Exogenous Application of the Functional Metabolite Rosmarinic Acid Can Simulate the Effects of AMF Inoculation on Plant Growth Promotion and Cd Passivation

Previous studies revealed that inoculation with Glomus mosseae (AMF) significantly increased biomass and reduced cadmium (Cd) accumulation in Cd-sensitive Medicago sativa, which was linked to AMF-induced root exudates recruiting a functional microbiome enriched with plant growth-promoting and Cd-passivating bacteria. Here, we employed microcosm and greenhouse pot experiments to identify key metabolites that enhance the α diversity and functional potential of this microbiome and evaluated their exogenous application effects. Microcosm screening, based on α diversity and functional profiling of growth-promoting and Cd-passivating bacterial communities, identified rosmarinic acid, myristic acid, and indole-3-acetic acid (IAA) as critical functional metabolites. Greenhouse experiments demonstrated that all three metabolites significantly increased plant biomass (rosmarinic acid: 28.8%; myristic acid: 35.8%; IAA: 22.2%; P < 0.05) compared to the control. However, only rosmarinic acid significantly reduced rhizosphere Cd availability (14.4%, P < 0.05), with effects comparable to AMF inoculation (P > 0.05). Single-strain recruitment assays further confirmed that rosmarinic acid exhibited superior capability in enriching functional bacteria, demonstrating stronger chemotaxis and biofilm formation abilities.