IDA: a peptide ligand regulating cell separation processes in Arabidopsis

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1 Journal of Experimental Botany, Vol. 64, No. 17, pp , 2013 doi: /jxb/ert338 Advance Access publication 22 October, 2013 Review paper IDA: a peptide ligand regulating cell separation processes in Arabidopsis Reidunn B. Aalen, Mari Wildhagen, Ida M. Stø and Melinka A. Butenko* Department of Biosciences, University of Oslo, N-0316 Oslo, Norway * To whom correspondence should be addressed. m.a.butenko@ibv.uio.no Received 17 June 2013; Revised 30 August 2013; Accepted 2 September 2013 Abstract In contrast to animals, plants continuously produce new organs, such as leaves, flowers, and lateral roots (LRs), and may shed organs that have served their purpose. In the model plant Arabidopsis thaliana the peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) signals through the leucine-rich repeat-receptor-like kinases (LRR-RLKs) HAESA (HAE), and HAESA-LIKE2 (HSL2) to control the abscission of floral organs after pollination. Recent work from other plant species indicates that this signalling system is conserved and could regulate leaf abscission in soybean and tomato. Abscission is a cell separation process involving the breakdown of cell walls between adjacent files of abscission zone (AZ) cells at the base of organs to be shed. The emergence of new lateral root primordia (LRP), initiated deep inside the root under the influence of the phytohormone auxin, is similarly dependent on cell wall dissolution to separate cells in the overlying tissues. It has been shown that this process also requires IDA, HAE, and HSL2. The receptors are redundant in function during floral organ abscission, but during lateral root emergence (LRE) they are differentially involved in regulating cell wall remodelling (CWR) genes. An overview is given here of the similarities and differences of IDA signalling during floral organ abscission and LRE. Key words: Abscission, cell separation, HAE, HSL2, IDA, lateral root emergence. The IDA-HAE/HSL2 signalling system regulates floral organ abscission Cell separation processes are critical for the development of a plant and play key roles from sculpting the form of the plant to detachment (or abscission) of unwanted, damaged or senescent organs (Patterson, 2001). In addition, abscission also plays an important role in plant reproduction by the release of ripe fruits and the dispersal of seeds (Estornell et al., 2013). Different plant species display distinct patterns and timing of organ shedding, most probably adapted during evolution to their diverse life styles. However, premature loss of organs can occur as a result of numerous environmental factors including deficit or surplus of water, temperature extremes, nutritional deficiencies, and pest and pathogen attacks (Taylor and Whitelaw, 2001; Garner and Lovatt, 2008). In an agricultural context, abscission can reduce crop productivity and, in fact, during crop domestication, there has been an intentional selection towards cultivars of fruits and grains that have reduced the abscission of fruits and seeds (Pickersgill, 2007). For some plant species, the molecular basis underlying these traits have been unravelled, for example, the reduced shattering varieties of rice (Oryza sativa) (Konishi Abbreviations/acronyms: AZ, abscission zone; aa, amino acid; ARF, Auxin Response Factors; cellulases, β-1,4-glucanases; BOP1, BOP2, BLADE-ON-PETIOLE 1 and 2; CLE, CLV3/ENDOSPERM SURROUNDING REGION (ESR)-related; CWR, cell wall remodelling; CWR, cell wall remodelling genes; dab5, delayed in abscission 5; EXPs, expansins;hae, HAESA; HSL2, HAESA-LIKE2; IDA, INFLORESCENCE DEFICIENT IN ABSCISSION; IDL, IDA-LIKE; LAX3, LIKE AUX1-3;LR, lateral root; LRE, lateral root emergence; LRP, lateral root primordia; LRR-RLKs, leucine-rich repeat-receptor-like kinases; LRs, lateral roots; TF, transcription factors; XTHs, xyloglucan endotransglucosylase/hydrolases; nev, nevershed; OX, over-expressing; PGLR; PG LATERAL ROOT; PGAZAT, PG ABSCISSION ZONE ARABIDOPSIS THALIANA; PGs, polygalacturonases; PI, propidium iodide; SLR1/IAA14, SOLITARY ROOT1 /IAA14; wt, wild-type; XTR6, 23/XYLOGLUCAN ENDOTRANSGLYCOSYLASE6. The Author Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved. For permissions, please journals.permissions@oup.com

