Tools and Strategies to Match Peptide-Ligand Receptor Pairs

The Structural Basis of Ligand Perception and Signal Activation by Receptor Kinases

Plants have evolved a family of unique membrane receptor kinases to orchestrate the growth and development of their cells, tissues, and organs. Receptor kinases also form the first line of defense of the plant immune system and allow plants to engage in symbiotic interactions. Here, we discuss recent advances in understanding, at the molecular level, how receptor kinases with lysin-motif or leucine-rich-repeat ectodomains have evolved to sense a broad spectrum of ligands. We summarize and compare the established receptor activation mechanisms for plant receptor kinases and dissect how ligand binding at the cell surface leads to activation of cytoplasmic signaling cascades. Our review highlights that one family of plant membrane receptors has diversified structurally to fulfill very different signaling tasks.

Chemiluminescence-Based Detection of Peptide Activity and Peptide-Receptor Binding in Plants

In living organisms, physical interaction of ligand molecules with their cognate receptors is an indispensable requirement for the initiation of cellular signaling pathways. To technically prove the biochemical interaction of ligands with their corresponding receptor, a biologically active but labeled peptide is required. Easily scorable bioassays, such as the production of reactive oxygen species, can be used to quantify the activity of a peptide. By using chemiluminescent probes, such as acridinium esters, as conjugates to label peptide ligands of interest, quantitative measurements of ligand-receptor binding can be performed in standard luminometers. Here we describe how this binding approach can be used to reveal peptide ligand-receptor binding in plant material.

Bound by Fate: The Role of Reactive Oxygen Species in Receptor-Like Kinase Signaling

In plants, receptor-like kinases (RLKs) and extracellular reactive oxygen species (ROS) contribute to the communication between the environment and the interior of the cell. Apoplastic ROS production is a frequent result of RLK signaling in a multitude of cellular processes; thus, by their nature, these two signaling components are inherently linked. However, it is as yet unclear how ROS signaling downstream of receptor activation is executed. In this review, we provide a broad view of the intricate connections between RLKs and ROS signaling and describe the regulatory events that control and coordinate extracellular ROS production. We propose that concurrent initiation of ROS-dependent and -independent signaling linked to RLKs might be a critical element in establishing cellular responses. Furthermore, we discuss the possible ROS sensing mechanisms in the context of the biochemical environment in the apoplast. We suggest that RLK-dependent modulation of apoplastic and intracellular conditions facilitates ROS perception and signaling. Based on data from plant and animal models, we argue that specific RLKs could be components of the ROS sensing machinery or ROS sensors. The importance of the crosstalk between RLK and ROS signaling is discussed in the context of stomatal immunity. Finally, we highlight challenges in the understanding of these signaling processes and provide perspectives for future research.

Regulatory Peptides in Plants

Many different peptides regulating cell differentiation, growth, and development are found in plants. Peptides participate in regulation of plant ontogenesis starting from pollination, pollen tube growth, and the very early stages of embryogenesis, including formation of embryo and endosperm. They direct differentiation of meristematic stem cells, formation of tissues and individual organs, take part in regulation of aging, fruit maturation, and abscission of plant parts associated with apoptosis. Biological activity of peptides is observed at very low concentrations, and it has mainly signal nature and hormonal character. "Mature" peptides appear mainly due to processing of protein precursors with (or without) additional enzymatic modifications. Plant peptides differ in origin, structure, and functional properties. Their specific action is due to binding with respective receptors and interactions with various proteins and other factors. Peptides can also regulate physiological functions by direct peptide-protein interactions. Peptide action is coordinated with the action of known phytohormones (auxins, cytokinins, and others); thus, peptides control phytohormonal signal pathways.

