The biogenesis of CLEL peptides involves several processing events in consecutive compartments of the secretory pathway

The lowdown on breakdown: Open questions in plant proteolysis

Proteolysis, including post-translational proteolytic processing as well as protein degradation and amino acid recycling, is an essential component of the growth and development of living organisms. In this article, experts in plant proteolysis pose and discuss compelling open questions in their areas of research. Topics covered include the role of proteolysis in the cell cycle, DNA damage response, mitochondrial function, the generation of N-terminal signals (degrons) that mark many proteins for degradation (N-terminal acetylation, the Arg/N-degron pathway, and the chloroplast N-degron pathway), developmental and metabolic signaling (photomorphogenesis, abscisic acid and strigolactone signaling, sugar metabolism, and postharvest regulation), plant responses to environmental signals (endoplasmic-reticulum-associated degradation, chloroplast-associated degradation, drought tolerance, and the growth-defense trade-off), and the functional diversification of peptidases. We hope these thought-provoking discussions help to stimulate further research.

More than meets the eye: knowns and unknowns of the trafficking of small secreted proteins in Arabidopsis

Small proteins represent a significant portion of the cargo transported through plant secretory pathways, playing crucial roles in developmental processes, fertilization, and responses to environmental stresses. Despite the importance of small secreted proteins, substantial knowledge gaps persist regarding the regulatory mechanisms governing their trafficking along the secretory pathway, and their ultimate localization or destination. To address these gaps, we conducted a comprehensive literature review, focusing particularly on trafficking and localization of Arabidopsis small secreted proteins with potential biochemical and/or signaling roles in the extracellular space, typically those within the size range of 101-200 amino acids. Our investigation reveals that while at least six members of the 21 mentioned families have a confirmed extracellular localization, eight exhibit intracellular localization, including cytoplasmic, nuclear, and chloroplastic locations, despite the presence of N-terminal signal peptides. Further investigation into the trafficking and secretion mechanisms of small protein cargo could not only deepen our understanding of plant cell biology and physiology but also provide a foundation for genetic manipulation strategies leading to more efficient plant cultivation.

Transcription factor LBD16 targets cell wall modification/ion transport genes in peach lateral root formation

LATERAL ORGAN BOUNDARIES DOMAIN/ASYMMETRIC LEAVES2-LIKEs (LBDs/ASLs) are plant-specific transcription factors that function downstream of auxin-regulated lateral root (LR) formation. Our previous research found that PpLBD16 positively regulates peach (Prunus persica) LR formation. However, the downstream regulatory network and target genes of PpLBD16 are still largely unknown. Here, we constructed a PpLBD16 homologous overexpression line and a PpLBD16 silenced line. We found that overexpressing PpLBD16 promoted peach root initiation, while silencing PpLBD16 inhibited peach root formation. Through RNA sequencing (RNA-seq) analysis of roots from PpLBD16 overexpression and silenced lines, we discovered that genes positively regulated by PpLBD16 were closely related to cell wall synthesis and degradation, ion/substance transport, and ion binding and homeostasis. To further detect the binding motifs and potential target genes of PpLBD16, we performed DNA-affinity purification sequencing (DAP-seq) analysis in vitro. PpLBD16 preferentially bound to CCNGAAANNNNGG (MEME-1), [C/T]TTCT[C/T][T/C] (MEME-2), and GCGGCGG (ABR1) motifs. By combined analysis of RNA-seq and DAP-seq data, we screened candidate target genes for PpLBD16. We demonstrated that PpLBD16 bound and activated the cell wall modification-related genes EXPANSIN-B2 (PpEXPB2) and SUBTILISIN-LIKE PROTEASE 1.7 (PpSBT1.7), the ion transport-related gene CYCLIC NUCLEOTIDE-GATED ION CHANNEL 1 (PpCNGC1) and the polyphenol oxidase (PPO)-encoding gene PpPPO, thereby controlling peach root organogenesis and promoting LR formation. Moreover, our results displayed that PpLBD16 and its target genes are involved in peach LR primordia development. Overall, this work reveals the downstream regulatory network and target genes of PpLBD16, providing insights into the molecular network of LBD16-mediated LR development.

