Photoreceptors of the phytochrome family control a multitude of responses in plants. Phytochrome A (phyA) is essential for far-red light perception, which is important for germination and seedling establishment in strong canopy shade. Translocation of phyA from the cytosol into nucleus is a key step in far-red light signaling and requires FAR-RED ELONGATED HYPOCOTYL 1 (FHY1) and FHY1-LIKE (FHL). FHY1/FHL bind to phyA downstream signaling components. Therefore, it has been suggested that FHY1/FHL also have a function in assembling phyA transcription factor complexes in the nucleus. Yet, in this study, we show that constitutively nuclear-localized phyA is active in the absence of FHY1 and FHL. Furthermore, an artificial FHY1, consisting of an SV40 NLS, a phyA binding site, and a YFP tag as spacer between them, complements the fhy1-3 fhl-1 double mutant. These findings show that FHY1 and FHL are not required for phyA downstream signaling in the nucleus. However, we found that lines expressing phyA-NLS-YFP are hypersensitive to red and far-red light and that slightly increased levels of constitutively nuclear-localized phyA result in photomorphogenic development in the dark. Thus, restricting phyA to the cytosol and inducing nuclear transport in light by interaction with FHY1/FHL might be important to suppress photomorphogenesis in the dark.

光敏色素家族的光感受器控制着植物的多种反应。光敏色素 A (phyA) 对远红光感知至关重要，这对于在浓荫下发芽和幼苗建立很重要。phyA 从细胞质到细胞核的易位是远红光信号传导的关键步骤，需要远红细长下胚轴 1 (FHY1) 和 FHY1-LIKE (FHL)。FHY1/FHL 与 phyA 下游信号成分结合。因此，有人提出 FHY1/FHL 也具有在细胞核中组装 phyA 转录因子复合物的功能。然而，在这项研究中，我们表明在没有 FHY1 和 FHL 的情况下，组成性核定位的 phyA 是活跃的。此外，人工 FHY1 由 SV40 NLS、phyA 结合位点和作为它们之间的间隔物的 YFP 标签组成，补充了fhy1-3 fhl-1双突变体。这些发现表明 FHY1 和 FHL 不是细胞核中 phyA 下游信号传导所必需的。然而，我们发现表达 phyA-NLS-YFP 的细胞系对红光和远红光过敏，并且组成性核定位 phyA 的水平略有增加，导致在黑暗中光形态发育。因此，通过与 FHY1/FHL 的相互作用将 phyA 限制在细胞质中并在光下诱导核转运对于抑制黑暗中的光形态发生可能很重要。