ʟ-Tyrosine as a Precursor of Flavonoids in Buckwheat Cotyledons

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Abstract
Exogenous ʟ-tyrosine was incorporated 13 -14 times less effectively than exogenous L-phenylalanine into flavonoids in excised cotyledons of 3 days old buckwheat seedlings but proved to be an only 5 times poorer precursor when the experiments were carried out with 5 days old material. Simultaneous administration of ʟ-phenylalanine markedly reduced incorporation of ʟ-tyrosine into flavonoids. A similar decrease in the utilization of exogenous ʟ-tyrosine for flavonoid bio­ synthesis occurred in kinetin-treated cotyledons. However, in cotyledons treated with high doses of glyphosate, an inhibitor of the shikimic acid pathway, an increase in the formation of flavonoids from exogenously supplied ʟ-tyrosine was observed. Under all conditions the relative incorporation rate of exogenous L-tyrosine was highest for anthocyanins and lowest for C-glycosylflavones while within the latter class of compounds the luteolinic derivatives orientin and isoorientin incorporated more label than their apigeninic analogues vitexin and isovitexin. PAL and TAL activities were found to be present in the cotyledons in a ratio of 50:1. The possible role of ʟ-tyrosine as an alternative natural precursor for the biosynthesis of flavonoids and other related polyphenols is discussed.

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