IDL6-HAE/HSL2 impacts pectin degradation and resistance to Pseudomonas syringae pv tomato DC3000 in Arabidopsis leaves

Plant cell walls undergo dynamic structural and chemical changes during plant development and growth. Floral organ abscission and lateral root emergence are both accompanied by cell-wall remodeling, which involves the INFLORESCENCE DEFICIENT IN ABSCISSION (IDA)-derived peptide and its receptors, HAESA (HAE) and HAESA-LIKE2 (HSL2). Plant cell walls also act as barriers against pathogenic invaders. Thus, the cell-wall remodeling during plant development could have an influence on plant resistance to phytopathogens. Here, we identified IDA-like 6 (IDL6), a gene that is prominently expressed in Arabidopsis leaves. IDL6 expression in Arabidopsis leaves is significantly upregulated when the plant is suffering from attacks of the bacterial Pseudomonas syringae pv. tomato (Pst) DC3000. IDL6 overexpression and knockdown lines respectively decrease and increase the Arabidopsis resistance to Pst DC3000, indicating that the gene promotes the Arabidopsis susceptibility to Pst DC3000. Moreover, IDL6 promotes the expression of a polygalacturonase (PG) gene, ADPG2, and increases PG activity in Arabidopsis leaves, which in turn reduces leaf pectin content and leaf robustness. ADPG2 overexpression restrains Arabidopsis resistance to Pst DC3000, whereas ADPG2 loss-of-function mutants increase the resistance to the bacterium. Pst DC3000 infection elevates the ADPG2 expression partially through HAE and HSL2. Taken together, our results suggest that IDL6-HAE/HSL2 facilitates the ingress of Pst DC3000 by promoting pectin degradation in Arabidopsis leaves, and Pst DC3000 might enhance its infection by manipulating the IDL6-HAE/HSL2-ADPG2 signaling pathway.

Quantitative Detection of Oxidative Burst upon Activation of Plant Receptor Kinases

The oxidative burst or the production of reactive oxygen species (ROS) is a typical cellular response of both plants and animals to diverse abiotic and biotic stresses. Mainly, the (re-)active oxygen species include the superoxide anion (O2-), hydrogen peroxide (H2O2), and the hydroxyl radical (OH•). Here, we outline the detection of extracellularly produced ROS in plant leaf pieces using a chemiluminescence-based bioassay with the luminol L-012 as a substrate being oxidized in the presence of ROS. Since this type of assay is in use in many laboratories, e.g., as a readout for activation of plant receptor kinases, we include a discussion on the interpretation of results and points addressing problems with the buffers at suboptimal pH values that negatively influence the chemiluminescence production.

Two SERK Receptor-Like Kinases Interact with EMS1 to Control Anther Cell Fate Determination

Cell signaling pathways mediated by leucine-rich repeat receptor-like kinases (LRR-RLKs) are essential for plant growth, development, and defense. The EMS1 (EXCESS MICROSPOROCYTES1) LRR-RLK and its small protein ligand TPD1 (TAPETUM DETERMINANT1) play a fundamental role in somatic and reproductive cell differentiation during early anther development in Arabidopsis (Arabidopsis thaliana). However, it is unclear whether other cell surface molecules serve as coregulators of EMS1. Here, we show that SERK1 (SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE1) and SERK2 LRR-RLKs act redundantly as coregulatory and physical partners of EMS1. The SERK1/2 genes function in the same genetic pathway as EMS1 in anther development. Bimolecular fluorescence complementation, Förster resonance energy transfer, and coimmunoprecipitation approaches revealed that SERK1 interacted biochemically with EMS1. Transphosphorylation of EMS1 by SERK1 enhances EMS1 kinase activity. Among 12 in vitro autophosphorylation and transphosphorylation sites identified by tandem mass spectrometry, seven of them were found to be critical for EMS1 autophosphorylation activity. Furthermore, complementation test results suggest that phosphorylation of EMS1 is required for its function in anther development. Collectively, these data provide genetic and biochemical evidence of the interaction and phosphorylation between SERK1/2 and EMS1 in anther development.

