Cell wall integrity in Magnaporthe oryzae is weakened by proteins secreted by Bacillus licheniformis BL06

Abstract Rice blast disease is one of the most destructive diseases in cultivated rice in the world. The causal agent of rice blast disease has been identified as the hemibiotrophic filamentous fungus Magnaporthe oryzae. Biological control agents for rice blast have been investigated for several decades. However, the mechanisms by which these agents control rice blast have rarely been reported. Here, we show that Bacillus licheniformis BL06-secreted proteins (BL06-SP) extracted from the supernatant of a B. licheniformis BL06 solution had a strong antimicrobial effect on M. oryzae strain guy11. To better exploit BL06-SP as a biocontrol agent, the mechanism by which BL06-SP suppress rice blast pathogens was studied in this work. Conidial germination in strain guy11 was significantly suppressed by BL06-SP, and germination inhibition reached 97.33% at a BL06-SP concentration of 5 mg per ml. Few appressoria were formed due to germ tube swelling caused by BL06-SP. The transcription levels of chitin synthase genes, including chitin synthase 1, chitin synthase 2, chitin synthase 4, chitin synthase 6 and chitin synthase 7, which are key genes involved in the synthesis of chitin in the cell walls of conidia and mycelia, were downregulated in treatments with BL06-SP at high concentrations. We also found that the chitin content of mycelia and conidia decreased significantly in M. oryzae treated with BL06-SP at 5 mg per ml. The relative fluorescence intensity of chitin in M. oryzae treated with BL06-SP at 5 mg per ml determined with CFW-28 fluorescence staining showed an approximately 3-fold reduction in conidia compared with that in the control. Overall, our results indicate that B. licheniformis BL06-SP at high concentrations reduce chitin content, inhibit appressorium formation, and weaken the pathogenicity of M. oryzae. These results suggest that BL06-SP may inhibit M. oryzae by interfering with its cell wall integrity and indicate that BL06-SP may provide a potential new pathway for controlling this devastating pathogen by targeting specific chitin synthesis genes.