ULTRAVIOLET-LIGHT INHIBITION OF PHYTOCHROME-INDUCED FLAVONOID BIOSYNTHESIS AND DNA PHOTOLYASE FORMATION IN MUSTARD COTYLEDONS (SINAPIS-ALBAL)

In cotyledons of etiolated mustard (Sinapis alba L.) seedlings, phytoc hrome-far-red-absorbing form-induced flavonoid biosynthesis was found to be inhibited by short-term ultraviolet (UV) irradiations. UV inhibi tion was shown for the synthesis of quercetin, anthocyanin, and also f or the accumulation of the mRNA for chalcone synthase, the key enzyme of this pathway. The UV effect was more pronounced on flavonoid biosyn thesis, a process that selectively occurs in the epidermal layers, tha n on the synthesis of mRNA for chlorophyll a/b-binding protein localiz ed in the mesophyll tissue. These UV inhibitory effects were accompani ed by cyclobutane pyrimidine dimer (CPD) formation showing a linear fl uence-response relationship. CPD formation and UV inhibition of flavon oid biosynthesis was found to be partially reversible by blue/UV-A lig ht via DNA photolyase (PRE), allowing photoreactivation of the DNA by splitting of CPDs, which are the cause of the UV effect. Like flavonoi d formation PRE was also induced by the Car-red-absorbing form of phyt ochrome and induction was inhibited by UV. A potential risk of inhibit ion, in response to solar UV-B irradiation, was shown for anthocyanin formation. This inhibition, however, occurred only if photoreactivatio n was experimentally reduced. The PRE activity present in the etiolate d seedlings (further increasing about 5-fold during light acclimatizat ion) appears to be sufficient to prevent the persistence of CPDs even under conditions of high solar irradiation.