Fujimura-Kamada, K. | bioRxiv Science

bioRxiv2022

Phosphorylation of light-harvesting complex II controls excitation energy spillover between photosystems

Land plants and microalgae convert solar energy into electrochemical energy by using cooperative two photosystems (PSI and PSII). To maintain optimal photosynthetic rates under variable light conditions in nature, phosphorylation of light-harvesting complex for PSII (LHCII) balances the excitation energy distribution between the two photosystems. Here, we investigated the mechanism of this balancing in a green alga. We show that phospho-LHCIIs physically bind to both photosystems. The energy transfer from the LHCIIs to the PSII core complexes becomes less efficient, whereas the excitation level of PSI increases. The time-resolved fluorescence spectra showed an increase in delayed PSI fluorescence, which represents energetical spillover from PSII to PSI. In addition, the spillover is likely mediated by phospho-LHCIIs and PSI antennas. We hypothesize that the spillover explains the larger extent of phospho-LHCIIs dependent energy balancing in the green alga than land plants, which is important for the short-term photoadaptaion in the algal habitat.

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↗