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

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.