Perception of Arabidopsis AtPep peptides, but not bacterial elicitors, accelerates starvation-induced senescence

Effects of Nitrogen Application Strategies on Yield, Nitrogen Uptake and Leaching in Spring Maize Fields in Northwest China

Nitrogen (N) is an essential nutrient for crop growth, as N fertilizer application regulates crop nitrogen uptake, affecting leaf photosynthetic rates, crop growth, and yield formation. However, both N deficiency and excess can reduce corn yields. Hence, optimizing the N fertilizer application strategy is crucial for crop production. In this study, a field plot trial with five N fertilization application strategies was conducted in the maize field from 2021 to 2022 in the Ningxia Yellow Irrigation District, Northwest China. These strategies contain zero N application rates (CK, 0 kg ha-1), the farmer practical N fertilizer application strategy (FP, 420 kg ha-1), the optimized N fertilizer application strategy (OPT, 360 kg ha-1), organic fertilizer and chemical fertilizer combination application (ON, 300 kg ha-1), and controlled-release N fertilizer and 33 urea application (CN, 270 kg ha-1). The maize yield and N balance under each treatment were investigated to propose the optimized N application strategy. The results showed that the CN treatment's grain yield (15,672 kg ha-1) was the highest in both years, which was 109.97% and 8.92% higher than the CK and FP treatments, respectively. The apparent utilization rate and partial productivity of N fertilizer decreased with the increase in the N application rate. Also, the apparent utilization rate of N fertilizer in CN was 23.02%, 19.41%, and 13.02% higher than the FP, OPT, and ON, respectively. Applying controlled-release urea and organic fertilizers improved the physical and chemical properties of the soil, increased the organic matter content and soil fertility, and ultimately increased the spring maize yield. Meanwhile, the TN, NO3--N, and NH4+-N concentrations in leaching water significantly correlated with the N application rate. With the extension of the maize growth period, the concentrations of TN, NO3--N, and NH4+-N in leaching water gradually decreased. The N leaching amount in FP was the highest, while the CN was the lowest. The NO3--N is the primary N leaching form, accounting for 46.78~54.68% of the TN leaching amount. Compared with the CN, the ON significantly increased the inorganic N content in the 0-40 cm soil layer, and it reduced the residual inorganic N content below 40 cm soil depths compared with FP and OPT treatments. Considering the relatively high spring maize yield and N utilization efficiency, as well as the relatively low N leaching amount and soil inorganic N residues, the ON and CN treatments with 270-300 kg ha-1 N application rate were the optimized N application strategies in the spring maize field in the study area.

SCOOP10 and SCOOP12 peptides act through MIK2 receptor-like kinase to antagonistically regulate Arabidopsis leaf senescence

Leaf senescence plays a critical role in a plant's overall reproductive success due to its involvement in nutrient remobilization and allocation. However, our current understanding of the molecular mechanisms controlling leaf senescence remains limited. In this study, we show that the receptor-like kinase MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2 (MIK2) functions as a negative regulator of leaf senescence. We found that the SERINE-RICH ENDOGENOUS PEPTIDE 12, previously known to physically interact with MIK2, competes with SCOOP10 to regulate MIK2-dependent leaf senescence. We observed that increased expression of SCOOP10 or the application of exogenous SCOOP10 peptides accelerated leaf senescence in a MIK2-dependent manner. Conversely, SCOOP12 acted as a suppressor of MIK2-dependent leaf senescence regulation. Biochemical assays showed that SCOOP12 enhances while SCOOP10 diminishes MIK2 phosphorylation. Thus, the SCOOP12-MIK2 module might function antagonistically on SCOOP10-MIK2 signaling at late senescing stages, allowing for fine-tuned modulation of the leaf senescence process. Our study sheds light on the complex mechanisms underlying leaf senescence and provides valuable insights into the interplay between receptors, peptides, and the regulation of plant senescence.