Precursor processing for plant peptide hormone maturation by subtilisin-like serine proteinases

Peptide hormones that regulate plant growth and development are derived from larger precursor proteins by proteolytic processing. Our study addressed the role of subtilisin-like proteinases (SBTs) in this process. Using tissue-specific expression of proteinase inhibitors as a tool to overcome functional redundancy, we found that SBT activity was required for the maturation of IDA (INFLORESCENCE DEFICIENT IN ABSCISSION), a peptide signal for the abscission of floral organs in Arabidopsis We identified three SBTs that process the IDA precursor in vitro, and this processing was shown to be required for the formation of mIDA (the mature and bioactive form of IDA) as the endogenous signaling peptide in vivo. Hence, SBTs act as prohormone convertases in plants, and several functionally redundant SBTs contribute to signal biogenesis.

The pattern-recognition receptor CORE of Solanaceae detects bacterial cold-shock protein

Plants and animals recognize microbial invaders by detecting microbe-associated molecular patterns (MAMPs) by cell surface receptors. Many plant species of the Solanaceae family detect the highly conserved nucleic acid binding motif RNP-1 of bacterial cold-shock proteins (CSPs), represented by the peptide csp22, as a MAMP. Here, we exploited the natural variation in csp22 perception observed between cultivated tomato (Solanum lycopersicum) and Solanum pennellii to map and identify the leucine-rich repeat (LRR) receptor kinase CORE (cold shock protein receptor) of tomato as the specific, high-affinity receptor site for csp22. Corroborating its function as a genuine receptor, heterologous expression of CORE in Arabidopsis thaliana conferred full sensitivity to csp22 and, importantly, it also rendered these plants more resistant to infection by the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Our study also confirms the biotechnological potential of enhancing plant immunity by interspecies transfer of highly effective pattern-recognition receptors such as CORE to different plant families.

Disrupting ER-associated protein degradation suppresses the abscission defect of a weak hae hsl2 mutant in Arabidopsis

In Arabidopsis thaliana, the process of abscission, or the shedding of unwanted organs, is mediated by two genes, HAESA (HAE) and HAESA-LIKE 2 (HSL2), encoding receptor-like protein kinases (RLKs). The double loss-of-function mutant hae-3 hsl2-3 is completely deficient in floral abscission, but, interestingly, the hae-3 hsl2-9 mutant displays a less severe defect. This mutant was chosen for an ethyl methanesulfonate (EMS) screen to isolate enhancer and suppressor mutants, and two such suppressors are the focus of this study. Pooled DNA from the F₂ generation of a parental backcross was analyzed by genome sequencing to reveal candidate genes, two of which complement the suppressor phenotype. These genes, EMS-MUTAGENIZED BRI1 SUPPRESSOR 3 (EBS3) and EBS4, both encode mannosyltransferases involved in endoplasmic reticulum (ER)-associated degradation (ERAD) of proteins. Further analysis of these suppressor lines revealed that suppressor mutations are acting solely on the partially functional hsl2-9 mutant receptor to modify the abscission phenotype. Expressing a hsl2-9-yellow fluorescent protein (YFP) transgene in ebs3 mutants yields a higher fluorescent signal than in EBS3/ebs3, suggesting that these mutants restore abscission by disrupting ERAD to allow accumulation of the hsl2-9 receptor, which probably escapes degradation to be trafficked to the plasma membrane to regain signaling.

Q&A: How does peptide signaling direct plant development?

A significant part of the communication between plant cells is mediated by signaling peptides and their corresponding plasma membrane-localized receptor-like kinases. This communication mechanism serves as a key regulatory unit for coordination of plant growth and development. In the past years more peptide–receptor signaling pathways have been shown to regulate developmental processes, such as shoot and root meristem maintenance, seed formation, and floral abscission. More detailed understanding of the processes behind this regulation might also be helpful to increase the yield of crop plants.

