Any of a class of photosensitive flavoproteins that are sensitive to blue light (wavelength 400–500 nm) and trigger reorientating growth of a plant towards a light source (i.e. phototropism). The bound flavin mononucleotide acts as a chromophore, absorbing light and causing photoexcitation of a light-sensitive domain of the protein. This initiates autophosphorylation of the protein and triggers a signal cascade resulting in cell elongation. Phototropins also cause light-induced opening of stomata and movement of chloroplasts inside leaves. Light activates phototropins by inducing autophosphorylation.