2 5254 Aalen et al. et al., 2006; Li et al., 2006; Onishi et al., 2007; Zhou et al., 2012) and the jointless-stem tomato (Solanum lycopersicum) (Szymkowiak and Irish, 1999; Mao et al., 2000). Common to these cultivars is the absence of differentiated, small, isodiametric, cytoplasmically dense, abscission zone (AZ) cells which are responsive to signals promoting abscission and which undergo cell wall degradation during shattering or fruit loss (Addicott, 1982). In both cases, the absence of abscission is due to mutations in genes encoding transcription factors (TF). As the trait of reduced organ abscission is still pursued in both horticulture and agriculture, understanding the underlying molecular regulation is important and the use of Arabidopsis thaliana as a model has been valuable in this respect. In contrast to tomato, where abscission takes place at the pedicel resulting in the loss of flowers, Arabidopsis shed floral organs after pollination has taken place. The progressive changes that the Arabidopsis AZ cells undergo during abscission are, however, common to other species. The abscission process is divided into four distinct developmental stages and serves as a model for cell separation processes where cell wall material between adjacent cells is broken down (Patterson, 2001; Aalen et al., 2006). The first stage, which occurs early and simultaneously with the development of lateral organs from the apical meristem, is the differentiation of AZ cells (Addicott, 1982; Sexton and Roberts, 1982; Osborne, 1989; Roberts et al., 2002). In Arabidopsis floral organs and in the primary leaf of bean (Phaseolus vulgaris) the AZ region consists of one to a few layers of cells (Wright and Osborne, 1974; Bleecker and Patterson, 1997), while the leaflet of elderberry (Sambucus nigra) contains cell layers (Osborne, 1976) and here cell separation is restricted to a narrow band of cells which comprise the separation layer (Addicott, 1982). Although the knowledge of the genetics regulating AZ differentiation is limited, studies on mutants showing defects in abscission have started to shed light on the process. In tomato, the MADS box TFs JOINTLESS and MACROCALYX have been suggested to form a complex that regulates the formation of the pedicel AZ together with the VHIID protein LATERAL SUPPRESSOR (Schumacher et al., 1999; Nakano et al., 2012). In Arabidopsis, the two genes BLADE- ON-PETIOLE 1 and 2 (BOP1, BOP2), encoding proteins belonging to a family containing BTB/POZ domains and ankyrin repeats, are important for AZ development. In a bop1 bop2 double mutant, the AZ cells are not differentiated and floral abscission does not take place (McKim et al., 2008). The second phase starts as the organs prepare to separate from the plant and the AZ cells become competent to respond to abscission signals. The regulatory effects of plant hormones are of major importance, as are environmental cues. In general, ethylene and jasmonic acid accelerate abscission while auxin, gibberellins, and brassinosteroids act as inhibiting signals (reviewed in Estornell et al., 2013). The current view of the influence of hormones is that auxin and ethylene levels establish the timing and progression of organ abscission. There are several experiments indicating that changes in auxin gradients may signal the onset of abscission. The application of auxin at the distal end of AZ explants delays abscission while application at the proximal end accelerates it (Addicott and Lynch, 1951). In Arabidopsis,the timing of floral abscission is closely regulated by ethylene as seen in the ethylene-insensitive mutant etr-1 which is significantly delayed in abscission and senescence (Bleecker and Patterson, 1997). However, etr-1 flowers ultimately go through all the stages of floral abscission showing that ethylene is not an essential component driving the abscission process and that other signals are necessary to activate cell separation (Bleecker and Patterson, 1997; Butenko et al., 2006). Once the abscission process is activated, the pectin in the cell walls between the AZ cell layers starts to degrade followed by an expansion in the size of the AZ cells. A concerted effort from research on many plant species have identified enzymes