Development of abiotic stress tolerance in inoculated apple plantlets with a plant growth-promoting rhizobacterium, Bacillus velezensis strain MWS28 in Korea
This study investigated the potential of the plant growth-promoting rhizobacterium (PGPR) Bacillus velezensis strain MWS28 (MWS28) to enhance abiotic stress tolerance in apple (Malus domestica) plantlets in Korea. Treatments with B. velezensis MWS28 or B. vallismortis EXTN-1 (positive control) by both foliar spraying and dipping methods significantly reduced cold injury symptoms, such as browning and tissue necrosis, compared to untreated controls. MWS28 treatment, especially via foliar spray, suppressed chilling injury by over 85%, outperforming conventional agents like benzothiadiazole (BTH) and streptomycin. Under drought conditions, both MWS28 and EXTN-1 treatments improved relative moisture content (RMC) and minimized leaf wilting throughout the stress period; MWS28-treated plants exhibited the strongest RMC after 15 days of water deprivation. Biochemical analysis showed MWS28-treated plants had significantly elevated activities of antioxidant enzymes (APX, CAT, SOD, and POD) and reduced the malondialdehyde (MDA) levels, indicating enhanced oxidative stress protection and membrane stability. MWS28 treatment also increased endogenous indole-3-acetic acid (IAA) and abscisic acid (ABA) concentrations under stress. Gene expression analysis demonstrated that MWS28 notably upregulated stress-related genes, particularly those involved in antioxidant defense, flavonoid biosynthesis (notably DFR, ANS, and UFGT), and pathogenesis-related (PR5 and PR8) genes in “Fuji” apple leaves using both dipping and foliar spray methods. These results collectively demonstrate that the MWS28 strain confers robust abiotic stress tolerance in apples by enhancing physiological resilience and activating key molecular defenses. This approach offers a promising, sustainable strategy for mitigating climate-related stresses in apple cultivation.