CONTROL OF VIRULENCE GENE-EXPRESSION BY PLANT CALCIUM IN THE PHYTOPATHOGEN ERWINIA-CAROTOVORA

Plant calcium can modulate a particular plant-pathogen interaction and have a decisive role in disease development. Enhanced resistance to t he phytopathogenic enterobacterium Erwinia carotovora, the causal agen t of bacterial soft rot disease, is observed in high-calcium plants. O ne of the main virulence determinants of E. carotovora, the PehA endop olygalacturonase, is specifically required in the early stages of the infection. Production of PehA was found to be dependent on the calcium concentration in the bacterial environment. An increase in extracellu lar calcium to mM concentrations repressed pehA gene expression withou t reducing or even enhancing expression of other extracellular enzyme- encoding genes of this pathogen. An increase in plant calcium levels c ould be correlated to enhanced resistance to E. carotovora infection a nd to an inhibition of in planta production of PehA. Ectopic expressio n of pehA from a calcium-insensitive promoter allowed E. carotovora to overcome this calcium-induced resistance. The results imply that plan t calcium can constitute an important signal molecule in plant-pathoge n interaction, which acts by modulating the expression of virulence ge nes of the pathogen.