acting on structural polysaccharides leading to the hydrolysis of the middle lamella and cell walls of the AZ cells (Estornell et al., 2013). These include β-1,4-glucanases (cellulases), polygalacturonases (PGs), xyloglucan endotransglucosylase/ hydrolases (XTHs), and expansins (EXPs). After the actual separation of the organ, a lignified protective layer develops on the distal side of the AZ to protect the plant from pathogen attack. To identify key genetic components regulating cell separation during floral organ abscission, Arabidopsis mutants deficient in abscission were identified from a genetic screen based on random mutagenesis. In wild-type (wt) Arabidopsis flowers, the shedding of flower petals, sepals, and filaments occurs shortly after pollination or anthesis. By contrast, one of the mutant lines identified retained its flowers indefinitely (Butenko et al., 2003). Morphological studies of the floral organ AZ region showed that, unlike the jointless-stem tomato, this mutant had differentiated AZ cells but these cells were incapable of undergoing cell wall degradation and organ separation (Butenko et al., 2003). The affected gene, INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), encodes a short protein with an N-terminal secretory signal peptide which, subcellular localization studies have shown, direct the protein to the extracellular space. In silico approaches followed by gene expression analysis enabled the identification of five IDA- LIKE (IDL) transcripts from Arabidopsis (Butenko et al., 2003). Amino acid (aa) sequence comparison between IDA and IDL family members disclosed a conserved 20 aa C-terminal proline-rich domain which was named the EPIP motif (Stenvik et al., 2008). The structure of the IDA and IDL proteins with the N-terminal secretion signal, a variable region, and the short proline-rich EPIP domain resembled CLAVATA3 (CLV3) and suggested that IDA functioned as a small signalling peptide (Butenko et al., 2003; Stenvik et al., 2008). Recent protein alignment studies revealed that the EPIP domain is present in IDA and IDL sequences of bean (Phaseolus vulgaris), tomato (Solanum lycopersicum), and soybean (Glycine max) (Tucker and Yang, 2012). Based on gene expression studies, it is possible that these IDA orthologues maintain the same function in regulating cell

3 The IDA peptide controls cell separation 5255 separation during leaf abscission of soybean and tomato (Tucker and Yang, 2012). As for other small secreted peptides (Matsubayashi, 2011) the IDA protein undergoes post-translational processing (Stenvik et al., 2008) and EPIP domain swap experiments between IDA and the IDL genes indicated that as for the CLV3/ENDOSPERM SURROUNDING REGION (ESR)-related (CLE) peptide proteins (Jun et al., 2008), the C-terminally located EPIP domain is sufficient for function. In accordance with this, in vivo data where deletion constructs of the IDA protein were used to complement the ida mutant phenotype, showed that the EPIP domain is essential and sufficient for the function of the IDA protein. Consistent with this, a synthetic IDA-EPIP peptide was capable of inducing floral organ abscission in Arabidopsis (Stenvik et al., 2008). However, relatively high amounts (10 μm) of peptide were needed in this bioassay, indicating that the mature active peptide in vivo could be shorter than the EPIP domain, modified, or both. Furthermore, the EPIP peptide was not capable of inducing abscission in explants of soybean (Tucker and Yang, 2012). Both a promoter:gus reporter of IDA (pida:gus) and in situ experiments have shown that IDA is expressed shortly after the AZ cells become competent to respond to abscission signals and the initial loosening of the cell walls takes place (Butenko et al., 2003; Butenko et al., 2006). In addition, opposite to what is observed in the ida mutant, Arabidopsis plants over-expressing (OX) IDA under the control of the cauliflower mosaic virus 35S promoter (35S) display precocious floral organ abscission and an enlargement of the AZ cells at the point where the organs detach from the plant (Stenvik et al., 2006). Taken together, these results indicate that the IDA peptide regulates initial cell wall loosening and separation of AZ cells, probably by controlling the expression of cell wall