Transcriptome and metabolome analyses reveal the regulation effect of ultraviolet-B irradiation on secondary metabolites in pakchoi

Ultraviolet-B (UV-B, 280-315 nm) is considered to be an environmental stressor or regulation factor of plant metabolism depending on UV-B irradiance dose, duration, and wavelength. However, to date, the information related to transcription regulating and signaling cascades of secondary metabolites shift and biosynthesis in crop response to UV-B environmental conditions largely remains elusive. In this study, the change of phytochemical profiles and the association of transcriptome and metabolome were conducted under different UV-B durations in pakchoi. The content of total anthocyanins and total flavonoids increased from 100 to 159 (U kg-1) and 61.153-121.665 (g kg-1) under UV-B treatment for three days, respectively. Besides, the amounts of cinnamic acid, naringenin chalcone, kaempferin, and dihydroquercetin, as well as the expression levels of the flavonoid biosynthesis gene, were higher after one day of UV-B exposure. Under UV-B treatment for two and three days, the transcription level of indolic glucosinolate biosynthesis genes increased significantly compared to those of CK, while the gene expression of aliphatic glucosinolates and flavonoids biosynthesis decreased. In the KEGG database, pathways were enriched involving flavonoid biosynthesis under UV-B treatment for one and two days, glucosinolate biosynthesis under UV-B treatment for two and three days. The gene expression of MYB28 and MYB29 decreased under UV-B treatment for two and three days, while MYB51 and MYB122 increased compared to those of CK, respectively. Surprisingly, these changes in secondary metabolite biosynthesis and accumulation were closely related to a variety of phytohormones, including IAA, ABA, SA, and JA. These findings revealed that a variation in UV-B duration generated a change in secondary metabolite, which was influenced by the phytohormone trade-off and irradiation signal cascades.