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.

Jasmonate-Dependent and -Independent Pathways Mediate Specific Effects of Solar Ultraviolet B Radiation on Leaf Phenolics and Antiherbivore Defense

“…Increased accumulation of phenolic compounds is one of the best characterized responses to UV-B radiation; these compounds contribute to filter out UV-B photons before they reach sensitive molecules in the mesophyll (Braun and Tevini, 1993;Landry et al, 1995;Reuber et al, 1996;Bilger et al, 1997;Barnes et al, 2000;Mazza et al, 2000). Soluble phenolic compounds are often also induced in response to insect herbivory, and they are thought to play a role as direct antiherbivore defenses (Elliger et al, 1981;Stamp and Osier, 1998;Hoffland et al, 2000;Leitner et al, 2005;Izaguirre et al, 2006).…”

Effects of Epidermal Cell Shape and Pigmentation on Optical Properties of Antirrhinum Petals at Visible and Ultraviolet Wavelengths

“…Optimally, a leaf's structure would allow the penetration of photosynthetically active light to the interior while limiting the penetration of harmful UV rays. In fact, considerable evidence exists that penetration of UV-B radiation is strongly reduced by UV-absorbing compounds, chiefly flavonoids, in epidermal tissues (Robberecht andCaldwell, 1978, 1983;Tevini et al, 1991;Braun and Tevini, 1993;Day et al, 1993;Stapleton and Walbot, 1994). However, in herbaceous annuals UV-screening pigments are restricted mainly to the vacuoles of the epidermal cells, and UV-B can leak into the interior of the leaf along the anticlinal walls (Day et al, 1993).…”

Ultraviolet-B-Responsive Anthocyanin Production in a Rice Cultivar Is Associated with a Specific Phase of Phenylalanine Ammonia Lyase Biosynthesis

“…The UV-B light-absorbing flavonoids are implicated as protective pigments in shoots and leaves exposed to UV-B light, and their specific location in the epidermal layer protects intemal cell layers by attenuating the impinging UV-B radiation at the epidermis (Tevini et al, 1991;Braun and Tevini, 1993). It has been shown that the photoinduced accumulation of these flavonoids is preceded by an indixtion of severa1 enzymes of phenylpropanoid biosynthetic pathway such as PAL and chalcone synthase of the flavonoid biosynthetic pathway (Hahlbrock and Scheel, 1989;Schmelzer et al, 1989).…”