remodelling (CWR) enzymes (Niederhuth et al., 2013). These enzymes include PGs that catalyse the hydrolysis of the β-1,4-d-galacturonan backbone of pectins and are of particular importance in cell separation events (Ogawa et al., 2009). However, the expansion of AZ cells in lines OX IDA could also be a consequence of increased turgor pressure. During leaf abscission of deciduous trees the turgor pressure in parenchyma cells increases as the cell wall interconnections are weakened, causing these cells to expand. The forces caused by the expanding cells are thought to aid in the abscission process (Sexton and Roberts, 1982). The IDA OX phenotype is dependent on the N-terminal secretion signal of the protein. Plants OX IDA lacking the secretion signal do not portray any OX phenotypes, supporting the notion that export to the apoplastic space is essential for IDA function where it is believed that an active, mature IDA peptide interacts with plasma-membrane bound receptor proteins (Stenvik et al., 2008). The leucine-rich repeat-receptor-like kinase (LRR-RLK) genes HAESA (HAE) and HAESA-LIKE2 (HSL2) are expressed in AZ cells during the time-course of floral abscission (Jinn et al., 2000; Cai and Lashbrook, 2008; Cho et al., 2008) and are functionally redundant in controlling cell separation during floral organ abscission (Stenvik et al., 2006; Cho et al., 2008). The hae hsl2 double mutant shares a similar phenotype to plants carrying a mutation in the IDA gene. Genetic interaction studies identified IDA as a likely peptide ligand partner for HAE and/or HSL2 (Stenvik et al., 2006; Cho et al., 2008) activating a MAP kinases cascade including MKK4, MKK5, MPK3, and MPK6 (Cho et al., 2008). A suppression screen of ida disclosed the homeobox KNOX transcription factors KNAT1, KNAT2, and KNAT6 acting downstream of the IDA-HAE/HSL2 signalling pathway. KNAT1 acts as an inhibitor of cell wall loosening and restricts the expression of KNAT2 and KNAT6 which are necessary for the induction of cell separation (Shi et al., 2011). The IDA-HAE/HSL2 signalling system facilitates lateral root emergence For other Arabidopsis peptide ligands and plasma membrane-bound receptors the same or part of the same, signalling module is employed to regulate similar biological processes in different plant organs and developmental stages (Stahl et al., 2013). Based on gene expression studies it has previously been proposed that the IDL genes may share a common role with IDA in regulating other cell-to-cell separation events during plant development (Stenvik et al., 2008), but the finding that IDA and HAE are expressed in dehiscence zones of mature siliques which undergo cell separation to allow seed shedding (Butenko et al., 2006) brought forth the hypothesis that IDA-HAE/HSL2 signalling might control cell-separation processes elsewhere than in AZ cells (Butenko et al., 2009). Prominent expression of pida:gus in cells surrounding lateral roots (LRs) prompted investigation of the role of IDA, HAE, and HSL2 in lateral root emergence (Kumpf et al., 2013). A significant reduction of the densities of emerged LRs was shown for both the ida mutant and the hae hsl2 double mutant compared with the wt (Kumpf et al., 2013). Interestingly, the single receptor mutants also showed reduced LR densities, thus HAE and HSL2 are not fully functionally redundant during LR development. This is in contrast to the abscission process where no obvious delay or deficiency in abscission has been noted in the single mutants (Cho et al., 2008; Stenvik et al., 2008). Consistent with the difference in redundancy, the two receptor genes have overlapping spatial and temporal expression in the floral AZ region while they portray a different spatial distribution in the root. During floral abscission a promoter:gus reporter of HAE and HSL2 shows expression in the AZ cells of the stamens, petals, and sepals shortly before the organs separate from the receptacle of the flower. During root development, however, HAE and HSL2 show differentially timed expression in the endodermal, cortical, and epidermal cell layers that overlay the LR primordia (LRP) and, in addition. spatially different expression patterns in the LRP with prominent expression of HSL2 at the later stages of LRP development (Kumpf et al., 2013).

