Flavonoids and UV photoprotection in Arabidopsis mutants

Wild-type Arabidopsis L. leaves exposed to low ultraviolet-B (UVB) conditio ns contained predominantly kaempferol glycosides, with low levels of querce tin glycosides. The flavonoid level doubled on treatment with UVB and an in crease in the ratio of quercetin: kaempferol was observed. These results su ggest that flavonols protect Arabidopsis plants from UVB damage, and indica te that the flavonoid 3'-hydroxylase (F3'H) enzyme, which converts dihydrok aempferol to dihydroquercetin, may play a crucial role. The tt7 mutant lack s this gene and, after treatment with sub-ambient U-VB, contained kaempfero l glycosides exclusively, to a level of total flavonols similar to that in wild-type Arabidopsis. Total flavonols after enhanced UVB treatment were hi gher in tt7 than in similarly treated wild-type plants, and only kaempferol glycosides were detected. Despite this high level, tt7 plants were less to lerant of UVB radiation than wild-type plants. These observations suggests that kaempferol is a less effective photoprotectant than quercetin. The cha lcone isomerase (CHI) mutant (tt5) surprisingly did not accumulate naringen in chalcone, and this suggests that the mutation may not be restricted to t he CHI gene alone. The concentration of hydroxycinnamic. acid derivatives d id not change with UVB treatment in most varieties indicating that their ro le in UV photoprotection may be subordinate to that of the flavonoids.