The Effects of Bacillus subtilis Expressing a Plant Elicitor Peptide on Nematode Infection on Soybean

There is a pressing need to develop alternative management strategies for the soybean cyst nematode (Heterodera glycines), the most costly pathogen to soybeans. Plant elicitor peptides (PEPs), which are produced by plants in response to stress and stimulate broad-spectrum disease resistance, were previously shown to reduce soybean cyst nematode infection on soybeans when applied as a seed treatment. Here, we introduce an alternative method to deliver PEPs to soybean using a common plant growth-promoting rhizobacterium, Bacillus subtilis, as a bacterial expression system. Similar to the empty vector control, B. subtilis engineered to express a PEP from soybean (GmPEP3) was able to colonize soybean roots and persisted on roots more than a month after treatment. Compared with water or the empty vector control, plants that received a seed treatment with B. subtilis expressing GmPEP3 (B.+GmPEP3) were significantly taller early in vegetative growth (V1 stage) and had lower chlorophyll content in the reproductive stage (R3/R4); these results suggest that GmPEP3 may hasten growth and subsequent senescence. When plants were inoculated with soybean cyst nematode at the V1 stage, those pretreated with B.+GmPEP3 supported significantly fewer nematode eggs at the reproductive stage (R3/R4) than plants treated with water or the empty vector. The effects of B.+GmPEP3 on nematode infection and plant growth appeared to be due primarily to the peptide itself because no significant differences were observed between plants treated with water or with B. subtilis expressing the empty vector. These results indicate the ability of B. subtilis to deliver defense activators for nematode management on soybean.

Arabidopsis WRKY71 regulates ethylene-mediated leaf senescence by directly activating EIN2, ORE1 and ACS2 genes

Leaf senescence is a pivotal step in the last stage of the plant life cycle and is influenced by various external and endogenous cues. A series of reports have indicated the involvement of the WRKY transcription factors in regulating leaf senescence, but the molecular mechanisms and signaling pathways remain largely unclear. Here we provide evidence demonstrating that WRKY71 acts as a positive regulator of leaf senescence in Arabidopsis. WRKY71-1D, an overexpressor of WRKY71, exhibited early leaf senescence, while wrky71-1, the WRKY71 loss-of-function mutant, displayed delayed leaf senescence. Accordingly, a set of senescence-associated genes (SAGs) were substantially elevated in WRKY71-1D but markedly decreased in wrky71-1. Chromatin immunoprecipitation assays indicated that WRKY71 can bind directly to the promoters of SAG13 and SAG201. Transcriptome analysis suggested that WRKY71 might mediate multiple cues to accelerate leaf senescence, such as abiotic stresses, dark and ethylene. WRKY71 was ethylene inducible, and treatment with the ethylene precursor 1-amino-cyclopropane-1-carboxylic acid enhanced leaf senescence in WRKY71-1D but caused only a marginal delay in leaf senescence in wrky71-1. In vitro and in vivo assays demonstrated that WRKY71 can directly regulate ETHYLENE INSENSITIVE2 (EIN2) and ORESARA1 (ORE1), genes of the ethylene signaling pathway. Consistently, leaf senescence of WRKY71-1D was obviously retarded in the ein2-5 and nac2-1 mutants. Moreover, WRKY71 was also proved to interact with ACS2 in vitro and in vivo. Treatment with AgNO₃ and aminoethoxyvinylglycine and acs2-1 could greatly arrest the leaf senescence of WRKY71-1D. In conclusion, our data revealed that WRKY71 mediates ethylene signaling and synthesis to hasten leaf senescence in Arabidopsis.

Prunus persica plant endogenous peptides PpPep1 and PpPep2 cause PTI-like transcriptome reprogramming in peach and enhance resistance to Xanthomonas arboricola pv. pruni

BACKGROUND

Rosaceae species are economically highly relevant crops. Their cultivation systems are constrained by phytopathogens causing severe losses. Plants respond to invading pathogens through signaling mechanisms, a component of which are of them being plant elicitor peptides (Peps). Exogenous application of Peps activates defense mechanisms and reduces the symptoms of pathogen infection in various pathosystems. We have previously identified the Rosaceae Peps and showed, in an ex vivo system, that their topical application efficiently enhanced resistance to the bacterial pathogen Xanthomonas arboricola pv. pruni (Xap).