4 5256 Aalen et al. To disclose the role of HAE and HSL2 further during LRE, the root gravitropic response was exploited to induce LR organogenesis. When vertical plates with seedlings are turned 90, the roots reorient their direction of growth and a new LRP will form in at the resulting bend (De Smet et al., 2007; Lucas et al., 2008; Moreno-Risueno et al., 2010). This facilitates the investigation of LRs initiated at the same time. After 42 h, more than 40% of the wt LRPs had emerged compared with about 20% of the LRs of the hae, hsl2, and hae hsl2 mutants (Kumpf et al., 2013). Even more pronounced was the significantly delayed progression of the developing LR in relation to the layers that overlay the emerging LRP. When registering the position of each LRP tip relative to the overlying layers at the different stages of LRP development, differences in phenotypes were observed for the different mutants. In the ida, hae, and hsl2 single mutants and the hae hsl2 double mutant the LRP was delayed in penetrating each of the overlying layers (Kumpf et al., 2013). The abnormal progression of the LRP through the endodermal, cortical, and epidermal cell layers was indicative of a defect in cell wall degradation that is needed for the LRP to penetrate the overlying tissues. This view was further substantiated when investigating the morphology of wt epidermal cells compared with those of the ida and hae mutants. Whereas wt LRPs emerge between intact neighbouring epidermal cells with a smooth surface, the epidermal cells surrounding the emerged LRs in the ida and hae mutants were ruptured (Kumpf et al., 2013). The IDA-HAE/HSL2 signalling module triggers cell-wall remodelling and degradation during abscission and LR emergence The process leading to the emergence of LRs can, like the abscission process, be divided into distinct steps. Starting with the initiation of LRP from founder cells in the pericycle, the LRs go through seven developmental stages defined by the number of cell layers in the LRP. Before emergence the LRs have to pass the endodermal, cortical, and epidermal cell layers (Malamy and Benfey, 1997). The emergence of the LR, as the loss of organs, requires the dissolution of the middle lamella between adjacent cells. During abscission, initial cell-wall loosening occurs by enzymes like XTHs and EXPs (Cai and Lashbrook, 2008; Ogawa et al., 2009). EXPs have been shown to be involved during elderberry leaflet abscission (Belfield et al., 2005), petal abscission in Rosa bourboniana (Sane et al., 2007), and in pedicel loosening in Arabidopsis (Cho and Cosgrove, 2000). The loosening of the cell walls facilitates the access of cell-wall degradation enzymes like cellulases and PGs that hydrolyse the major polymer of pectin polygalacturonic acid (Osborne, 1989; Sander et al., 2001). Specific members of the cellulase and PG gene family are up-regulated during organ abscission in different species and, for some, mutation in the corresponding genes leads to abscission defects (Estornell et al., 2013). One example is the PG gene PGAZAT/ADPG2 from Arabidopsis that is implicated in cell separation during dehiscence and floral organ abscission (Ogawa et al., 2009; Gonzalez-Carranza et al., 2012). The use of quantitative expression measurements of candidate CWR genes were used to assay how IDA signalling affected the expression of CWR genes during LRE. For members of the XTH and EXP gene families expressed in cell layers that overlay the LRP [i.e. XTH 23/XYLOGLUCAN ENDOTRANSGLYCOSYLASE6 (XTR6) and EXP17] (Laskowski et al., 2006; Gonzalez-Carranza et al., 2007, 2012; Swarup et al., 2008), a substantial reduction in transcript levels was seen in the hae hsl2 double mutant but not in the hae or hsl2 single mutants (Kumpf et al., 2013). However, for two representatives of the PG family [PG LATERAL ROOT (PGLR), and PG ABSCISSION ZONE ARABIDOPSIS THALIANA (PGAZAT)/(ADPG2)] (Swarup et al., 2008; Gonzalez-Carranza et al., 2012) the expression levels were reduced in hae and hae hsl2 but not in the hsl2 single mutant (Kumpf et al., 2013). The difference in expression levels of PGLR and PGAZAT observed in hae and hsl2 could account for the phenotypic differences observed between hae and hsl2 during LRE. It is likely that IDA signalling through HAE primarily induces the expression