Resistance (R) proteins function as important immune receptors (sensors) to induce strong immune responses in plants. Although almost a quarter of a century has passed since the first R gene was identified, the molecular mechanisms of R protein activation are largely unknown because direct downstream signaling molecules of R proteins remain obscure.

Their previous study has revealed that the rice small GTPase OsRac1 acts as a switch protein, working downstream of the Pit, and triggers reactive oxygen species (ROS) production and localized cell death, which are typical immune responses of plants against pathogens.

In this study, researchers identified a direct signaling molecule of the R proteins, Pit and RGA4, named OsSPK1 that plays important roles in rice immunity.

OsSPK1, a GDP/GTP exchange factor protein, was revealed as a direct downstream target of Pit. It functions as an activator protein for OsRac1, and the Pit-OsSPK1-OsRac1 module thus plays key roles in Pit-mediated immunity.

They further discovered that OsSPK1 also participates in a signaling pathway of another rice R protein, RGA4.

Taken together, the results raise the possibility that the R protein-OsSPK1-OsRac1 modules are the common key machineries in R protein-mediated immunity.

This study presents an explanation for how R proteins trigger ROS production and localized cell death through the direct binding partner OsSPK1. It provides insight into understanding the molecular mechanisms of plant immunity.

"R proteins exist in all plants and contribute to protecting plants from pathogens. If we can control R proteins properly then we will be able to manage diseases caused by pathogens in cereals, fruits, vegetables, and flowers in addition to rice.” said Dr. Kawano.

Crop losses caused by pathogens inflicting diseases such as rice blast disease, potato plague disease, Chinese cabbage root disease are enormous. To improve disease resistance of rice is an important issue as rice is the most important food crop that supports half of the world's population.