The novel peptide NbPPI1 identified from Nicotiana benthamiana triggers immune responses and enhances resistance against Phytophthora pathogens

Litchi aspartic protease LcAP1 enhances plant resistance via suppressing cell death triggered by the pectate lyase PlPeL8 from Peronophythora litchii

Plant cell death is regulated in plant-pathogen interactions. While some aspartic proteases (APs) participate in regulating programmed cell death or defense responses, the defense functions of most APs remain largely unknown. Here, we report on a virulence factor, PlPeL8, which is a pectate lyase found in the hemibiotrophic pathogen Peronophythora litchii. Through in vivo and in vitro assays, we confirmed the interaction between PlPeL8 and LcAP1 from litchi, and identified LcAP1 as a positive regulator of plant immunity. PlPeL8 induced cell death associated with NbSOBIR1 and NbMEK2. The 11 conserved residues of PlPeL8 were essential for inducing cell death and enhancing plant susceptibility. Twenty-three LcAPs suppressed cell death induced by PlPeL8 in Nicotiana benthamiana depending on their interaction with PlPeL8. The N-terminus of LcAP1 was required for inhibiting PlPeL8-triggered cell death and susceptibility. Furthermore, PlPeL8 led to higher susceptibility in NbAPs-silenced N. benthamiana than the GUS-control. Our results indicate the crucial roles of LcAP1 and its homologs in enhancing plant resistance via suppression of cell death triggered by PlPeL8, and LcAP1 represents a promising target for engineering disease resistance. Our study provides new insights into the role of plant cell death in the arms race between plants and hemibiotrophic pathogens.

Secreted Peptide SpPIP1 Modulates Disease Resistance and Salt Tolerance in Tomato

Tomato is a globally important horticultural and economic crop, but its productivity is severely affected by various stresses. Plant small secretory peptides have been identified as crucial mediators in plant resistance. Here, we conducted a comparative transcriptome analysis and identified the prePIP1 gene from Solanum pimpinellifolium (SpprePIP1), as an ortholog of Arabidopsis prePIP1 encoding the precursor protein of PAMP-induced SSP 1. The expression level of SpprePIP1 is transcriptionally induced in tomato upon infection with Phytophthora infestans (P. infestans), the pathogen responsible for late blight. Overexpression of SpprePIP1 resulted in enhanced tomato resistance to P. infestans. In addition, exogenous application of SpPIP1, whether through spraying or irrigation, improved tomato resistance by enhancing the transcript accumulations of pathogenesis-related proteins, as well as reactive oxygen species and the jasmonic acid (JA) levels. Integrated analysis of transcriptomics and metabolomics revealed the potential contributions of JA and phenylpropanoid biosynthesis to SpPIP1-induced tomato immunity. Additionally, SpPIP1 may strengthen tomato resistance to salt stress through the ABA signaling pathway. Overall, our findings demonstrate that SpPIP1 positively regulates tomato tolerance to P. infestans and salt stress, making it a potential plant elicitor for crop protection in an environmentally friendly way.

The Pythium periplocum elicitin PpEli2 confers broad-spectrum disease resistance by triggering a novel receptor-dependent immune pathway in plants

Elicitins are microbe-associated molecular patterns produced by oomycetes to elicit plant defense. It is still unclear whether elicitins derived from non-pathogenic oomycetes can be used as bioactive molecules for disease control. Here, for the first time we identify and characterize an elicitin named PpEli2 from the soil-borne oomycete Pythium periplocum, which is a non-pathogenic mycoparasite colonizing the root ecosystem of diverse plant species. Perceived by a novel cell surface receptor-like protein, REli, that is conserved in various plants (e.g. tomato, pepper, soybean), PpEli2 can induce hypersensitive response cell death and an immunity response in Nicotiana benthamiana. Meanwhile, PpEli2 enhances the interaction between REli and its co-receptor BAK1. The receptor-dependent immune response triggered by PpEli2 is able to protect various plant species against Phytophthora and fungal infections. Collectively, our work reveals the potential agricultural application of non-pathogenic elicitins and their receptors in conferring broad-spectrum resistance for plant protection.

