A bHLH transcription factor RrUNE12 regulates salt tolerance and promotes ascorbate synthesis.

RrUNE12 binds to the RrGGP2 promoter to facilitate biosynthesis of AsA in Rosa roxburghii fruit. Furthermore, RrUNE12 upregulates antioxidant-related genes and maintains ROS homeostasis, thereby improving tolerance to salt stress. L-ascorbic acid (AsA) plays an essential role in stress defense as a major antioxidant in plant cells. GDP-L-galactose pyrophosphatase 2 (RrGGP2) has been previously identified as the key structural gene operating in AsA overproduction in Rosa roxburghii fruit. However, the transcriptional regulation of RrGGP2 in response to abiotic stress is not fully elucidated. In this study, we identified a bHLH transcription factor, RrUNE12, whose transcription level significantly correlated with RrGGP2 abundance and AsA accumulation in developing fruit. RrUNE12 is localized in the nucleus and specifically binds to the promoter of RrGGP2 to promote its transcription. The overexpression or silencing of RrUNE12 in R. roxburghii fruit and fruit callus further confirmed that RrUNE12 positively regulated RrGGP2 transcription and AsA level. Different abiotic stress treatments indicated that RrUNE12 was greatly induced by salt. Exogenous NaCl treatment on the RrUNE12-overexpressing or RrUNE12-silencing fruits also led to enhanced transcripts abundance of both RrUNE12 and RrGGP2, compared to the treatment without adding NaCl. RrUNE12 overexpression in fruit callus alleviated salt stress damage by upregulating the expression of RrGGP2 and antioxidant-related genes. Additionally, stable overexpression of RrUNE12 in tomato plants resulted in a significant increase in AsA content and antioxidant capacity, accompanied by an increased resistance to the salt stress. Collectively, the results suggest that RrUNE12 functions as an activator of AsA biosynthesis in R. roxburghii fruit and plays a positive role in mitigating salt stress by increasing both AsA level and the oxidation resistance.