Involvement of TSP9 Phosphoprotein in Balancing the Photosynthetic Light Harvesting Process in Arabidopsis thaliana

TSP9 is the thylakoid soluble phosphoprotein of 9 kDa, an intrinsically unstructured plant specific protein that has earlier been found interacting with light harvesting proteins and both photosystems in photosynthetic thylakoid membranes from spinach. We investigated the function of this protein in Arabidopsis thaliana using reverse genetics, biochemical and fluorometric methods.\ Recombinant TSP9 was used as a substrate for light-induced phosphorylation by thylakoid membranes isolated from wild type and two mutant lines lacking either STN7 or STN8 protein kinases. TSP9 was found phosphorylated by STN7 kinase. Phosphorylation of light-harvesting complex II was found reduced in knockout plants incapable of expressing TSP9. Chlorophyll fluorescence studies showed reduced ability of the TSP9 lacking mutant to perform the photosynthetic state transitions, as well as decreased non-photochemical quenching as compared with the wild type plants. We propose a\ role for this small soluble protein in the regulation of light harvesting in Arabidopsis.