GIGANTEA (GI) is involved in the promotion of flowering by long days, in light input to the circadian clock, and in seedling de-etiolation under continuous red light or blue light but not under continuous far-red light (FR). Since red-light effects are mediated largely by phytochrome B and those of FR by phytochrome A (phyA), GI was considered not to affect phyA signaling. However, recent observations using brief FR pulses indicate that GI does affect the very-low-fluence response (VLFR) and not the high-irradiance response (HIR) pathway of phyA. Seed germination, seedling de-etiolation and gene expression showed rhythmic sensitivity to FR pulses. GI affected the magnitude but not the rhythmic pattern of the responses, indicating that GI regulation of phyA signaling does not derive from its effects on the clock. Here we show that despite de above divergence between VLFR and HIR, both pathways require the bZip transcription factor HY5. Furthermore, the blue-light phenotype of gi is not mediated by phyA, indicating a role of GI in the control of cryptochrome signaling. These findings illustrate features of plant signaling networks.
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