PLANT PHYSIOLOGY , Vol 108, Issue 1 227-234, Copyright © 1995 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Ultraviolet Light Inhibition of Phytochrome-Induced Flavonoid Biosynthesis and DNA Photolyase Formation in Mustard Cotyledons (Sinapis alba L.)
G. Buchholz, B. Ehmann and E. Wellmann
Institut fur Biologie II, Universitat Freiburg, Schanzlestrasse 1, 79104 Freiburg, Germany
In cotyledons of etiolated mustard (Sinapis alba L.) seedlings,
phytochrome-far-red-absorbing form-induced flavonoid biosynthesis was found
to be inhibited by short-term ultraviolet (UV) irradiations. UV inhibition
was shown for the synthesis of quercetin, anthocyanin, and also for the
accumulation of the mRNA for chalcone synthase, the key enzyme of this
pathway. The UV effect was more pronounced on flavonoid biosynthesis, a
process that selectively occurs in the epidermal layers, than on the
synthesis of mRNA for chlorophyll a/b-binding protein localized in the
mesophyll tissue. These UV inhibitory effects were accompanied by
cyclobutane pyrimidine dimer (CPD) formation showing a linear
fluence-response relationship. CPD formation and UV inhibition of flavonoid
biosynthesis was found to be partially reversible by blue/UV-A light via
DNA photolyase (PRE), allowing photoreactivation of the DNA by splitting of
CPDs, which are the cause of the UV effect. Like flavonoid formation PRE
was also induced by the far-red-absorbing form of phytochrome and induction
was inhibited by UV. A potential risk of inhibition, in response to solar
UV-B irradiation, was shown for anthocyanin formation. This inhibition,
however, occurred only if photoreactivation was experimentally reduced. The
PRE activity present in the etiolated seedlings (further increasing about
5-fold during light acclimatization) appears to be sufficient to prevent
the persistence of CPDs even under conditions of high solar irradiation.