Characterization of Prolyl-4-Hydroxylase Substrate Specificity Using Pichia pastoris as an Efficient Eukaryotic Expression System

The use of eukaryotic expression systems facilitates the heterologous expression of complex eukaryotic proteins in their post-translationally modified and biologically active state, as a prerequisite for subsequent biochemical characterization and functional analysis. Here we describe the complete workflow for the expression of Arabidopsis thaliana prolyl-4-hydroxylases (P4Hs) in the methylotrophic yeast Pichia pastoris (renamed as Komagataella phaffii), for the extraction of the recombinant enzymes, purification by affinity chromatography, and characterization of P4H activity and specificity toward oligopeptide substrates by mass spectrometry. We expressed eight of the 13 Arabidopsis P4Hs and show that they are all active against proline-rich extensin-derived peptides. However, three of them differed in substrate specificity and were also able to hydroxylate the CLEL9 signaling peptide, featuring a single proline within its mature peptide sequence.

Small secreted proteins and exocytosis regulators: do they go along?

Small secreted proteins play an important role in plant development, as well as in reactions to changes in the environment. In Arabidopsis thaliana, they are predominantly members of highly expanded families, such as the pathogenesis-related (PR) 1-like protein family, whose most studied member PR1 is involved in plant defense responses by a so far unknown mechanism, or Clavata3/Endosperm Surrounding Region (CLE) protein family, whose members' functions in the development are well described. Our survey of the existing literature for the two families showed a lack of details on their localization, trafficking, and exocytosis. Therefore, in order to uncover the modes of their secretion, we tested the hypothesis that a direct link between the secreted cargoes and the secretion regulators such as Rab GTPases, SNAREs, and exocyst subunits could be established using in silico co-expression and clustering approaches. We employed several independent techniques to uncover that only weak co-expression links could be found for limited numbers of secreted cargoes and regulators. We propose that there might be particular spatio-temporal requirements for PR1 and CLE proteins to be synthesized and secreted, and efforts to experimentally cover these discrepancies should be invested along with functional studies.

Subtilase-mediated biogenesis of the expanded family of SERINE RICH ENDOGENOUS PEPTIDES

Plant signalling peptides are typically released from larger precursors by proteolytic cleavage to regulate plant growth, development and stress responses. Recent studies reported the characterization of a divergent family of Brassicaceae-specific peptides, SERINE RICH ENDOGENOUS PEPTIDES (SCOOPs), and their perception by the leucine-rich repeat receptor kinase MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2 (MIK2). Here, we reveal that the SCOOP family is highly expanded, containing at least 50 members in the Columbia-0 reference Arabidopsis thaliana genome. Notably, perception of these peptides is strictly MIK2-dependent. How bioactive SCOOP peptides are produced, and to what extent their perception is responsible for the multiple physiological roles associated with MIK2 are currently unclear. Using N-terminomics, we validate the N-terminal cleavage site of representative PROSCOOPs. The cleavage sites are determined by conserved motifs upstream of the minimal SCOOP bioactive epitope. We identified subtilases necessary and sufficient to process PROSCOOP peptides at conserved cleavage motifs. Mutation of these subtilases, or their recognition motifs, suppressed PROSCOOP cleavage and associated overexpression phenotypes. Furthermore, we show that higher-order mutants of these subtilases show phenotypes reminiscent of mik2 null mutant plants, consistent with impaired PROSCOOP biogenesis, and demonstrating biological relevance of SCOOP perception by MIK2. Together, this work provides insights into the molecular mechanisms underlying the functions of the recently identified SCOOP peptides and their receptor MIK2.

Phytaspase Is Capable of Detaching the Endoplasmic Reticulum Retrieval Signal from Tobacco Calreticulin-3

Soluble chaperones residing in the endoplasmic reticulum (ER) play vitally important roles in folding and quality control of newly synthesized proteins that transiently pass through the ER en route to their final destinations. These soluble residents of the ER are themselves endowed with an ER retrieval signal that enables the cell to bring the escaped residents back from the Golgi. Here, by using purified proteins, we showed that Nicotiana tabacum phytaspase, a plant aspartate-specific protease, introduces two breaks at the C-terminus of the N. tabacum ER resident calreticulin-3. These cleavages resulted in removal of either a dipeptide or a hexapeptide from the C-terminus of calreticulin-3 encompassing part or all of the ER retrieval signal. Consistently, expression of the calreticulin-3 derivative mimicking the phytaspase cleavage product in Nicotiana benthamiana cells demonstrated loss of the ER accumulation of the protein. Notably, upon its escape from the ER, calreticulin-3 was further processed by an unknown protease(s) to generate the free N-terminal (N) domain of calreticulin-3, which was ultimately secreted into the apoplast. Our study thus identified a specific proteolytic enzyme capable of precise detachment of the ER retrieval signal from a plant ER resident protein, with implications for the further fate of the escaped resident.

