Synergistic effects of Rhodopseudomonas palustris and Bacillus subtilis on cadmium accumulation in Forsythia suspensa

Cadmium (Cd(II)) contamination threatens agricultural systems and the safety of traditional Chinese medicinal herbs. While beneficial microbes can alleviate Cd(II) stress in plants, the synergistic mechanisms underlying microbial co-inoculation in medicinal plant species remain unclear. This study investigated the individual and combined effects of Rhodopseudomonas palustris and Bacillus subtilis on Forsythia suspensa under Cd(II) stressed. The results showed that co-inoculation achieved markedly better effects than single-strain inoculation. Specifically, the HRB treatment enhanced plant height, belowground fresh weight, and aboveground fresh weight by 148.87%, 207.91%, and 188.81%, respectively, compared to the Cd(II) group. Furthermore, microbial inoculation enhanced chlorophyll content and strengthened the antioxidant defense system. The activities of SOD, POD, and CAT, as well as GSH content, were greatly improved: SOD activity increased by 3.2-6.3 folds, POD activity by 73.4%-173.9%, CAT activity by 66.2%-156.0%, and GSH content by 131.81% in the HRB group. Meanwhile, oxidative damage was alleviated, with MDA content decreasing by 62.61% in the HRB group. Crucially, co-inoculation modified the rhizosphere microenvironment: it improved bacterial diversity, enhanced soil enzyme activities (S-UE, S-CAT, S-ALP, and S-SC increased by 44.92%, 10.82%, 12.12%, and 10.75%, respectively, in the HRB group), and reduced Cd(II) bioavailability (acid-extractable Cd(II) decreased by 36.77% in the HRB group). Compared to the Cd(II) group, all inoculation treatments significantly increased the relative abundances of beneficial bacterial phyla including Proteobacteria, Actinobacteria, Gemmatimonadetes, and Bacteroidetes, while decreasing the abundances of Chloroflexi and Rokubacteria. These variations ultimately reduced Cd(II) accumulation in plant tissues. Pearson correlation analysis indicated that beneficial bacterial taxa and soil enzyme activities were positively correlated with plant physiological indices, whereas these parameters were negatively correlated with Cd(II) bioavailability and plant Cd(II) accumulation. Collectively, this study demonstrates that R. palustris and B. subtilis work synergistically to mitigate Cd(II) phytotoxicity and reduce Cd(II) accumulation in F. suspensa. The findings provide a promising bioremediation strategy for cultivating safer medicinal plants in Cd(II)-contaminated soils.