REGULATION OF UV‐PROTECTIVE PIGMENT SYNTHESIS IN THE EIPERMAL LAYER OF RYE SEEDLINGS (Secale cereale L. cv. KUSTRO)

Abstract Trans‐cinnamic acid is the product of the L‐phenylalanine ammonia lyase (PAL, EC 4.3.1.5) enzyme reaction and might regulate the subsequent phenylpropanoid synthesis by feedback inhibition of PAL activity. The partial conversion of trans‐cinnamic acids (e.g. trans‐p‐coumaric acid) and of trans‐p‐coumaryl‐glucoside (chemically synthesized model substance for cinnamic acid derivatives) to the cis isomers upon UV irradiation leads to reduced inhibition of PAL activity within minutes. In rye leaves this rapid enhancement of PAL activity (within minutes) causes a rapid increase in phenylpropanoids, which are predominantly located in the epidermal layer. Because phenylpropanoid derivatives are precursors of flavonoid biosynthesis, the observed rapid increase in flavonoid content after short‐term UV irradiation can be explained by this UV‐induced regulation mechanism. Consequently, transphenylpropanoids can be regarded as photoreceptors and regulators of the phenylpropanoid pathway during UV‐B (280–320nm) irradiation. In rye leaves an additional slower UV response, obviously by induction of genetic transcription as described for other plant systems, leads to a much greater flavonoid accumulation compared to the rapid induction.