Overexpression of tobacco UBQ2 increases Cd tolerance by decreasing Cd accumulation and oxidative stress in tobacco and Arabidopsis

Ubiquitin (Ub)-extension protein (UBQ) functions as a Ub-donor in the Ub/26S proteasome system, which is widely engaged in degrading target proteins and, thus, participates in a broad range of physiological responses. We previously reported that the overexpression of NtUBQ2 confers cadmium (Cd) tolerance by increasing 20S proteasome activity and reducing ubiquitinated protein levels in NtUBQ2 tobacco (Lee and Hwang, 2015a). Here, we show that higher Cd tolerance in NtUBQ2 tobacco is ascribed to decreased Cd accumulation and oxidative stress. The lower accumulation of Cd is attributed to lower expression levels of HMA-A, HMA-B, IRT1 and ZIP1, and the lower oxidative stress is ascribed to higher activities of antioxidative enzymes and lower Cd accumulation. To further confirm the role of NtUBQ2 in Arabidopsis, we generated transgenic Arabidopsis lines expressing NtUBQ2. These plants exhibited higher Cd tolerance and lower Cd accumulation, which may be caused by enhanced proteasome activity. The reduced Cd accumulation is ascribed to the higher expression of PDR8 and lower expression levels of IRT1, HMA-2 and HMA-4. Furthermore, atubq2 knockout Arabidopsis showed a reduction in Cd tolerance and proteasome activity. Taken together, our results indicate that NtUBQ2 expression elevates Cd tolerance in transgenic tobacco and in Arabidopsis by increasing 20S proteasome activity, reducing oxidative stress through activating antioxidant enzymes and decreasing Cd accumulation by regulating the expression of Cd/metal transporters. This study can be applied to creating metal phytoremediators or metal hypo-accumulating crops.