Temporal and spatial expression of a polygalacturonase during leaf and flower abscission in oilseed rape and Arabidopsis

Preventing unwanted breakups: using polygalacturonases to regulate cell separation

Cell separation is an important biological process in plants that is precisely regulated both spatially and temporally. Key separation events include abscission of organs such as leaves and fruit and dehiscence events such as pod shatter in canola and other Brassicas. Polygalacturonases (PGs) are enzymes essential for the degradation of pectin, an important component of the adhesive material between cells. Although there are around 70 PG genes with overlapping expression domains, recent analysis has revealed the function of several PGs in specific aspects of Arabidopsis reproductive development. Upstream regulators that control the expression domain of some of these PGs during reproductive development have also been identified. This information provides new strategies to control unwanted cell separation events in various crops.

Overexpression of AtDOF4.7, an Arabidopsis DOF family transcription factor, induces floral organ abscission deficiency in Arabidopsis

After flower pollination, a programmed process called abscission occurs in which unwanted floral organs are actively shed from the main plant body. We found that a member of the DOF (for DNA binding with one finger) transcription factor family, Arabidopsis (Arabidopsis thaliana) DOF4.7, was expressed robustly in the abscission zone. The Arabidopsis 35S::AtDOF4.7 lines with constitutive expression of AtDOF4.7 exhibited an ethylene-independent floral organ abscission deficiency. In these lines, anatomical analyses showed that the formation of the abscission zone was normal. However, dissolution of the middle lamella failed to separate between the cell walls. AtDOF4.7 was identified as a nucleus-localized transcription factor. This protein had both in vitro and in vivo binding activity to typical DOF cis-elements in the promoter of an abscission-related polygalacturonase (PG) gene, PGAZAT. Overexpression of AtDOF4.7 resulted in down-regulation of PGAZAT. AtDOF4.7 interacted with another abscission-related transcription factor, Arabidopsis ZINC FINGER PROTEIN2. Taken together, our results suggest that AtDOF4.7 participates in the control of abscission as part of the transcription complex that directly regulates the expression of cell wall hydrolysis enzymes.

Molecular analysis and expression of a floral organ-specific polygalacturonase gene isolated from rapeseed (Brassica napus L.)

High throughput screening of stage-specific differentially expressed genes in a Brassica napus two-line Rs1046A/B subtractive library was used to identify the BnQRT3 gene associated with cell wall metabolism. Phylogenetic analysis indicates the protein product of BnQRT3 is polygalacturonase. According to cytological comparisons of Rs1046 sterile and fertile anthers, RT-PCR studies and in situ hybridizations, BnQRT3 is expressed most strongly in floral organs and may play an essential role in pollen maturation. Analysis of the histological staining pattern of BnQRT3 promoter-GUS constructs in transgenic Arabidopsis and Brassica napus revealed that proximal part of 5'-flanking region directed expression in the vascular tissue of filaments, veins in sepal and petals, stigma, branch connective and the floral organ abscission zone during the open flower stage. In the meanwhile, Activity of BnQRT3 was detected in the anthers, which commences at the microsporocyte stage and persists as anther approaches dehiscence. Strong GUS expression also can be observed in the vascular tissue of leaves and stem by compression with forceps or excision, suggesting that the BnQRT3 promoter is responsive to wounding.

Characterization and structural analysis of wild type and a non-abscission mutant at the development funiculus (Def) locus in Pisum sativum L

BACKGROUND

In pea seeds (Pisum sativum L.), the Def locus defines an abscission event where the seed separates from the funicle through the intervening hilum region at maturity. A spontaneous mutation at this locus results in the seed failing to abscise from the funicle as occurs in wild type peas. In this work, structural differences between wild type peas that developed a distinct abscission zone (AZ) between the funicle and the seed coat and non-abscission def mutant were characterized.

