Indirect pathogen recognition in plant innate immunity

Plants possess a sophisticated and robust immune system that is able to detect and ward off many pathogenic organisms. The initial immune recognition event is often due to the ability of a single disease resistance protein in the plant to identify and respond to a specific protein produced by the potential pathogen. This type of specific immune recognition system has been referred to as “gene-for-gene” disease resistance, since a successful immune response requires the presence of a certain plant disease resistance protein that specifically recognizes a cognate pathogen-derived protein. In cases where the parasite is a gram-negative bacterium, the recognized protein is most often a virulence factor delivered directly into the plant cell by a type-III secretion system. The Pseudomonas syringae AvrRpt2 protein is such a virulence factor, and can be recognized in a gene-for-gene manner by the Arabidopsis thaliana disease resistance protein RPS2.; Mutational analyses demonstrate that the C-terminal portion of AvrRpt2 is most important for RPS2 recognition, and that conserved regions of the RPS2 protein are also critical. Biochemical analyses show that the RPS2-AvrRpt2 recognition event takes place at the plasma membrane of the plant cell, where RPS2 is physically bound to the Arabidopsis RIN4 protein. Delivery of AvrRpt2 during P. syringae infection causes the destruction of RIN4 protein. There is overwhelming evidence to suggest that AvrRpt2 is a novel and sequence-divergent cysteine protease whose proteolytic activity is required for RIN4 destruction during infection. It is postulated that RPS2-mediated recognition of AvrRpt2 is achieved indirectly by perception and reaction to RIN4 disappearance, rather than a direct ligand-receptor interaction with AvrRpt2. It is further postulated that, in the absence of RPS2-mediated recognition, the AvrRpt2-mediated destruction of RIN4 acts to modify plant cell physiology to the benefit of the invading pathogen.