NON-DAMAGING AND POSITIVE EFFECTS OF UV RADIATION ON HIGHER PLANTS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 155

sensitivity to UV-B radiation 21-25 . Mazza et al 26 demonstrated the importance of
phenylpropanoids in reducing UV penetration into leaf tissue using Arabidopsis
transparent testa mutants in the field. The potential antioxidant role of these pigments is
discussed in this volume in the chapter by Bornman.
Although there are nine major sub-groups produced in the various branches of
the flavonoid biosynthesis pathways, it is common practice in the literature to use the
term flavonoid to refer to all flavonoids with the exception of anthocyanins 27 and this
approach will be followed here. Anthocyanins are 3- or 3,5-glycosides of
anthocyanidins and are a major subgroup of flavonoid biosynthesis [see Winkel-
Shirley 28 for a recent update]. The main function of anthocyanins appears to be to
provide the red (R), purple and blue (B) colouration of flowers and other organs, but
there is also evidence for them having a protective function against UV-B radiation 29 ,
drought and cold 30 . Specific biological reponses are often controlled by more than one
photoreceptor, and UV-B appears to be involved in the control of several of these
reponses as the following examples for flavonoid and anthocyanin synthesis
demonstrate.
Beggs et al 31 have demonstrated that there is a specific UV-B induction of
anthocyanin and/or flavonoid production in at least 11 species. Using parsley cell
suspension cultures and seedlings, Wellmann (in Beggs and Wellmann 27 ) defined a
specific photomorphogenetic UV-B effect on flavonoid synthesis. He observed that
phytochrome and the BAP were only effective if the tissue had received UV-B
radiation. A detailed action spectrum for flavonoid synthesis in parsley cell suspension
cultures (Fig. 1) showed a clear action maximum near 295 nm with rapidly declining
effectiveness at both shorter and longer wavelengths; had this been a damage response,
increasing effectiveness at shorter wavelengths would have been expected. It has also
been shown that the synthesis of chalcone synthase in Arabidopsis , which plays a key
early role in the formation of flavonoids, is regulated by UV-B, UV-A/B and R/FR
photoreceptors 32 . Wade et al 33 have shown that phytochrome B regulates the
cryptochrome and UV-B signalling pathways.
A different spectral response is seen in the action spectrum for isoflavonoid
synthesis in bean ( Phaseolus vulgaris L.) is clearly different from that for flavonoid
synthesis in parsley (Fig. 1 34 ). Not only is the shape different with no decrease in
effectiveness at lower wavelengths, but the response can be photorepaired by
subsequent irradiation with UV-A or B radiation 34 thereby indicating that the radiation
is perceived by DNA rather than the UV-B photoreceptor. Nevertheless, the effect of
UV radiation acting through DNA is positive rather than negative because it results in
the production of protective pigments
Photocontrol of anthocyanin production can involve several photoreceptors and
the photoreceptors involved differ between species and even between organs, as the
following examples show. UV-B radiation has long been known to induce anthocyanin
synthesis 38 . Anthocyanin synthesis in mustard ( Sinapis alba L.) cotyledons, for example,
is controlled by phytochrome alone and is R/FR reversible (data of Schmidt in Mohr 39 ).
In the cotyledons of mustard, however, a specific promotory B effect on anthocyanin
production exists (data of Drumm-Herrel in Mohr 39 ) which indicates that photoreceptor
responses can be organ-specific within the same species. The role of UV-B radiation
varies between organs and species.