The NB ‐ LRR proteins RGA 4 and RGA 5 interact functionally and physically to confer disease resistance

Plant resistance proteins of the class of nucleotide‐binding and leucine‐rich repeat domain proteins ( NB ‐ LRR s) are immune sensors which recognize pathogen‐derived molecules termed avirulence ( AVR ) proteins. We show that RGA 4 and RGA 5, two NB ‐ LRR s from rice, interact functionally and physically to mediate resistance to the fungal pathogen Magnaporthe oryzae and accomplish different functions in AVR recognition. RGA 4 triggers an AVR ‐independent cell death that is repressed in the presence of RGA 5 in both rice protoplasts and Nicotiana benthamiana . Upon recognition of the pathogen effector AVR ‐Pia by direct binding to RGA 5, repression is relieved and cell death occurs. RGA 4 and RGA 5 form homo‐ and hetero‐complexes and interact through their coiled‐coil domains. Localization studies in rice protoplast suggest that RGA 4 and RGA 5 localize to the cytosol. Upon recognition of AVR ‐Pia, neither RGA 4 nor RGA 5 is re‐localized to the nucleus. These results establish a model for the interaction of hetero‐pairs of NB ‐ LRR s in plants: RGA 4 mediates cell death activation, while RGA 5 acts as a repressor of RGA 4 and as an AVR receptor.