Phytochrome A-specific signaling in Arabidopsis thaliana

Abstract

Among the five phytochromes in Arabidopsis thaliana, phytochrome A (phyA) plays a major role in seedling de-etiolation. Until now more then ten positive and some negative components acting downstream of phyA have been identified. However, their site of action and hierarchical relationships are not completely understood yet.

Figure 1.

A simplified model for phyA (phytochrome A) signaling pathways. After de novo synthesis in the cytosol, assembly of the chromophor and dimerization, light establishes a photoequilibrium (designated as λ) between P_r (red light absorbing form of phytochromes) and P_fr (far red light absorbing form of phytochromes). The P_fr form of phyA interacts with the transport helper proteins FHY1 (FARRED ELONGATED HYPOCOTYL 1) and FHL (FHY1-LIKE PROTEIN) and is imported into the nucleus by a piggyback mechanism. FHY1 and FHL can re-enter the cytosol and thus function as shuttle-proteins for phyA (see Figure 2 for details). Residual cytosolic phyA together with the blue light receptors PHOT1 and 2 (PHOTOTROPIC 1 and 2) trigger the phototropic response potentially via the PKS1 protein. In cytosol and nucleus phyA can become ubiquitinated in its P_fr form and is subsequently degraded, irrespective of its actual conformational state. Within the nucleus several phyA signal transduction pathways have been described. Phytochromes repress the (photomorphogenic repressor) COP1/SPA complex and thus enhance the nuclear accumulation of positive regulators such as HY5 (HYPOCOTYL 5), LAF1 (LONG AFTER FAR RED LIGHT1) and HFR1 (LONG HYPOCOTYL IN FAR RED 1). HY5 directly targets a number of genes important for photomorphogenesis. A second branch is that of the PIFs (PHYTOCHROME INTERACTING FACTORs), negative regulators of photomorphogenesis. P_fr induces the rapid degradation of these bHLH factors. A third pathway nearly exclusively responsible for the HIR (HIGH IRRADIANCE RESPONSE) includes the F-Box-protein EID1 (EMPFINDLICHER IM DUNKELROTEN LICHT 1). This protein targets a still unknown component of the phyA signaling network for degradation and functions as a repressor of phyA signaling. Not shown are the proposed effects on signaling of hypo- and hyperphosporylated forms of phyA.

Figure 2.

Light-induced nuclear transport of PHYA. FHY1 and FHL are distributed in the cytosol and nucleus in the dark. Upon light irradiation phyA interacts with FHY1/FHL hetero- or homodimers in the cytosol. Functional NLS motifs of the FHY1/FHL are recognized by a cytosolic importin α protein. The IMPα-FHY1/FHL-phyA(Pfr) complex is transported through the nuclear pores to the nuclei, where importin dissociates from the complex. In the nucleus two proteins, FHY3 and FAR1, related to Mutator-like transposases, act together to modulate phyA signaling by directly activating the transcription of FHY1 and FHL in the dark, whose products are essential for light-induced phyA nuclear accumulation. Subsequent light responses mediated by phyA include the attenuation of COP1 level, accumulation of HY5 and feedback regulation of FHY3 and FAR1 transcript levels. HY5 downregulates FHY1/FHL transcription by modulating the activities of FHY3 and FAR1.