Variation in the substrate specificity of allozymes catalyzing flavone-O-glucoside biosynthesis in Silene plants.
The 7-O- and 2"-O-glycosylation of the flavone isovitexin (6-C-glucosylapigenin) in the petals of Silene plants is accomplished by allozymes which differ in their specificity toward the sugar to be transferred. The g locus controls the 7-O-glycosylation; allele gG controls the binding of glucose, and allele gX that of xylose. In the present paper it is shown that at least two different forms of gG exist. The enzyme activities encoded by these two different alleles differ with respect to the flavone acceptor to which glucose is transferred. Allele gGm encodes a 7-O-glucosyltransferase that transfers glucose to isovitexin but that is not able to glycosylate isovitexin 2"-O-rhamnoside. The 7-O-glucosyltransferase encoded by allele gGd preferentially transfers glucose to isovitexin 2"-O-rhamnoside and not to isovitexin. The allozymes encoded by gGm and gGd were partly purified. Linearity of incorporation, pH optimum, effect of divalent cations and EDTA, apparent molecular weight, substrate specificity, and Michaelis enzyme kinetic parameters were determined for both enzyme activities. The simultaneous presence within a plant of gene glR, which controls the biosynthesis of isovitexin 2"-O-rhamnoside, with either gGm or gGd leads to different glycosylation types. In gGm/glR plants two monoglycosides accumulate in the petals, isovitexin 7-O-glucoside and isovitexin 2"-O-rhamnoside, respectively, whereas in gGd/glR plants the corresponding diglycoside, isovitexin 7-O-glucose 2"-O-rhamnoside, is synthesized. The distribution of the two alleles over chemical races of Silene pratensis in Europe is described; possible evolutionary relations between the various glycosyltransferases in Silene are discussed.