OsRap2.6 transcription factor contributes to rice innate immunity through its interaction with Receptor for Activated Kinase-C 1 (RACK1)

Background
The rice small GTPase OsRac1 is a molecular switch in rice innate immunity. The Receptor for Activated Kinase C-1 (RACK1) interacts with OsRac1 to suppress the growth of the rice blast fungus, <i>Magnaporthe oryzae</i>. RACK1 has two homologs in rice, RACK1A and RACK1B. Overexpressing RACK1A enhances resistance to the rice blast fungus. However, RACK1A downstream signals are largely unknown.

Results
Here, we report the identification of OsRap2.6, a transcription factor that interacts with RACK1A. We found a 94% similarity between the OsRap2.6 AP2 domain and Arabidopsis Rap2.6 (AtRap2.6). Bimolecular fluorescence complementation (BiFC) assays in rice protoplasts using tagged OsRap2.6 and RACK1A with the C-terminal and N-terminal fragments of Venus (Vc/Vn) indicated that OsRap2.6 and RACK1A interacted and localized in the nucleus and the cytoplasm. Moreover, OsRap2.6 and OsMAPK3/6 interacted in the nucleus and the cytoplasm. Expression of defense genes <i>PAL1</i> and <i>PBZ1</i> as well as <i>OsRap2.6</i> was induced after chitin treatment. Disease resistance analysis using <i>OsRap2.6</i> RNAi and overexpressing (Ox) plants infected with the rice blast fungus indicated that <i>OsRap2.6</i> RNAi plants were highly susceptible, whereas <i>OsRap2.6</i> Ox plants had an increased resistance to the compatible blast fungus.

Conclusions
OsRap2.6 contributes to rice innate immunity through its interaction with RACK1A in compatible interactions.