Characterization of mutations leading to constitutively active quenching in Arabidopsis thaliana

Alizée Malnoëa,b,d, Cynthia Amstutza,b, Rikard Fridstedte, Emily J. Sylak-Glassmanc,d, Alex Schultinkb, Roberta Crocee, Sabeeha Merchantf, Graham Flemingc,d, Krishna K. Niyogia,b,d
aHoward Hughes Medical Institute; bDepartment of Plant and Microbial Biology, University of California, Berkeley, CA 94720-3102, USA; cDepartment of Chemistry, University of California, Berkeley, CA 94720; dMolecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA; eDepartment of Physics and Astronomy, Vrije University of Amsterdam, The Netherlands; fDepartment of Chemistry and Biochemistry, UCLA, Los Angeles, CA 90095, USA

Photosynthetic organisms require light for photosynthesis, yet the amount of light absorbed often exceeds what can be utilized and can lead to photo-oxidative damage. Non-photochemical quenching (NPQ) mechanisms safely dissipate excess absorbed light energy as heat. The SOQ1 protein maintains the efficiency of light harvesting by preventing a slowly reversible antenna NPQ mechanism that is independent of the PsbS protein, the pH gradient, zeaxanthin formation and the STN7 kinase [1]. To identify molecular players in the SOQ1-related quenching pathway, we isolated and characterized otk1 mutations that, in combination with the soq1 mutation, exhibit severely quenched maximum and initial fluorescence in the dark suggesting that quenching processes are constitutively turned on. Average chlorophyll fluorescence lifetime in dark-acclimated leaves is substantially decreased (0.05 ns) compared to wild type (1.35 ns). soq1 otk1 also displays higher stacked (two-fold) and enlarged (three-fold) thylakoid grana and higher level of light-harvesting antenna pigments. This altered ultrastructure and pigment content might compensate for the decrease in light absorption under such severe quenching, which nevertheless permits photoautotrophic growth, although at slower rate than wild type. The OTK1 gene encodes a previously uncharacterized atypical short chain dehydrogenase/reductase belonging to the NAD(P)-binding Rossmann-fold protein superfamily. We found that this protein is chloroplast-localized in the stroma and conditionally co-purifies with the thylakoid membrane fraction. Experiments are underway to identify the site and mechanism of this quenching and to further elucidate the function of OTK1 and its relationship to SOQ1.

Acknowledgement

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division.

Reference

[1] Brooks, M.D. et al. PNAS 2013, 110, 2733–2740.