Characteristic rhizosphere metabolites of low-cadmium-accumulating rice drive microbially induced soil cadmium speciation transformation

Introduction: Cadmium (Cd) contamination in paddy soils threatens global rice safety. However, the mechanisms by which interactions between rhizosphere metabolites and microorganisms regulate Cd bioavailability in the rhizosphere microecosystem of low-Cd-accumulating rice (LAR) remain unclear. Methods: = 3) groups. Results: ) through antimicrobial activity. These interactions establish a microecological mechanism that suppresses Cd mobilization and enhances immobilization. Moreover, LAR metabolites increase the abundance of microbial genes encoding enzymes for sinapic acid and biotin biosynthesis-including caffeoyl-CoA O-methyltransferase (EC: 2.1.1.104) and 6-carboxyhexanoate-CoA ligase (EC: 6.2.1.14)-and promote the enrichment of related functional microorganisms such as Candidatus Sulfobium mesophilum and Deltaproteobacteria bacterium, thereby further regulating Cd speciation. Validation experiments revealed that a consortium of 23 characteristic LAR metabolites (e.g., sinapic acid, biotin, isosteviol, hydroxyisocaproic acid, 2-hydroxyhexanoic acid, lumichrome, and hypoxanthine) reduced exchangeable Cd by 10-21% and increased Fe-Mn oxide-bound Cd by 28-56% in both natural and sterilized soils. Conclusion: These findings reveal that characteristic LAR metabolites directly promote Cd immobilization and drive directional microbial assembly and functional optimization, thereby providing novel insights that can facilitate the establishment of a rhizosphere ecological barrier that can increase resistance to Cd stress in rice.