FHL is required for full phytochrome A signaling and shares overlapping functions with FHY1

Abstract

SummaryPhytochrome A (phyA) plays a primary role in initiating seedling de-etiolation and is the only plant photoreceptor known to be activated by far-red light (FR). The signaling intermediate FHY1 appears to either participate directly in relaying the phyA signal or to positively regulate a critical signaling event(s) downstream of phyA activation. Here we identify a homolog of FHY1 named FHL (FHY1-like) as a novel signaling factor essential for complete responsiveness to phyA. FHL possesses functional nuclear localization and nuclear export signals. Lines in which FHL function was abolished by insertional mutagenesis or attenuated by RNAimediated suppression displayed a weaker hyposensitivity to continuous FR than fhy1 null mutants and most reported phyA signaling mutants. However, hypocotyl elongation assays indicated that suppression of FHL expression in fhy1-3 caused an insensitivity of hypocotyl elongation to FR and blue light (B) indistinguishable from that seen in phyA. Real-time PCR indicates that in FR, FHY1 transcripts are approximately 15-fold more abundant than FHL transcripts. Although both FHY1 and FHL are capable of homo-and hetero-interaction via their C-termini, the ability of FHL overexpression to restore wild-type (WT) morphological and molecular phenotypes to fhy1-3 seedlings suggests that the extreme insensitivity to FR associated with suppression of FHL expression in fhy1-3 cannot be accounted for by a critical role for FHY1-FHL heterodimers in phyA signal transmission. Rather, we suggest that the relative abundances of FHY1 and FHL in WT plants account for the differences in the severity of fhy1 and fhl mutations. As for FHY1, FHL transcript accumulation is dependent on FHY3 and is decreased after exposure to FR, R or B light. These findings reiterate the prevalence of partial degeneracy in plant signaling networks that regulate responses crucial to survival.

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

“…For the nuclear transport of phyBN a light dependency was observed. When a point mutation (G767 to R) was introduced in the C-terminal phyB-fragment, PIF3 did not induce nuclear transport of PHYBC- a transport mechanism should only have a minor influence of the in vivo situation, because the phenotype of fhy1/fhl double-mutants resemble the phyA mutant in far-red light (18). However, circumstantial evidence indicated the existence of phyA-mediated responses also in the fhy1/fhl double-mutants.…”

“…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).…”

DWA1 and DWA2, Two Arabidopsis DWD Protein Components of CUL4-Based E3 Ligases, Act Together as Negative Regulators in ABA Signal Transduction

“…For example, FAR-RED ELONGATED HYPOCOTYL1 (FHY1) and its homolog FHY1-LIKE (FHL) have been shown to form homodimers and heterodimers and then act together to regulate downstream events in Phytochrome A (PHYA) signaling, such as nuclear accumulation of PHYA and inhibition of hypocotyl elongation (Zhou et al, 2005;Hiltbrunner et al, 2006;Yang et al, 2009). Similarly, SPAs (for SUPPRESSOR OF PHYA) 1, 2, 3, and 4 are highly homologous proteins that can form homo-or heterodimers.…”

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).…”