Recognition of the inducible, secretory small protein OsSSP1 by the membrane receptor OsSSR1 and coreceptor OsBAK1 confers rice resistance to the blast fungus

The plant apoplast, which serves as the frontline battleground for long-term host-pathogen interactions, harbors a wealth of disease resistance resources. However, identification of these disease resistance proteins in the apoplast is relative lacking. In this study, we identified a rice secretory protein OsSSP1 (Oryza sativa secretory small protein 1). The OsSSP1 protein can be secreted into the plant apoplast, and both in vitro treatment and overexpression in rice can trigger plant immune response. The expression of OsSSP1 is suppressed significantly during Magnaporthe oryzae infection in susceptible rice TP309, and OsSSP1-overexpressing lines all show strong resistance to M. oryzae. Combining the knockout and overexpression results, we found that OsSSP1 positively regulates plant immunity in response to fungal infection. Moreover, the recognition and immune response triggered by OsSSP1 depend on an uncharacterized transmembrane OsSSR1 (secretory small protein receptor 1) and the key coreceptor OsBAK1 since most of the induced immune response and resistance are lost in the absence of OsSSR1 or OsBAK1. Moreover, the OsSSP1 protein is relatively stable and can still induces plant resistance after one week of storage in the open environment, and exogenous OsSSP1 treatment with 2-week period did not affect rice yield. Together, our data demonstrate that the OsSSP1 protein is secreted into the apoplast and precepted by the plasma membrane receptors OsSSR1 and OsBAK1 during fungal infection, subsequently triggering the immune response to enhance plant resistance to M. oryzae, providing novel resources and clues for crop breeding and green pest control.