AMF and UPB Synergistically Enhanced Cadmium Tolerance in Maize via Hormonal Regulation, Photosynthetic Optimization,...

Arbuscular mycorrhizal fungi (AMF) and urease-producing bacteria (UPB) ubiquitously coexist in cadmium (Cd)-contaminated soils, yet their combined physiological effects on crop growth under Cd stress remain poorly understood. Through sand culture experiments (25 mg kg^-1 Cd), this study examined the individual and coinoculation impacts of AMF and UPB on maize growth and Cd tolerance. Compared to noninoculated controls, coinoculation significantly increased maize Cd tolerance, resulting in an 81%-168% increase in root length, surface area, and volume; a 111% increase in biomass; and a 51% decrease in root Cd content. While raising the root endogenous hormones (indole-3-acetic acid, abscisic acid, zeatin, and gibberellins), leaf photosynthetic parameters (photosynthesis rate, transpiration rate, and stomatal conductance), and root/leaf antioxidant enzymes activity (superoxide dismutase, peroxidase, and catalase) by 12%-27%, 82%-91%, and 33%-85%, respectively. Individual AMF inoculation enhanced the root length and surface area by 271% and 100%, respectively; elevated the shoot/root potassium content by 19% and 61%; increased the plant antioxidant enzymes activity and endogenous hormones (auxins, zeatin, and gibberellins) by 24%-91% and 7%-28%, respectively. UPB inoculation increased root antioxidant enzymes by 58%-89% and hormones (auxins, zeatin, and gibberellins) by 8%-28%. Two-factor ANOVA revealed that AMF and UPB exerted appreciable positive impacts on maize photosynthesis, endogenous hormones, and antioxidant enzymes. The results of partial least squares structural equation modeling (PLS-SEM) showed that AMF-UPB synergism directly modulated the hormonal balance and photosynthetic efficiency, thereby enhancing the antioxidant capacity and promoting maize growth and Cd tolerance collectively.