Mechanistic insight into a peptide hormone signaling complex mediating floral organ abscission

Plants constantly renew during their life cycle and thus require to shed senescent and damaged organs. Floral abscission is controlled by the leucine-rich repeat receptor kinase (LRR-RK) HAESA and the peptide hormone IDA. It is unknown how expression of IDA in the abscission zone leads to HAESA activation. Here we show that IDA is sensed directly by the HAESA ectodomain. Crystal structures of HAESA in complex with IDA reveal a hormone binding pocket that accommodates an active dodecamer peptide. A central hydroxyproline residue anchors IDA to the receptor. The HAESA co-receptor SERK1, a positive regulator of the floral abscission pathway, allows for high-affinity sensing of the peptide hormone by binding to an Arg-His-Asn motif in IDA. This sequence pattern is conserved among diverse plant peptides, suggesting that plant peptide hormone receptors may share a common ligand binding mode and activation mechanism.

Mechanistic insight into a peptide hormone signaling complex mediating floral organ abscission

Plants constantly renew during their life cycle and thus require to shed senescent and damaged organs. Floral abscission is controlled by the leucine-rich repeat receptor kinase (LRR-RK) HAESA and the peptide hormone IDA. It is unknown how expression of IDA in the abscission zone leads to HAESA activation. Here we show that IDA is sensed directly by the HAESA ectodomain. Crystal structures of HAESA in complex with IDA reveal a hormone binding pocket that accommodates an active dodecamer peptide. A central hydroxyproline residue anchors IDA to the receptor. The HAESA co-receptor SERK1, a positive regulator of the floral abscission pathway, allows for high-affinity sensing of the peptide hormone by binding to an Arg-His-Asn motif in IDA. This sequence pattern is conserved among diverse plant peptides, suggesting that plant peptide hormone receptors may share a common ligand binding mode and activation mechanism.

DOI:http://dx.doi.org/10.7554/eLife.15075.001

Plants can shed their leaves, flowers or other organs when they no longer need them. But how does a leaf or a flower know when to let go? A receptor protein called HAESA is found on the surface of the cells that surround a future break point on the plant. When its time to shed an organ, a hormone called IDA instructs HAESA to trigger the shedding process. However, the molecular details of how IDA triggers organ shedding are not clear.

The shedding of floral organs (or leaves) can be easily studied in a model plant called Arabidopsis. Santiago et al. used protein biochemistry, structural biology and genetics to uncover how the IDA hormone activates HAESA. The experiments show that IDA binds directly to a canyon shaped pocket in HAESA that extends out from the surface of the cell. IDA binding to HAESA allows another receptor protein called SERK1 to bind to HAESA, which results in the release of signals inside the cell that trigger the shedding of organs.

The next step following on from this work is to understand what signals are produced when IDA activates HAESA. Another challenge will be to find out where IDA is produced in the plant and what causes it to accumulate in specific places in preparation for organ shedding.

DOI:http://dx.doi.org/10.7554/eLife.15075.002

Crystal structure of PXY-TDIF complex reveals a conserved recognition mechanism among CLE peptide-receptor pairs

Plants can achieve amazing lifespans because of their continuous and repetitive formation of new organs by stem cells present within meristems. The balance between proliferation and differentiation of meristem cells is largely regulated by the CLAVATA3/ENDOSPERM SURROUNDING REGION (CLE) peptide hormones. One of the well-characterized CLE peptides, CLE41/TDIF (tracheary elements differentiation inhibitory factor), functions to suppress tracheary element differentiation and promote procambial cell proliferation, playing important roles in vascular development and wood formation. The recognition mechanisms of TDIF or other CLE peptides by their respective receptors, however, remain largely elusive. Here we report the crystal structure of TDIF in complex with its receptor PXY, a leucine-rich repeat receptor kinase (LRR-RK). Our structure reveals that TDIF mainly adopts an “Ω”-like conformation binding to the inner surface of the LRR domain of PXY. Interaction between TDIF and PXY is predominately mediated by the relatively conserved amino acids of TDIF. Structure-based sequence alignment showed that the TDIF-interacting motifs are also conserved among other known CLE receptors. Our data provide a structural template for understanding the recognition mechanism of CLE peptides by their receptors, offering an opportunity for the identification of receptors of other uncharacterized CLE peptides.

