Cadmium (Cd) is toxic to plant cells even at low concentrations. Root is the first barrier of Cd uptake in plant. Roots secrete an enormous range of compounds into the surrounding soil to adapt various environments (i.e., heavy metal stress). Rice tends to accumulate high quantity of Cd, and is the major source of dietary Cd intake in rice-eating populations. However, the impacts of root exudates on Cd uptake and transport in rice have largely been overlooked. Here, we hypothesize that metabolic up-regulation of the key root exudates to sequester/exclude Cd would inhibit Cd uptake in Cd-tolerant rice by forming Cd-complex. To test this hypothesis, two rice cultivars with contrasting Cd tolerance and accumulation were selected to conduct root exudations collection and metabolic profiling analysis by hydroponic culture under 25 μM Cd stress.

The results showed that Cd inhibited the growth of the sensitive cultivar while the tolerant one was promoted compared with the control because of the lower Cd uptake, translocation and oxidative damage. Under Cd stress, the tolerant cultivar tended to develop more adventitious roots, and have greater root length, surface area, root volume and number of tips while those root parameters of the sensitive one were reduced, compared to the control. Cadmium addition inhibited the uptake of trace elements (Cu, Mn, Mg and Fe), the concentration of which had a significant negative correlation with shoot Cd concentration. Metabolomics data revealed that Cd triggered significant metabolic changes in root exudates which are significantly different between the two rice cultivars, and three compounds were identified and assumed to be potential biomarkers. The root exudate metabolic changes revealed an active defence mechanism against Cd stress. Together, this work demonstrates that root exudates metabolomics is a good tool to reveal the detoxification mechanism of Cd tolerance in rice.

This work was supported by the National Key R&D Program of China (2018YFD0800700), National Natural Science Foundation of China (41877121), and the 111 Project(B18060).