Hypsochromic shift in phytochrome C Pr absorption complements phytochrome B-mediated inhibition of hypocotyl elongation.

Phytochromes are red/far-red light photoreceptors that regulate diverse aspects of plant photomorphogenesis. Among angiosperm phytochromes, phytochrome C (phyC) is the least well characterized, despite its conservation across monocots and dicots. Previous studies have suggested that phyC exhibits absorption properties distinct from those of phyA and phyB, but the physiological significance of these differences has remained unclear. Here, we show that the photosensory module of phyC exhibits a hypsochromically shifted Pr absorption maximum that is conserved across monocots and dicots. This spectral property is primarily determined by the GAF domain rather than by individual amino acid substitutions. Physiological analyses revealed that phyC contributes to the inhibition of hypocotyl elongation under red light conditions lacking shorter wavelength components, whereas its contribution is limited under white light conditions. These findings indicate that the hypsochromic shift in phyC provides a complementary mechanism for phytochrome signaling under specific light environments. Spectral overlap analysis further indicated that the hypsochromically shifted Pr absorption of phyC enhances photon capture under both direct sunlight and leaf-filtered light, although the magnitude of this effect is modest. Together, these findings indicate that the conserved hypsochromic shift in Pr absorption of phyC provides a complementary, context-dependent mechanism for phytochrome signaling, particularly under light environments in which phyB-mediated responses are less effective.