Ligand-Induced Receptor-like Kinase Complex Regulates Floral Organ Abscission in Arabidopsis

Abscission is a developmental process that enables plants to shed unwanted organs. In Arabidopsis, the floral organ abscission is regulated by a signaling pathway consisting of the peptide ligand IDA, the receptor-like kinases (RLKs) HAE and HSL2, and a downstream MAP kinase (MAPK) cascade. However, little is known about the molecular link between ligand-receptor pairs and intracellular signaling. Here, we report that the SERK family RLKs function redundantly in regulating floral organ abscission downstream of IDA and upstream of the MAPK cascade. IDA induces heterodimerization of HAE/HSL2 and SERKs, which transphosphorylate each other. The SERK3 residues mediating its interaction with the immune receptor FLS2 and the brassinosteroid receptor BRI1 are also required for IDA-induced HAE/HSL2-SERK3 interaction, suggesting SERKs serve as co-receptors of HAE/HSL2 in perceiving IDA. Thus, our study reveals the signaling activation mechanism in floral organ abscission by IDA-induced HAE/HSL2-SERK complex formation accompanied by transphosphorylation.

Analysis of Phosphorylation of the Receptor-Like Protein Kinase HAESA during Arabidopsis Floral Abscission

Receptor-like protein kinases (RLKs) are the largest family of plant transmembrane signaling proteins. Here we present functional analysis of HAESA, an RLK that regulates floral organ abscission in Arabidopsis. Through in vitro and in vivo analysis of HAE phosphorylation, we provide evidence that a conserved phosphorylation site on a region of the HAE protein kinase domain known as the activation segment positively regulates HAE activity. Additional analysis has identified another putative activation segment phosphorylation site common to multiple RLKs that potentially modulates HAE activity. Comparative analysis suggests that phosphorylation of this second activation segment residue is an RLK specific adaptation that may regulate protein kinase activity and substrate specificity. A growing number of RLKs have been shown to exhibit biologically relevant dual specificity toward serine/threonine and tyrosine residues, but the mechanisms underlying dual specificity of RLKs are not well understood. We show that a phospho-mimetic mutant of both HAE activation segment residues exhibits enhanced tyrosine auto-phosphorylation in vitro, indicating phosphorylation of this residue may contribute to dual specificity of HAE. These results add to an emerging framework for understanding the mechanisms and evolution of regulation of RLK activity and substrate specificity.

Conservation of the abscission signaling peptide IDA during Angiosperm evolution: withstanding genome duplications and gain and loss of the receptors HAE/HSL2

The peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), which signals through the leucine-rich repeat receptor-like kinases HAESA (HAE) and HAESA-LIKE2 (HSL2), controls different cell separation events in Arabidopsis thaliana. We hypothesize the involvement of this signaling module in abscission processes in other plant species even though they may shed other organs than A. thaliana. As the first step toward testing this hypothesis from an evolutionarily perspective we have identified genes encoding putative orthologs of IDA and its receptors by BLAST searches of publically available protein, nucleotide and genome databases for angiosperms. Genes encoding IDA or IDA-LIKE (IDL) peptides and HSL proteins were found in all investigated species, which were selected as to represent each angiosperm order with available genomic sequences. The 12 amino acids representing the bioactive peptide in A. thaliana have virtually been unchanged throughout the evolution of the angiosperms; however, the number of IDL and HSL genes varies between different orders and species. The phylogenetic analyses suggest that IDA, HSL2, and the related HSL1 gene, were present in the species that gave rise to the angiosperms. HAE has arisen from HSL1 after a genome duplication that took place after the monocot-eudicots split. HSL1 has also independently been duplicated in the monocots, while HSL2 has been lost in gingers (Zingiberales) and grasses (Poales). IDA has been duplicated in eudicots to give rise to functionally divergent IDL peptides. We postulate that the high number of IDL homologs present in the core eudicots is a result of multiple whole genome duplications (WGD). We substantiate the involvement of IDA and HAE/HSL2 homologs in abscission by providing gene expression data of different organ separation events from various species.