of genes involved in cell wall hydrolysis and pectin degradation, while both receptors are involved in the regulation of genes encoding enzymes allowing cell extensibility. GUS reporter lines for XTR6, PGAZAT/ADPG2, and PGLR indicated that IDA signalling also regulates the expression of these genes during both LRE and floral organ abscission (Ogawa et al., 2009; Gonzalez-Carranza et al., 2012; Kumpf et al., 2013). and Fig. 1A E). The absence of GUS expression in the floral AZ and during LRE in ida and hae hsl2 mutants (Fig. 1A E) points towards a functional conservation of IDA signalling during LRE and floral organ abscission. A functional role for PGAZAT during floral abscission in Arabidopsis has previously been reported in that silencing of this gene causes a delay in abscission (Gonzalez-Carranza et al., 2007). During abscission, XTR6 could play a role in cell expansion which occurs prior to cell separation and has been shown to be important for the timing of organ loss in Arabidopsis (Leslie et al., 2010; Shi et al., 2011). Propidium iodide (PI) commonly used to stain root cell walls, binds the negatively charged carboxyl and hydroxyl groups on homogalacturonases of pectic polysaccharides (Rounds et al., 2011). When PI was used to stain the walls of cells directly overlying the LRP in wt, hae, hsl2, hae hsl2, and ida roots a reduced PI staining was seen in wt and hsl2 cells compared with the surrounding cell walls, while the intensity of the staining was unaltered in the ida, hae, and hae hsl2 mutants (Kumpf et al., 2013). This, together with the reduction of PG gene expression in the hae mutant, indicates an aberration in the dissolution of the pectinaceous middle lamella at the interphase of adjacent plant cells that overlay the LRP in ida and hae mutants. In the hsl2 mutant, IDA signalling through HAE may lead to reduced PI staining, as in the wt, substantiating the difference in outcome of IDA signalling through the two receptors. However, to analyse fully the difference in cell wall composition of the cells covering the LRP in hae and hsl2 mutants, a cell wall analysis

5 The IDA peptide controls cell separation 5257 Fig. 1. Regulation of cell wall remodelling genes by IDA signalling during floral abscission and lateral root emergence. (A) ppgazat:gus, (B) ppglr, and (C) pxtr6:gus in wt, ida, and hae hsl2 mutant backgrounds during lateral root emergence. (D) ppgazat:gus and (E) ppglr:gus in wt and hae hsl2 during floral organ abscission. (This figure is modified from Kumpf et al., Floral organ abscission peptide IDA and its HAE/HSL2 receptors control cell separation during lateral root emergence. Proceedings of the National Academy of Sciences, USA 110, ) would need to be performed followed by immunohistochemical studies using antibodies against cell wall components (Pilarska et al., 2013). The IDA-HAE/HSL2 signalling system is induced by auxin Auxin, accumulating in the central cells of LRPs and later in the tip of the LRP, is a key regulator of LR development and emergence (Peret et al., 2009). Auxin is assumed to diffuse from the LRP into the endodermis and cortex and, in the latter tissue, this induces expression of the auxin influx carrier LIKE AUX1-3 (LAX3) which is responsible for the further import of auxin into cortex and epidermal layers. In these tissues, auxin-induced degradation of SOLITARY ROOT1 (SLR1)/IAA14 releases the AUXIN RESPONSE FACTOR 7 (ARF7) and ARF19 TFs, leading to the induction of CWR genes like EXP17, XTR6, and PGLR (Laskowski et al., 2006; Swarup et al., 2008). During LRE, IDA expression is dependent on auxin. Evidence for this comes from expression studies where the pida:gus transgene is activated in all cortex and epidermal cells by exogenous IAA or NAA (1 µm). Furthermore, the IDA expression level increases more than 100-fold from 6 24 h after the addition of 1 µm IAA. In an arf19 background the induction level was reduced, while in the arf7 mutant normal IDA induction was totally abolished, positioning IDA downstream of ARF7 (Kumpf et al., 2013). However, there is no direct evidence that IDA is directly regulated by these factors and the effect of auxin on IDA expression levels is likely to be indirect. HAE and HSL2 were also induced by 1 µm IAA in an ARF7-dependent manner. The induction was, however, transient