Tobacco NtUBC1 and NtUBQ2 enhance salt tolerance by reducing sodium accumulation and oxidative stress through proteasome activation in Arabidopsis.

The ubiquitin/proteasome system plays a crucial role in the regulation of plant responses to environmental stress. Here, we studied the involvement of the UBC1 and UBQ2 genes encoding a ubiquitin conjugating enzyme (E2) and ubiquitin extension protein, respectively, in the response to salt stress. Our results showed that the constitutive expression of tobacco NtUBC1 and NtUBQ2 in Arabidopsis thaliana improved salt tolerance, along with the lower Na level and higher K/Na ratio compared to control plants. Moreover, the expression levels of sodium transporters, including AtHKT1 (High-Affinity K Transporter1) and AtSOS1 (Salt Overly Sensitive 1), were higher in NtUBC1- and NtUBQ2-Arabidopsis. However, the transcript level of AtNHX1 (Na/H Exchanger 1) was similar between control and transgenic plants. After salt exposure, the activity of the 26S proteasome markedly increased in NtUBC1- and NtUBQ2-expressing plants; however, ubiquitinated protein levels decreased compared to control plants. Furthermore, higher activity of antioxidant enzymes and lower ROS production were observed in UBC1- and UBQ2-expressing plants. We further challenged atubc1, atubc2, and atubq2 single mutants and atubc1ubc2 double mutant lines with salt stress; interestingly, the salt sensitivity and sodium levels of the studied mutants were enhanced, while the potassium levels were reduced. However, the atubc1ubc2 double mutant illustrated a more severe phenotype than the single mutants, probably due to the redundant function of UBC1 and UBC2 in Arabidopsis. Taken together, NtUBC1 and NtUBQ2 enhance salt tolerance by enhancing 26S proteasome activity and reducing Na accumulation, ROS, and ubiquitinated/salt-denatured proteins.