Nucleotide-binding leucine-rich repeat receptor homologs Pib and PibH8 interact and contribute to immunity in rice

Plant nucleotide-binding leucine-rich repeat receptors (NLRs) sense pathogen effectors and activate effector-triggered immunity (ETI). Many plant NLRs form pairs with other NLRs to recognize effectors and initiate ETI. PIRICULARIA ORYZAE RESISTANCE IN BL1 (Pib), an NLR protein in rice (Oryza sativa), activates resistance by recognizing the rice blast effector AvrPib. The activation of Pib is suppressed by SH3 DOMAIN-CONTAINING PROTEIN 2 (OsSH3P2) in the absence of AvrPib. However, how Pib triggers defense responses and whether Pib pairs with another NLR are not clear. In this study, we identified Pib by map-based cloning and showed that a homolog of Pib, PIB HOMOLOGUE 8 (PibH8), interacts with Pib. Pib and PibH8 mediate resistance to the Magnaporthe oryzae isolate Guy11, a rice blast strain carrying AvrPib. Interestingly, the pib/pibh8 double mutant exhibited enhanced susceptibility to Guy11 compared to the single mutant. Furthermore, PibH8 can oligomerize through its coiled-coil (CC) domain, which also contributes to the Pib-PibH8 interaction, suggesting that Pib and PibH8 may form a complex to recognize AvrPib. OsSH3P2 inhibited the interaction of Pib and PibH8 through association with the CC domain of PibH8. Taken together, these results indicate that both Pib and PibH8 are required for rice blast resistance to M. oryzae carrying AvrPib, which is negatively regulated by OsSH3P2. This study not only identifies an NLR that functions in rice blast resistance but also reveals a possible complex immune strategy in which homologous NLR proteins may regulate the complete activation of plant immunity.