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

Abstract

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, Bacillus 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 Bacillus 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 Bacillus sp. SYM-4. Therefore, Bacillus 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 (Bacillus 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% Bacillus sp. SYM-4 microbial fertilizer and 80% chemical fertilizer showed significantly increased height. Substitution of chemical fertilizer (20%) with microbial fertilizer (Bacillus 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.

Keywords: Bacillus spp.; IAA; PGPR; maize; microbial fertilizer.

Figure 1

Determination of IAA-producing ability and movement ability of the strains. (A–C) Qualitative and quantitative analyses of IAA-producing ability of different bacterial strains. (D–F) Qualitative diagram of the movement ability of SYM-4, SYM-5, and SYM-6. (G) Quantitative analysis of the movement ability of SYM-4, SYM-5, and SYM-6. Scale bar = 1 cm. The different small letters represent significant differences at the p < 0.05 level.

Figure 2

Phylogenetic tree reconstructed using DNA sequences from Bacillus sp. SYM-4 and sequences downloaded from NCBI.

Figure 3

Growth promotion effects of Bacillus sp. SYM-4 inoculation on potted maize seedlings. (A) Effects of SYM-4 on the growth of maize seedlings at 7 dpi. (B) Effects of SYM-4 on the growth of maize seedlings at 14 dpi. (C–E) Quantitative parameters, including leaf area, stem diameter, and biomass, measured at 14 dpi. (F) Temporal dynamics of plant height from 4 dpi to 20 dpi. Triple asterisks indicate significant differences between the control group and other treatments, *** p < 0.001, ** p < 0.05, * p < 0.01.

Figure 4

Effect of inoculation with Bacillus sp. SYM-4 on physiological and biochemical parameters in maize seedlings. (A–C) Effect of SYM-4 on the antioxidant enzyme activity of maize seedlings at 14 dpi. (D–F) Effect of SYM-4 on the chlorophyll content, malondialdehyde, and soluble protein of maize seedlings at 14 dpi. (G) Effect of SYM-4 on IAA content in shoots of maize seedlings at 7 dpi and 14 dpi. (H) Effect of SYM-4 on IAA content in roots of maize seedlings at 7 dpi and 14 dpi. Triple asterisks indicate significant differences between the control group and other treatments, *** p < 0.001, ** p < 0.05, * p < 0.01.

Figure 5

Plant height (A–D) and leaf area (E–H) of maize seedlings at four growth stages in the field, following application of fertilizer, where different proportions (0%, 20%, 40%, 60%, 80%, or 100%) of chemical fertilizer were replaced with microbial fertilizer (SYM-4). The legend shows the percentage of chemical fertilizer replaced by microbial fertilizer in each treatment. Mean differences were compared using one-way ANOVA with Tukey’s test. The different small letters represent significant differences at the p < 0.05 level. The results shown represent the means ± standard deviation.

Figure 6

The effects of replacement of different proportions of chemical fertilizer with microbial fertilizer on maize kernels and panicles. The effects of control group (A) and 20% Bacillus sp. SYM-4 microbial fertilizer replacement of chemical fertilizer (B) on corn. The concentrations of soluble protein (C) and soluble sugars (D) in the maize kernels were determined. In panels C and D, the legend shows the proportions (0%, 20%, 40%, 60%, 80%, or 100%) of chemical fertilizer replaced by microbial fertilizer. Scale bar = 1 cm. Mean differences were compared using one-way ANOVA with Tukey’s test. The different small letters represent significant differences at the p < 0.05 level. The results shown represent the means ± standard deviation.