RESULTS

Here we demonstrate the effectiveness of Prunus persica peptides PpPep1 and PpPep2 in protecting peach plants in vivo at nanomolar doses, with 40% reduction of the symptoms following Xap massive infection. We used deep sequencing to characterize the transcriptomic response of peach plants to preventive treatment with PpPep1 and PpPep2. The two peptides induced highly similar massive transcriptomic reprogramming in the plant. One hour, 1 day and 2 days after peptide application there were changes in expression in up to 8% of peach genes. We visualized the transcriptomics dynamics in a background knowledge network and detected the minor variations between plant responses to PpPep1 and PpPep2, which might explain their slightly different protective effects. By designing a P. persica Pep background knowledge network, comparison of our data and previously published immune response datasets was possible.

CONCLUSIONS

Topical application of P. persica Peps mimics the PTI natural response and protects plants against massive Xap infection. This makes them good candidates for deployment of natural, targeted and environmental-friendly strategies to enhance resistance in Prunus species and prevent important biotic diseases.

Differential activities of maize plant elicitor peptides as mediators of immune signaling and herbivore resistance

Plant elicitor peptides (Peps) are conserved regulators of defense responses and models for the study of damage-associated molecular pattern-induced immunity. Although present as multigene families in most species, the functional relevance of these multigene families remains largely undefined. While Arabidopsis Peps appear largely redundant in function, previous work examining Pep-induced responses in maize (Zm) implied specificity of function. To better define the function of individual ZmPeps and their cognate receptors (ZmPEPRs), activities were examined by assessing changes in defense-associated phytohormones, specialized metabolites and global gene expression patterns, in combination with heterologous expression assays and analyses of CRISPR/Cas9-generated knockout plants. Beyond simply delineating individual ZmPep and ZmPEPR activities, these experiments led to a number of new insights into Pep signaling mechanisms. ZmPROPEP and other poaceous precursors were found to contain multiple active Peps, a phenomenon not previously observed for this family. In all, seven new ZmPeps were identified and the peptides were found to have specific activities defined by the relative magnitude of their response output rather than by uniqueness. A striking correlation was observed between individual ZmPep-elicited changes in levels of jasmonic acid and ethylene and the magnitude of induced defense responses, indicating that ZmPeps may collectively regulate immune output through rheostat-like tuning of phytohormone levels. Peptide structure-function studies and ligand-receptor modeling revealed structural features critical to the function of ZmPeps and led to the identification of ZmPep5a as a potential antagonist peptide able to competitively inhibit the activity of other ZmPeps, a regulatory mechanism not previously observed for this family.

Hydrogen sulphide alleviates iron deficiency by promoting iron availability and plant hormone levels in Glycine max seedlings

BACKGROUND

Hydrogen sulphide (H2S) is involved in regulating physiological processes in plants. We investigated how H2S ameliorates iron (Fe) deficiency in soybean (Glycine max L.) seedlings. Multidisciplinary approaches including physiological, biochemical and molecular, and transcriptome methods were used to investigate the H2S role in regulating Fe availability in soybean seedlings.

RESULTS

Our results showed that H2S completely prevented leaf interveinal chlorosis and caused an increase in soybean seedling biomass under Fe deficiency conditions. Moreover, H2S decreased the amount of root-bound apoplastic Fe and increased the Fe content in leaves and roots by regulating the ferric-chelate reductase (FCR) activities and Fe homeostasis- and sulphur metabolism-related gene expression levels, thereby promoting photosynthesis in soybean seedlings. In addition, H2S changed the plant hormone concentrations by modulating plant hormone-related gene expression abundances in soybean seedlings grown in Fe-deficient solution. Furthermore, organic acid biosynthesis and related genes expression also played a vital role in modulating the H2S-mediated alleviation of Fe deficiency in soybean seedlings.

