Missense Mutation in the PAS2 Domain of Phytochrome A Impairs Subnuclear Localization and a Subset of Responses

Abstract

Phytochrome A signaling shows two photobiologically discrete outputs: so-called very-low-fluence responses (VLFR) and high-irradiance responses (HIR). By modifying previous screening protocols, we isolated two Arabidopsis mutants retaining VLFR and lacking HIR. Phytochrome A negatively or positively regulates phytochrome B signaling, depending on light conditions. These mutants retained the negative but lacked the positive regulation. Both mutants carry the novel phyA-302 allele, in which Glu-777 (a residue conserved in angiosperm phytochromes) changed to Lys in the PAS2 motif of the C-terminal domain. The phyA-302 mutants showed a 50% reduction in phytochrome A levels in darkness, but this difference was compensated for by greater stability under continuous far-red light. phyA-302:green fluorescent protein fusion proteins showed normal translocation from the cytosol to the nucleus under continuous far-red light but failed to produce nuclear spots, suggesting that nuclear speckles could be involved in HIR signaling and phytochrome A degradation. We propose that the PAS2 domain of phytochrome A is necessary to initiate signaling in HIR but not in VLFR, likely via interaction with a specific partner.

Characterization of the requirements for localization of phytochrome B to nuclear bodies

“…For example, this model does not explain why most phyB PAS domain mutations from our studies and reports (17,36) do not form phyB NBs but are loss-of-function alleles and why no loss-of-function mutation has been isolated with normal or enhanced phyB NB formation. Therefore, we propose an alternative model, in which phyB is active both in the nucleoplasm and in NBs with different signaling mechanisms to respond to low and high intensities of R, respectively.…”

Complementation of phytochrome chromophore-deficient Arabidopsis by expression of phycocyanobilin:ferredoxin oxidoreductase

“…The occurrence of the Lm-2 allele of phyA in natural populations of Arabidopsis implicates spectral tuning of phytochromes to be of ecological significance, especially for seedlings grown under FR-enriched canopy shade (49). PCB substitution for P⌽B also enabled us to distinguish between phyA-mediated VLFR and FR-HIR, as has also been reported for another mutant allele of phyA (50). It is therefore likely that spectral tuning of phytochromes has been going on for millions of years, caused by mutations in apoprotein and various chromophore substitutions, and that many novel spectrally shifted mutant alleles of phytochromes will be uncovered in years to come.…”

Lysine 206 inArabidopsisphytochrome A is the major site for ubiquitin-dependent protein degradation

“…We analysed the effect of mutating K206 of phyA to R206 on the inhibition of hypocotyl elongation and cotyledon opening under far-red light. PhyA null mutant seedlings are characterized by loss of inhibition of hypocotyl elongation and cotyledon opening under far-red light conditions, which can be rescued by introduction of full-length phytochrome A (25,42,44). Stable phyA mutants exhibit hypersensitive responses to light due to increased phyA activity (41).…”