Innate immunity in plants: an arms race between pattern recognition receptors in plants and effectors in microbial pathogens

Abstract

For many years, research on a suite of plant defense responses that begin when plants are exposed to general microbial elicitors was underappreciated, for a good reason: There has been no critical experimental demonstration of their importance in mediating plant resistance during pathogen infection. Today, these microbial elicitors are named pathogen- or microbe-associated molecular patterns (PAMPs or MAMPs) and the plant responses are known as PAMP-triggered immunity (PTI). Recent studies provide an elegant explanation for the difficulty of demonstrating the role of PTI in plant disease resistance. It turns out that the important contribution of PTI to disease resistance is masked by pathogen virulence effectors that have evolved to suppress it.

Fig. 1

Concept of activation and suppression of PTI during pathogen infection. (A) An Arabidopsis plant showing disease symptoms (in the foreground; natural size) after infection by P. syringae bacteria (electron microscopy image in the background; magnification, 10,000x). (B) A conceptual diagram of PRR signaling and action of several P. syringae effectors for which the plant targets and immune suppression function have been characterized. Green and purple colors indicate plant targets and P. syringae effectors, respectively. Numbers in circles denote six steps targeted by effectors: MAMP perception (PRRs), the MAPK cascade (MPK3 and MPK6), RNA metabolism (GRP7), vesicle traffic (MIN7), regulators of PTI (RIN4 and RAR1), and chloroplast function (Hsp70) (22).

Fig. 1

Concept of activation and suppression of PTI during pathogen infection. (A) An Arabidopsis plant showing disease symptoms (in the foreground; natural size) after infection by P. syringae bacteria (electron microscopy image in the background; magnification, 10,000x). (B) A conceptual diagram of PRR signaling and action of several P. syringae effectors for which the plant targets and immune suppression function have been characterized. Green and purple colors indicate plant targets and P. syringae effectors, respectively. Numbers in circles denote six steps targeted by effectors: MAMP perception (PRRs), the MAPK cascade (MPK3 and MPK6), RNA metabolism (GRP7), vesicle traffic (MIN7), regulators of PTI (RIN4 and RAR1), and chloroplast function (Hsp70) (22).