Transcriptomic and functional evidence for a CiHY5-CiMYB12-like module associated with UV-B-enhanced drought acclimation in Chrysanthemum indicum var. aromaticum.

An aromatic ornamental and medicinal plant resource, Chrysanthemum indicum var. aromaticum grows at elevations above 2000 m, where it is exposed to strong ultraviolet-B (UV-B) radiation and other environmental constraints. Our previous research indicated that supplemental UV-B improves drought acclimation in C. indicum var. aromaticum. However, the underlying molecular mechanisms remain unclear. This study performed transcriptome sequencing of C. indicum var. aromaticum leaves following 4 h of supplemental UV-B exposure. The response involved multiple signaling and defense-related processes, while genes in the UV-B signaling and phenylpropanoid-flavonoid pathways were prominently induced. Heterologous overexpression of the UV-B photoreceptor CiUVR8 in Arabidopsis thaliana enhanced drought acclimation. Furthermore, virus-induced gene silencing of CiUVR8 in C. indicum var. aromaticum compromised UV-B-induced drought acclimation. Focusing on the MYB transcription factor SG7 subgroup responsive to UV-B, we identified CiMYB12-like as the most significantly upregulated member. As a central component of the UV-B signaling pathway, CiHY5 bound to the CiMYB12-like promoter in yeast one-hybrid assays and activated its transcription in dual-luciferase assays. CiMYB12-like also bound to and activated the promoter of the flavonoid biosynthetic gene CiCHI. Transient expression of CiMYB12-like increased the expression of flavonoid biosynthetic genes and total flavonoid content. A candidate co-expression network associated with CiHY5 and CiMYB12-like contained 28 genes related to transcriptional regulation, secondary metabolism, antioxidant defense, and transmembrane transport. Together, these findings establish a functional link between UV-B perception and drought acclimation in a native high-altitude species. The CiHY5-CiMYB12-like flavonoid module and its associated candidate co-expression network extend the canonical UV-B signaling framework to an ecologically relevant multi-stress context and provide targets for further investigation of high-altitude plant acclimation.