Revealing plant growth-promoting mechanisms of Bacillus strains in elevating rice growth and its interaction with salt stress

Abstract

Soil salinity is a major environmental stress that has been negatively affecting the growth and productivity of rice. However, various salt-resistant plant growth-promoting rhizobacteria (PGPR) have been known to promote plant growth and alleviate the damaging effects of salt stress via mitigating physio-biochemical and molecular characteristics. This study was conducted to examine the salt stress potential of Bacillus strains identified from harsh environments of the Qinghai-Tibetan plateau region of China. The Bacillus strains NMTD17, GBSW22, and FZB42 were screened for their response under different salt stress conditions (1, 4, 7, 9, 11, 13, and 16%). The screening analysis revealed strains NMTD17, GBSW22, and FZB42 to be high-salt tolerant, moderate-salt tolerant, and salt-sensitive, respectively. The NMTD17 strain produced a strong biofilm, followed by GBSW22 and FZB42. The expression of salt stress-related genes in selected strains was also analyzed through qPCR in various salt concentrations. Further, the Bacillus strains were used in pot experiments to study their growth-promoting ability and antioxidant activities at various concentrations (0, 100, 150, and 200 mmol). The analysis of growth-promoting traits in rice exhibited that NMTD17 had a highly significant effect and GSBW22 had a moderately significant effect in comparison with FZB42. The highly resistant strain NMTD17 that stably promoted rice plant growth was further examined for its function in the composition of rhizobacterial communities. The inoculation of NMTD17 increased the relative abundance and richness of rhizobacterial species. These outcomes propose that NMTD17 possesses the potential of PGPR traits, antioxidants enzyme activities, and reshaping the rhizobacterial community that together mitigate the harmful effects of salinity in rice plants.

Keywords: PGPR; antioxidant enzymes; biofilm; cell physiology; cellular interactions; rice.

Figure 1

The Bacillus spp. grown on LB media with salt stress incubated for 96 h at 30°C. (A) The growth of strains on solid LB media with different salt concentrations (a-g represent 1, 4, 7, 9, 11, 13, and 16%, respectively). Each number showing Bacillus strain: (1) CK, (2) FZB42, (3) NMTD17, (4) GBSW22, (5) NMSW10 and (6) GBSW11. (B) The graphical representation of the optical density of each strain at the same salt concentrations (1 to 16%) at different time intervals measured by spectrophotometer.

Figure 2

The biofilm formation of Bacillus strains grown under different salt conditions (1, 4, 7, 9, 11, 13, and 16%) up to 96 h at 37°C. The biofilm formation ability was observed in selected Bacillus strains (A). The higher amount of green fluorescence in the FZB42 strain cultured for 4 days at 37°C indicates a higher quantity of reactive oxygen species (ROS). When compared to GBSW22 and FZB42, the reduced fluorescence indicates that NMTD17 cells produce less ROS (B).

Figure 3

Relative expression levels of various Bacillus strains grown under salt and normal conditions for 96 h. (A) OpuAC, (B) SodB, (C) DegS, (D) DegU, (E) HPII, (F) OpuD, (G) ComA, and (H) SodA. Vertical bars on graphs indicate the standard deviation of the mean (n = 3). Tuckey’s HSD test was used to recognize a significant difference at p ≤ 0.05 between the treatments.

Figure 4

Relative expression levels of six possible DEGs in inoculated and un-inoculated rice plants under salt stress and control conditions (A) Ossamdc2, (B) Osdreb1f, (C) Oserebp2, (D) Oslea3-1 (E) Oserf104, (F) Oscyp89g1. The rice plants were subjected to different treatments. Vertical bars on graphs indicate the standard deviation of the mean (n = 3). Tuckey’s HSD test was used to recognize a significant difference at p ≤ 0.05 between the treatments.

Figure 5

Taxonomic composition of the microbial communities in different inoculated and un-inoculated rice rhizosphere soils. (A) Microbial compositions at the phylum level. (B) Heatmap of compositions showing 43 microbial communities at the genus level.

Figure 6

Analysis of the bacterial communities in various inoculated and un-inoculated rice rhizosphere soils. The box diagram of alpha- and beta-diversity of the inoculated and un-inoculated rice rhizosphere soil in groups (A) ACE, (B) Observed value, (C) Chao1 index, (D) Shannon index, (E) Simpson index, (F) Venn diagram of bacterial communities of different inoculated and inoculated rice rhizosphere soils in groups based on OTUs.

Figure 7

(A) The Principal coordinate analysis (PCoA) ordinations of bacterial community composition based on total OTUs using the Bray–Curtis matrix from the rhizosphere of rice under inoculated and un-inoculated plants. (B) Spearman’s correlation coefficients between the relative abundance of dominant bacterial community at genera level with plant growth and antioxidant activity parameters. Root length (RL), superoxide dismutase (SOD), Root diameter (RD), Root tips (RT), Dry weight (DW), Shoot length (SL), catalase (CAT), Fresh weight (FW), Root surface (RS), Peroxidase (POD), ascorbate peroxidase (APX).