Colonized extremophile Deinococcus radiodurans alleviates toxicity of cadmium and lead by suppressing heavy metal accumulation and improving antioxidant system in rice.

Cadmium (Cd) and lead (Pb) are the major toxic heavy metals accumulated in rice and pose a serious threat to human health. The most important remediation strategy is to reduce the translocation of these heavy metals from polluted soil to rice. Bioremediation using microorganisms had been widely used for preventing environmental heavy metal pollution, and the interaction between microorganisms and plants is critical to reduce the heavy metal stress. In this study, we demonstrated that an extremophile Deinococcus radiodurans, especially its mutant strain-Δdr2577 which is deficient in cell surface-layer, could efficiently prevent the translocation and damages of Cd or Pb in rice. The bacterial cells efficiently removed Cd or Pb from culture medium. Following colonization of Δdr2577 cells in rice root, Cd level decreased to 71.6% in root and 60.9% in shoot, comparing to the plants treated with Cd alone; Pb level decreased to 73.3% in root and 56.9% in shoot, comparing to the plants treated with Pb alone. Meanwhile, the bacterial cells released their intracellular antioxidant-related molecules including glutamate and manganese ions into culture medium. Accumulation of glutamate and manganese ions detected in rice root and shoot ameliorate Cd/Pb-induced oxidative stress as indicated by reduced levels of ROS and enhanced activities of antioxidant enzymes in rice. Our results provide a potential application of an extremophile bacterium in alleviating heavy metal toxicity in rice. The main findings of the work reveal the interaction between the D. radiodurans and rice, as well as the alleviating mechanism of Cd and Pb toxicity through suppressing heavy metal accumulation and improving the antioxidant system in rice by the extremophile bacterium.