CONCLUSION

Our results indicated that Fe deficiency was alleviated by H2S through enhancement of Fe acquisition and assimilation, thereby regulating plant hormones and organic acid synthesis in plants.

Grain Yields and Nitrogen Use Efficiencies in Different Types of Stay-Green Maize in Response to Nitrogen Fertilizer

The stay-green leaf phenotype is typically associated with increased yields and improved stress resistance in maize breeding, due to higher nitrogen (N) nutrient levels that prolong greenness. The application of N fertilizer can regulate the N status of plants, and furthermore, impact the photosynthetic rates of leaves at the productive stage; however, N deficiencies and N excesses will reduce maize yields. Consequently, it is necessary to develop N fertilizer management strategies for different types of stay-green maize. For this study, the senescent cultivar Lianchuang 808 (LC808), moderate-stay-green cultivar Zhengdan 958 (ZD958), and over stay-green cultivar Denghai 685 (DH685) were selected as experimental models. Our results revealed that yields of ZD958 were slightly higher than DH685 and notably improved over than LC808. Compared with a non-stay-green cultivar LC808, ZD958 and DH685 still maintained higher chlorophyll contents and cell activities following the silking stage, while efficiently slowing the senescence rate. The supply of N fertilizer significantly prolonged leaf greenness and delayed senescence for ZD958 and DH685; however, the effect was not obvious for LC808. The stem remobilization efficiency of N was higher in the moderate-stay-green cultivar ZD958, in contrast to LC808, while the transfer of leaf N was lower than LC808, which guaranteed high leaf N levels, and that sufficient N was transferred to grains in ZD958. To obtain the highest yields, the optimal N fertilizer rates were 228.1 kg hm⁻² for LC0808, 180 kg hm⁻² for ZD958, and 203.8 kg hm⁻² for DH685. In future, the selection of stay-green type crops might serve as an important agricultural strategy to reduce the quantity of N fertilizer and increase N efficiency.

Type-II Metacaspases Mediate the Processing of Plant Elicitor Peptides in Arabidopsis

Plants can produce animal cytokine-like immune peptides, among which plant elicitor peptides (Peps) derive from the C termini of their precursors (PROPEPs). Recently, the functions of Peps have been expanded beyond plant immunity. However, a long-standing enigma is how PROPEPs are processed into Peps. Here, we report that the Ca²⁺-dependent type-II metacaspases (MCs) constitute the proteolytic enzymes to mediate PROPEP processing in Arabidopsis. In protoplasts, co-expression of PROPEP1 with type-II MCs, including MC4 to MC9, can promote the generation of processed Pep1. Destruction of the catalytic cysteine residue in MC4 or the conserved arginine residue preceding the Pep1 sequence blocks PROPEP1 cleavage, whereas the bacterial elicitor flg22 enhances the MC4-mediated PROPEP1 processing. MC4 cleaves PROPEP1 in vitro and also cleaves PROPEP2 to PROPEP8, but, surprisingly, not PROPEP6 in protoplasts. Domain swapping between PROPEP1 and PROPEP6 suggests a hidden role of the sequence context upstream of the Pep sequence for PROPEP processing. flg22-induced PROPEP1 processing and Botrytis cinerea resistance are severely impaired in the mc4/5/6/7 quadruple-mutant plants. Taken together, our study identifies the type-II MCs as new players in Pep signaling, and lays the foundation for understanding the regulation of multifaceted functions of Peps in plant immunity and beyond.