and weak (<3-fold), suggesting that the receptors, and not the IDA peptide, are the limiting factors controlling cell separation during LRE. If the receptors were as widely induced by auxin as the peptide one would expect cell separation to occur in all cells where IDA was induced by auxin, which is not observed at this auxin concentration. The HAE and HSL2 receptors are also limiting factors for above-ground organ shedding. This is apparent in lines OX IDA where cell separation and loss of organs only occur in cells expressing HAE and HSL2 (Jinn et al., 2000; Cho et al., 2008; Stenvik et al., 2008). In contrast to the promoting effect of auxin on IDA expression and LRE, exogenous auxin commonly delays organ shedding and counteracts the abscission-promoting effect of ethylene (Aalen, 2011; Sexton and Roberts, 1982). Although it is not known whether auxin is involved in the regulation of IDA or its receptors in the floral organ AZs, recent work where AZ-specific promoters were used to induce auxin production during abscission showed that increased levels of auxin delayed organ shedding (Basu et al., 2013). Moreover, mutations in the ARF genes, ARF1 and ARF2, have a delay in floral abscission which is enhanced by mutations in ARF7 and ARF19 (Ellis et al., 2005). During the specification of the valve margin separation layer in Arabidopsis fruits a local auxin minimum is necessary. The valve margin identity factor INDEHISCENT is responsible for forming this auxin minimum by preventing polar PIN3 localization in this region (Sorefan et al., 2009). It is possible that a similar mechanism is in place to ensure auxin minima in AZ cells.

6 5258 Aalen et al. The IDA-HAE/HSL2 signalling module in inflorescences and the root: variations over a theme The IDA-HAE/HSL2 signalling system share many similarities during floral abscission and LRE (Fig. 2). During both processes specialized cells capable of undergoing programmed cell separation respond to elevated or reduced levels of auxin and ethylene (Roberts et al., 2000). Hormones influence the competence of these cells to undergo separation. Floral organ AZ cells become competent to respond to abscission signals, in part, by the influence of ethylene (Bleecker and Patterson, 1997; Butenko et al., 2006), while auxin diffusing from the LRP differentiate the cells overlying the LPR. During floral abscission, IDA initiates a signalling cascade through the HAE and HSL2 receptors which leads to the repression of the KNOX TF KNAT1 and the de-repression of KNAT2 and KNAT6. It is believed that KNAT2 and KNAT6 induce the transcription of CWR genes (Shi et al., 2011). During LRE, IDA signalling induces a similar set of CWR genes, however, here the signalling is differentiated and activation of HAE predominantly induces the transcription of cell- wall-degrading enzymes, while HSL2 regulates genes responsible for cell expansion (Kumpf et al., 2013). Mechanical strain may promote cell separation during abscission by the enlargement of AZ cells on the plant body side and, during LR emergence, by the growing LRP. It has recently been shown that mechanical constraints exerted by the overlying layers influences the shape of the LRP (Lucas et al., 2013) and, vice versa, it is likely that the mechanical pressure from the growing LR facilitates the emergence between overlying cells where cell wall loosening has been initiated. However, IDA- and HAE-dependent pectin dissolution is needed for the final separation step, as emergence is delayed and penetration of the overlying cells by the emerging LRP leads to cell rupture when these genes are mutated (Kumpf et al., 2013). Mechanical pressure may similarly facilitate the abscission process, as AZ cells on the plant body side of the AZ expand prior to organ loss. Although the relationship between AZ cell expansion and organ separation in Arabidopsis is unclear (Patterson, 2001) the contribution of AZ cell expansion to organ abscission might be analogous to that of lignified fruit cells to Arabidopsis pod dehiscence where differential cell shrinkage causes internal tension leading to pod shattering (Liljegren et al., 2004). In other plant species, such as in the leaf abscission of bean and Impatiens sultani, cell expansion leads to mechanical forces necessary to rupture the xylem