Phya

PhyA, or phytochrome A, is a red/far-red light photoreceptor in plants and a member of the phytochrome family, which in Arabidopsis includes PHYA through PHYE. It is encoded by the PHYA gene and operates as part of a family of bilin-based sensors that regulate plant development in response to light. Phytochromes contain a photosensory module bound to a linear tetrapyrrole chromophore (phytochromobilin) and exist in two interconvertible forms: Pr, which absorbs red light, and the biologically active Pfr, which absorbs far-red light. In darkness, phyA accumulates mainly as Pr; upon illumination, it is converted to Pfr and initiates signaling before being progressively degraded, making phyA notably light-labile.

In seedlings, phyA is the primary mediator of very low fluence and continuous far-red light responses, promoting photomorphogenesis, chloroplast development, and the expression of light-responsive genes. It also contributes to seed germination and various shade-related responses, particularly under canopy shade where the light is rich in far-red wavelengths. phyA signaling generally leads to growth inhibition of etiolation and activation of gene networks that enable plants to adapt to light environments that are poor in red light.

Signal transduction involves the Pfr form translocating to the nucleus, where it interacts with transcription factors such as PIFs and with regulators like HY5, influencing gene expression. phyA can form nuclear photobodies and rapidly alters signaling dynamics. In light, phyA is rapidly degraded via the ubiquitin-proteasome pathway, a mechanism that helps terminate the signal and enables plants to respond to changing light conditions.

PhyA is a widely used model in plant light signaling research and a key component of photomorphogenic development across higher plants.