NPH3

NPH3, or Non-Phototropic Hypocotyl 3, is a plant signaling protein first identified in Arabidopsis thaliana as part of the blue-light phototropism pathway. The protein is part of the NPH3/RPT2 family and is thought to act downstream of the blue-light receptors phototropins (PHOT1 and PHOT2). NPH3 helps translate perception of blue light into growth toward the light by regulating the asymmetric distribution of the plant hormone auxin, which drives differential cell elongation.

In blue light, phototropins activate signaling that leads to phosphorylation and/or relocalization of NPH3, coordinating changes in auxin transport. Genetic studies show that nph3 mutants display reduced or altered phototropic bending of hypocotyls, especially under low-intensity blue light, indicating NPH3’s role in early signaling events that favor phototropism. The protein is primarily cytosolic but associates with the plasma membrane and may function at sites where auxin transport components, such as PIN-FORMED (PIN) carriers, are regulated.

NPH3 is part of a small gene family with homologs in many plant species, including RPT2. Functional redundancy and diversification within this family contribute to phototropic responses in various tissues and developmental stages. Overall, NPH3 is considered a key component connecting blue-light perception to directed growth through regulation of auxin transport.