Bacillus thuringiensis PM25 ameliorates oxidative damage of salinity stress in maize via regulating growth, leaf pigments, antioxidant defense system, and stress responsive gene expression

Abstract

Soil salinity is the major abiotic stress that disrupts nutrient uptake, hinders plant growth, and threatens agricultural production. Plant growth-promoting rhizobacteria (PGPR) are the most promising eco-friendly beneficial microorganisms that can be used to improve plant responses against biotic and abiotic stresses. In this study, a previously identified B. thuringiensis PM25 showed tolerance to salinity stress up to 3 M NaCl. The Halo-tolerant Bacillus thuringiensis PM25 demonstrated distinct salinity tolerance and enhance plant growth-promoting activities under salinity stress. Antibiotic-resistant Iturin C (ItuC) and bio-surfactant-producing (sfp and srfAA) genes that confer biotic and abiotic stresses were also amplified in B. thuringiensis PM25. Under salinity stress, the physiological and molecular processes were followed by the over-expression of stress-related genes (APX and SOD) in B. thuringiensis PM25. The results detected that B. thuringiensis PM25 inoculation substantially improved phenotypic traits, chlorophyll content, radical scavenging capability, and relative water content under salinity stress. Under salinity stress, the inoculation of B. thuringiensis PM25 significantly increased antioxidant enzyme levels in inoculated maize as compared to uninoculated plants. In addition, B. thuringiensis PM25-inoculation dramatically increased soluble sugars, proteins, total phenols, and flavonoids in maize as compared to uninoculated plants. The inoculation of B. thuringiensis PM25 significantly reduced oxidative burst in inoculated maize under salinity stress, compared to uninoculated plants. Furthermore, B. thuringiensis PM25-inoculated plants had higher levels of compatible solutes than uninoculated controls. The current results demonstrated that B. thuringiensis PM25 plays an important role in reducing salinity stress by influencing antioxidant defense systems and abiotic stress-related genes. These findings also suggest that multi-stress tolerant B. thuringiensis PM25 could enhance plant growth by mitigating salt stress, which might be used as an innovative tool for enhancing plant yield and productivity.

Keywords: PGPR—plant growth-promoting rhizobacteria; abiotic stress; antioxiants; plant-microbe interactions; qRT-PCR.

Figure 1

Growth curve analysis of Bacillus thuringiensis PM25 under salinity stress (0, 1, 2, and 3 M NaCl).

Figure 2

Effects of NaCl on salinity tolerance traits of PM25 (A) Bacterial population (B) Flocculation yield (C) Bacterial Na+ uptake (D) Biofilm formation. Bars sharing different letter (s) for each parameter are significantly different from each other according to the 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 PGP traits of PM25 under salinity stress: (A) IAA (B) Siderophore (C) ACCD (D) EPS. Bars sharing different letter (s) for each parameter are significantly different from each other according to the 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 Bacillus thuringiensis PM25 on levels of enzymatic and non-enzymatic antioxidants: (A) APX (B) POD (C) SOD (D) Ascorbic Acid. Bars sharing different letter (s) for each parameter are significantly different from each other according to the 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 5

Effects of Bacillus thuringiensis PM25 on (A) Total soluble sugars (B) Protein content (C) Flavonoid content (D) Phenolic content. Bars sharing different letter (s) for each parameter are significantly different from each other according to the 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

Expression levels of antioxidant genes of maize in the absence and presence of B. thuringiensis PM25 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 the 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 7

PCA biplot showing the categorization of PM25 based on its effects on maize growth-promoting characteristics under salinity stress (A) Cluster analysis (B) PCA Biplot analysis.

Figure 8

Pearson correlation between antioxidants and biochemical traits with plant biomass parameters under salinity stress; 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 (Peroxidases), 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), EL (Electrolyte leakage), H₂O₂ (Hydrogen peroxide), MDA (Malondialdehyde), FAA (Free amino acids), GB (Glycine betaine). The treatments exhibit (*) within rows that represent a significance (p ≤ 0.05) level.