Antagonistic peptide technology for functional dissection of CLE peptides revisited

In the Arabidopsis thaliana genome, over 1000 putative genes encoding small, presumably secreted, signalling peptides can be recognized. However, a major obstacle in identifying the function of genes encoding small signalling peptides is the limited number of available loss-of-function mutants. To overcome this, a promising new tool, antagonistic peptide technology, was recently developed. Here, this antagonistic peptide technology was tested on selected CLE peptides and the related IDA peptide and its usefulness in the context of studies of peptide function discussed. Based on the analyses, it was concluded that the antagonistic peptide approach is not the ultimate means to overcome redundancy or lack of loss-of-function lines. However, information collected using antagonistic peptide approaches (in the broad sense) can be very useful, but these approaches do not work in all cases and require a deep insight on the interaction between the ligand and its receptor to be successful. This, as well as peptide ligand structure considerations, should be taken into account before ordering a wide range of synthetic peptide variants and/or generating transgenic plants.

Beyond the meristems: similarities in the CLAVATA3 and INFLORESCENCE DEFICIENT IN ABSCISSION peptide mediated signalling pathways

Plants form new organs throughout their lives; this requires a balance between cell proliferation and differentiation, and between the generation and loss of organs. To do this, plants must maintain a population of stem cells within the meristems, and at the same time, closely control the identity and position of cells at the meristem boundaries as they differentiate to new leaf or flower primordia. Once developed, organs may need to be shed, either as a controlled developmental decision-such as floral abscission after pollination, or as a response to disease, environmental stress, and predators. Cell wall degradation at specialized abscission zone (AZ) cells needs to occur for this to take place, but since there is little cell rearrangement in plants, cell separation events are also important for plant architecture. In this Opinion paper we discuss the role of two peptide ligand signalling systems that control stem cell homeostasis and cell separation, respectively. We draw parallels between the signalling pathways and explore on the commonalities of the downstream components activated and controlled by the signalling peptides. We provide evidence for AZ cells having a meristem identity and discuss the role of identical KNOTTED-LIKE HOMEOBOX (KNOX) transcription factors in meristem maintenance and abscission. Lastly we explore the evolutionary relationship between the pathways.

The IDA/IDA-LIKE and PIP/PIP-LIKE gene families in Arabidopsis: phylogenetic relationship, expression patterns, and transcriptional effect of the PIPL3 peptide

Peptide ligands play crucial roles in the life cycle of plants by modulating the innate immunity against pathogens and regulating growth and developmental processes. One well-studied example is INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), which controls floral organ abscission and lateral root emergence in Arabidopsis thaliana. IDA belongs to a family of five additional IDA-LIKE (IDL) members that have all been suggested to be involved in regulation of Arabidopsis development. Here we present three novel members of the IDL subfamily and show that two of them are strongly and rapidly induced by different biotic and abiotic stresses. Furthermore, we provide data that the recently identified PAMP-INDUCED SECRETED PEPTIDE (PIP) and PIP-LIKE (PIPL) peptides, which show similarity to the IDL and C-TERMINALLY ENCODED PEPTIDE (CEP) peptides, are not only involved in innate immune response in Arabidopsis but are also induced by abiotic stress. Expression patterns of the IDA/IDL and PIP/PIPL genes were analysed using in silico data, qRT-PCR and GUS promoter lines. Transcriptomic responses to PIPL3 peptide treatment suggested a role in regulation of biotic stress responses and cell wall modification.

