Heterologous expression of rice OsEXO70FX1 confers tolerance to cadmium in Arabidopsis thaliana and fission yeast

Cadmium (Cd) is an environmental toxicant that accumulates in grains, which greatly increases the risk of human exposure to Cd via food chain. The exocytosis of Cd is one of the essential detoxification mechanisms in plants. OsEXO70s, which facilitate the fusion of secretory vesicles and target membranes, has undergone significant expansion in rice. Here, we uncovered 40 OsEXO70 genes characterized by genome-wide profiling and focused on the potential functions of OsEXO70s, especially OsEXO70FX1, in Cd stress. Overexpression of OsEXO70FX1 enhanced both diamide and Cd tolerances in Schizosaccharomyces pombe (S. pombe), and in Arabidopsis resulted in 11% more seedlings survival rate and about 70% longer primary roots under Cd treatment compared with WT (empty vector). Meanwhile, Cd treatment upregulated the expression levels of some exocyst subunits in over-expression lines. Trichomes isolated from overexpression lines were observed to accumulate more Cd. Also, reactive oxygen species (ROS) induced by Cd stress reflected less sensitivity of OsEXO70FX1 overexpression lines to Cd stress, which was evidenced in the Cd determination assay. These results provide the fundament to future research on rice EXO70 family and suggest that it may have evolved a specialized role in response to Cd stress.