Phototropine

Phototropine, an alternate spelling of phototropin, refers to a family of blue-light photoreceptors that mediate several light-guided responses in green tissues. In many plants, including Arabidopsis, two phototropins, PHOT1 and PHOT2, contribute to blue-light perception with overlapping but distinct roles. Phototropins are soluble cytosolic proteins associated with the plasma membrane and feature two N-terminal LOV (light, oxygen, voltage) sensing domains and a C-terminal serine/threonine kinase domain. Each LOV domain binds flavin mononucleotide (FMN) as a chromophore.

Upon blue light, a covalent adduct forms between FMN and a conserved cysteine in the LOV2 domain, triggering a conformational change that relieves autoinhibition of the kinase domain. The activated kinase autophosphorylates and initiates signaling cascades that modulate auxin transport by regulating PIN-FORMED transporters, leading to phototropic bending toward the light. Phototropins also control chloroplast movement, enabling chloroplasts to accumulate under low light and to avoid damage under high light, as well as stomatal opening and adjustments in leaf development under favorable illumination.

Genetic analyses show that phototropin activity is essential for typical blue-light responses; PHOT1 primarily drives rapid early phototropism, while PHOT2 contributes strongly under high-intensity blue light and in chloroplast avoidance movements. The signaling network involves NPH3-like/RPT2 family proteins and other downstream factors linking phototropin activity to cytoskeletal dynamics and hormone transport.

Phototropins are widely distributed among land plants and some algae, representing a key component of the plant light-signaling repertoire and enabling optimization of photosynthesis and growth in fluctuating light environments.