Chlorophyll catabolism precedes changes in chloroplast structure and proteome during leaf senescence

The earliest visual changes of leaf senescence occur in the chloroplast as chlorophyll is degraded and photosynthesis declines. Yet, a comprehensive understanding of the sequence of catabolic events occurring in chloroplasts during natural leaf senescence is still missing. Here, we combined confocal and electron microscopy together with proteomics and biochemistry to follow structural and molecular changes during Arabidopsis leaf senescence. We observed that initiation of chlorophyll catabolism precedes other breakdown processes. Chloroplast size, stacking of thylakoids, and efficiency of PSII remain stable until late stages of senescence, whereas the number and size of plastoglobules increase. Unlike catabolic enzymes, whose level increase, the level of most proteins decreases during senescence, and chloroplast proteins are overrepresented among these. However, the rate of their disappearance is variable, mostly uncoordinated and independent of their inherent stability during earlier developmental stages. Unexpectedly, degradation of chlorophyll-binding proteins lags behind chlorophyll catabolism. Autophagy and vacuole proteins are retained at relatively high levels, highlighting the role of extra-plastidic degradation processes especially in late stages of senescence. The observation that chlorophyll catabolism precedes all other catabolic events may suggest that this process enables or signals further catabolic processes in chloroplasts.

Differential and tissue-specific activation pattern of the AtPROPEP and AtPEPR genes in response to biotic and abiotic stress in Arabidopsis thaliana

In Arabidopsis thaliana AtPEPR1 and AtPEPR2 act as the receptors for the endogenous AtPROPEP-derived Pep peptides and subsequently initiate defense-signaling cascades. In the previous work,⁹ the expression pattern of the genes encoding the PEPR receptors and the AtPROPEP peptide precursor proteins was studied using promoter-GUS reporter constructs. Here, using the same constructs to study their expression pattern under biotic and abiotic stress, including AtPep1, flg22, methyl jasmonate (MeJA), and NaCl treatments, we observed that in response to AtPep1 and flg22, the activation of AtPEPR1 promoter was different from AtPEPR2. We also found that these promoters were differentially activated in response to NaCl. Remarkably, we showed that it is possible to classify the genes of the AtPROPEP family, based on the response of their promoters to the various stimuli employed: thus, we classify AtPROPEP1 in one group; AtPROPEP2 and AtPROPEP3 in a second group; AtPROPEP4, AtPROPEP7 and AtPROPEP8 in a third group and AtPROPEP5 in a fourth group. Our finding, confirm non-redundant roles among the members of the AtPROPEP family and their corresponding receptors.

Integration of danger peptide signals with herbivore-associated molecular pattern signaling amplifies anti-herbivore defense responses in rice

Plant defense against herbivores is modulated by herbivore-associated molecular patterns (HAMPs) from oral secretions (OS) and/or saliva of insects. Furthermore, feeding wounds initiate plant self-damage responses modulated by danger-associated molecular patterns (DAMPs) such as immune defense-promoting plant elicitor peptides (Peps). While temporal and spatial co-existence of both patterns during herbivory implies a possibility of their close interaction, the molecular mechanisms remain undetermined. Here we report that exogenous application of rice (Oryza sativa) peptides (OsPeps) can elicit multiple defense responses in rice cell cultures. Specific activation of OsPROPEP3 gene transcripts in rice leaves by wounding and OS treatments further suggests a possible involvement of the OsPep3 peptide in rice-herbivore interactions. Correspondingly, we found that simultaneous application of OsPep3 and Mythimna loreyi OS significantly amplifies an array of defense responses in rice cells, including mitogen-activated protein kinase activation, and generation of defense-related hormones and metabolites. The induction of OsPROPEP3/4 by OsPep3 points to a positive auto-feedback loop in OsPep signaling which may contribute to additional enhancement of defense signal(s). Finally, the overexpression of the OsPep receptor OsPEPR1 increases the sensitivity of rice plants not only to the cognate OsPeps but also to OS signals. Our findings collectively suggest that HAMP-DAMP signal integration provides a critical step in the amplification of defense signaling in plants.