Sulfopeptide CLEL6 inhibits anthocyanin biosynthesis in Arabidopsis thaliana

Posttranslationally modified peptides are now recognized as important regulators of plant stress responses. Here, we identified the small sulfated CLE-LIKE6 (CLEL6) peptide as a negative regulator of anthocyanin biosynthesis in etiolated and in light-stressed Arabidopsis (Arabidopsis thaliana) seedlings. CLEL6 function depends on proteolytic processing of the CLEL6 precursor by subtilisin-like serine proteinase 6.1 (SBT6.1) and on tyrosine sulfation by tyrosylprotein sulfotransferase (TPST). Loss-of-function mutants of either sbt6.1 or tpst showed significantly higher anthocyanin accumulation than the wild type upon light stress. The anthocyanin overaccumulation phenotype of sbt6.1 and tpst was suppressed by application of mature CLEL6. Overexpression and external application of CLEL6 inhibited the expression of anthocyanin biosynthesis genes in etiolated and light-stressed seedlings, confirming the role of CLEL6 as an inhibitor of anthocyanin biosynthesis. Small posttranslationally modified peptides are perceived by leucine-rich repeat receptor-like kinases. Using a quintuple mutant of ROOT MERISTEM GROWTH FACTOR 1 INSENSITIVE (RGI) receptors, we showed the essential function of the RGI receptor family in CLEL6 signaling. Our data indicate that overexpression or application of CLEL6 inhibits anthocyanin biosynthesis through RGI receptors. We propose that CLEL6 inhibits anthocyanin biosynthesis in etiolated seedlings, and that anthocyanin biosynthesis is derepressed when CLEL6 expression is downregulated upon light exposure. Hyperaccumulation of anthocyanins in light-stressed tpst and sbt6.1 mutant seedlings suggests that CLEL6, or related sulfopeptides, continues to act as negative regulators to limit pigment accumulation in the light.

The Research Process of PSK Biosynthesis, Signaling Transduction, and Potential Applications in Brassica napus

Phytosulfokine (PSK) is a disulfated pentapeptide that acts as a growth regulator to control plant growth and development as well as adaptability to biotic and abiotic stress. In the last three decades, PSK has drawn increasing attention due to its various functions. Preproproteins that have been tyrosine sulfonylated and then cleaved by specific enzymes contribute to mature PSK. To transfer a signal from the apoplast to the inner cells, the PSK peptide must bind to the PSK receptors (PSKR1 and PSKR2) at the cell surface. The precise mechanism of PSK signal transduction is still unknown, given that PSKR combines receptor and kinase activity with a capacity to bind calmodulin (CaM). The binding of PSK and PSKR stimulates an abundance of cGMP downstream from PSKR, further activating a cation-translocating unit composed of cyclic nucleotide-gated channel 17 (CNGC17), H+-ATPases AHA1 and AHA2, and BRI-associated receptor kinase 1 (BAK1). Recently, it has been revealed that posttranslational ubiquitination is closely related to the control of PSK and PSKR binding. To date, the majority of studies related to PSK have used Arabidopsis. Given that rapeseed and Arabidopsis share a close genetic relationship, the relevant knowledge obtained from Arabidopsis can be further applied to rapeseed.

pH regulates peptide-receptor perception

Diverse plant small peptides are perceived by their corresponding receptors to mediate local or long-distance intercellular communications in various developmental and adaptive programs; notably, the mechanisms of peptide-receptor perception remain largely unrevealed. Two reports (Liu et al.; Diaz-Ardila et al.) shed light on how pH regulates peptide-receptor perception.

