CsERF21-l-CsASA2 module positively regulates cold tolerance by promoting tryptophan biosynthesis in tea plants.

Cold stress is a widely distributed abiotic stress that severely limits the yield and quality of tea plants. Tryptophan (Trp) and downstream indole compounds play an important role in plant growth and stress response. However, beyond its involvement in indole compounds synthesis, the other physiological functions of Trp in tea plants remain unknown. In this study, anthranilate synthetase (ASA2) was a positive regulator to enhance cold stress tolerance in Camellia sinensis. The expression of CsASA2 was strongly induced by cold stress. Suppression of CsASA2 expression in C. sinensis reduced the accumulation of Trp, lowered reactive oxygen species (ROS) scavenging capacity, and ultimately impaired cold stress tolerance. Heterologous overexpression of CsASA2 increased the endogenous Trp and melatonin content in response to cold stress and then showed the opposite cold resistance trend. Further study revealed that CsASA2 expression was positively regulated by the transcription factor CsERF21-l. Low temperature induced and kept CsERF21-l expression at a relatively high level. Suppression of CsERF21-l expression in tea plant reduced the accumulation of IAA and melatonin, increased the extent of cytoplasmic membrane damage, and weakened the cold tolerance of tea plant. Further, our study demonstrated that CsERF21-l enhanced cold tolerance in tea plants by promoting CsASA2 transcription. Overall, our results showed that the CsERF21-l-CsASA2 model balances the growth and cold tolerance in tea plants by regulating melatonin and IAA levels by recruiting Trp under cold stress.