Nuclear Localization of HopA1 Pss61 Is Required for Effector-Triggered Immunity.

Plant resistance proteins recognize cognate pathogen avirulence proteins (also named effectors) to implement the innate immune responses called effector-triggered immunity. Previously, we reported that hopA1 from Pseudomonas syringae pv. syringae strain 61 was identified as an avr gene for Arabidopsis thaliana . Using a forward genetic screen approach, we cloned a hopA1- specific TIR-NBS-LRR class disease resistance gene, RESISTANCE TO PSEUDOMONAS SYRINGAE6 ( RPS6 ). Many resistance proteins indirectly recognize effectors, and RPS6 is thought to interact with HopA1 _Pss61 indirectly by surveillance of an effector target. However, the involved target protein is currently unknown. Here, we show RPS6 is the only R protein that recognizes HopA1 _Pss61 in Arabidopsis wild-type Col-0 accession. Both RPS6 and HopA1 _Pss61 are co-localized to the nucleus and cytoplasm. HopA1 _Pss61 is also distributed in plasma membrane and plasmodesmata. Interestingly, nuclear localization of HopA1 _Pss61 is required to induce cell death as NES- HopA1 _Pss61 suppresses the level of cell death in Nicotiana benthamiana . In addition, in planta expression of hopA1 _Pss61 led to defense responses, such as a dwarf morphology, a cell death response, inhibition of bacterial growth, and increased accumulation of defense marker proteins in transgenic Arabidopsis. Functional characterization of HopA1 _Pss61 and RPS6 will provide an important piece of the ETI puzzle.