Calcium Signaling Is Suppressed inMagnaporthe oryzaeConidia byBacillus cereusHS24

Rice yield is greatly reduced owing to rice blast, a polycyclic fungal disease caused by the ascomycete Magnaporthe oryzae. Previously, Bacillus cereus HS24, isolated from a rice farm, showed a strong antimicrobial effect toward M. oryzae. To better exploit it as a biocontrol agent, HS24 was studied for the mechanism that it uses to suppress rice blast. Conidium germination in M. oryzae was significantly inhibited by HS24, whereby inhibition reached 97.8% at the concentration of 10 7 CFU/ml. The transcription levels of Ca 2+ /calmodulin-dependent protein kinase II, PMC1, and CCH1, key genes involved in the M. oryzae Ca 2+ signaling pathway, were significantly decreased in HS24-treated conidia at high concentration. The treatment of M. oryzae with the corresponding Ca 2+ signaling pathway inhibitors KN-93, verapamil, and cyclopiazonic acid significantly reduced conidium germination. This inhibitory effect was found to be concentration dependent, similar to the HS24 treatment. We also found that HS24 was able to decrease the intracellular free Ca 2+ concentration in M. oryzae conidia significantly. The addition of exogenous Ca 2+ did not diminish the inhibitory effect of HS24 on the reduction of intracellular free Ca 2+ concentration and the level of conidium germination. In conclusion, B. cereus HS24 at high concentration prevents extracellular Ca 2+ from entering the conidia in M. oryzae, causes a significant reduction of intracellular free Ca 2+ concentration, and results in the inhibition of conidium germination.