Protein phosphatase inhibitors activate defense responses in rice (Oryza sativa) leaves

The protein phosphatase inhibitor cantharidin activates defense responses i n rice leaves when applied exogenously at concentrations ranging from 100 t o 500 muM, Responses include the accumulation of the major rice phenolic ph ytoalexin sakuranetin and the lactone phytoalexin momilactome A, Accumulati on of sakuranetin was preceded by an induction of phenylalanine ammonia lya se (PAL) activity and an increase in the activity of naringenin 7-O-methylt ransferase (NOMT), the key enzyme in sakuranetin biosynthesis. Cantharidin also strongly induced accumulation of the probenazole (PBZ)-inducible prote in (PBZ1) and two novel, related proteins named PBZ2 and PBZ3, Endothall, a herbicide and potent protein phosphatase inhibitor, but not its inactive a nalog (1,4-dimethylendothall) also induced sakuranetin accumulation, increa sed activity of NOMT and accumulation of the 3 PBZ proteins, In contrast, t wo other protein phosphatase inhibitors, calyculin A and microcystin LR, di d not activate these defense responses, Induction of NOMT and PAL activity, and sakuranetin accumulation, was completely blocked by cycloheximide. Lea f segments treated with cantharidin and endothall skewed brownish and orang e colored lesions, respectively, similar to the lesion mimic mutants of ric e, These results indicate a direct role for protein phosphorylation/dephosp horylation events in the activation of defense responses in rice, in partic ular on the accumulation of antifungal phytoalexins and the PBZ proteins.