Mechanisms controlling plant proteases and their substrates

In plants, proteolysis is emerging as an important field of study due to a growing understanding of the critical involvement of proteases in plant cell death, disease and development. Because proteases irreversibly modify the structure and function of their target substrates, proteolytic activities are stringently regulated at multiple levels. Most proteases are produced as dormant isoforms and only activated in specific conditions such as altered ion fluxes or by post-translational modifications. Some of the regulatory mechanisms initiating and modulating proteolytic activities are restricted in time and space, thereby ensuring precision activity, and minimizing unwanted side effects. Currently, the activation mechanisms and the substrates of only a few plant proteases have been studied in detail. Most studies focus on the role of proteases in pathogen perception and subsequent modulation of the plant reactions, including the hypersensitive response (HR). Proteases are also required for the maturation of coexpressed peptide hormones that lead essential processes within the immune response and development. Here, we review the known mechanisms for the activation of plant proteases, including post-translational modifications, together with the effects of proteinaceous inhibitors.

Biogenesis of post-translationally modified peptide signals for plant reproductive development

Post-translationally modified peptides (PMPs) are important regulators of plant growth and development. They are derived from larger inactive precursors by post-translational modification (PTM) and proteolytic processing to result in the bioactive peptide signals. We discuss how and why these modifications contribute to the bioactivity of inflorescence deficient in abscission (IDA), phytosulfokine (PSK), and peptides of the Casparian strip integrity factor (CIF) family, as signaling molecules during reproductive development. The emerging picture suggests that PTMs evolved to increase the specificity of interaction of PMPs with cognate receptors and of PMP precursors with processing proteases. Cleavage sites in PMP precursors are recognized by subtilases (SBTs) in a highly specific manner. SBT-mediated processing results in the activation of PMP signals regulating stress-induced flower drop, the formation of the embryonic cuticle, and pollen development.

Spatial range, temporal span, and promiscuity of CLE-RLK signaling

CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) signaling through receptor-like kinases (RLKs) regulates developmental transitions and responses to biotic and abiotic inputs by communicating the physiological state of cells and tissues. CLE peptides have varying signaling ranges, which can be defined as the distance between the source, i.e., the cells or tissue that secrete the peptide, and their destination, i.e., cells or tissue where the RLKs that bind the peptide and/or respond are expressed. Case-by-case analysis substantiates that CLE signaling is predominantly autocrine or paracrine, and rarely endocrine. Furthermore, upon CLE reception, the ensuing signaling responses extend from cellular to tissue, organ and whole organism level as the downstream signal gets amplified. CLE-RLK-mediated effects on tissue proliferation and differentiation, or on subsequent primordia and organ development have been widely studied. However, studying how CLE-RLK regulates different stages of proliferation and differentiation at cellular level can offer additional insights into these processes. Notably, CLE-RLK signaling also mediates diverse non-developmental effects, which are less often observed; however, this could be due to biased experimental approaches. In general, CLEs and RLKs, owing to the sequence or structural similarity, are prone to promiscuous interactions at least under experimental conditions in which they are studied. Importantly, there are regulatory mechanisms that suppress CLE-RLK cross-talk in vivo, thereby eliminating the pressure for co-evolving binding specificity. Alternatively, promiscuity in signaling may also offer evolutionary advantages and enable different CLEs to work in combination to activate or switch off different RLK signaling pathways.

Regulation of pattern-triggered immunity and growth by phytocytokines

Endogenous signalling peptides play diverse roles during plant growth, development and stress responses. Research in recent years has unravelled peptides with previously known growth-regulatory function as immune-modulatory agents that fine-tune pattern-triggered immunity (PTI). Moreover, peptides that are long known as endogenous danger signals were recently implicated in growth and development. In analogy to metazoan systems these peptides are referred to as phytocytokines. In this review we will highlight recent progress made on our understanding of phytocytokines simultaneously regulating growth and PTI which shows the complex interplay of peptide signalling pathways regulating multiple aspects of a plant's life.

RGI-GOLVEN signaling promotes cell surface immune receptor abundance to regulate plant immunity

Plant immune responses must be tightly controlled for proper allocation of resources for growth and development. In plants, endogenous signaling peptides regulate developmental and growth‐related processes. Recent research indicates that some of these peptides also have regulatory functions in the control of plant immune responses. This classifies these peptides as phytocytokines as they show analogies with metazoan cytokines. However, the mechanistic basis for phytocytokine‐mediated regulation of plant immunity remains largely elusive. Here, we identify GOLVEN2 (GLV2) peptides as phytocytokines in Arabidopsis thaliana. GLV2 signaling enhances sensitivity of plants to elicitation with immunogenic bacterial elicitors and contributes to resistance against virulent bacterial pathogens. GLV2 is perceived by ROOT MERISTEM GROWTH FACTOR 1 INSENSITIVE (RGI) receptors. RGI mutants show reduced elicitor sensitivity and enhanced susceptibility to bacterial infection. RGI3 forms ligand‐induced complexes with the pattern recognition receptor (PRR) FLAGELLIN SENSITIVE 2 (FLS2), suggesting that RGIs are part of PRR signaling platforms. GLV2‐RGI signaling promotes PRR abundance independent of transcriptional regulation and controls plant immunity via a previously undescribed mechanism of phytocytokine activity.

