UV/blue light signal transduction regulating gene expression in Arabidopsis

Plants have evolved a number of mechanisms to protect themselves against environmental stresses. For example, in response to potentially harmful levels of UV light plants can accumulate flavonoids which, among other functions, act as UV-protective pigments. Chalcone synthase (CHS) is the key enzyme which commits the phenylpropanoid pathway to flavonoid biosynthesis. As the expression of CHS and other flavonoid biosynthesis genes is controlled by UV/blue light in higher plants, these genes provide ideal subjects for investigation of the cellular and molecular mechanisms involved in coupling UV/blue photoreception to transcription. To investigate the signal transduction processes concerned with the induction of CHS expression by UV/blue light, the effects of specific agonists and inhibitors were examined in a photomixotrophic Arabidopsis cell suspension culture. This cell culture behaves similarly to mature Arabidopsis leaves in the light regulation of CHS. That is CHS transcript levels are induced by UV-B and UV-A/blue light but not phytochrome. Moreover, experiments with a hy4 mutant cell suspension culture demonstrate that the effects of UV-B and UV-A/blue light on CHS expression are mediated by separate detection systems, the latter involving the CRY 1 photoreceptor. Pharmacological studies with Arabidopsis cell culture indicate that the UV-B and UV-A/blue phototransduction processes require cellular calcium. The inhibitors used suggest that an intracellular pool of calcium may be involved. However, the artificial elevation of cytosolic calcium using an ionophore is insufficient on its own to stimulate CHS expression. Similarly, in preliminary experiments with transgenic Arabidopsis containing cytosolic aequorin, no significant rapid increase in calcium is observed in response to either UV-B or UV-A/blue light. Possible reasons for this discrepancy are discussed. The UV-B induction of CHS expression appears to involve calmodulin because it is strongly inhibited by the antagonist W-7. In contrast, W-7 has little, if any, effect on the UV-A/blue response, indicating that the UV-B and UV-A/blue light signal transduction pathways regulating CHS are, at least in part, distinct. Further evidence shows that both pathways involve protein phosphorylation and require cytoplasmic protein synthesis. The involvement of chromophore excitation, ion fluxes and heterotrimeric G proteins in UV/blue light signal transduction have also been investigated. From the above pharmacological experiments, it is evident that the UV-B and UV-A/blue phototransduction pathways are distinct from the phytochrome signal transduction pathway regulating CHS expression in other species. Support for this conclusion comes from the observation that activators/inhibitors of phytochrome signalling have no effect on CHS expression in the Arabidopsis cell culture. (Abstract shortened by ProQuest.)