Genetic and molecular analysis of the Arabidopsis copine I gene in plant defense responses

The copines are a ubiquitous class of calcium-dependent, phospholipid binding proteins of undetermined biological function. Mutation of the Arabidopsis COPINE 1 gene (CPN1) causes a humidity sensitive lesion mimic phenotype with increased resistance to a bacterial and an oomyceteous pathogen, constitutive pathogenesis-related (PR) gene expression, and an accelerated hypersensitive cell death defense response (HR). Low temperature also induces the lesion-mimic phenotype, increased disease resistance and constitutive PR gene expression in the cpn1-1 mutant. CPN1 is a constitutively expressed gene in wild-type plants with increased expression under low humidity and low temperature conditions. Strong expression of CPN1 is observed in wild-type plants when infected with avirulent and virulent pathogens and by salicylic acid treatment. Copine proteins consist of a unique combination of two C2 domains at its N terminus and a VWA domain at the C terminus. Over-expressing VWA domain fragment in wild-type plants resulted in a lesion-mimic phenotype with increased resistance to a bacterial pathogen. The lesion-mimic phenotype could be due to the inhibition of the function of endogenous full length CPN1 by introduced excess VWA domain. The npr1-1 mutation in cpn1-1 mutants did not affect the PR gene expression but delayed the lesion development. These results provide evidence that CPN1 in Arabidopsis may act as a suppressor of cell death and defense responses. These results also show that CPN1 may play a role in the signaling events during plant acclimation to low humidity and low temperature conditions. Taken together, these data suggest that CPN1 may represent a link between plant biotic and abiotic stress responses.