Phytophthora infestans RxLR Effector PiAvr3b Targets NADP-Malic Enzyme to Promote Infection.

Phytophthora infestans, the causal agent of potato late blight, employs divergent RxLR effectors to undermine host immunity. One such effector, PiAvr3b, is known to activate resistance in plants carrying the R3b gene; however, its virulence mechanism in susceptible hosts remains unclear. Here, we demonstrate that PiAvr3b is delivered from haustoria through a brefeldin A (BFA)-insensitive pathway, bypassing the traditional endoplasmic reticulum-Golgi trafficking process. Through yeast two-hybrid screening and functional validation, we identified potato NADP-malic enzyme 3 (StME3) and its Nicotiana benthamiana ortholog NbME3 as the direct host target of PiAvr3b. StME3/NbME3 acts as a susceptibility factor; its overexpression enhanced pathogen colonisation, while silencing it confered resistance. PiAvr3b stabilises StME3 independently of the 26S proteasome and autophagy pathways, thereby extending the half-life of this metabolic enzyme. Importantly, although PiAvr3b retained immune-suppressive activity in NbME3-silenced plants, it failed to promote infection of P. infestans, revealing that ME3 stabilisation is indispensable for PiAvr3b's virulence function. Binding to PiAvr3b did not alter the enzyme activity of StME3. Our findings uncover a strategy by which an oomycete effector determines a key metabolic enzyme turnover without disturbing enzymatic activity to reprogramme host susceptibility and establish disease.