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 the phytopathogenic enterobacterium Erwinia carotovora , the causal agent of bacterial soft rot disease, is observed in high-calcium plants. One of the main virulence determinants of E. carotovora , the PehA endopolygalacturonase, 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 extracellular calcium to mM concentrations repressed pehA gene expression without reducing or even enhancing expression of other extracellular enzyme-encoding genes of this pathogen. An increase in plant calcium levels could be correlated to enhanced resistance to E. carotovora infection and to an inhibition of in planta production of PehA. Ectopic expression of pehA from a calcium-insensitive promoter allowed E. carotovora to overcome this calcium-induced resistance. The results imply that plant calcium can constitute an important signal molecule in plant–pathogen interaction, which acts by modulating the expression of virulence genes of the pathogen.