Nuclear Accumulation of the Phytochrome A Photoreceptor Requires FHY1

Abstract

The phytochrome family of red/far-red (R/FR)-responsive photoreceptors plays a key role throughout the life cycle of plants . Arabidopsis has five phytochromes, phyA-phyE, among which phyA and phyB play the most predominant functions . Light-regulated nuclear accumulation of the phytochromes is an important regulatory step of this pathway, but to this date no factor specifically required for this event has been identified . Among all phyA signaling mutants, fhy1 and fhy3 (far-red elongated hypocotyl 1 and 3) have the most severe hyposensitive phenotype, indicating that they play particularly important roles . FHY1 is a small plant-specific protein of unknown function localized both in the nucleus and the cytoplasm . Here we show that FHY1 is specifically required for the light-regulated nuclear accumulation of phyA but not phyB. Moreover, phyA accumulation is only slightly affected in fhy3, indicating that the diminished nuclear accumulation of phyA observed in fhy1 seedlings is not simply a general consequence of reduced phyA signaling. By in vitro pull-down and yeast two-hybrid analyses, we demonstrate that FHY1 physically interacts with phyA, preferentially in its active Pfr form. Furthermore, FHY1 and phyA colocalize in planta. We therefore identify the first component required for light-regulated phytochrome nuclear accumulation.

Interaction with plant transcription factors can mediate nuclear import of phytochrome B

“…Although the mechanism of the nuclear transport of phyA was intensively studied in recent years (13)(14)(15)(16)21), the underlying mechanism of phyB nuclear import remained less understood. Until now, an intrinsic NLS in the phyB molecule itself was proposed (22)(23)(24).…”

“…The higher affinity to the Pfr-form could be the driving force for the light-dependent accumulation in the nucleus. For phyA, with concentrations almost two orders-ofmagnitude higher than those of the other phytochromes (3), the phyA specific FHY1-transport-system evolved that interacts with N terminus of phyA (13,14,37).…”

“…Both proteins interact in a light-dependent manner with phyA and bear a functional nuclear localization signal (NLS) and nuclear exclusion signal (13,14,(17)(18)(19)(20). In the absence of FHY1 and FHL phyA remains in the cytoplasm and nuclear accumulation of phyA is no longer detected after light irradiation (13).…”

“…Recent studies showed that the nuclear transport of phyA depends on the two paralogous proteins far red elongated hypocotyl 1 (FHY1) and fhy1-like (FHL) (13)(14)(15)(16). Both proteins interact in a light-dependent manner with phyA and bear a functional nuclear localization signal (NLS) and nuclear exclusion signal (13,14,(17)(18)(19)(20).…”

OWL1: An Arabidopsis J-Domain Protein Involved in Perception of Very Low Light Fluences

“…In addition to these VLFR-specific components, several other signaling intermediates have been demonstrated to be important for both FR-HIR and VLFR. These include FHY1 and FHY1-LIKE, two proteins necessary for the import of phyA into the nucleus upon light activation (see Supplemental Figure 1 online; Zeidler et al, 2004;Hiltbrunner et al, 2005Hiltbrunner et al, , 2006Zhou et al, 2005;Rosler et al, 2007;Genoud et al, 2008) and some members of the PKS1 and SPA1 protein family. The cytoplasmic PKS1 and 2 proteins, which can interact with phyA, show enhanced phyA-mediated VLFR and are also involved in blue light-dependent phototropism (Fankhauser et al, 1999;Lariguet et al, 2003).…”