PLANT PHYSIOLOGY , Vol 108, Issue 3 903-911, Copyright © 1995 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Flavonol 3-O-Glycosyltransferases Associated with Petunia Pollen Produce Gametophyte-Specific Flavonol Diglycosides
T. Vogt and L. P. Taylor
Department of Genetics and Cell Biology and Program in Plant Physiology, Washington State University, Pullman, Washington 99164-6414
Wild-type petunia pollen accumulates high levels of flavonol
3-O-glycosides. Pollen from conditionally male-fertile petunia has no
flavonols and is unable to germinate. Pollen function is restored both in
vivo and in vitro by providing flavonol aglycones, but not flavonol
glycosides, to the pollen. In the present study, incubation of an in vitro
suspension of conditionally male-fertile pollen with kaempferol or
quercetin resulted in the accumulation of kaempferol and quercetin
3-O-glycosides in the pollen. We identified two glycosyltransferase
activities associated with the intact pollen grain that catalyze the
formation of a gametophyte-specific class of flavonol glycosides. Feeding
studies showed that product formation was highly specific for flavonols
with an unsubstituted 3-hydroxyl group and was not dependent on an external
source of UDP-hexose. Ultraviolet spectral analysis, fast atom bombardment
mass spectrometry, 1H-nuclear magnetic resonance, and 13C-nuclear magnetic
resonance identified the products as kaempferol and quercetin
3-O-(2"-O-[beta]-D-glucopyranosyl)-[beta]-D-galactopyranoside, identical
with the flavonol 3-O-glycosides present in wild-type pollen. The sugars
are linked in a 1->2 configuration that results in a pollen-specific
class of compounds. To retain both glycosyltransferase activities in a
cell-free extract, it was necessary to add Triton X-100, suggesting that
one or both of the proteins may be associated with a pollen membrane. A
model for flavonol glycoside biosynthesis and uptake into the pollen is
discussed in terms of the germination requirement for flavonols.