RESULTS

A clear abscission event was observed in wild type pea seeds that were associated with a distinct double palisade layers at the junction between the seed coat and funicle. Generally, mature seeds fully developed an AZ, which was not present in young wild type seeds. The AZ was formed exactly below the counter palisade layer. In contrast, the palisade layers at the junction of the seed coat and funicle were completely absent in the def mutant pea seeds and the cells in this region were seen to be extensions of surrounding parenchymatous cells.

CONCLUSION

The Def wild type developed a distinct AZ associated with palisade layer and counterpalisade layer at the junction of the seed coat and funicle while the def mutant pea seed showed non-abscission and an absence of the double palisade layers in the same region. We conclude that the presence of the double palisade layer in the hilum of the wild type pea seeds plays an important structural role in AZ formation by delimiting the specific region between the seed coat and the funicle and may play a structural role in the AZ formation and subsequent detachment of the seed from the funicle.

Transcriptional activation of a 37 kDa ethylene responsive cysteine protease gene, RbCP1, is associated with protein degradation during petal abscission in rose

Cysteine proteases play an important role in several developmental processes in plants, particularly those related to senescence and cell death. A cysteine protease gene, RbCP1, has been identified that encodes a putative protein of 357 amino acids and is expressed in the abscission zone (AZ) of petals in rose. The gene was responsive to ethylene in petals, petal abscission zones, leaves, and thalamus. The expression of RbCP1 increased during both ethylene-induced as well as natural abscission and was inhibited by 1-MCP. Transcript accumulation of RbCP1 was accompanied by the appearance of a 37 kDa cysteine protease, a concomitant increase in protease activity and a substantial decrease in total protein content in the AZ of petals. Agro-injection of rose petals with a 2.0 kb region upstream of the RbCP1 gene could drive GUS expression in an abscission zone-specific manner and was blocked by 1-MCP. It is concluded that petal abscission is associated with a decrease in total protein content resulting from rapid transcription of RbCP1 and the expression of a 37 kDa protease.

ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE1 (ADPG1), ADPG2, and QUARTET2 are Polygalacturonases required for cell separation during reproductive development in Arabidopsis

Cell separation is thought to involve degradation of pectin by several hydrolytic enzymes, particularly polygalacturonase (PG). Here, we characterize an activation tagging line with reduced growth and male sterility caused by increased expression of a PG encoded by QUARTET2 (QRT2). QRT2 is essential for pollen grain separation and is part of a small family of three closely related endo-PGs in the Arabidopsis thaliana proteome, including ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE1 (ADPG1) and ADPG2. Functional assays and complementation experiments confirm that ADPG1, ADPG2, and QRT2 are PGs. Genetic analysis demonstrates that ADPG1 and ADPG2 are essential for silique dehiscence. In addition, ADPG2 and QRT2 contribute to floral organ abscission, while all three genes contribute to anther dehiscence. Expression analysis is consistent with the observed mutant phenotypes. INDEHISCENT (IND) encodes a putative basic helix-loop-helix required for silique dehiscence, and we demonstrate that the closely related HECATE3 (HEC3) gene is required for normal seed abscission and show that IND and HEC3 are required for normal expression of ADPG1 in the silique dehiscence zone and seed abscission zone, respectively. We also show that jasmonic acid and ethylene act together with abscisic acid to regulate floral organ abscission, in part by promoting QRT2 expression. These results demonstrate that multiple cell separation events, including both abscission and dehiscence, require closely related PG genes.

