PHYTOCHROME A is a regulatory photoreceptor responsible for response to red/far-red light. The protein has two forms: Pr (absorbs red light maximally) and Pfr (absorbs far-red light maximally). Photoconversion from Pr to Pfr induces morphological responses as Pfr controls the expression of gene such as: small subunit of ribulose-bisphosphate carboxylase, chlorophyll A/B binding protein, and protochlorophyllide reductase.

The protein is important in the de-etiolation of seedlings during emergence, preventing excessive elongation growth, sensing degree of shade, and perception of daylength. It has also been implicated in flowering time regulation and gene regulation in Arabidopsis thaliana.