Comparative transcriptome analysis reveals different functions of Kandelia obovata superoxide dismutases in regulation of cadmium translocation.

Kandelia obovata is a dominant mangrove species in southeastern of China. This species has a high tolerance to heavy metal stress that is mainly ascribed to the thickening of the secondary exodermis and high activity of superoxide dismutase (SOD) enzymes in the roots. The thickened exodermis inhibits entry of heavy metals into the roots, and oxidative stress due to Cd can be reduced efficiently by SOD activity. Although there are several proposed mechanisms by which genes encoding SODs in K. obovata (KoSODs) could induce lignification that contributes to thickening of the root exodermis, the exact functions of FeSOD2 (KoFSD2) and Cu/ZnSOD3 (KoCSD3) remain unclear. In this study we investigated the role of a thickened exodermis in K. obovata root tissues in response to Cd treatment, and determined functions of KoFSD2 and KoCSD3 by performing comparative transcriptome analysis on tobacco lines that overexpress these two enzymes. we found that a thickened exodermis can reduce Cd uptake, and that exposure to high concentrations Cd can promote secondary thickening processes in K. obovata roots exodermis. Transgenic Nicotiana benthamiana overexpressing of KoFSD2 or KoCSD3 exhibit different responses to Cd stress. KoCSD3 can promote thickening of the secondary cell wall of root vascular tissues to impede Cd entry. Meanwhile, KoFSD2 and KoCSD3 can also regulate translocation of copper and iron in response to Cd accumulation. Taken together, our results expand our understanding of the physiological roles of SODs in K. obovata against Cd stress.