UV-A, and Blue Light Signal Transduction Pathways lnteract Synergistically to Regulate Chalcone Synthase Gene Expression in Arabidopsis

UV and blue light stimulate transcription of key flavonoid biosynthesis genes in a range of higher plants. Here, we provide evidence that several distinct "inductive" and ''synergistic" UV/blue phototransduction pathways regulate chalcone synthase (CHS) gene transcription and transcript accumulation in Arabidopsis leaf tissue. Experiments with the long-hypocotyl hy4-2.23N mutant showed that separate inductive pathways mediate responses to UV-B and UV-Alblue light. Only the UV-Alblue light induction of CHS expression involved the CRYl photoreceptor. In addition, UV-A and blue light each act synergistically with UV-8 to stimulate CHS transcript accumulation and b-glucuronidase activity driven by a CHS promoter in transgenic leaf tissue. The UV-A and blue phototransduction pathways responsible for synergism are distinct because they produce transient and relatively stable signals, respectively, and can function additively to stimulate CHS promoter function. The hy4-2.23N mutant retains the synergistic interactions between UV-B and both UV-A and blue light, indicating that neither synergism pathway involves the CRYl photoreceptor. Our findings reveal considerable complexity in both photoreception and signal transduction in regulating CHS gene expression by UV and blue light.