Perception of the pathogen-induced peptide RGF7 by the receptor-like kinases RGI4 and RGI5 triggers innate immunity in Arabidopsis thaliana

Signaling peptides play crucial roles in plant growth, development and defense. We report here that the Arabidopsis thaliana secreted peptide, ROOT MERISTEM GROWTH FACTOR 7 (RGF7), functions as an endogenous elicitor to trigger plant immunity. Expression of the RGF7 precursor-encoding gene (preRGF7) is highly induced in Arabidopsis leaves upon infection by the bacterial pathogen Pseudomonas syringae. The pathogen-responsive preRGF7 expression is regulated by the transcription factor WRKY33 and its upstream mitogen-activated protein kinases MPK3/MPK6 and calcium-dependent protein kinases CPK5/CPK6. In the absence of pathogen attack, chemically induced expression of preRGF7 in transgenic Arabidopsis plants was sufficient to trigger immune responses. Pre-induction of preRGF7 expression in transgenic Arabidopsis also led to enhanced immune responses and increased resistance to P. syringae infection. Biochemical and genetic analyses demonstrated that RGF7 is perceived by the leaf-expressed RGF1 INSENSITIVE (RGI) family receptors RGI4 and RGI5. The SOMATIC EMBRYOGENESIS RECEPTOR KINASES (SERKs) BAK1 and SERK4 are also involved in RGF7 perception via forming RGF7-induced receptor-complexes with RGI4 and RGI5. These results indicate that the pathogen-induced RGF7 peptide, perceived by the RGI4/RGI5-BAK1/SERK4 receptor complexes, acts as a new damage-associated molecular pattern (DAMP) and plays a significant role in Arabidopsis immunity.

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.

The family of Peps and their precursors in Arabidopsis: differential expression and localization but similar induction of pattern-triggered immune responses

In Arabidopsis thaliana, the endogenous danger peptides, AtPeps, have been associated with plant defences reminiscent of those induced in pattern-triggered immunity. AtPeps are perceived by two homologous receptor kinases, PEPR1 and PEPR2, and are encoded in the C termini of the PROPEP precursors. Here, we report that, contrary to the seemingly redundant AtPeps, the PROPEPs fall at least into two distinct groups. As revealed by promoter-β-glucuronidase studies, expression patterns of PROPEP1-3, -5, and -8 partially overlapped and correlated with those of the PEPR1 and -2 receptors, whereas those of PROPEP4 and -7 did not share any similarities with the former. Moreover, bi-clustering analysis indicated an association of PROPEP1, -2, and -3 with plant defence, whereas PROPEP5 expression was related to patterns of plant reproduction. In addition, at the protein level, PROPEPs appeared to be distinct. PROPEP3::YFP (fused to yellow fluorescent protein) was present in the cytosol, but, in contrast to previous predictions, PROPEP1::YFP and PROPEP6::YFP localized to the tonoplast. Together with the expression patterns, this could point to potentially non-redundant roles among the members of the PROPEP family. By contrast, their derived AtPeps, including the newly reported AtPep8, when applied exogenously, provoked activation of defence-related responses in a similar manner, suggesting a high level of functional redundancy between the AtPeps. Taken together, our findings reveal an apparent antagonism between AtPep redundancy and PROPEP variability, and indicate new roles for PROPEPs besides plant immunity.

An endogenous peptide signal in Arabidopsis activates components of the innate immune response

Innate immunity is initiated in animals and plants through the recognition of a variety of pathogen-associated molecules that in animals are called pathogen-associated molecular patterns and in plants are called elicitors. Some plant pathogen-derived elicitors have been identified as peptides, but peptide elicitors derived from the plant itself that activate defensive genes against pathogens have not been previously identified. Here, we report the isolation and characterization of a 23-aa peptide from Arabidopsis, called AtPep1, which activates transcription of the defensive gene defensin (PDF1.2) and activates the synthesis of H(2)O(2), both being components of the innate immune response. The peptide is derived from a 92-aa precursor encoded within a small gene that is inducible by wounding, methyl jasmonate, and ethylene. Constitutive expression of the AtPep1 precursor gene PROPEP1 in transgenic Arabidopsis plants causes a constitutive transcription of PDF1.2. When grown in soil, the transgenic plants exhibited an increased root development compared with WT plants and an enhanced resistance toward the root pathogen Pythium irregulare. Six paralogs of PROPEP1 are present in Arabidopsis, and orthologs have been identified in species of several agriculturally important plant families, where they are of interest for their possible use in crop improvement.