PHYTOCHROMEINTERACTING

PhytochromeInteracting is a descriptor used in plant biology to refer to proteins and factors that physically bind phytochrome photoreceptors and modulate red/far-red light signaling. Phytochromes (phyA–phyE in many plants) switch between inactive Pr and active Pfr forms in response to light, and their interaction partners translate these light cues into changes in gene expression and development.

Interactions with phytochromes can influence receptor stability, subcellular localization, and the activity of downstream signaling networks. Some partners promote degradation of target proteins through the COP1-SPA ubiquitin ligase pathway under certain light conditions, while others stabilize transcription factors that drive light-responsive growth programs. Overall, phytochrome-interacting proteins help regulate processes such as seed germination, seedling de-etiolation, shade avoidance, and flowering by shaping the transcriptional response to light.

Canonical examples of phytochrome-interacting proteins include the Phytochrome Interacting Factors (PIFs), a family of basic helix-loop-helix transcription factors, and FHY1/FHY3, which mediate phyA signaling in far-red light. Additional interactions involve components of the COP1/SPA complex and various transcriptional co-regulators, illustrating the diversity of mechanisms by which phytochromes influence gene expression.

Research on phytochrome-interacting proteins employs genetic, biochemical, and imaging approaches to map interactions, determine light-dependent binding, and delineate signaling networks across plant species. The term emphasizes an interaction-centric view of phytochrome signaling, complementing studies focused on the receptor itself.