27Rice yield is greatly reduced due to rice blast, a worldwide 28 multi-cycle fungal disease caused by the ascomycete Magnaporthe 29 oryzae. Previously, Bacillus cereus HS24 was isolated from rice growing 30 area, which showed a strong bio-control effect on M. oryzae. In order to 31 better exploit it as a bio-control agent, HS24 has been studied for its 32 mechanism of controlling rice blast. Our results showed that conidial 33 germination of M. oryzae was significantly inhibited by HS24 suspension, 34 and the inhibition rate reached to 97.83% at the concentration of 10 7 35 CFU/ml. The transcriptional level of CAMKII, PMC1 and CCH1, which 36 are key genes involved in Ca 2+ signaling pathway, were significantly 37 decreased in HS24-treated conidia. The treatment of M. oryzae with Ca 2+ 38 signaling pathway inhibitors KN-93, Verapamil, and cyclopiazonic acid 39 (CPA) significantly reduced conidial germination rate and inhibited germ 40 tube elongation. This inhibition effect was found to be 41 concentration-dependent, similar to the HS24 treatment. By quantifying 42 free Ca 2+ in M. oryzae conidia, a significant reduction of intracellular free 43 Ca 2+ concentration in HS24-treated conidia in comparison to sterile 44 water-treated conidia was found. The addition of exogenous Ca 2+ did not 45 abolish the inhibitory effect of HS24 on the reduction of intracellular free 46 Ca 2+ concentration and conidial germination. In conclusion, B. cereus 47 48 oryzae, and thus offers a great potential as a bio-control agent in rice blast 49 management. 50 51 Importance 52 In the bio-control of rice blast, most of researches focused on the 53 inhibitory effect of bio-control agents on development of M. oryzae, 54 including inhibition of conidia germination, germ tube deformity, 55 mycelium growth inhibition, etc, while little work has elucidated the 56 molecular mechanisms underlying the various phenotypic change in M. 57oryzae. In order to better exploit HS24 as a potential bio-control agent, 58 we studied the molecular mechanisms underlying the suppression of B. 59 cereus HS24 on M. oryzae Guy11, and draw the conclusion that HS24 60 may inhibit conidia germination and germ tube elongation through 61 inhibiting the Ca 2+ signaling pathway. In this study, we characterized the 62 morphological and physiological changes of M. oryzae when interacting 63 with B. cereus HS24, and further investgated the responses of M. oryzae 64 Ca 2+ signallig pathway to HS24 treatment. Thus we have provided one 65 more piece for the B. cereus / M. oryzae supression puzzle at the 66 molecular level. 67 68 of the world's population. However, rice yield is greatly reduced due to 71 rice blast, a worldwide multi-cycle fungal disease caused by the 72 ascomycete Magnaporthe oryzae (M. oryzae). Infection by the pathogen 73 is composed of a series of distinct processes (1). First, conidia adhere to 74 the rice epidermis, and then germinate to form germ tubes under suitable 75 environmental conditions. When the tips of germ tubes receive 76 hy...