PLANT PHYSIOLOGY , Vol 103, Issue 4 1377-1384, Copyright © 1993 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Juvenile-Specific Localization and Accumulation of a Rhamnosyltransferase and Its Bitter Flavonoid in Foliage, Flowers, and Young Citrus Fruits
M. Bar-Peled, R. Fluhr and J. Gressel
Department of Plant Genetics, The Weizmann Institute of Science, Rehovot 76100, Israel
1-2-Rhamnosyltransferase catalyzes the production of
disaccharide-flavonoids that accumulate to 75% of dry weight. Vast energy
is expended in a short time span to produce these flavonoids. The highest
rhamnosyltransferase activities and immunodetected concentrations were
observed in early development of Citrus grandis (pummelo), coinciding with
up to 13% of fresh weight as naringin. The concentration of naringin in
leaves, petals, receptacles, filaments, albedo, and flavedo drops
drastically during development and correlates directly with a decrease in
the activity and amounts of 1-2-rhamnosyltransferase. Anthers had minute
rhamnosyltransferase activities and low concentrations of naringin.
Conversely, high 1-2-rhamnosyltransferase activity and naringin
concentrations appeared in both young and mature ovaries, as well as in
young fruits. The total amounts of naringin in mature leaves decreased
without detectable in vitro degradation of naringin in leaves. There was
still a net accumulation of naringin in the albedo and flavedo of older
fruit even though these tissues had only traces of
1-2-rhamnosyltransferase. Traces of enzyme synthesis in fruits, or import
of the product from leaves, may explain the net accumulation of naringin in
growing fruits. Unlike the late-expressed genes for glycosyltransferases in
anthocyanin biosynthesis, the rhamnosyltransferases from Citrus are active
only in juvenile stages of development.