Bacillus cereusHS24 Suppresses Conidia Germination ofMagnaporthe oryzaeby Inhibiting the Ca2+Signaling Pathway

Rice yield is greatly reduced due to rice blast, a worldwide multi-cycle fungal disease caused by the ascomycete Magnaporthe oryzae. Previously, Bacillus cereus HS24 was isolated from rice growing area, which showed a strong bio-control effect on M. oryzae. In order to better exploit it as a bio-control agent, HS24 has been studied for its mechanism of controlling rice blast. Our results showed that conidial germination of M. oryzae was significantly inhibited by HS24 suspension, n and the inhibition rate reached to 97.83% at the concentration of 10 CFU/ml. The transcriptional level of CAMKII, PMC1 and CCH1, which are key genes involved in Ca 2+ signaling pathway, were significantly decreased in HS24-treated conidia. The treatment of M. oryzae with Ca 2+ signaling pathway inhibitors KN-93, Verapamil, and cyclopiazonic acid (CPA) significantly reduced conidial germination rate and inhibited germ tube elongation. This inhibition effect was found to be concentration-dependent, similar to the HS24 treatment. By quantifying free Ca 2+ in M. oryzae conidia, a significant reduction of intracellular free Ca 2+ concentration in HS24-treated conidia in comparison to sterile water-treated conidia was found. The addition of exogenous Ca 2+ did not abolish the inhibitory effect of HS24 on the reduction of intracellular free Ca 2+ concentration and conidial germination. In conclusion, B. cereus HS24 can inhibit conidial germination by suppressing Ca 2+ signaling in M. oryzae, and thus offers a great potential as a bio-control agent in rice blast management. Importance In the bio-control of rice blast, most of researches focused on the inhibitory effect of bio-control agents on development of M. oryzae , including inhibition of conidia germination, germ tube deformity, mycelium growth inhibition, etc, while little work has elucidated the molecular mechanisms underlying the various phenotypic change in M. oryzae. In order to better exploit HS24 as a potential bio-control agent, we studied the molecular mechanisms underlying the suppression of B. cereus HS24 on M. oryzae Guy11, and draw the conclusion that HS24 may inhibit conidia germination and germ tube elongation through inhibiting the Ca 2+ signaling pathway. In this study, we characterized the morphological and physiological changes of M. oryzae when interacting with B. cereus HS24, and further investgated the responses of M. oryzae Ca 2+ signallig pathway to HS24 treatment. Thus we have provided one more piece for the B. cereus / M. oryzae supression puzzle at the molecular level.