Expression analysis of the BFN1 nuclease gene promoter during senescence, abscission, and programmed cell death-related processes

Little is known about the biological role of nucleases induced during plant senescence and programmed cell death (PCD). Arabidopsis BFN1 has been identified as a senescence-associated type I nuclease, whose protein sequence shares high homology with some other senescence- or PCD-associated plant nucleases. To learn about BFN1 regulation, its expression pattern was analysed. A 2.3 kb portion of the 5' promoter sequence of BFN1 was cloned and its ability to activate the GUS reporter gene was examined. Transgenic Arabidopsis and tomato plants harbouring this chimeric construct were analysed for GUS expression. In both, the BFN1 promoter was able specifically to direct GUS expression in senescent leaves, differentiating xylem and the abscission zone of flowers. Thus, at least part of the regulation of BFN1 is mediated at the transcriptional level, and the regulatory elements are recognized in the two different plants. In tomato, specific expression was observed in the leaf and the fruit abscission zones. The BFN1 promoter was also active in other tissues, including developing anthers and seeds, and in floral organs after fertilization. PCD has been implicated in all of these processes, suggesting that in addition to senescence, BFN1 is involved in PCD associated with different development processes in Arabidopsis.

Ethylene-induced differential gene expression during abscission of citrus leaves

The main objective of this work was to identify and classify genes involved in the process of leaf abscission in Clementina de Nules (Citrus clementina Hort. Ex Tan.). A 7 K unigene citrus cDNA microarray containing 12 K spots was used to characterize the transcriptome of the ethylene-induced abscission process in laminar abscission zone-enriched tissues and the petiole of debladed leaf explants. In these conditions, ethylene induced 100% leaf explant abscission in 72 h while, in air-treated samples, the abscission period started later and took 240 h. Gene expression monitored during the first 36 h of ethylene treatment showed that out of the 12 672 cDNA microarray probes, ethylene differentially induced 725 probes distributed as follows: 216 (29.8%) probes in the laminar abscission zone and 509 (70.2%) in the petiole. Functional MIPS classification and manual annotation of differentially expressed genes highlighted key processes regulating the activation and progress of the cell separation that brings about abscission. These included cell-wall modification, lipid transport, protein biosynthesis and degradation, and differential activation of signal transduction and transcription control pathways. Expression data associated with the petiole indicated the occurrence of a double defensive strategy mediated by the activation of a biochemical programme including scavenging ROS, defence and PR genes, and a physical response mostly based on lignin biosynthesis and deposition. This work identifies new genes probably involved in the onset and development of the leaf abscission process and suggests a different but co-ordinated and complementary role for the laminar abscission zone and the petiole during the process of abscission.

The BLADE-ON-PETIOLE genes are essential for abscission zone formation in Arabidopsis

The Arabidopsis BLADE-ON-PETIOLE 1 (BOP1) and BOP2 genes encode redundant transcription factors that promote morphological asymmetry during leaf and floral development. Loss-of-function bop1 bop2 mutants display a range of developmental defects, including a loss of floral organ abscission. Abscission occurs along specialised cell files, called abscission zones (AZs) that develop at the junction between the leaving organ and main plant body. We have characterized the bop1 bop2 abscission phenotype to determine how BOP1 and BOP2 contribute to the known abscission developmental framework. Histological analysis and petal breakstrength measurements of bop1 bop2 flowers show no differentiation of floral AZs. Furthermore, vestigial cauline leaf AZs are also undifferentiated in bop1 bop2 mutants, suggesting that BOP proteins are essential to establish AZ cells in different tissues. In support of this hypothesis, BOP1/BOP2 activity is required for both premature floral organ abscission and the ectopic abscission of cauline leaves promoted by the INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) gene under the control of the constitutive CaMV 35S promoter. Expression of several abscission-related marker genes, including IDA, is relatively unperturbed in bop1 bop2 mutants, indicating that these AZ genes respond to positional cues that are independent of BOP1/BOP2 activity. We also show that BOP1 and BOP2 promote growth of nectary glands, which normally develop at the receptacle adjacent to developing AZs. Taken together, these data suggest that BOP1/BOP2 activity is required for multiple cell differentiation events in the proximal regions of inflorescence lateral organs.

