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Developing primary leaves of Secale cereale L. exhibit a dynamic metabolism of the major flavonoid luteolin 7-O-[β-ᴅ-glucuronosyl(1→2)β-ᴅ-glucuronide]-4′-O-β-ᴅ-glucuronide (R 1 ). Final steps of R, biosynthesis are sequential glucuronidations of luteolin, which are catalyzed by three specific UDP-glucuronate: flavone glucuronosyltransferases. These enzymes reach highest activities at the fourth and fifth day of leaf development, coinciding with maximal R, accumulation. The activities decrease with advancing age of the leaves. In contrast, a R 1 -specific β-glucuronidase, responsible for the hydrolysis of glucuronic acid in position 4′, shows increasing activity up to the 5 th  or 6 th  day; but this activity, leading to luteolin 7-O-diglucuronide (R 2 ), is not reduced in later developmental stages. In this phase of leaf development, the amount of R, drastically drops, whereas R 2  accumulates only slightly. From in vitro results and from feeding experiments using [ 14 C]cinnamic acid, a precursor of R, biosynthesis, we conclude that the anabolic sequential glucuronidation takes place in young and expanding leaf tissue, whereas deglucuronidation occurs in nearly mature and mature tissue. The three glucuronosyltransferases as well as the β-glucuronidase, and the flavonoids R 1  and R 2  are localized in the mesophyll.

Dynamics of the Tissue-Specific Metabolism of Luteolin Glucuronides in the Mesophyll of Rye Primary Leaves (Secale cereale)