Hypsochromic shift of phyC complements the inhibition of hypocotyl elongation under moderate red/far-red light

Phytochrome (phy) is a plant photoreceptor that regulates various photomorphogenesis, and occurs in two forms, a red light (R)-absorbing form (Pr) and a far-red light (FR)-absorbing form (Pfr). Absorption spectral analyses of the photosensory module (PSM) showed that phyC in the Pr of Arabidopsis thaliana, Solanum lycopersicum and Zea mays exhibited the absorption maxima shift toward shorter wavelengths (hypsochromic shift) compared with those of phyA and phyB. Substitution of the chromophore-binding domain complemented the hypsochromic shift in the spectra of phyC in the Pr. The effect of the hypsochromic shift on the inhibition of hypocotyl elongation was studied under R/FR ratio from 0.5-10. PhyB was revealed to play a major role in inhibition, and phyC showed a complementary role under R/FR <2.0. This may result from the activation peak of the phyC PSM, which was hypsochromically shifted compared with that of the phyB PSM from the Pr to Pfr. The leaf-filtered light measurement suggested that phyC enables plants to receive more R and contributes to survival in the field. Under low R/FR conditions, the activation efficiency of phyC was greater than that of phyB, suggesting that the hypsochromic shift of phyC is necessary for the robust growth of angiosperms. HighlightAngiosperm phyC with hypsochromically shifted activation inhibit hypocotyl elongation under relatively low red/far-red light conditions, in which phyB is not fully functional.