and cuticle and, ultimately, to cause the separation of the organ (Sexton and Redshaw, 1981). However, in Arabidopsis, cell expansion is not sufficient for cell separation to occur. This is, for instance, demonstrated in the delayed Fig. 2. IDA-dependent cell separation during floral organ abscission and lateral root emergence. During abscission (upper panel) specialized cells capable of undergoing programmed cell separation respond to reduced levels of auxin and augmented levels of ethylene and become competent to respond to abscission signals. IDA, which has a broader expression pattern than the receptor genes HAE and HSL2, encodes a peptide ligand that signals through HAE and HSL2. Activation of the receptors leads ultimately to the induction of transcription of cell wall remodelling genes which cause cell wall expansion. The mechanical strain caused by the cell expansion aids the following cell separation. During lateral root emergence (lower panel) auxin diffusing from the lateral root primordia differentiate the overlying cells. IDA, which is expressed in the overlying cells, is translated to an active peptide which signals through HAE and HSL2 primarily expressed in the same cells as IDA. IDA signalling in the root induces the transcription of a similar set of cell wall remodelling genes as those induced during abscission. The cell wall remodelling enzymes lead to cell wall loosening and the growing lateral root facilitates the emergence between overlying cells.

7 The IDA peptide controls cell separation 5259 in abscission 5 (dab5) mutant that shows enlargement of AZ cells but a significant delay in organ separation (Patterson and Bleecker, 2004). Furthermore, genetic interaction studies between the mutant nevershed (nev), which is deficient in floral abscission, and an OX line of IDA supports the notion that cell expansion is not sufficient for floral organ separation (Liu et al., 2013). NEV encodes an ARF-GAP protein and mutations in NEV alter the organization of the Golgi apparatus, thereby blocking the movement of molecules required for cell separation (Liljegren et al., 2009). When IDA is OX in a nev background there is an excessive AZ cell enlargement but the floral organs remain attached (Liu et al., 2013). When the IDA gene was identified (Butenko, 2003) only a limited number of peptide ligand-receptor signalling systems had been identified in Arabidopsis. Bioinformatic approaches have predicted that several thousand unanotated small genes important in morphogenesis are present in the Arabidopsis genome, many of which are likely to be ligands to the large family of RLKs (Shiu and Bleecker, 2001; Hanada et al., 2013). Future challenges lie in identifying additional ligand-receptor pairs, dissecting their downstream signalling components and, ultimately, the signalling output in the form of gene regulation (Murphy et al., 2012). The IDA signalling pathway is an example of how the same or part of the same, signalling module can be employed to regulate similar developmental processes in different plant organs and developmental stages. Such a conservation of function makes it easier to predict signalling components and target genes, as exemplified by the regulation of common CWR genes with shared functions by IDA signalling in the root and flower. Furthermore, the evolutionary conservation of peptides and receptor proteins between different plant species (Lehti-Shiu et al., 2009; Matsubayashi, 2011) makes it likely that a similar signalling system is conserved across plant species. Acknowledgements This work was supported by Grants 13785/F20 (to MAB and RBA) and /F20 (to MW IMS, and RBA) from the Research Council of Norway. References Aalen RB Flower and floral organ abscission: control, gene expression and hormone interaction. In: Yaish M, ed. The flowering process and its control in plants: gene expression and hormone interaction. Kerala: Research Signpost/Transworld Research Network, Aalen RB, Butenko MA, Stenvik G-E, Tandstad NM, Patterson SE Genetic control of floral abscission. In: Texeira da Silva JA, ed. Floriculture, ornamental and plant biotechnology: advances and topical issues. London: Global Science Books Ltd., Addicott FT Abscission. Berkeley: University of California Press. 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