Pseudomonas SyringaeTypeIII‐Secreted Proteins and Their Activities and Effects on Plant Innate Immunity

Abstract

Bacterial pathogens of both plants and animals use type III protein secretion systems (T3SSs) to inject bacterial proteins called type III effectors into host cells to alter host physiology and favor the pathogen. The bacterial plant pathogenPseudomonas syringaeis dependent on the Hrp T3SS to cause disease on plants. It has become apparent over the past several years that a primary target forP. syringaetype III effectors is the plant innate immune system. The innate immune system of plants and animals use specific pattern recognition receptors (PRRs) to recognize conserved molecules of microorganisms called pathogen‐associated molecular patterns (PAMPs) and induce a basal defense now known as PAMP‐triggered immunity (PTI). SeveralP. syringaetype III effectors have been shown to suppress PTI. A second tier of the plant innate immune system uses resistance (R) proteins to detect pathogen effectors, including bacterial type III effectors, and induce an immune response now known as effector‐triggered immunity (ETI). SeveralP. syringaetype III effectors can also suppress ETI. The existence of R protein‐recognized type III effectors and type III effectors that suppress ETI and PTI have produced a molecular arms war of sorts in which the bacterial pathogen has evolved an assortment of offensive type III effectors that suppress plant immunity and plants a myriad of defensive PRRs and R proteins that detect the presence of pathogens. This chapter covers what is known about proteins that are secreted by theP. syringaeT3SS. In addition to type III effectors, type III‐secreted helper proteins assist in type III effector delivery. Since a primary target of type III effectors is the plant innate immune system, an introduction to it is also included. Recent progress suggests thatP. syringaetype III effectors can be used as tools to identify new components of plant immunity. Increasing our understanding of the activities and targets of type III effectors will likely reveal how plants defend themselves against bacterial invasion.