Novel plant-specific cyclin-dependent kinase inhibitors induced by biotic and abiotic stresses

The EL2 gene of rice ( Oryza sativa), previously classified as early response gene against the potent biotic elicitor N-acetylchitoheptaose and encoding a short polypeptide with unknown function, was identified as a novel cell cycle regulatory gene related to the recently reported SIAMESE ( SIM) gene of Arabidopsis thaliana. Iterative two-hybrid screens, in vitro pull-down assays, and fluorescence resonance energy transfer analyses showed that Orysa; EL2 binds the cyclin-dependent kinase ( CDK) CDKA1; 1 and D-type cyclins. No interaction was observed with the plant-specific B-type CDKs. The amino acid motif ELERFL was identified to be essential for cyclin, but not for CDK binding. Orysa; EL2 impaired the ability of Orysa; CYCD5; 3 to complement a budding yeast ( Saccharomyces cerevisiae) triple CLN mutant, whereas recombinant protein inhibited CDK activity in vitro. Moreover, Orysa; EL2 was able to rescue the multicellular trichome phenotype of sim mutants of Arabidopsis, unequivocally demonstrating that Orysa; EL2 operates as a cell cycle inhibitor. Orysa; EL2 mRNA levels were induced by cold, drought, and propionic acid. Our data suggest that Orysa; EL2 encodes a new type of plant CDK inhibitor that links cell cycle progression with biotic and abiotic stress responses.