Figure 10.

A Working Model Depicting How HY5 Functions in the Feedback Regulation of phyA Signaling.

In the absence of light, FHY3 and FAR1 induce the expression of FHY1 and FHL in anticipation of the upcoming light signal. Accumulation of FHY1 and FHL proteins in dark-grown seedlings may serve to ensure rapid and sufficient phyA nuclear accumulation upon FR light exposure to jump start phyA signaling events in the nucleus. Upon light exposure, the Pfr form of phyA is imported into the nucleus by FHY1/FHL and thus triggers phyA signaling leading to multiple light responses, including the reduction of COP1 in the nucleus and accumulation of HY5 (Osterlund and Deng, 1998; Osterlund et al., 2000), and feedback regulation of FHY3 and FAR1 transcript levels (Lin et al., 2007). HY5 plays dual roles in phyA signaling: promoting photomorphogenesis and downregulating FHY1/FHL transcript levels by modulating the activities of the transcriptional activators FHY3 and FAR1. FHY3 and FHY1 (indicated by larger letters) are the more predominant players in the phyA signaling process compared with their respective homologs FAR1 and FHL. Pr, R-absorbing form of phyA (inactive); Pfr, FR-absorbing form of phyA (active). Arrow, positive regulation; bar, negative regulation.