The Plant Peptidome: An Expanding Repertoire of Structural Features and Biological Functions

Peptides fulfill a plethora of functions in plant growth, development, and stress responses. They act as key components of cell-to-cell communication, interfere with signaling and response pathways, or display antimicrobial activity. Strikingly, both the diversity and amount of plant peptides have been largely underestimated. Most characterized plant peptides to date acting as small signaling peptides or antimicrobial peptides are derived from nonfunctional precursor proteins. However, evidence is emerging on peptides derived from a functional protein, directly translated from small open reading frames (without the involvement of a precursor) or even encoded by primary transcripts of microRNAs. These novel types of peptides further add to the complexity of the plant peptidome, even though their number is still limited and functional characterization as well as translational evidence are often controversial. Here, we provide a comprehensive overview of the reported types of plant peptides, including their described functional and structural properties. We propose a novel, unifying peptide classification system to emphasize the enormous diversity in peptide synthesis and consequent complexity of the still expanding knowledge on the plant peptidome.

Modulation of Arabidopsis and monocot root architecture by CLAVATA3/EMBRYO SURROUNDING REGION 26 peptide

Plant roots are important for a wide range of processes, including nutrient and water uptake, anchoring and mechanical support, storage functions, and as the major interface with the soil environment. Several small signalling peptides and receptor kinases have been shown to affect primary root growth, but very little is known about their role in lateral root development. In this context, the CLE family, a group of small signalling peptides that has been shown to affect a wide range of developmental processes, were the focus of this study. Here, the expression pattern during lateral root initiation for several CLE family members is explored and to what extent CLE1, CLE4, CLE7, CLE26, and CLE27, which show specific expression patterns in the root, are involved in regulating root architecture in Arabidopsis thaliana is assessed. Using chemically synthesized peptide variants, it was found that CLE26 plays an important role in regulating A. thaliana root architecture and interacts with auxin signalling. In addition, through alanine scanning and in silico structural modelling, key residues in the CLE26 peptide sequence that affect its activity are pinpointed. Finally, some interesting similarities and differences regarding the role of CLE26 in regulating monocot root architecture are presented.

Development of schizogenous intercellular spaces in plants

Gas exchange is essential for multicellular organisms. In contrast to the circulatory systems of animals, land plants have tissues with intercellular spaces (ICSs), called aerenchyma, that are critical for efficient gas exchange. Plants form ICSs by two different mechanisms: schizogeny, where localized cell separation creates spaces; and lysogeny, where cells die to create ICSs. In schizogenous ICS formation, specific molecular mechanisms regulate the sites of cell separation and coordinate extensive reorganization of cell walls. Emerging evidence suggests the involvement of extracellular signaling, mediated by peptide ligands and leucine-rich repeat receptor-like kinases, in the regulation of cell wall remodeling during cell separation. Recent work on the liverwort Marchantia polymorpha has demonstrated a critical role for a plasma membrane-associated plant U-box E3 ubiquitin ligase in ICS formation. In this review, I discuss the mechanism of schizogenous ICS formation, focusing on the potential role of extracellular signaling in the regulation of cell separation.

Polypeptide signaling molecules in plant development

Intercellular communication mediated by small signaling molecules is a key mechanism for coordinating plant growth and development. In the past few years, polypeptide signals have been shown to play prominent roles in processes as diverse as shoot and root meristem maintenance, vascular differentiation, lateral root emergence, and seed formation. Signaling components such as CLV1 and the IDA-HAE/HSL2 signaling module have been discovered to regulate distinct developmental processes in different tissues. Recent studies have also uncovered novel polypeptide-receptor interactions, intracellular components and downstream target genes, adding complexity to our picture of polypeptide signaling networks. Finally, new families of plant polypeptides, such as the GLV/RGF/CLEL and ESF factors, are being identified, the functions of which we are only beginning to understand.

