Rhizobacteria mitigate salinity stress in maize by modulating photosynthesis, antioxidant defense, and rhizosphere microbial diversity

Abstract

Soil salinization poses challenges to agricultural growth; nonetheless, plant growth-promoting rhizobacteria (PGPR) can enhance the potential of plants to withstand salt stress. However, further investigation is needed to understand the specific mechanisms through which PGPR influence rhizosphere microbial diversity and root exudates in improving maize (Zea mays) salt tolerance under field conditions. This study utilized four treatments under field conditions: normal soil (NN), normal soil with PGPR (NP), salt-stressed soil (SN), and salt-stressed soil with PGPR (SP) to explore the influence of PGPR on maize rhizosphere microbial diversity and root exudates. The results showed that salt stress reduced maize's net photosynthetic rate (Pn), leaf area index (LAI), leaf K⁺ concentration, and yield while increasing Na⁺ concentration and Na⁺/K⁺ ratio. PGPR alleviated these effects, improving Pn, K⁺ levels, and yield while reducing Na⁺ and Na⁺/K⁺ ratios. Under salinity, Pn, stomatal conductance, transpiration rate, LAI, and chlorophyll content dropped by 52.71 %, 37.14 %, 29.53 %, 14.66 %, and 60.20 %, respectively, but PGPR increased them by 60.75 %, 37.14 %, 69.60 %, 13.20 %, and 27.17 %. Salt stress disrupted carbohydrate metabolism and antioxidant enzyme activities, reducing sucrose synthase and ADP-glucose pyrophosphorylase activities. PGPR restored these and boosted antioxidant defenses. PGPR minimized Na+ uptake, improved ion balance, and mitigated yield losses, achieving a maximum yield of 21,852.43 kg ha^-1 (NP treatment) versus 15,859.95 kg ha^-1 under SN. Additionally, SN decreased bacterial Shannon and Chao1 indices, whereas PGPR resulted in higher indices than SN. The SN treatment enriched salt-tolerant and pathogenic microorganisms (e.g., Rhodanobacter, Alternaria, Tausonia), while PGPR-enriched beneficial microorganisms that mitigate salt stress effects (e.g., Streptomyces, Sphingomonas, MND1). Additionally, root exudates from PGPR were enriched with metabolites classified as benzenoids, which positively correlated with Streptomyces and Sphingomonas. The study shows that adding PGPR reduces Na⁺ absorption in maize roots, lowers leaf Na⁺/K⁺ ratios, and mitigates yield loss under salt stress. Further research is needed to apply these findings in agriculture and evaluate their long-term impacts on soil quality and crop yields.