The IAA-Producing Rhizobacterium Bacillus sp. SYM-4 Promotes Maize Growth and Yield

The application of microbial fertilizers derived from plant growth-promoting rhizobacteria (PGPR) is an important approach to increase crop yield while reducing the use of chemical fertilizers. Here, UPLC-MS/MS analyses were used to identify a bacterium, <i>Bacillus</i> sp. SYM-4, with a strong ability to secrete IAA. The strain was identified from 36 bacteria obtained from the rhizospheric soil of maize. Further inoculation experiments showed that <i>Bacillus</i> sp. SYM-4 was able to colonize the maize rhizosphere, resulting in a significant increase in IAA concentrations in seedlings. In addition, the antioxidant enzyme activity and chlorophyll content of maize seedlings were also significantly increased after inoculation with <i>Bacillus</i> sp. SYM-4. Therefore, <i>Bacillus</i> sp. SYM-4 was determined to be a PGPR for maize seedling growth. After further making it into microbial fertilizer, we found that, when 20% of the normal amount of chemical fertilizer was replaced with microbial fertilizer (<i>Bacillus</i> sp. SYM-4) and applied to field-cultivated maize seedlings, the growth of the maize plants at different stages was significantly promoted. Compared with the maize grown following application of a commercial microbial fertilizer (Pathfinder pioneer), seedlings grown using 20% <i>Bacillus</i> sp. SYM-4 microbial fertilizer and 80% chemical fertilizer showed significantly increased height. Substitution of chemical fertilizer (20%) with microbial fertilizer (<i>Bacillus</i> sp. SYM-4) treatment resulted in increases in maize yield over several measures (numbers of kernel rows on each ear, bald tip length, 100-grain weight and yield, and kernel nutrient content) compared to plants treated with pure chemical fertilizer. In this study, the replacement of a proportion of conventional fertilizer with a microbial substitute demonstrates a new technique with great potential for the green and efficient cultivation of maize.