Zhou, Jinggeng (2013-08). Bacterial Effector HopF2 Suppresses Arabidopsis Immunity by Targeting BAK1. Master's Thesis.
Pseudomonas syringae delivers a plethora of effector proteins into host cells to sabotage host immune responses and physiology to favor infection. We have previously shown that P. syringae pv. tomato DC3000 effector HopF2 suppresses Arabidopsis innate immunity triggered by multiple pathogen-associated molecular patterns (PAMP) at the plasma membrane. We show here that HopF2 possesses distinct mechanisms in the suppression of two branches of PAMP-activated MAP kinase cascades. In contrast to blocking MKK5 in MEKK1-MKK4/5-MPK3/6 cascade, HopF2 targets additional component(s) upstream of MEKK1 in MEKK1-MKK1/2-MPK4 cascade and plasma membrane-localized receptor-like cytoplasmic kinase BIK1 and its homologs. We further show that HopF2 directly targets BAK1, a plasma membrane-localized receptor-like kinase involved in multiple PAMP signaling. The interaction between BAK1 and HopF2 or two additional P. syringae effectors AvrPto and AvrPtoB, was confirmed in vivo and in vitro. Consistent with BAK1 as a physiological target of HopF2, the lethality of overexpression of HopF2 in wild-type Arabidopsis transgenic plants was largely alleviated in bak1 mutant plants. Identification of BAK1 as an additional HopF2 virulence target not only explains HopF2 suppression of multiple PAMP signaling at the plasma membrane, but also supports the notion that pathogen virulence effectors have multiple targets in host cells.