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-A/blue light. Only the UV-A/blue light induction of CHS expression involved the CRY1 photoreceptor. In addition, UV-A and blue light each act synergistically with UV-B to stimulate CHS transcript accumulation and beta-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 CRY1 photoreceptor. Our findings reveal considerable complexity in both photoreception and signal transduction in regulating CHS gene expression by UV and blue light.

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-A/blue light. Only the UV-A/blue light induction of CHS expression involved the CRY1 photoreceptor. In addition, UV-A and blue light each act synergistically with UV-B to stimulate CHS transcript accumulation and beta-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 CRY1 photoreceptor. Our findings reveal considerable complexity in both photoreception and signal transduction in regulating CHS gene expression by UV and blue light.

The hy4 mutant of Arabidopsis thaliana (L.) Heynh. was previously shown to be impaired in the suppression of hypocotyl extension specifically by blue light. We report here that hy4 is altered in a range of blue-light-mediated extension-growth responses in various organs in seedlings and mature plants: it shows greater length of bolted stems, increased petiole extension and increased leaf width and area in blue light compared to the wild type. The hy4 mutant shows decreased cotyledon expansion in both red and blue light compared to the wild type. Anthocyanin formation and the expression of several flavonoid biosynthesis genes is stimulated by blue light in the wild type but to a much lower extent in hy4. The results indicate that the HY4 gene product is concerned with the perception of blue light in a range of extension-growth and gene-expression responses in Arabidopsis.

No abstract

No abstract