Examination of the Abscission-Associated Transcriptomes for Soybean, Tomato, and Arabidopsis Highlights the Conserved Biosynthesis of an Extensible Extracellular Matrix and Boundary Layer

Abscission zone (AZ) development and the progression of abscission (detachment of plant organs) have been roughly separated into four stages: first, AZ differentiation; second, competence to respond to abscission signals; third, activation of abscission; and fourth, formation of a protective layer and post-abscission trans-differentiation. Stage three, activation of abscission, is when changes in the cell wall and extracellular matrix occur to support successful organ separation. Most abscission research has focused on gene expression for enzymes that disassemble the cell wall within the AZ and changes in phytohormones and other signaling events that regulate their expression. Here, transcriptome data for soybean, tomato and Arabidopsis were examined and compared with a focus not only on genes associated with disassembly of the cell wall but also on gene expression linked to the biosynthesis of a new extracellular matrix. AZ-specific up-regulation of genes associated with cell wall disassembly including cellulases (beta-1,4-endoglucanases, CELs), polygalacturonases (PGs), and expansins (EXPs) were much as expected; however, curiously, changes in expression of xyloglucan endotransglucosylase/hydrolases (XTHs) were not AZ-specific in soybean. Unexpectedly, we identified an early increase in the expression of genes underlying the synthesis of a waxy-like cuticle. Based on the expression data, we propose that the early up-regulation of an abundance of small pathogenesis-related (PR) genes is more closely linked to structural changes in the extracellular matrix of separating cells than an enzymatic role in pathogen resistance. Furthermore, these observations led us to propose that, in addition to cell wall loosening enzymes, abscission requires (or is enhanced by) biosynthesis and secretion of small proteins (15-25 kDa) and waxes that form an extensible extracellular matrix and boundary layer on the surface of separating cells. The synthesis of the boundary layer precedes what is typically associated with the post-abscission synthesis of a protective scar over the fracture plane. This modification in the abscission model is discussed in regard to how it influences our interpretation of the role of multiple abscission signals.

The IDA Peptide Controls Abscission in Arabidopsis and Citrus

Organ abscission is an important process in plant development and reproduction. During abscission, changes in cellular adhesion of specialized abscission zone cells ensure the detachment of infected organs or those no longer serving a function to the plant. In addition, abscission also plays an important role in the release of ripe fruits. Different plant species display distinct patterns and timing of organ shedding, most likely adapted during evolution to their diverse life styles. However, it appears that key regulators of cell separation may have conserved function in different plant species. Here, we investigate the functional conservation of the citrus ortholog of the Arabidopsis peptide ligand INFLORESCENCE DEFICIENT IN ABSCISSION (AtIDA), controlling floral organ abscission. We discuss the possible implications of modifying the citrus IDA ortholog for citrus fruit production.

Growth Control: A Saga of Cell Walls, ROS, and Peptide Receptors

Despite an increasingly detailed understanding of endogenous and environmental growth-controlling signals and their signaling networks, little is known on how these networks are integrated with the cell expansion machinery. Members of the CrRLK1L family control cell wall properties and cell expansion in a variety of developmental and environmental contexts. Two recent reports provide exciting new insights into the mode of action of these RLKs. One study shows that one family member, FERONIA (FER), is required for the production of hydroxyl radicals in the female gametophyte, which causes pollen tube rupture and sperm cell release during fertilization. Another study shows that FER is a receptor for a signaling peptide (Rapid Alkalinization Factor 1 [RALF1]) that triggers cell wall alkalinization and growth arrest, possibly through the inhibition of plasma membrane H⁺-ATPase activity. RALF1 belongs to a large gene family, with a wide range of expression patterns. Other CrRLK1L family members therefore may also be receptors for RALF-like peptides. These findings have important implications for our understanding of the control of cell wall integrity during growth and raise new intriguing questions.