Abstract
Plants have evolved a two-layered immune system consisting of pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). PTI and ETI are functionally linked, but also have distinct characteristics. Unraveling how these immune systems coordinate plant responses against pathogens is crucial for understanding the regulatory mechanisms underlying plant defense. Here we report integrative proteomic and phosphoproteomic analyses of the tomato-Pseudomonas syringae (Pst) pathosystem with different Pst mutants that allow the dissection of PTI and ETI. A total of 225 proteins and 79 phosphopeptides differentially accumulated in tomato leaves during Pst infection. The abundances of many proteins and phosphoproteins changed during PTI or ETI, and some responses were triggered by both PTI and ETI. For most proteins, the ETI response was more robust than the PTI response. The patterns of protein abundance and phosphorylation changes revealed key regulators involved in Ca2+ signaling, mitogen-activated protein kinase cascades, reversible protein phosphorylation, reactive oxygen species (ROS) and redox homeostasis, transcription and protein turnover, transport and trafficking, cell wall remodeling, hormone biosynthesis and signaling, suggesting their common or specific roles in PTI and/or ETI. A NAC (NAM, ATAF, and CUC family) domain protein and lipid particle serine esterase, two PTI-specific genes identified from previous transcriptomic work, were not detected as differentially regulated at the protein level and were not induced by PTI. Based on integrative transcriptomics and proteomics data, as well as qRT-PCR analysis, several potential PTI and ETI-specific markers are proposed. These results provide insights into the regulatory mechanisms underlying PTI and ETI in the tomato-Pst pathosystem, and will promote future validation and application of the disease biomarkers in plant defense.
Highlights
Plants are vulnerable to infection by a variety of microbial pathogens
To profile protein expression and phosphorylation events during pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) activation, we applied a multiplexed proteomics approach to quantify the proteome and phosphoproteome of tomato leaves infiltrated by different Pst DC3000 mutants, which allow for the dissection of the specific plant immune responses
It is known that Rio Grande-PtoR resistant plants (RG-PtoR) plants infiltrated with Pst DC3000 fliC show no disease symptoms, but develop speck disease when infiltrated with a Pst strain lacking effectors AvrPto and AvrPtoB
Summary
Plants are vulnerable to infection by a variety of microbial pathogens. During evolution, plants have developed two interlinked immune systems: pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). PTI responses include production of reactive oxygen species (ROS), activation of mitogen-activated protein kinase (MAPK) cascades, changes in the intracellular calcium concentration and transcriptional reprogramming of immunity-related genes, cell wall callose deposition, stomatal closure, and moderate inhibition of pathogen growth (Couto and Zipfel, 2016; Li et al, 2016, 2020). Plants have evolved intracellular resistance proteins (mainly pathogen-specific nucleotide-binding leucine-rich repeat (NLR)type immune receptors) capable of sensing specific effectors This layer of defense, termed ETI, initiates an incompatible interaction characterized by induction of salicylic acid and systemic acquired resistance, defense gene transcription, as well as a hypersensitive response involving programmed cell death that limits pathogen spread (Cui et al, 2015; Li et al, 2020; Lu and Tsuda, 2021)
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