RGI-GOLVEN signaling promotes cell surface immune receptor abundance to regulate plant immunity

Plant immune responses must be tightly controlled for proper allocation of resources for growth and development. In plants, endogenous signaling peptides regulate developmental and growth-related processes. Recent research indicates that some of these peptides also have regulatory functions in the control of plant immune responses. This classifies these peptides as phytocytokines as they show analogies with metazoan cytokines. However, the mechanistic basis for phytocytokine-mediated regulation of plant immunity remains largely elusive. Here, we identify GOLVEN2 (GLV2) peptides as phytocytokines in Arabidopsis thaliana. GLV2 signaling enhances sensitivity of plants to elicitation with immunogenic bacterial elicitors and contributes to resistance against virulent bacterial pathogens. GLV2 is perceived by ROOT MERISTEM GROWTH FACTOR 1 INSENSITIVE (RGI) receptors. RGI mutants show reduced elicitor sensitivity and enhanced susceptibility to bacterial infection. RGI3 forms ligand-induced complexes with the pattern recognition receptor (PRR) FLAGELLIN SENSITIVE 2 (FLS2), suggesting that RGIs are part of PRR signaling platforms. GLV2-RGI signaling promotes PRR abundance independent of transcriptional regulation and controls plant immunity via a previously undescribed mechanism of phytocytokine activity.

Knowing me, knowing you: Self and non-self recognition in plant immunity

Perception of non-self molecules known as microbe-associated molecular patterns (MAMPs) by host pattern recognition receptors (PRRs) activates plant pattern-triggered immunity (PTI). Pathogen infections often trigger the release of modified-self molecules, termed damage- or danger-associated molecular patterns (DAMPs), which modulate MAMP-triggered signaling to shape the frontline of plant immune responses against infections. In the context of advances in identifying MAMPs and DAMPs, cognate receptors, and their signaling, here, we focus on the most recent breakthroughs in understanding the perception and role of non-self and modified-self patterns. We highlight the commonalities and differences of MAMPs from diverse microbes, insects, and parasitic plants, as well as the production and perception of DAMPs upon infections. We discuss the interplay between MAMPs and DAMPs for emerging themes of the mutual potentiation and attenuation of PTI signaling upon MAMP and DAMP perception during infections.

Immunomodulatory peptides-A promising source for novel functional food production and drug discovery

Immunomodulatory peptides are a complex class of bioactive peptides that encompasses substances with different mechanisms of action. Immunomodulatory peptides could also be used in vaccines as adjuvants which would be extremely desirable, especially in response to pandemics. Thus, immunomodulatory peptides in food of plant origin could be regarded both as valuable suplements of novel functional food preparation and/or as precursors or possible active ingredients for drugs design for treatment variety of conditions arising from impaired function of immune system. Given variety of mechanisms, different tests are required to assess effects of immunomodulatory peptides. Some of those effects show good correlation with in vivo results but others, less so. Certain plant peptides, such as defensins, show both immunomodulatory and antimicrobial effect, which makes them interesting candidates for preparation of functional food and feed, as well as templates for design of synthetic peptides.

Identification of Cognate Protease/Substrate Pairs by Use of Class-Specific Inhibitors

Many proteins are regulated post-translationally by proteolytic processing. This includes plant signaling peptides that are proteolytically released from larger precursor proteins. The proteases involved in the biogenesis of signaling peptides and in regulation of other proteins by limited proteolysis are largely unknown. Here we describe how protease inhibitors that are specific for a certain class of proteases can be employed for the identification of proteases that are responsible for the processing of a given target protein. After having identified the protease family to which the processing enzyme belongs, candidate proteases and the GFP-tagged target protein are agro-infiltrated for transient expression in N. benthamiana leaves. Cleavage products are analyzed on immuno-blots and specificity of cleavage is confirmed by co-expression of class-specific inhibitors. For the identification of processing sites within the target protein, cleavage product(s) are purified by immunoprecipitation followed by polyacrylamide gel electrophoresis and analyzed by mass spectrometry.

