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

This article focuses on the molecular and physiological mechanisms underlying phototropism in plants, specifically how they recognize the direction of light through photosensory adaptation. It reviews evidence supporting the photoproduct-gradient model, which proposes that differences in a hypothetical photoproduct—now considered to be the dissociated state of the phototropin1 (phot1)–NONPHOTOTROPIC HYPOCOTYL3 (NPH3) complex—between the irradiated and shaded sides regulate differential cell elongation and bending toward light. Key components involved in photosensory adaptation include phot1, ROOT PHOTOTROPISM2 (RPT2), NPH3, and phytochromes, which modulate phot1 activity and NPH3 phosphorylation to maintain sensitivity across varying light intensities. The review also discusses how red-light pretreatment via phytochrome signaling enhances phototropic responses by regulating RPT2 expression and auxin transport, while noting that the role of phototropin2 (phot2) in phototropism remains less understood and requires further study.