Novel Rosaceae plant elicitor peptides as sustainable tools to control Xanthomonas arboricola pv. pruni in Prunus spp

Fruit crops are regarded as important health promoters and constitute a major part of global agricultural production, and Rosaceae species are of high economic impact. Their culture is threatened by bacterial diseases, whose control is based on preventative treatments using compounds of limited efficacy and negative environmental impact. One of the most economically relevant examples is the pathogen Xanthomonas arboricola pv. pruni (Xap) affecting Prunus spp. The plant immune response against pathogens can be triggered and amplified by plant elicitor peptides (Peps), perceived by specific receptors (PEPRs). Although they have been described in various angiosperms, scarce information is available on Rosaceae species. Here, we identified the Pep precursor (PROPEP), Pep and PEPR orthologues of 10 Rosaceae species and confirmed the presence of the Pep/PEPR system in this family. We showed the perception and elicitor activity of Rosaceae Peps using the Prunus-Xap pathosystem as proof-of-concept. Treatment with nanomolar doses of Peps induced the corresponding PROPEP and a set of defence-related genes in Prunus leaves, and enhanced resistance against Xap. Peps from the same species had the highest efficiencies. Rosaceae Peps could potentially be used to develop natural, targeted and environmentally friendly strategies to enhance the resistance of Prunus species against biotic attackers.

Expression of tomato prosystemin gene in Arabidopsis reveals systemic translocation of its mRNA and confers necrotrophic fungal resistance

Systemin (SYS), an octadecapeptide hormone processed from a 200-amino-acid precursor (prosystemin, PS), plays a central role in the systemic activation of defense genes in tomato in response to herbivore and pathogen attacks. However, whether PS mRNA is transferable and its role in systemic defense responses remain unknown. We created the transgenic tomato PS gene tagged with the green fluorescent protein (PS-GFP) using a shoot- or root-specific promoter, and the constitutive 35S promoter in Arabidopsis. Subcellular localization of PS-/SYS-GFP was observed using confocal laser scanning microscopy and gene transcripts were determined using quantitative real-time PCR. In Arabidopsis, PS protein can be processed and SYS is secreted. Shoot-/root-specific expression of PS-GFP in Arabidopsis, and grafting experiments, revealed that the PS mRNA moves in a bi-directional manner. We also found that ectopic expression of PS improves Arabidopsis resistance to the necrotrophic fungus Botrytis cinerea, consistent with substantial upregulation of the transcript levels of specific pathogen-responsive genes. Our results provide novel insights into the multifaceted mechanism of SYS signaling transport and its potential application in genetic engineering for increasing pathogen resistance across diverse plant families.

In roots of Arabidopsis thaliana, the damage-associated molecular pattern AtPep1 is a stronger elicitor of immune signalling than flg22 or the chitin heptamer

Plants interpret their immediate environment through perception of small molecules. Microbe-associated molecular patterns (MAMPs) such as flagellin and chitin are likely to be more abundant in the rhizosphere than plant-derived damage-associated molecular patterns (DAMPs). We investigated how the Arabidopsis thaliana root interprets MAMPs and DAMPs as danger signals. We monitored root development during exposure to increasing concentrations of the MAMPs flg22 and the chitin heptamer as well as of the DAMP AtPep1. The tissue-specific expression of defence-related genes in roots was analysed using a toolkit of promoter::YFPN lines reporting jasmonic acid (JA)-, salicylic acid (SA)-, ethylene (ET)- and reactive oxygen species (ROS)- dependent signalling. Finally, marker responses were analysed during invasion by the root pathogen Fusarium oxysporum. The DAMP AtPep1 triggered a stronger activation of the defence markers compared to flg22 and the chitin heptamer. In contrast to the tested MAMPs, AtPep1 induced SA- and JA-signalling markers in the root and caused a severe inhibition of root growth. Fungal attack resulted in a strong activation of defence genes in tissues close to the invading fungal hyphae. The results collectively suggest that AtPep1 presents a stronger danger signal to the Arabidopsis root than the MAMPs flg22 and chitin heptamer.

