Photosensory adaptation mechanisms in hypocotyl phototropism: how plants recognize the direction of a light source.

Plants recognize the light source direction and exhibit phototropic responses. Physiological studies have predicted that a difference in the light intensity received by the cells on the irradiated and shaded sides of a coleoptile/hypocotyl causes differences in the amounts of photoproduct. This hypothetical photoproduct appears to regulate a signaling pathway that controls cell elongation, in which cells with lower light intensity elongate by more than those with higher light intensity. This results in a bending growth toward a light source and has been proposed as the photoproduct-gradient model of phototropism. In this review, we summarize recent findings on the photosensory adaptation mechanisms involving a blue-light photoreceptor phototropin1 (phot1), ROOT PHOTOTROPISM2 (RPT2), NONPHOTOTROPIC HYPOCOTYL3 (NPH3), and another photoreceptor family, the phytochromes (phys). The current evidence demonstrates that, in addition to the transition of the phot1-NPH3 photoreceptor complexes to their active state, the presence of a certain population of the phot1-NPH3 complexes showing a steady state, even in a light environment, is essential to recognition of the light source direction in phototropism. This is consistent with the idea of the photoproduct-gradient model, and a dissociation state of the phot1-NPH3 complex would be considered an entity of the hypothetical photoproduct in this model.