Stamen abscission zone transcriptome profiling reveals new candidates for abscission control: enhanced retention of floral organs in transgenic plants overexpressing Arabidopsis ZINC FINGER PROTEIN2

Organ detachment requires cell separation within abscission zones (AZs). Physiological studies have established that ethylene and auxin contribute to cell separation control. Genetic analyses of abscission mutants have defined ethylene-independent detachment regulators. Functional genomic strategies leading to global understandings of abscission have awaited methods for isolating AZ cells of low abundance and very small size. Here, we couple laser capture microdissection of Arabidopsis thaliana stamen AZs and GeneChip profiling to reveal the AZ transcriptome responding to a developmental shedding cue. Analyses focus on 551 AZ genes (AZ(551)) regulated at the highest statistical significance (P < or = 0.0001) over five floral stages linking prepollination to stamen shed. AZ(551) includes mediators of ethylene and auxin signaling as well as receptor-like kinases and extracellular ligands thought to act independent of ethylene. We hypothesized that novel abscission regulators might reside in disproportionately represented Gene Ontology Consortium functional categories for cell wall modifying proteins, extracellular regulators, and nuclear-residing transcription factors. Promoter-beta-glucuronidase expression of one transcription factor candidate, ZINC FINGER PROTEIN2 (AtZFP2), was elevated in stamen, petal, and sepal AZs. Flower parts of transgenic lines overexpressing AtZFP2 exhibited asynchronous and delayed abscission. Abscission defects were accompanied by altered floral morphology limiting pollination and fertility. Hand-pollination restored transgenic fruit development but not the rapid abscission seen in wild-type plants, demonstrating that pollination does not assure normal rates of detachment. In wild-type stamen AZs, AtZFP2 is significantly up-regulated postanthesis. Phenotype data from transgene overexpression studies suggest that AtZFP2 participates in processes that directly or indirectly influence organ shed.

Physiological roles of plant glycoside hydrolases

The functions of plant glycoside hydrolases and transglycosidases have been studied using different biochemical and molecular genetic approaches. These enzymes are involved in the metabolism of various carbohydrates containing compounds present in the plant tissues. The structural and functional diversity of the carbohydrates implies a vast spectrum of enzymes involved in their metabolism. Complete genome sequence of Arabidopsis and rice has allowed the classification of glycoside hydrolases in different families based on amino acid sequence data. The genomes of these plants contain 29 families of glycoside hydrolases. This review summarizes the current research on plant glycoside hydrolases concerning their principal functional roles, which were attributed to different families. The majority of these plant glycoside hydrolases are involved in cell wall polysaccharide metabolism. Other functions include their participation in the biosynthesis and remodulation of glycans, mobilization of energy, defence, symbiosis, signalling, secondary plant metabolism and metabolism of glycolipids.

Xyloglucan-pectin linkages are formed intra-protoplasmically, contribute to wall-assembly, and remain stable in the cell wall

We tested two hypotheses for the mechanism by which xyloglucan-pectin covalent bonds are formed in Arabidopsis cell cultures. Hypothesis 1 proposed hetero-transglycosylation, with xyloglucan as donor substrate and a rhamnogalacturonan-I (RG-I) side-chain as acceptor. We looked for enzyme activities that catalyse this reaction using alpha-(1-->5)-L-[(3)H]arabino- or beta-(1-->4)-D-[(3)H]galacto-oligosaccharides as model acceptor substrates. The (3)H-oligosaccharides were supplied (with or without added xyloglucans) to living Arabidopsis cell-cultures, permeabilised cells, cell-free extracts, or four authentic XTHs. No hetero-transglycosylation occurred. Therefore, we cannot support hypothesis 1. Hypothesis 2 proposed that some xyloglucan is manufactured de novo as a side-chain on RG-I. To test this, we pulse-labelled Arabidopsis cell-cultures with [(3)H]arabinose and monitored the radiolabelling of anionic (pectin-bonded) xyloglucan, which was resolved from free xyloglucan by ion-exchange chromatography. [(3)H]Xyloglucan-pectin complexes were detectable <4 min after [(3)H]arabinose feeding, which is shorter than the transit-time for polysaccharide secretion, indicating that xyloglucan-pectin bonds were formed intra-protoplasmically. Thereafter, the proportion of the wall-bound [(3)H]xyloglucan that was anionic remained almost constant at approximately 50% for > or =6 days, showing that the xyloglucan-pectin bond was stable in vivo. Some [(3)H]xyloglucan was rapidly sloughed into the medium instead of becoming wall-bound. Only approximately 30% of the sloughed [(3)H]xyloglucan was anionic, indicating that bonding to pectin promoted the integration of xyloglucan into the wall. We conclude that approximately 50% of xyloglucan in cultured Arabidopsis cells is synthesised on a pectic primer, then secreted into the apoplast, where the xyloglucan-pectin bonds are stable and the pectic moiety aids wall-assembly.