Subtilase activity in intrusive cells mediates haustorium maturation in parasitic plants

Parasitic plants that infect crops are devastating to agriculture throughout the world. These parasites develop a unique inducible organ called the haustorium that connects the vascular systems of the parasite and host to establish a flow of water and nutrients. Upon contact with the host, the haustorial epidermal cells at the interface with the host differentiate into specific cells called intrusive cells that grow endophytically toward the host vasculature. Following this, some of the intrusive cells re-differentiate to form a xylem bridge (XB) that connects the vasculatures of the parasite and host. Despite the prominent role of intrusive cells in host infection, the molecular mechanisms mediating parasitism in the intrusive cells remain poorly understood. In this study, we investigated differential gene expression in the intrusive cells of the facultative parasite Phtheirospermum japonicum in the family Orobanchaceae by RNA-sequencing of laser-microdissected haustoria. We then used promoter analyses to identify genes that are specifically induced in intrusive cells, and promoter fusions with genes encoding fluorescent proteins to develop intrusive cell-specific markers. Four of the identified intrusive cell-specific genes encode subtilisin-like serine proteases (SBTs), whose biological functions in parasitic plants are unknown. Expression of SBT inhibitors in intrusive cells inhibited both intrusive cell and XB development and reduced auxin response levels adjacent to the area of XB development. Therefore, we propose that subtilase activity plays an important role in haustorium development in P. japonicum.

Phytosulfokine (PSK) precursor processing by subtilase SBT3.8 and PSK signaling improve drought stress tolerance in Arabidopsis

Increasing drought stress poses a severe threat to agricultural productivity. Plants, however, have evolved numerous mechanisms to cope with such environmental stress. Here we report that the stress-induced production of a peptide signal contributes to stress tolerance. The expression of phytosulfokine (PSK) peptide precursor genes, and transcripts of three subtilisin-like serine proteases, SBT1.4, SBT3.7, and SBT3.8, were found to be up-regulated in response to osmotic stress. Stress symptoms were more pronounced in sbt3.8 loss-of-function mutants and could be alleviated by PSK treatment. Osmotic stress tolerance was improved in plants overexpressing the PSK1 precursor (proPSK1) or SBT3.8, resulting in higher fresh weight and improved lateral root development in transgenic plants compared with wild-type plants. We further showed that SBT3.8 is involved in the biogenesis of the bioactive PSK peptide. ProPSK1 was cleaved by SBT3.8 at the C-terminus of the PSK pentapeptide. Processing by SBT3.8 depended on the aspartic acid residue directly following the cleavage site. ProPSK1 processing was impaired in the sbt3.8 mutant. The data suggest that increased expression of proPSK1 in response to osmotic stress followed by the post-translational processing of proPSK1 by SBT3.8 leads to the production of PSK as a peptide signal for stress mitigation.

The PSY Peptide Family-Expression, Modification and Physiological Implications

Small post-translationally modified peptides are gaining increasing attention as important signaling molecules in plant development. In the family of plant peptides containing tyrosine sulfation (PSYs), only PSY1 has been characterized at the mature level as an 18-amino-acid peptide, carrying one sulfated tyrosine, and involved in cell elongation. This review presents seven additional homologs in Arabidopsis all sharing high conservation in the active peptide domain, and it shows that PSY peptides are found in all higher plants and mosses. It is proposed that all eight PSY homologs are post-translationally modified to carry a sulfated tyrosine and that subtilisin-like subtilases (SBTs) are involved in the processing of PSY propeptides. The PSY peptides show differential expression patterns indicating that they serve several distinct functions in plant development. PSY peptides seem to be at least partly regulated at the transcriptional level, as their expression is greatly influenced by developmental factors. Finally, a model including a receptor in addition to PSY1R is proposed.

