The PSY Peptide Family-Expression, Modification and Physiological Implications

Comparative Genomics Screens Identify a Novel Small Secretory Peptide, SlSolP12, which Activates Both Local and Systemic Immune Response in Tomatoes and Exhibits Broad-Spectrum Activity

Small secreted peptides (SSPs) are essential for defense mechanisms in plant-microbe interactions, acting as danger-associated molecular patterns (DAMPs). Despite the first discovery of SSPs over three decades ago, only a limited number of SSP families, particularly within Solanaceae plants, have been identified due to inefficient approaches. This study employed comparative genomics screens with Solanaceae proteomes (tomato, tobacco, and pepper) to discover a novel SSP family, SolP. Bioinformatics analysis suggests that SolP may serve as an endogenous signal initiating the plant PTI response. Interestingly, SolP family members from tomato, tobacco, and pepper share an identical sequence (VTSNALALVNRFAD), named SlSolP12 (also referred to as NtSolP15 or CaSolP1). Biochemical and phenotypic analyses revealed that synthetic SlSolP12 peptide triggers multiple defense responses: ROS burst, MAPK activation, callose deposition, stomatal closure, and expression of immune defense genes. Furthermore, SlSolP12 enhances systemic resistance against Botrytis cinerea infection in tomato plants and interferes with classical peptides, flg22 and Systemin, which modulate the immune response. Remarkably, SolP12 activates ROS in diverse plant species, such as Arabidopsis thaliana, soybean, and rice, showing a broad spectrum of biological activities. This study provides valuable approaches for identifying endogenous SSPs and highlights SlSolP12 as a novel DAMP that could serve as a useful target for crop protection.

Wound-induced small-peptide-mediated signaling cascade, regulated by OsPSKR, dictates balance between growth and defense in rice

Wounding is a general stress in plants that results from various pest and pathogenic infections in addition to environment-induced mechanical damages. Plants have sophisticated molecular mechanisms to recognize and respond to wounding, with those of monocots being distinct from dicots. Here, we show the involvement of two distinct categories of temporally separated, endogenously derived peptides, namely, plant elicitor peptides (PEPs) and phytosulfokine (PSK), mediating wound responses in rice. These peptides trigger a dynamic signal relay in which a receptor kinase involved in PSK perception named OsPSKR plays a major role. Perturbation of OsPSKR expression in rice leads to compromised development and constitutive autoimmune phenotypes. OsPSKR regulates the transitioning of defense to growth signals upon wounding. OsPSKR displays mutual antagonism with the OsPEPR1 receptor involved in PEP perception. Collectively, our work indicates the presence of a stepwise peptide-mediated signal relay that regulates the transition from defense to growth upon wounding in monocots.

Small Peptides: Orchestrators of Plant Growth and Developmental Processes

Small peptides (SPs), ranging from 5 to 100 amino acids, play integral roles in plants due to their diverse functions. Despite their low abundance and small molecular weight, SPs intricately regulate critical aspects of plant life, including cell division, growth, differentiation, flowering, fruiting, maturation, and stress responses. As vital mediators of intercellular signaling, SPs have garnered significant attention in plant biology research. This comprehensive review delves into SPs' structure, classification, and identification, providing a detailed understanding of their significance. Additionally, we summarize recent findings on the biological functions and signaling pathways of prominent SPs that regulate plant growth and development. This review also offers a perspective on future research directions in peptide signaling pathways.

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.

Sulfated peptides and their receptors: Key regulators of plant development and stress adaptation

Four distinct types of sulfated peptides have been identified in Arabidopsis thaliana. These peptides play crucial roles in regulating plant development and stress adaptation. Recent studies have revealed that Xanthomonas and Meloidogyne can secrete plant-like sulfated peptides, exploiting the plant sulfated peptide signaling pathway to suppress plant immunity. Over the past three decades, receptors for these four types of sulfated peptides have been identified, all of which belong to the leucine-rich repeat receptor-like protein kinase subfamily. A number of regulatory proteins have been demonstrated to play important roles in their corresponding signal transduction pathways. In this review, we comprehensively summarize the discoveries of sulfated peptides and their receptors, mainly in Arabidopsis thaliana. We also discuss their known biological functions in plant development and stress adaptation. Finally, we put forward a number of questions for reference in future studies.