Ethylene induced cotton leaf abscission is associated with higher expression of cellulase (GhCel1) and increased activities of ethylene biosynthesis enzymes in abscission zone

Ethylene induced cotton (Gossypium hirsutum var RST-39) leaf abscission has been characterized by measuring the activities of ACC synthase (ACS, E.C. 4.4.1.14), ACC oxidase (ACO, E.C. 1.14.17.4) and cellulase (E.C. 3.2.1.4). In addition, a leaf abscission specific cDNA (GhCel1) has been cloned from cotton, which belongs to the alpha(2) subgroup of cellulases that possess a C-terminus carbohydrate-binding domain. Measurement of enzyme activity in the abscission zones of cotton leaf explants exposed to ethylene for 48h compared to non-treated controls indicated a more than 5-fold increase in the activity of ACS, 1.2-fold increase in the activity of ACO and about 2.7-fold increase in the activity of cellulase in the ethylene treated explants. This increase was accompanied by a substantial decrease in the force required to separate the petiole from the stem (break strength) and an increased accumulation of cellulase transcript in the abscission zone. Treatment of explants with 1-Methylcyclopropene (1-MCP) prior to ethylene resulted in significant inhibition of enzyme activities and transcript accumulation. It is concluded that ethylene response of cotton leaf abscission leads to higher cellulase expression and increased activities of ethylene biosynthesis enzymes in the abscission zone.

Expression of polygalacturonases and evidence to support their role during cell separation processes in Arabidopsis thaliana

Polygalacturonases (PGs) have been proposed to play an important role in the process of cell separation. The Arabidopsis thaliana genome contains 69 annotated genes that by amino acid homology and transcript organization could be classified as putative PGs and these can be grouped into multiple clades. An analysis of five members located in two separate clades, using reporter fusion constructs and reverse transcription-PCR, revealed that whilst these PGs exhibit high sequence similarity they have distinct patterns of spatial and temporal expression. Sites of expression include the aleurone and endosperm cells surrounding the emerging radicle in a germinating seed, the cortical cells adjacent to the developing lateral root, the abscission zones of floral organs, the dehiscence zone of anthers and siliques, and pollen grains. Silencing of an abscission-related PG (At2g41850), using a T-DNA insertion strategy, delayed the time-course of floral organ loss but did not prevent shedding from taking place. These observations are discussed with regard to the contribution that PGs may play during the life cycle of a plant.

Hawaiian skirt: an F-box gene that regulates organ fusion and growth in Arabidopsis

