Bacillus mycoides PM35 Reinforces Photosynthetic Efficiency, Antioxidant Defense, Expression of Stress-Responsive Genes, and Ameliorates the Effects of Salinity Stress in Maize

Abstract

Soil salinity is one of the abiotic constraints that imbalance nutrient acquisition, hampers plant growth, and leads to potential loss in agricultural productivity. Salt-tolerant plant growth-promoting rhizobacteria (PGPR) can alleviate the adverse impacts of salt stress by mediating molecular, biochemical, and physiological status. In the present study, the bacterium Bacillus mycoides PM35 showed resistance up to 3 M NaCl stress and exhibited plant growth-promoting features. Under salinity stress, the halo-tolerant bacterium B. mycoides PM35 showed significant plant growth-promoting traits, such as the production of indole acetic acid, siderophore, ACC deaminase, and exopolysaccharides. Inoculation of B. mycoides PM35 alleviated salt stress in plants and enhanced shoot and root length under salinity stress (0, 300, 600, and 900 mM). The B. mycoides PM35 alleviated salinity stress by enhancing the photosynthetic pigments, carotenoids, radical scavenging capacity, soluble sugars, and protein content in inoculated maize plants compared to non-inoculated plants. In addition, B. mycoides PM35 significantly boosted antioxidant activities, relative water content, flavonoid, phenolic content, and osmolytes while reducing electrolyte leakage, H₂O₂, and MDA in maize compared to control plants. Genes conferring abiotic stress tolerance (CzcD, sfp, and srfAA genes) were amplified in B. mycoides PM35. Moreover, all reactions are accompanied by the upregulation of stress-related genes (APX and SOD). Our study reveals that B. mycoides PM35 is capable of promoting plant growth and increasing agricultural productivity.

Keywords: abiotic stress; bio-surfactant; plant growth-promoting bacteria; plant–microbe interactions; salinity stress.

Figure 1

Growth curve analysis of B. mycoides PM35 under salinity stress (0, 1, 2 and 3 M NaCl).

Figure 2

Effects of NaCl on salinity tolerance traits of B. mycoides PM35 (a) Bacterial Population (b) Flocculation Yield (c) Bacterial Sodium Uptake (d) Biofilm Formation. Bars sharing different letter(s) for each parameter are significantly different from each other according to Least Significant Difference (LSD) test (p ≤ 0.05). All the data represented are the average of three replications (n = 3). Error bars represent the standard errors (SE) of three replicates.

Figure 3

Quantitative estimation of the following PGP traits of B. mycoides PM35: (a) Indole-3- acetic acid (IAA) (b) Siderophore (c) ACC deaminase (ACCD) (d) Exopolysaccharides (EPS). Bars sharing different letter(s) for each parameter are significantly different from each other according to Least Significant Difference (LSD) test (p ≤ 0.05). All the data represented are the average of three replications (n = 3). Error bars represent the standard errors (SE) of three replicates.

Figure 4

Effects of B. mycoides PM35 on plant growth promotion of Zea mays L. under salinity stress.

Figure 5

Effects of B. mycoides PM35 on levels of enzymatic and non-enzymatic antioxidants; (a) Superoxide dismutase (SOD) (b) Peroxidases (POD) (c) Ascorbate peroxidase (APX) (d) Ascorbic Acid. Bars sharing different letter(s) for each parameter are significantly different from each other according to Least Significant Difference (LSD) test (p ≤ 0.05). All the data represented are the average of three replications (n = 3). Error bars represent the standard errors (SE) of three replicates.

Figure 6

Amplification of abiotic stress-related genesin B. mycoides PM35: (a) CzcD-gene (b) sfp-gene (c) srfAA-gene.

Figure 7

Expression levels of antioxidant genes of maize in the absence and presence of B. mycoides PM35 under salinity stress, (a) Ascorbate peroxidase (APX) (b) Superoxide dismutase (SOD). Bars sharing different letter(s) for each parameter are significantly different from each other according to Least Significant Difference (LSD) test (p ≤ 0.05). All the data represented are the average of three replications (n = 3). Error bars represent the standard errors (SE) of three replicates.

Figure 8

PCA biplot showing the categorization of B. mycoides PM35 based on its effects on maize growth-promoting characteristics under salinity stress.

Figure 9

Pearson correlation between antioxidants and biochemical traits with plant biomass parameters under various salt stresses; Pro, (proline), SL (shoot length), RL (root length), PH (plant height), FW (fresh weight), DW (dry weight), LA (leaf area), Chl a (chlorophyll a), Chl b (chlorophyll b), T. chl (total chlorophyll), Caro (carotenoids), DPPH (radical scavenging capacity), SOD (superoxide dismutase), POD (peroxidase), APX (ascorbate peroxidase), AA (ascorbic acid), TPC (total phenolic content), TFC (total flavonoid content), TSS (total soluble sugars), TP (total protein), RWC (relative water content), ELL (electrolyte leakage), H₂O₂ (hydrogen peroxide), MDA (malondialdehyde), FAA (free amino acids), and GB (glycine betaine).