Detection of the Arabidopsis Proteome and Its Post-translational Modifications and the Nature of the Unobserved (Dark) Proteome in PeptideAtlas

This study describes a new release of the Arabidopsis thaliana PeptideAtlas proteomics resource (build 2023-10) providing protein sequence coverage, matched mass spectrometry (MS) spectra, selected post-translational modifications (PTMs), and metadata. 70 million MS/MS spectra were matched to the Araport11 annotation, identifying ∼0.6 million unique peptides and 18,267 proteins at the highest confidence level and 3396 lower confidence proteins, together representing 78.6% of the predicted proteome. Additional identified proteins not predicted in Araport11 should be considered for the next Arabidopsis genome annotation. This release identified 5198 phosphorylated proteins, 668 ubiquitinated proteins, 3050 N-terminally acetylated proteins, and 864 lysine-acetylated proteins and mapped their PTM sites. MS support was lacking for 21.4% (5896 proteins) of the predicted Araport11 proteome: the "dark" proteome. This dark proteome is highly enriched for E3 ligases, transcription factors, and for certain (e.g., CLE, IDA, PSY) but not other (e.g., THIONIN, CAP) signaling peptides families. A machine learning model trained on RNA expression data and protein properties predicts the probability that proteins will be detected. The model aids in discovery of proteins with short half-life (e.g., SIG1,3 and ERF-VII TFs) and for developing strategies to identify the missing proteins. PeptideAtlas is linked to TAIR, tracks in JBrowse, and several other community proteomics resources.

PSY-PSYR peptide-receptor pairs control the trade-off between plant growth and stress response

Leucine-rich repeat-receptor kinases (LRR-RKs) perceive various endogenous peptide hormones that control plant growth and development. However, the majority of corresponding ligands and their direct ligand-binding receptors have not been identified yet. A recent study demonstrated that three LRR-RK PLANT PEPTIDE CONTAINING SULFATED TYROSINE RECEPTORS (PSYRs) act as ligand-receptors of the PSY family peptides that mediate the trade-off between the optimal plant growth and stress tolerance responses. The genetic, biochemical, and transcriptome analyses suggested that PSYR1, PSYR2, and PSYR3 function as negative regulators of plant growth in the absence of PSY peptides and induce stress tolerance responses, whereas the PSY family peptides repress PSYR signaling, allowing plant growth. This trade-off mechanism between plant growth and stress responses mediated by the PSY-PSYR signaling module allows plants to survive under ever changing environmental stresses.

Peptides from conserved tandem direct repeats of SHORT-LEAF regulate gametophore development in moss P. patens

Tandem direct repeat (TDR)-containing proteins, present across all domains of life, play crucial roles in plant development and defense mechanisms. Previously, we identified that disruption of a bryophyte-specific protein family, SHORT-LEAF (SHLF), possessing the longest reported TDRs, is the cause of the shlf mutant phenotype in Physcomitrium patens. shlf exhibits reduced apical dominance, altered auxin distribution, and 2-fold shorter leaves. However, the molecular role of SHLF was unclear due to the absence of known conserved domains. Through a series of protein domain deletion analyses, here, we demonstrate the importance of the signal peptide and the conserved TDRs and report a minimal functional protein (miniSHLF) containing the N-terminal signal peptide and first two TDRs (N-TDR1-2). We also demonstrate that SHLF behaves as a secretory protein and that the TDRs contribute to a pool of secreted peptides essential for SHLF function. Further, we identified that the mutant secretome lacks SHLF peptides, which are abundant in WT and miniSHLF secretomes. Interestingly, shlf mutants supplemented with the secretome or peptidome from WT or miniSHLF showed complete or partial phenotypic recovery. Transcriptomic and metabolomic analyses revealed that shlf displays an elevated stress response, including high ROS activity and differential accumulation of genes and metabolites involved in the phenylpropanoid pathway, which may affect auxin distribution. The TDR-specific synthetic peptide SHLFpep3 (INIINAPLQGFKIA) also rescued the mutant phenotypes, including the altered auxin distribution, in a dosage-dependent manner and restored the mutant's stress levels. Our study shows that secretory SHLF peptides derived from conserved TDRs regulate moss gametophore development.