A fast neutron-mutagenized population of Arabidopsis (Arabidopsis thaliana) Columbia-0 wild-type plants was screened for floral phenotypes and a novel mutant, termed hawaiian skirt (hws), was identified that failed to shed its reproductive organs. The mutation is the consequence of a 28 bp deletion that introduces a premature amber termination codon into the open reading frame of a putative F-box protein (At3g61590). The most striking anatomical characteristic of hws plants is seen in flowers where individual sepals are fused along the lower part of their margins. Crossing of the abscission marker, Pro(PGAZAT):beta-glucuronidase, into the mutant reveals that while floral organs are retained it is not the consequence of a failure of abscission zone cells to differentiate. Anatomical analysis indicates that the fusion of sepal margins precludes shedding even though abscission, albeit delayed, does occur. Spatial and temporal characterization, using Pro(HWS):beta-glucuronidase or Pro(HWS):green fluorescent protein fusions, has identified HWS expression to be restricted to the stele and lateral root cap, cotyledonary margins, tip of the stigma, pollen, abscission zones, and developing seeds. Comparative phenotypic analyses performed on the hws mutant, Columbia-0 wild type, and Pro(35S):HWS ectopically expressing lines has revealed that loss of HWS results in greater growth of both aerial and below-ground organs while overexpressing the gene brings about a converse effect. These observations are consistent with HWS playing an important role in regulating plant growth and development.

Overexpression of INFLORESCENCE DEFICIENT IN ABSCISSION activates cell separation in vestigial abscission zones in Arabidopsis

Plants may shed organs when they have been injured or served their purpose. The differential pattern of organ abscission in different species is most likely the result of evolutionary adaptation to a variety of life styles and environments. The final step of abscission-related cell separation in floral organs of wild-type Arabidopsis thaliana, which only abscises sepals, petals, and stamens, is controlled by INFLORESCENCE DEFICIENT IN ABSCISSION (IDA). Here, we demonstrate that Arabidopsis 35S:IDA lines constitutively overexpressing IDA exhibit earlier abscission of floral organs, showing that the abscission zones are responsive to IDA soon after the opening of the flowers. In addition, ectopic abscission was observed at the bases of the pedicel, branches of the inflorescence, and cauline leaves. The silique valves also dehisced prematurely. Scanning electron microscopy indicated a spread of middle lamella degradation from preformed abscission zone cells to neighboring cells. A transcript encoding an arabinogalactan protein (AGP) was upregulated in the 35S:IDA lines, and large amounts of AGP were secreted at the sites of abscission. AGP was shown to be a constituent of wild-type floral abscission zones during and soon after cell separation had been completed. We suggest that the restricted expression pattern of IDA precludes abscission of nonfloral organs in Arabidopsis.

Ethylene biosynthesis and perception in apple fruitlet abscission (Malus domestica L. Borck)

Abscission was studied in immature apple fruits (cv. Golden Delicious) during the physiological drop. Fruitlet populations, characterized by different abscission potential, were analysed. Non-abscising fruitlets (NAF) were obtained from central flowers borne in clusters where all the lateral flowers had been removed at bloom while abscising fruitlets (AF) were derived from lateral fruitlets of trees sprayed with benzylaminopurine (BAP) at 200 ppm, 17 d after petal fall (APF), when the fruit cross diameter was about 10-12 mm. Fruit shedding, monitored at the end of the June drop, was significantly different in the two populations, being less than 10%, and more than 90%, in NAF and AF, respectively. In AF, fruit drop peaked around 33 d after petal fall (APF) and was preceded by an increase in ethylene around 20 d APF. Transcript analysis was performed from 17-24 d APF, since preliminary experiments pointed out that major changes in expression of abscission related genes occurred within this period. Transcript accumulation of genes involved in ethylene biosynthesis (MdACS5B and MdACO) and action (MdERS1, MdETR1, and MdCTR1) was studied in the seed, cortex, peduncle, and abscission zone (AZ) of the two fruit populations. MdACS5B and MdACO transcripts accumulated along the experimental period in AF population, even though at a different magnitude, while ethylene evolution declined after peaking at day three. MdETR1, MdERS1, and MdCTR1 expression patterns depended on tissue and/or population. The ERS/ETR ratio was higher in AF than in NAF populations. Overall results pointed out that apple fruitlet drop is preceded by a stimulation of ethylene biosynthesis and a gain in sensitivity to the hormone.

