Trigo, 2023). ...
Plants shed organs like leaves, petals or fruits through the process of abscission. Monitoring cues like age, resource availability, biotic and abiotic stresses allows plants to abscise organs in a timely manner. How these signals are integrated in the molecular pathways that drive abscission is largely unknown. The INFLORESCENCE DEFICIENT IN ABSCISSION ( IDA ) gene is one of the main drivers of floral organ abscission in Arabidopsis and is known to transcriptionally respond to most abscission-regulating cues. Interrogating the IDA promoter in silico and in vitro we identified transcription factors that can potentially modulate IDA expression. We functionally characterized the importance of ERF and WRKY binding sites for IDA expression during floral organ abscission, with WRKYs being of special relevance to mediate IDA upregulation in response to biotic stress in tissues destined for separation. We further characterized WRKY57 as a positive regulator of IDA and IDA - like gene expression in abscission zones. Our findings highlight the promise of promoter element-targeted approaches to modulate the responsiveness of the IDA signaling pathway to harness controlled abscission timing for improved crop productivity.
Highlight
ERF and WRKY transcription factors distinctly contribute to the regulation of IDA expression and thereby abscission timing. WRKY57 modulates abscission via redundant IDA/IDA-like peptides.
... IDA binding to HAE and HSL2 promotes receptor association with members of the co-receptor SOMATIC EMBRYOGENESIS RECEPTOR KINASE (SERK) family and further downstream signaling leading to cell separation events. (Cho et al., 2008;Santiago et al., 2016;Stenvik et al., 2008). A deficiency in the IDA signaling pathway prevents the expression of genes encoding secreted cell wall remodeling and hydrolase enzymes thus hindering floral organs to abscise (Butenko et al., 2003;Cho et al., 2008;Kumpf et al., 2013;Niederhuth et al., 2013;Stenvik et al., 2008;Kumpf et al., 2013). ...
... (Cho et al., 2008;Santiago et al., 2016;Stenvik et al., 2008). A deficiency in the IDA signaling pathway prevents the expression of genes encoding secreted cell wall remodeling and hydrolase enzymes thus hindering floral organs to abscise (Butenko et al., 2003;Cho et al., 2008;Kumpf et al., 2013;Niederhuth et al., 2013;Stenvik et al., 2008;Kumpf et al., 2013). In addition, IDL1 signals through HSL2 to regulate cell separation during root cap sloughing (Shi et al., 2018). ...
... These results are in stark contrast to what has previously been reported, where cell separation during floral abscission was shown to be dependent on RBOHD and RBOHF (Lee et al., 2018). We used a stress transducer to quantify the force needed to remove petals, the petal breakstrength (pBS) (Stenvik et al., 2008), from the receptacle of gradually older flowers along the inflorescence of WT and rbohd rbohf flowers. Measurements showed a significant lower pBS value for the rbohd rbohf mutant compared to WT at the developmental stage where cell loosening normally occurs, indicating premature cell wall loosening (Sup Fig. 5e). ...
The abscission of floral organs and emergence of lateral roots in Arabidopsis is regulated by the peptide ligand INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) and the receptor protein kinases HAESA (HAE) and HAESA-LIKE 2 (HSL2). During these cell separation processes, the plant induces defense-associated genes to protect against pathogen invasion. However, the molecular coordination between abscission and immunity has not been thoroughly explored. Here we show that IDA induces a receptor-dependent release of cytosolic calcium ions (Ca2+) and apoplastic production of reactive oxygen species, which are signatures of early defense responses. In addition, we find that IDA promotes late defense responses by the transcriptional upregulation of genes known to be involved in immunity. When comparing the IDA induced early immune responses to known immune responses, such as those elicited by flagellin22 treatment, we observe both similarities and differences. We propose a molecular mechanism by which IDA promotes signatures of an immune response in cells destined for separation to guard them from pathogen attack.
... Key regulators controlling abscission have primarily been identified using Arabidopsis and tomato as model systems for abscission of floral organs and flowers, respectively. Arabidopsis floral organ abscission is triggered by the binding of the small peptide IN-FLORESCENCE DEFICIENT ABSCISION (IDA) to the receptor-like kinases HAE and HSL2 [3][4][5][6][7]. Then, MITOGEN-ACTIVATED PROTEIN KINASE KINASE 4/5 and MPK3/6 constitute the MAP kinase cascade, which can be activated by binding of co-receptors and HAE/HSL2 [4,5,7,8]. ...
... Arabidopsis floral organ abscission is triggered by the binding of the small peptide IN-FLORESCENCE DEFICIENT ABSCISION (IDA) to the receptor-like kinases HAE and HSL2 [3][4][5][6][7]. Then, MITOGEN-ACTIVATED PROTEIN KINASE KINASE 4/5 and MPK3/6 constitute the MAP kinase cascade, which can be activated by binding of co-receptors and HAE/HSL2 [4,5,7,8]. In tomatoes, activation of flower abscission can be triggered by interactions between the phytohormones ethylene and auxin, as well as abiotic stresses such as drought and low light intensity [9][10][11][12]. ...
The regulation of abscission has a significant impact on fruit yield and quality. Thus, understanding the mechanisms underlying abscission, particularly identifying key genes, is critical for improving fruit crop breeding and cultivation practices. Here, to explore the key genes involved in litchi fruitlet abscission, the two closest homologs of AGAMOUS-like 15/18 (LcAGL15 and LcAGL18) were identified. During the litchi fruitlet abscission process, LcAGL15 expression was reduced, whereas LcAGL18 expression was increased at the abscission zone. The abscission of floral organs was unaffected by ectopic expression of LcAGL15 in Arabidopsis. Moreover, high expression of LcAGL18 significantly delayed the abscission process of floral organs, particularly the sepals. Overexpression of LcAGL18 in Arabidopsis consistently repressed the expression of abscission-related genes, including HAESA (HAE) and HAESA-LIKE2 (HSL2), and cell wall remodeling genes at the abscission zone. Furthermore, LcAGL18 was localized in the nucleus and acted as a transcriptional inhibitor. Collectively, these results suggest that AGL18 homologs have conserved functions in Arabidopsis and litchi, and that LcAGL18 might function as a key regulator in litchi fruitlet abscission.
