CLONING AND REGULATION OF FLAVONOL 3-SULFOTRANSFERASE IN CELL-SUSPENSION CULTURES OF FLAVERIA-BIDENTIS

Flaveria spp. accumulate flavonol sulfate esters whose biosynthesis is catalyzed by a number of position-specific flavonol sulfotransferases . Although the accumulation of sulfated flavonols appears to be tissue specific and developmentally regulated and to vary among related spec ies, little is known about the mechanism of regulation controlling the synthesis of these metabolites. In the present work, we report the is olation of a cDNA clone from Flaveria bidentis (pBFST3) encoding flavo nol 3-sulfotransferase (FS-ST), which catalyzes the first step in the biosynthesis of flavonol polysulfates. This clone (pBFST3) was express ed in Escherichia coli and produced an F3-ST with high affinity for th e flavonol aglycones, quercetin, and its 7-methyl derivative, rhamneti n. In addition, the synthetic auxin 2,4-dichlorophenoxyacetic acid was shown to induce F3-ST enzyme activity and FS-ST mRNA transcript level s in cell cultures of F. bidentis. The F3-ST mRNA levels increased wit hin the first 3 h, reaching a maximum after 24 h of treatment, and rem ained elevated for up to 48 h. Treatments with either quercetin 3-sulf ate or quercetin 3,7,4'-trisulfate reduced F3-ST enzyme activity in ce ll cultures but had no effect an the transcript levels. These results are discussed in relation to the putative role of flavonoid conjugates in the regulation of auxin transport.