Microbial inoculation enhances salt tolerance by modulating secondary metabolites and key biosynthetic genes in...

Salinity stress significantly reduces plant growth, nutrient uptake, and physiological performance in M. officinalis. This study investigated the effects of inoculation with arbuscular mycorrhizal fungi (AMF; Glomus intraradices) and plant growth-promoting rhizobacteria (PGPR; Bacillus subtilis), individually and in combination, on the growth, biochemical responses, and stress tolerance of M. officinalis under different salinity levels, including control (0 mM NaCl), low salinity (40 mM NaCl), moderate salinity (80 mM NaCl), and severe salinity (120 mM NaCl). Salinity stress significantly reduced shoot and root length, biomass accumulation, and chlorophyll content while increasing oxidative stress markers such as hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and electrolyte leakage (ELI). However, microbial inoculation mitigated these effects, with co-inoculation (AMF+PGPR) providing the greatest improvements in plant growth, root colonization, and nutrient uptake (N, P, K/Na ratio). Under severe salinity, AMF+PGPR inoculation increased shoot length (57.7%), root length (31.6%), shoot dry weight (49.1%), and root dry weight (4.3%) compared to non-inoculated plants. Antioxidant enzyme activities, including catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), and superoxide dismutase (SOD), as well as phenylalanine ammonia-lyase (PAL), were significantly upregulated in inoculated plants, reducing oxidative damage. Key genes involved in rosmarinic acid (RSA) content biosynthesis (PAL, 4CL, RAS) exhibited differential expression in response to microbial inoculation. Under moderate salinity, combined AMF+PGPR inoculation upregulated PAL, 4CL, and RAS expression by 10.38-, 6.75-, and 9.48-fold, respectively, relative to control plants. Overall, AMF+PGPR co-inoculation mitigated salinity stress by enhancing secondary metabolism and upregulating key biosynthetic genes, providing a sustainable approach to improving salt tolerance in M. officinalis. These findings highlight the potential of microbial inoculants in optimizing plant resilience under saline conditions.