Juvenile-Specific Localization and Accumulation of a Rhamnosyltransferase and Its Bitter Flavonoid in Foliage, Flowers, and Young Citrus Fruits

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Abstract
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.