The RGF/GLV/CLEL Family of Short Peptides Evolved Through Lineage-Specific Losses and Diversification and Yet Conserves Its Signaling Role Between Vascular Plants and Bryophytes

Short secreted plant peptides act as key signaling molecules and control a plethora of developmental and physiological processes. The ROOT GROWTH FACTOR (RGF)/GOLVEN (GLV)/CLE-Like (CLEL) family of peptides was discovered to be involved in root development in Arabidopsis thaliana. In contrast to active research efforts, which have been revealing receptors and downstream signaling components, little attention has been paid to evolutionary processes that shaped the RGF signaling system as we know it in angiosperms today. As a first step toward understanding how RGF signaling emerged and evolved, this study aimed to elucidate the phylogenetic distribution and functional conservation of RGF-like sequences. Using publicly available, genome and transcriptome data, RGF-like sequences were searched in 27 liverworts, 22 mosses, 8 hornworts, 23 lycophytes, 23 ferns, 38 gymnosperms, and 8 angiosperms. This led to the identification of more than four hundreds of RGF-like sequences in all major extant land plant lineages except for hornworts. Sequence comparisons within and between taxonomic groups identified lineage-specific characters. Notably, one of the two major RGF subgroups, represented by A. thaliana RGF6/GLV1/CLEL6, was found only in vascular plants. This subgroup, therefore, likely emerged in a common ancestor of vascular plants after its divergence from bryophytes. In bryophytes, our results infer independent losses of RGF-like sequences in mosses and hornworts. On the other hand, a single, highly similar RGF-like sequence is conserved in liverworts, including Marchantia polymorpha, a genetically tractable model species. When constitutively expressed, the M. polymorpha RGF-like sequence (MpRGF) affected plant development and growth both in A. thaliana and M. polymorpha. This suggests that MpRGF can exert known RGF-like effects and that MpRGF is under transcriptional control so that its potent activities are precisely controlled. These data suggest that RGFs are conserved as signaling molecules in both vascular plants and bryophytes and that lineage-specific diversification has increased sequence variations of RGFs. All together, our findings form a basis for further studies into RGF peptides and their receptors, which will contribute to our understandings of how peptide signaling pathways evolve.

Processing and Formation of Bioactive CLE40 Peptide Are Controlled by Posttranslational Proline Hydroxylation

Small posttranslationally modified signaling peptides are proteolytically derived from larger precursor proteins and subject to several additional steps of modification, including Pro hydroxylation, Hyp glycosylation, and/or Tyr sulfation. The processing proteases and the relevance of posttranslational modifications for peptide biogenesis and activity are largely unknown. In this study these questions were addressed for the Clavata3/Endosperm Surrounding Region (CLE) peptide CLE40, a peptide regulator of stem cell differentiation in the Arabidopsis (Arabidopsis thaliana) root meristem. We identify three subtilases (SBT1.4, SBT1.7, and SBT4.13) that cleave the CLE40 precursor redundantly at two sites. C-terminal processing releases the mature peptide from its precursor and is thus required for signal biogenesis. SBT-mediated cleavage at a second site within the mature peptide attenuates the signal. The second cleavage is prevented by Pro hydroxylation, resulting in the formation of mature and bioactive CLE40 in planta. Our data reveal a role for posttranslational modification by Pro hydroxylation in the regulation of CLE40 formation and activity.

Extracellular proteolytic cascade in tomato activates immune protease Rcr3

Proteolytic cascades regulate immunity and development in animals, but these cascades in plants have not yet been reported. Here we report that the extracellular immune protease Rcr3 of tomato is activated by P69B and other subtilases (SBTs), revealing a proteolytic cascade regulating extracellular immunity in solanaceous plants. Rcr3 is a secreted papain-like Cys protease (PLCP) of tomato that acts both in basal resistance against late blight disease (Phytophthora infestans) and in gene-for-gene resistance against the fungal pathogen Cladosporium fulvum (syn. Passalora fulva) Despite the prevalent model that Rcr3-like proteases can activate themselves at low pH, we found that catalytically inactive proRcr3 mutant precursors are still processed into mature mRcr3 isoforms. ProRcr3 is processed by secreted P69B and other Asp-selective SBTs in solanaceous plants, providing robust immunity through SBT redundancy. The apoplastic effector EPI1 of P. infestans can block Rcr3 activation by inhibiting SBTs, suggesting that this effector promotes virulence indirectly by preventing the activation of Rcr3(-like) immune proteases. Rcr3 activation in Nicotiana benthamiana requires a SBT from a different subfamily, indicating that extracellular proteolytic cascades have evolved convergently in solanaceous plants or are very ancient in the plant kingdom. The frequent incidence of Asp residues in the cleavage region of Rcr3-like proteases in solanaceous plants indicates that activation of immune proteases by SBTs is a general mechanism, illuminating a proteolytic cascade that provides robust apoplastic immunity.