Primary cell wall metabolism: tracking the careers of wall polymers in living plant cells

Numerous examples have been presented of enzyme activities, assayed in vitro, that appear relevant to the synthesis of structural polysaccharides, and to their assembly and subsequent degradation in the primary cell walls (PCWs) of higher plants. The accumulation of the corresponding mRNAs, and of the (immunologically recognized) proteins, has often also (or instead) been reported. However, the presence of these mRNAs, antigens and enzymic activities has rarely been shown to correspond to enzyme action in the living plant cell. In some cases, apparent enzymic action is observed in vivo for which no enzyme activity can be detected in in-vitro assays; the converse also occurs. Methods are reviewed by which reactions involving structural wall polysaccharides can be tracked in vivo. Special attention is given to xyloglucan endotransglucosylase (XET), one of the two enzymic activities exhibited in vitro by xyloglucan endotransglucosylase/hydrolase (XTH) proteins, because of its probable importance in the construction and restructuring of the PCW's major hemicellulose. Attention is also given to the possibility that some reactions observed in the PCW in vivo are not directly enzymic, possibly involving the action of hydroxyl radicals. It is concluded that some proposed wall enzymes, for example XTHs, do act in vivo, but that for other enzymes this is not proven. Contents I. Primary cell walls: composition, deposition and roles 642 II. Reactions that have been proposed to occur in primary cell walls 645 III. Tracking the careers of wall components in vivo: evidence for action of enzymes in the walls of living plant cells 656 IV. Evidence for the occurrence of nonenzymic polymer scission in vivo? 666 VI. Conclusion 667 References 667.

Inflorescence deficient in abscission controls floral organ abscission in Arabidopsis and identifies a novel family of putative ligands in plants

Abscission is an active process that enables plants to shed unwanted organs. Because the purpose of the flower is to facilitate pollination, it often is abscised after fertilization. We have identified an Arabidopsis ethylene-sensitive mutant, inflorescence deficient in abscission (ida), in which floral organs remain attached to the plant body after the shedding of mature seeds, even though a floral abscission zone develops. The IDA gene, positioned in the genomic DNA flanking the single T-DNA present in the ida line, was identified by complementation. The gene encodes a small protein with an N-terminal signal peptide, suggesting that the IDA protein is the ligand of an unknown receptor involved in the developmental control of floral abscission. We have identified Arabidopsis genes, and cDNAs from a variety of plant species, that encode similar proteins, which are distinct from known ligands. IDA and the IDA-like proteins may represent a new class of ligands in plants.

Inflorescence deficient in abscission controls floral organ abscission in Arabidopsis and identifies a novel family of putative ligands in plants

Abscission is an active process that enables plants to shed unwanted organs. Because the purpose of the flower is to facilitate pollination, it often is abscised after fertilization. We have identified an Arabidopsis ethylene-sensitive mutant, inflorescence deficient in abscission (ida), in which floral organs remain attached to the plant body after the shedding of mature seeds, even though a floral abscission zone develops. The IDA gene, positioned in the genomic DNA flanking the single T-DNA present in the ida line, was identified by complementation. The gene encodes a small protein with an N-terminal signal peptide, suggesting that the IDA protein is the ligand of an unknown receptor involved in the developmental control of floral abscission. We have identified Arabidopsis genes, and cDNAs from a variety of plant species, that encode similar proteins, which are distinct from known ligands. IDA and the IDA-like proteins may represent a new class of ligands in plants.

Abscission, dehiscence, and other cell separation processes

Cell separation is a critical process that takes place throughout the life cycle of a plant. It enables roots to emerge from germinating seeds, cotyledons, and leaves to expand, anthers to dehisce, fruit to ripen, and organs to be shed. The focus of this review is to examine how processes such as abscission and dehiscence are regulated and the ways new research strategies are helping us to understand the mechanisms involved in bringing about a reduction in cell-to-cell adhesion. The opportunities for using this information to manipulate cell separation for the benefit of agriculture and horticulture are evaluated.