Bioassays for Identifying and Characterizing Plant Regulatory Peptides

Plant peptides are a new frontier in plant biology, owing to their key regulatory roles in plant growth, development, and stress responses. Synthetic peptides are promising biological agents that can be used to improve crop growth and protection in an environmentally sustainable manner. Plant regulatory peptides identified in pioneering research, including systemin, PSK, HypSys, RALPH, AtPep1, CLV3, TDIF, CLE, and RGF/GLV/CLEL, hold promise for crop improvement as potent regulators of plant growth and defense. Mass spectrometry and bioinformatics are greatly facilitating the discovery and identification of new plant peptides. The biological functions of most novel plant peptides remain to be elucidated. Bioassays are an essential part in studying the biological activity of identified and putative plant peptides. Root growth assays and cultivated plant cell cultures are widely used to evaluate the regulatory potential of plant peptides during growth, differentiation, and stress reactions. These bioassays can be used as universal approaches for screening peptides from different plant species. Development of high-throughput bioassays can facilitate the screening of large numbers of identified and putative plant peptides, which have recently been discovered but remain uncharacterized for biological activity.

Shining in the dark: the big world of small peptides in plants

Small peptides represent a subset of dark matter in plant proteomes. Through differential expression patterns and modes of action, small peptides act as important regulators of plant growth and development. Over the past 20 years, many small peptides have been identified due to technical advances in genome sequencing, bioinformatics, and chemical biology. In this article, we summarize the classification of plant small peptides and experimental strategies used to identify them as well as their potential use in agronomic breeding. We review the biological functions and molecular mechanisms of small peptides in plants, discuss current problems in small peptide research and highlight future research directions in this field. Our review provides crucial insight into small peptides in plants and will contribute to a better understanding of their potential roles in biotechnology and agriculture.

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.

Root-knot nematodes produce functional mimics of tyrosine-sulfated plant peptides

Root-knot nematodes (Meloidogyne spp.) are highly evolved obligate parasites threatening global food security. These parasites have a remarkable ability to establish elaborate feeding sites in roots, which are their only source of nutrients throughout their life cycle. A wide range of nematode effectors have been implicated in modulation of host pathways for defense suppression and/or feeding site development. Plants produce a diverse array of peptide hormones including PLANT PEPTIDE CONTAINING SULFATED TYROSINE (PSY)-family peptides, which promote root growth via cell expansion and proliferation. A sulfated PSY-like peptide RaxX (required for activation of XA21 mediated immunity X) produced by the biotrophic bacterial pathogen (Xanthomonas oryzae pv. oryzae) has been previously shown to contribute to bacterial virulence. Here, we report the identification of genes from root-knot nematodes predicted to encode PSY-like peptides (MigPSYs) with high sequence similarity to both bacterial RaxX and plant PSYs. Synthetic sulfated peptides corresponding to predicted MigPSYs stimulate root growth in Arabidopsis. MigPSY transcript levels are highest early in the infection cycle. Downregulation of MigPSY gene expression reduces root galling and egg production, suggesting that the MigPSYs serve as nematode virulence factors. Together, these results indicate that nematodes and bacteria exploit similar sulfated peptides to hijack plant developmental signaling pathways to facilitate parasitism.

Mapping the Arabidopsis thaliana proteome in PeptideAtlas and the nature of the unobserved (dark) proteome; strategies towards a complete proteome

This study describes a new release of the Arabidopsis thaliana PeptideAtlas proteomics resource providing protein sequence coverage, matched mass spectrometry (MS) spectra, selected PTMs, and metadata. 70 million MS/MS spectra were matched to the Araport11 annotation, identifying ∼0.6 million unique peptides and 18267 proteins at the highest confidence level and 3396 lower confidence proteins, together representing 78.6% of the predicted proteome. Additional identified proteins not predicted in Araport11 should be considered for building the next Arabidopsis genome annotation. This release identified 5198 phosphorylated proteins, 668 ubiquitinated proteins, 3050 N-terminally acetylated proteins and 864 lysine-acetylated proteins and mapped their PTM sites. MS support was lacking for 21.4% (5896 proteins) of the predicted Araport11 proteome - the 'dark' proteome. This dark proteome is highly enriched for certain ( e.g. CLE, CEP, IDA, PSY) but not other ( e.g. THIONIN, CAP,) signaling peptides families, E3 ligases, TFs, and other proteins with unfavorable physicochemical properties. A machine learning model trained on RNA expression data and protein properties predicts the probability for proteins to be detected. The model aids in discovery of proteins with short-half life ( e.g. SIG1,3 and ERF-VII TFs) and completing the proteome. PeptideAtlas is linked to TAIR, JBrowse, PPDB, SUBA, UniProtKB and Plant PTM Viewer.

Chemical Synthesis of Glycopeptides containing l-Arabinosylated Hydroxyproline and Sulfated Tyrosine

Post-translationally modified peptides are important regulating molecules for living organisms. Here, we report the stereoselective total synthesis of β-1,2-linked l-arabinosylated Fmoc-protected hydroxyproline building blocks and their incorporation, together with sulfated tyrosine and hydroxyproline, into the plant peptide hormone PSY1. Clean glycopeptides were obtained by performing acetyl removal from the l-arabinose groups prior to deprotection of the neopentyl-protected sulfated tyrosine.

