Leaf-to-root ROS signaling recruits rhizosphere Bacillus: a synbiotic mechanism for rice adaptability under foliar Cd stress.

In paddy fields near mining sites or transport corridors, elevated foliar cadmium (Cd) exposure is an emerging threat to rice (Oryza sativa) production. While rice recruits beneficial rhizosphere microbes against environmental stress, how this recruitment is coordinated under foliar Cd stress remains elusive. We assessed physiological responses to foliar Cd exposure in 26 rice cultivars and characterized rhizosphere bacterial communities and root exudates in six representative cultivars. Foliar Cd reduced leaf biomass by 6-29% and induced pronounced oxidative stress. In parallel, this foliar stress reshaped the rhizosphere microbiota, with consistent enrichment of Bacillus. This microbial shift was accompanied by increased root exudation of L-valine and L-leucine, which was associated with reactive oxygen species (ROS) signaling and increased root membrane permeability. Crucially, these amino acids promoted chemotaxis and root colonization by the representative Bacillus strains SRS50 and SRS107, selected from 11 candidates among 241 rhizosphere isolates, whereas colonization was reduced in ROS-deficient mutants. Functionally, application of a 'synbiotic' (amino acid-Bacillus combination) reduced grain Cd concentrations by 40.5-43.3%, associated with Bacillus-induced leaf lignification. These findings highlight a plant-microbe feedback loop that contributes to rice adaptation to foliar Cd stress.