Rhizospheric Bacillus amyloliquefaciens Protects Capsicum annuum cv. Geumsugangsan From Multiple Abiotic Stresses via Multifarious Plant Growth-Promoting Attributes

Abstract

Plant growth-promoting rhizobacteria (PGPR) are beneficial microorganisms that can be utilized to improve plant responses against biotic and abiotic stresses. In this study, we investigated whether PGPR (Bacillus amyloliquefaciens) isolated from the endorhizosphere of Sasamorpha borealis have the potential to sustain pepper growth under drought, salinity, and heavy metal stresses. The bacterial strain was determined based on 16S rDNA and gyrB gene sequencing and characterized based on the following biochemical traits: nitrogen fixation; 1-aminocyclopropane-1-carboxylate deaminase activity; indole acetic acid production; inorganic phosphate, potassium, zinc, and silicon solubilization; and siderophore production. Various abiotic stresses were applied to 28-day-old pepper seedlings, and the influence of the PGPR strain on pepper seedling growth under these stress conditions was evaluated. The application of PGPR improved survival of the inoculated pepper plants under stress conditions, which was reflected by higher seedling growth rate and improved physiochemical traits. The PGPR-treated plants maintained high chlorophyll, salicylic acid, sugar, amino acid, and proline contents and showed low lipid metabolism, abscisic acid, protein, hydrogen peroxide contents, and antioxidant activities under stress conditions. Gene expression studies confirmed our physiological and biochemical findings. PGPR inoculation led to enhanced expression of XTH genes and reduced expression of WRKY2, BI-1, PTI1, and binding immunoglobulin protein (BiP) genes. We conclude that the PGPR strain described in this study has great potential for use in the phytoremediation of heavy metals and for enhancing pepper plant productivity under stress conditions, particularly those involving salinity and drought.

Keywords: 1-aminocyclopropane-1-carboxylate deaminase; drought; endophyte; pepper; phytoremediation; plant growth-promoting rhizobacteria; salinity.

FIGURE 1

RAxML tree, based on partial 16s rDNA and gyrB nucleotide sequences and sequences from related Bacillus reference strains. The phylogram shows the relationship between the selected bacterial strain (B11) and the closely related taxa of Bacillus amyloliquefaciens. Bootstrap values (>50) are represented by numbers at the nodes based on 10,000 replications.

FIGURE 2

(A) Abscisic acid (ABA) and (B) salicylic acid (SA) content in leaves of pepper grown under normal and stress conditions and treated with plant growth-promoting rhizobacteria (PGPR) for 8 days (8DAT). Treatments: control + water, control + PGPR, control + 1% NaCl, PGPR + 1% NaCl, control + 10% polyethylene glycol (PEG), PGPR + 10% PEG, control + 0.1% Cd, and PGPR + 0.1% Cd. Values show the means ± SE (n = 3) and significant differences at p < 0.05 [least significant difference (LSD) test].

FIGURE 3

(A) Protein, (B) H₂O₂, (C) sugar, and (D) malondialdehyde (MDA) content in leaves of pepper grown under normal and stress conditions and treated with plant growth-promoting rhizobacteria (PGPR) for 8 days (8DAT). Treatments: control + water, control + PGPR, control + 1% NaCl, PGPR + 1% NaCl, control + 10% polyethylene glycol (PEG), PGPR + 10% PEG, control + 0.1% Cd, and PGPR + 0.1% Cd. Values show the means ± SE (n = 3) and significant differences at p < 0.05 [least significant difference (LSD) test].

FIGURE 4

Antioxidant content [DPPH, A; total polyphenol, B; superoxide dismutase (SOD), C; polyphenol oxidase (PPO), D; flavonoids, E; and peroxidase (POD), F] of pepper leaves grown under normal and stress conditions and treated with plant growth-promoting rhizobacteria (PGPR) for 8 days (8DAT). Treatments: control + water, control + PGPR, control + 1% NaCl, PGPR + 1% NaCl, control + 10% polyethylene glycol (PEG), PGPR + 10% PEG, control + 0.1% Cd, and PGPR + 0.1% Cd. Values show the means ± SE (n = 3) and significant differences at p < 0.05 [least significant difference (LSD) test].

FIGURE 5

Real-time expression analysis of CaBiPs (CaBiP1, A; CaBiP2, B; and CaBiP3, C), CaXTHs (CaXTH1, D; and CaXTH2, E), CaBI-1 (F), CaWRKY2 (G), CapTT1, and (H) in leaves of pepper grown under normal and stress conditions and treated with plant growth-promoting rhizobacteria (PGPR) after 8 days (8DAT). Treatment: control + water, control + PGPR, control + 1% NaCl, PGPR + 1% NaCl, control + 10% polyethylene glycol (PEG), PGPR + 10% PEG, control + 0.1% Cd, and PGPR + 0.1% Cd. Values show the means ± SE (n = 3) and significant differences at p < 0.05 [least significant difference (LSD) test].