Biology subjects

Okajima, K.

Publications and source records attributed to Okajima, K..

3 recordsLinked to original sources

bioRxiv2024

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.

plant biology↗

bioRxiv2020

UV-A radiation rapidly activates photoprotective mechanisms in Chlamydomonas reinhardtii

Conversion of light energy into chemical energy through photosynthesis in the chloroplasts of photosynthetic organisms is essential for photoautotrophic growth. However, the conversion of excess light energy into thermal energy by non-photochemical quenching (NPQ) is important for avoiding the generation of reactive oxygen species and maintaining efficient photosynthesis. In the unicellular green alga Chlamydomonas reinhardtii, NPQ is activated as a photoprotective mechanism through wavelength-specific light signaling pathways mediated by the phototropin (blue light) and UVR8 (ultra-violet light, UV) photoreceptors. NPQ-dependent photoprotection improves cell survival under high-light conditions; however, the biological significance of photoprotection being activated by light with different qualities remains poorly understood. Here, we demonstrate that NPQ-dependent photoprotection is activated more rapidly by UV than by visible light. We found that induction of gene expression and protein accumulation related to photoprotection was significantly faster and greater in magnitude under UV treatment compared to that under blue- or red-light treatment. Furthermore, the action spectrum of UV-dependent induction of photoprotective factors implied that Chlamydomonas sense relatively long-wavelength UV (including UV-A/B), whereas the model dicot plant Arabidopsis thaliana preferentially senses relatively short-wavelength UV (mainly UV-B/C) for induction of photoprotective responses. Therefore, we hypothesize that Chlamydomonas developed a UV response distinct from that of land plants. One-sentence summaryIn contrast to land plants, which sense short-wave UV light, the unicellular green alga Chlamydomonas senses long-wavelength UV light for photoprotective responses.

plant biology↗

bioRxiv2019

Arabidopsis ROOT PHOTOTROPISM2 is a Light-Dependent Dynamic Modulator of Phototropin1

Arabidopsis thaliana phototropin1 (phot1) is a blue-light photoreceptor, i.e. a blue-light-activated Ser/Thr-protein kinase that mediates various light responses including phototropism. Phot1 functions in hypocotyl phototropism dependent on the light induction of ROOT PHOTOTROPISM2 (RPT2) proteins within a broad range of blue light intensities. It is not yet known however how RPT2 contributes to the photosensory adaptation of phot1 to high intensity blue light and the second positive phototropism. We here show that RPT2 suppresses the activity of phot1. Yeast two-hybrid analysis indicated RPT2 binding to the LOV1 (light, oxygen or voltage sensing 1) domain of phot1 required for its high photosensitivity. Our biochemical analyses revealed that RPT2 inhibits the autophosphorylation of phot1, suggesting that it suppresses the photosensitivity and/or kinase activity of phot1 through the inhibition of LOV1 function. We found for the first time that RPT2 proteins are degraded via a ubiquitin-proteasome pathway when phot1 is inactive and stabilized under blue-light conditions in a phot1-dependent manner. We propose that RPT2 is a molecular rheostat that maintains a moderate activation level of phot1 under any light intensity conditions.

plant biology↗