Metabolome and Transcriptome Analysis Provide Insights into Flower Bud Color Variation in the Adaptation to UV-B Radiation of Litchi

Flower color variations have increasingly been recognized as playing an important role in the adaptation to UV-B radiation; however, the underlying mechanism is poorly understood in perennial fruit trees. Litchi is an important fruit tree, and extremely early maturing (EEM) and middle-to-late-maturing (MLM) cultivars that originated from regions with high and low UV-B radiation have dark brown and light yellow flower buds, respectively, while their hybrid early-maturing (EM) cultivars have an intermediate brown flower bud. This study comprehensively analyzed the metabolome and transcriptome of flower buds of litchi EEM, EM and MLM cultivars to explore the mechanism underlying flower color variation during the adaptation to UV-B radiation for the first time. Metabolomic analysis identified 72 flavonoids in litchi flower buds, among which a higher accumulation of flavonol glycosides was responsible for darker flower buds of EEM cultivars. And transcriptome analysis revealed key structural genes, including LcCHI, LcFLS and seven UGTs, together with two transcription factors (LcMYB12 and LcMYB111), which could be directly up-regulated by UV-B radiation, playing critical roles in regulating the differential accumulation of flavonol glycosides. These results provide new insights into the molecular mechanism underlying adaptation to UV-B radiation and provide a genetic basis for future breeding of stress-tolerant cultivars of litchi.