Abstract: Plant growth promoting rhizobacteria (PGPR) can decompose and transform organic substances and minerals in soil, prevent and control plant diseases and reduce the use of pesticides and fertilizers. Thus, PGPR are considered to be used as a new strategy to improve agricultural production. Bacillus amyloliquefaciens L-H15 is an excellent plant growth promoting rhizobacterium. Based on the results of previous studies of its full-length genomic sequence, we investigated the plant growth promoting properties of B. amyloliquefaciens L-H15 and optimized its culture conditions in order to develop a highly active solid starter culture. Our results indicated that B. amyloliquefaciens L-H15 produced a high level of siderophore (65.53%) and strong activity of 1-aminocyclopropane-1-carboxylate (ACC) deaminase showing a specific activity of 0.401 U/mg. This strain could also produce indole-3-acetic (IAA) and cytokinins (CTK). The ability to produce CTK was particularly significant, which was as high as 2.157 7 × 10-10 mol/L. Acetyl methyl alcohol, which can promote plant growth, was also detected in the volatiles generated by L-H15. These factors were able to effectively stimulate plant growth. We used response surface methodology based on Box-Behnken design to optimize the culture conditions of B. amyloliquefaciens L-H15 as follows: an inoculum size of 8%, a proportion of medium in a 500-mL volumetric flask of 67.8%, a culture temperature of 32 ℃, a culture time of 11.5 h, and a rotational speed of 196 r/min. Under these conditions, the viable cell count of B. amyloliquefaciens L-H15 was 1.21 × 109 CFU/mL. In conclusion, B. amyloliquefaciens L-H15 is an excellent PGPR, which has a broad prospect of development and utilization as a biofertilizer.

Key words: Bacillus amyloliquefaciens L-H15, plant growth promoting factors, fermentation process