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… Click to show full abstract
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.
               
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