Overexpression of Sedum alfredii cinnamyl alcohol dehydrogenase increases the tolerance and accumulation of cadmium in Arabidopsis

Plant cell walls protect plants from heavy metal stress by resistance, transport and the adsorption process. Cell walls are impregnated with lignin, which enables structural integrity, long-distance water transport and protects plants from many types of stress. Cinnamyl alcohol dehydrogenase (CAD) is an essential enzyme that participates in the final step of the phenylpropanoid lignin biosynthetic pathway and plays an important role in the stress resistance process. In this study, we demonstrated that a CAD-encoding gene in the hyper-accumulating Sedum alfredii Hance, SaCAD, is constitutively expressed in all tissues. While the abundance of the SaCAD transcript is affected by cadmium (Cd) stress, it is upregulated in roots, stems and leaves during Cd treatment. Overexpression of SaCAD in transgenic Arabidopsis thaliana significantly increased CAD activities. Compared with the wild type (WT) plants, the Cd concentrations of SaCAD-overexpressing plants increased in the leaves and roots under Cd stress. The increased fixation of Cd to the thickened cell wall in the SaCAD-overexpressing A. thaliana plants resulted in better growth when the plants were grown in Cd stress conditions. In agreement with these data, SaCAD-overexpressing plants exhibited higher Cd tolerance compared to the wild type (WT) with higher chlorophyll and proline (Pro) contents and antioxidant enzyme activity, as well as a lower methane dicarboxylic aldehyde (MDA) content, electric conductivity and reactive oxygen species when exposed to Cd stress due to a lower amount of Cd distributed in the cytoplasm which is the most site of cytosolic metabolism. SaCAD was found to localize to the cytoplasm of tobacco cells. NMT analysis of the root tips from transgenic A. thaliana lines during Cd stress confirmed that SaCAD-overexpressing plants were capable of retaining more Cd in the cell wall in the Cd-supplied growth medium, which provided additional evidence for the potential role of SaCAD in heavy metal ion compartmentation and detoxification. In summary, we concluded that SaCAD performs critical functions in plants: Cd absorption and fixation to lignified cell wall during stress conditions.