Dying to live: cell elimination as a developmental strategy in angiosperm seeds

The complete elimination of unwanted cells during development is a repeated theme in both multicellular animals and in plants. In plants, such events have been extensively studied and reviewed in terms of their molecular regulation, of marker genes and proteins expressed, and in terms of cellular changes associated with their progression. This review will take a slightly different view of developmental cell elimination and will concentrate specifically on the numerous elimination events that occur during ovule and seed development (here grouped together as seed development). It asks why this cell elimination occurs in specific seed tissues, in order to understand something about the commonalities underlying how seemingly disparate events are triggered and regulated. Finally, by placing the seed in its broader evolutionary context, the question of why cell elimination may have emerged as such a key component of the seed developmental toolbox will be considered.

Structural Insight into Recognition of Plant Peptide Hormones by Receptors

Secreted signaling peptides or peptide hormones play crucial roles in plant growth and development through coordination of cell-cell communication. Perception of peptide hormones in plants generally relies on membrane-localized receptor kinases (RKs). Progress has recently been made in structural elucidation of interactions between posttranslationally modified peptide hormones and RKs. The structural studies suggest conserved receptor binding and activation mechanisms of this type of peptide hormones involving their conserved C-termini. Here, we review these structural data and discuss how the conserved mechanisms can be used to match peptide-RK pairs.

Receptor-Like Kinases and Regulation of Plant Innate Immunity

Plants are sessile organisms exposed constantly to potential virulent microbes seeking for full pathogenesis in hosts. Different from animals employing both adaptive and innate immune systems, plants only rely on innate immunity to detect and fight against pathogen invasions. Plant innate immunity is proposed to be a two-tiered immune system including pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity. In PTI, PAMPs, the elicitors derived from microbial pathogens, are perceived by cell surface-localized proteins, known as pattern recognition receptors (PRRs), including receptor-like kinases (RLKs) and receptor-like proteins (RLPs). As single-pass transmembrane proteins, RLKs and RLPs contain an extracellular domain (ECD) responsible for ligand binding. Recognitions of signal molecules by PRR-ECDs induce homo- or heterooligomerization of RLKs and RLPs to trigger corresponding intracellular immune responses. RLKs possess a cytoplasmic Ser/Thr kinase domain that is absent in RLPs, implying that protein phosphorylations underlie key mechanism in transducing immunity signalings and that RLPs unlikely mediate signal transduction independently, and recruitment of other patterns, such as RLKs, is required for the function of RLPs in plant immunity. Receptor-like cytoplasmic kinases, resembling RLK structures but lacking the ECD, act as immediate substrates of PRRs, modulating PRR activities and linking PRRs with downstream signaling mediators. In this chapter, we summarize recent discoveries illustrating the molecular machines of major components of PRR complexes in mediating pathogen perception and immunity activation in plants.

Quo vadis, Pep? Plant elicitor peptides at the crossroads of immunity, stress, and development

The first line of inducible plant defence, pattern-triggered immunity (PTI), is activated by the recognition of exogenous as well as endogenous elicitors. Exogenous elicitors, also called microbe-associated molecular patterns, signal the presence of microbes. In contrast, endogenous elicitors seem to be generated and recognized under more diverse circumstances, making the evaluation of their biological relevance much more complex. Plant elicitor peptides (Peps) are one class of such endogenous elicitors, which contribute to immunity against attack by bacteria, fungi, as well as herbivores. Recent studies indicate that the Pep-triggered signalling pathways also operate during the response to a more diverse set of stresses including starvation stress. In addition, in silico data point to an involvement in the regulation of plant development, and a study on Pep-mediated inhibition of root growth supports this indication. Importantly, Peps are neither limited to the model plant Arabidopsis nor to a specific plant family like the previously intensively studied systemin peptides. On the contrary, they are present and active in angiosperms all across the phylogenetic tree, including many important crop plants. Here we summarize the progress made in research on Peps from their discovery in 2006 until now. We discuss the two main models which describe their likely function in plant immunity, highlight the studies supporting additional roles of Pep-triggered signalling and identify urgent research tasks to further uncover their biological relevance.

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.