Defense suppression by virulence effectors of bacterial phytopathogens.

Phytopathogenic bacteria and plants are locked in molecular struggles that determine the outcome of an infection. Bacteria make effector molecules that can induce defenses if recognized by specific host resistance (R) proteins. In susceptible hosts, however, effectors frequently promote virulence by suppressing host defenses. Defense-inducing and defense-suppressing activities are often related, as virulence-associated host modifications can elicit R protein activation. Thus, understanding of how an effector elicits defenses can translate into understanding of how it promotes virulence and vice versa. To control host cell functions, such as defense gene expression and vesicle trafficking, effectors use various biochemical activities, including protein modification, transcriptional regulation, and hormone mimicry. Progress with individual effectors will lead to an integrated view of how the activities of a collection of effectors intersect with genetically variable host plants to regulate susceptibility and resistance.