Peptide ligand-mediated trade-off between plant growth and stress response

Deciding whether to grow or to divert energy to stress responses is a major physiological trade-off for plants surviving in fluctuating environments. We show that three leucine-rich repeat receptor kinases (LRR-RKs) act as direct ligand-perceiving receptors for PLANT PEPTIDE CONTAINING SULFATED TYROSINE (PSY)-family peptides and mediate switching between two opposing pathways. By contrast to known LRR-RKs, which activate signaling upon ligand binding, PSY receptors (PSYRs) activate the expression of various genes encoding stress response transcription factors upon depletion of the ligands. Loss of PSYRs results in defects in plant tolerance to both biotic and abiotic stresses. This ligand-deprivation-dependent activation system potentially enables plants to exert tuned regulation of stress responses in the tissues proximal to metabolically dysfunctional damaged sites where ligand production is impaired.

Identification of Diverse Stress-Responsive Xylem Sap Peptides in Soybean

Increasing evidence has revealed that plant secretory peptides are involved in the long-distance signaling pathways that help to regulate plant development and signal stress responses. In this study, we purified small peptides from soybean (Glycine max) xylem sap via o-chlorophenol extraction and conducted an in-depth peptidomic analysis using a mass spectrometry (MS) and bioinformatics approach. We successfully identified 14 post-translationally modified peptide groups belonging to the peptide families CEP (C-terminally encoded peptides), CLE (CLAVATA3/embryo surrounding region-related), PSY (plant peptides containing tyrosine sulfation), and XAP (xylem sap-associated peptides). Quantitative PCR (qPCR) analysis showed unique tissue expression patterns among the peptide-encoding genes. Further qPCR analysis of some of the peptide-encoding genes showed differential stress-response profiles toward various abiotic stress factors. Targeted MS-based quantification of the nitrogen deficiency-responsive peptides, GmXAP6a and GmCEP-XSP1, demonstrated upregulation of peptide translocation in xylem sap under nitrogen-deficiency stress. Quantitative proteomic analysis of GmCEP-XSP1 overexpression in hairy soybean roots revealed that GmCEP-XSP1 significantly impacts stress response-related proteins. This study provides new insights that root-to-shoot peptide signaling plays important roles in regulating plant stress-response mechanisms.

Dissecting the genetic control of natural variation in sorghum photosynthetic response to drought stress

Drought stress causes crop yield losses worldwide. Sorghum is a C4 species tolerant to moderate drought stress, and its extensive natural variation for photosynthetic traits under water-limiting conditions can be exploited for developing cultivars with enhanced stress tolerance. The objective of this study was to discover genes/genomic regions that control the sorghum photosynthetic capacity under pre-anthesis water-limiting conditions. We performed a genome-wide association study for seven photosynthetic gas exchange and chlorophyll fluorescence traits during three periods of contrasting soil volumetric water content (VWC): control (30% VWC), drought (15% VWC), and recovery (30% VWC). Water stress was imposed with an automated irrigation system that generated a controlled dry-down period for all plants, to perform an unbiased genotypic comparison. A total of 60 genomic regions were associated with natural variation in one or more photosynthetic traits in a particular treatment or with derived variables. We identified 33 promising candidate genes with predicted functions related to stress signaling, oxidative stress protection, hormonal response to stress, and dehydration protection. Our results provide new knowledge about the natural variation and genetic control of sorghum photosynthetic response to drought with the ultimate goal of improving its adaptation and productivity under water stress scenarios.

Emerging roles of pathogen-secreted host mimics in plant disease development

Plant pathogens and parasites use multiple virulence factors to successfully infect plants. While most plant-pathogen interaction studies focus on pathogen effectors and their functions in suppressing plant immunity or interfering with normal cellular processes, other virulence factors likely also contribute. Here we highlight another important strategy used by pathogens to promote virulence: secretion of mimics of host molecules, including peptides, phytohormones, and small RNAs, which play diverse roles in plant development and stress responses. Pathogen-secreted mimics hijack the host endogenous signaling pathways, thereby modulating host cellular functions to the benefit of the pathogen and promoting infection. Understanding the mechanisms of pathogen-secreted host mimics will expand our knowledge of host-pathogen coevolution and interactions, while providing new targets for plant disease control.