Mediation of the transition from biotrophy to necrotrophy in hemibiotrophic plant pathogens by secreted effector proteins.

Hemibiotrophs, such as Phytophthora infestans, exhibit distinct phases of their life cycle: an early asymptomatic biotrophic phase and a late necrotrophic stage that is characterized by tissue degradation and disease symptoms. To date, little is known of the molecular mechanisms that promote each distinct phase, nor those that mediate the transition between the two. We hypothesized that these phytopathogens might secrete distinct classes of effector proteins that first suppress plant defense responses and associated programmed cell death (PCD), and later induce large scale necrosis. To this end, we have identified proteins that are secreted by P. infestans early or late in the infection cycle. Recently we described the characterization of SNE1, which is specifically expressed during early biotrophic growth in the host plant tomato (Solanum lycopersicum). We found that SNE1 suppresses the action of necrosis-inducing effectors (Nep1-like proteins), including PiNPP1.1 and PsojNIP, which are secreted by P. infestans and P. sojae, respectively, during necrotrophic growth, as well as PCD mediated by a broad spectrum of Avr-R protein interactions. This suggests that SNE1 and PiNPP1.1 act antagonistically, thereby providing a highly regulated means to control the transition from biotrophy to necrotrophy.