Bio-priming improves salt stress tolerance in Zea mays L. insights from phenotypic and metabolomic analysis

Salinity is a major abiotic stress that severely limits crop productivity worldwide by disrupting plant physiological and metabolic processes. Maize ( Zea mays L.) is considered a moderately salt-tolerant crop; however, high salinity can substantially impair its growth and productivity. This study evaluated the potential of rhizosphere-derived plant growth-promoting rhizobacteria (PGPR) to mitigate salt stress in maize through a bio-priming approach integrated with phenotypic and metabolomic analyses. Soil samples were collected from two maize fields, and 15 bacterial strains were isolated. Based on plant growth-promoting traits and salt tolerance, four promising PGPR strains were selected and evaluated under 250 mM NaCl stress. Bio-priming treatments consisted of UC (untreated control), SU (salt stress only), SB1 (PPB1 + salt stress), SB2 (PPB4 + salt stress), SB3 (PPB8 + salt stress), and SB4 (PPB12 + salt stress). Under salinity stress, PGPR inoculation produced strain-specific improvements in maize growth. Compared with the salinity-stressed treatment (SU), SB3 showed the strongest overall growth-promoting response, significantly increasing shoot and root length by approximately 20% and 45%, shoot and root fresh weight by approximately 95% and 50%, and shoot and root dry weight by approximately 20% and 133%, respectively ( P ≤ 0.05). SB2 also significantly improved shoot and root fresh weight and root dry weight relative to SU. Physiological and biochemical responses were likewise strain dependent. Compared with SU, SB1 significantly enhanced total chlorophyll content, whereas SB1 and SB2 significantly increased carotenoid content. SB2 produced the strongest antioxidant response, significantly enhancing POD, CAT, and APX activities relative to SU ( P ≤ 0.05). For LC–MS-based metabolomic profiling, four treatments were selected: UC, SU, SB2 (SU + PPB4; Bacillus aerius ), and SB3 (SU + PPB8; provisionally identified as a member of the Bacillus cereus group based on 16 S rRNA analysis). Metabolomic profiling revealed treatment-specific changes following PGPR inoculation. Relative to SU, L-citrulline and UDP-N-acetyl-α-D-glucosamine showed approximately 1.54- and 7.70-fold higher abundance, respectively, in SB2, whereas L-methionine showed approximately 5.11-fold higher abundance in SB3. These changes, together with alterations in other plant-related metabolites, indicated metabolic reprogramming associated with improved salinity adaptation. However, because PPB8 was identified only by 16 S rRNA sequencing, genome-based taxonomic confirmation and screening for toxin- and virulence-associated genes are required before considering this strain for agricultural application.