Plants fend off potentially damaging ultraviolet (UV)-

Flavonol synthase (FLS) (EC-number 1.14.11.23), the enzyme that catalyses the conversion of Xavonols into dihydroXavonols, is part of the Xavonoid biosynthesis pathway. In Arabidopsis thaliana, this activity is thought to be encoded by several loci. In addition to the FLAVONOL SYNTHASE1 (FLS1) locus that has been conWrmed by enzyme activity assays, loci displaying similarity of the deduced amino acid sequences to FLS1 have been identiWed. We studied the putative A. thaliana FLS gene family using a combination of genetic and metabolite analysis approaches. Although several of the FLS gene family members are expressed, only FLS1 appeared to inXuence Xavonoid biosynthesis. Seedlings of an A. thaliana Xs1 null mutant (Xs1-2) show enhanced anthocyanin levels, drastic reduction in Xavonol glycoside content and concomitant accumulation of glycosylated forms of dihydroXavonols, the substrate of the FLS reaction. By using a leucoanthocyanidin dioxygenase (ldox) Xs1-2 double mutant, we present evidence that the remaining Xavonol glycosides found in the Xs1-2 mutant are synthesized in planta by the FLS-like side activity of the LDOX enzyme.

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