Plant immunity: towards an integrated view of plant–pathogen interactions

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Abstract
Plant immunity depends on cell-autonomous events that are related to animal innate immunity, but plants have a greatly expanded recognition repertoire to compensate for their lack of an adaptive immune system. Ongoing research is revealing the recognition capacity of the plant immune system and concurrent studies on pathogen biology are beginning to unravel how these organisms manipulate host immunity to cause disease. Plants have evolved two strategies to detect pathogens. On the external face of the host cell, conserved microbial elicitors called pathogen-associated molecular patterns (PAMPs) are recognized by receptor proteins called pattern recognition receptors (PRRs); stimulation of PRRs leads to PAMP-triggered immunity (PTI). The second class of perception involves recognition by intracellular receptors of pathogen virulence molecules called effectors; this recognition induces effector-triggered immunity (ETI). PTI is generally effective against non-adapted pathogens in a phenomenon called non-host resistance, whereas ETI is active against adapted pathogens. However, these relationships are not exclusive and depend on the elicitor molecules present in each infection. Successful pathogens are able to suppress PTI responses and thereby multiply and cause disease. They achieve suppression through the deployment of 'effector' proteins. Plant receptor proteins can recognize pathogen effectors either by direct physical association or indirectly through an accessory protein. Our understanding of effector proteins and their host targets is at an early stage. Sophisticated biochemical screens for host protein targets that interact with the diverse suites of pathogen effectors is likely to lead to the identification of important components of host defence mechanisms, and teach us more about host immune pathways and pathogenicity strategies. It is crucially important for the deployment of existing and novel resistance genes in agriculture that we advance our knowledge of plant–pathogen molecular co-evolution.