NtMYB4 and NtCHS1 Are Critical Factors in the Regulation of Flavonoid Biosynthesis and Are Involved in Salinity Responsiveness

High levels of salinity induce serious oxidative damage in plants. Flavonoids, as antioxidants, have important roles in ROS scavenging. In the present study, the tobacco R2R3 MYBtype repressor, NtMYB4, was isolated and characterized. The expression of NtMYB4 was suppressed by salinity. Overexpression of NtMYB4 reduced the salt tolerance in transgenictobacco plants. NtMYB4 repressed the promoter activity of NtCHS1 and negatively regulated its expression. Rutinaccumulation was significantly decreased in NtMYB4 overexpressing transgenicplants and NtCHS1 RNAi silenced transgenicplants. Moreover, high H2O2 and O2- contents were detected in both types of rutin-reduced transgenicplants under high salt stress. In addition, exogenous rutinsupplementation effectively scavenged ROS (H2O2 and O2-) and improved the salt tolerance of the rutin-reduced transgenicplants. In contrast, NtCHS1 overexpressing plants had increased rutinaccumulation, lower H2O2 and O2- contents, and higher tolerance to salinity. These results suggested that tobacco NtMYB4 acts as a salinity response repressorand negatively regulates NtCHS1 expression, which results in the reduced flavonoid accumulation and weakened ROS-scavenging ability under salt stress.