Novel EIicitin from Pythium oligandrum Confers Disease Resistance against Phytophthora capsici in Solanaceae Plants

The mycoparasite Pythium oligandrum is a nonpathogenic oomycete that can boost plant immune responses. Elicitins are microbe-associated molecular patterns (MAMPs) specifically produced by oomycetes that activate plant defense. Here, we identified a novel elicitin, PoEli8, from P. oligandrum that exhibits immunity-inducing activity in plants. In vitro-purified PoEli8 induced strong innate immune responses and enhanced resistance to the oomycete pathogen Phytophthora capsici in Solanaceae plants, including Nicotiana benthamiana, tomato, and pepper. Cell death and reactive oxygen species (ROS) accumulation triggered by the PoEli8 protein were dependent on the plant coreceptors receptor-like kinases (RLKs) BAK1 and SOBIR1. Furthermore, REli from N. benthamiana, a cell surface receptor-like protein (RLP) was implicated in the perception of PoEli8 in N. benthamiana. These results indicate the potential value of PoEli8 as a bioactive formula to protect Solanaceae plants against Phytophthora.

Identification of Key Residues Essential for the Activation of Plant Immunity by Subtilisin From Bacillus velezensis LJ02

Subtilisin, a serine protease, can trigger defense responses in a wide variety of plants, both locally and systemically, to protect against pathogens. However, key residues of subtilisin to improve resistance to plant diseases remain unknown. In this study, Nicotiana benthamiana (N. benthamiana) leaves expressing subtilisin from Bacillus velezensis LJ02 were shown to improve protection against Botrytis cinerea (B. cinerea). Furthermore, the underlying mechanism that LJ02 subtilisin improved the protective effect was explored, and the direct inhibitory effect of subtilisin on B. cinerea was excluded in vitro. Subsequently, reactive oxygen species (ROS) burst and upregulation of resistance-related genes in systemic leaves of N. benthamiana further verified that subtilisin could induce systemic protection against B. cinerea. G307A/T308A and S213A/L214A/G215A subtilisin significantly reduced the ability to resist B. cinerea infection in N. benthamiana. Furthermore, the ROS content and expression levels of resistance-related genes of both mutants were significantly decreased compared with that of wild-type subtilisin. This work identified key residues essential for the activation function of subtilisin plant immunity and was crucial in inducing plant defense responses against B. cinerea.

Nep1-Like Proteins From the Biocontrol Agent Pythium oligandrum Enhance Plant Disease Resistance Independent of Cell Death and Reactive Oxygen Species

Microbial necrosis and ethylene-inducing peptide 1 (Nep1)-like proteins (NLPs) act as cytolytic toxins and immunogenic patterns in plants. Our previous work shows that cytolytic NLPs (i.e., PyolNLP5 and PyolNLP7) from the biocontrol agent Pythium oligandrum enhance plant resistance against Phytophthora pathogens by inducing the expression of plant defensins. However, the relevance between PyolNLP-induced necrosis and plant resistance activation is still unclear. Here, we find that the necrosis-inducing activity of PyolNLP5 requires amino acid residues D127 and E129 within the conserved "GHRHDLE" motif. However, PyolNLP5-mediated plant disease resistance is irrelevant to its necrosis-inducing activity and the accumulation of reactive oxygen species (ROS). Furthermore, we reveal the positive role of non-cytotoxic PyolNLPs in enhancing plant resistance against Phytophthora pathogens and the fugal pathogen Sclerotinia sclerotiorum. Similarly, non-cytotoxic PyolNLPs also activate plant defense in a cell death-independent manner and induce defensin expression. The functions of non-cytotoxic PyolNLP13/14 rely on their conserved nlp24-like peptide pattern. Synthetic Pyolnlp24s derived from both cytotoxic and non-cytotoxic PyolNLPs can induce plant defensin expression. Unlike classic nlp24, Pyolnlp24s lack the ability of inducing ROS burst in plants with the presence of Arabidopsis nlp24 receptor RLP23. Taken together, our work demonstrates that PyolNLPs enhance plant resistance in an RLP23-independent manner, which requires the conserved nlp24-like peptide pattern but is uncoupled with ROS burst and cell death.