Functional analysis identifies the novel kinase activity of rice OsTLP16.5 in the thylakoid lumen

• Rice OsTLP16.5 is a novel kinase. • OsTLP16.5 is the final link in the phosphorylation regulatory pathway within the thylakoid lumen. • OsTLP16.5 is able to phosphorylate OsPsbP1 in vitro and physically interact with it in vivo . Protein phosphorylation is considered to facilitate the PSII repair cycle. The kinases and phosphatases responsible for the modification of PSII core and LHCII subunits are well understood, however, knowledge is limited about the role of phosphorylation in the thylakoid lumen. The acid phosphatase AtTLP18.3 has been characterized for over a decade, but the kinase(s) still remains unidentified. Indeed, the processes of disassembly and reassembly of the OEC represent a largely unexplored area within the PSII repair cycle. Our findings establish OsTLP16.5, the rice ortholog of MPH2, as the first functionally validated lumenal kinase. OsTLP16.5 is able to phosphorylate OsPsbP1 in vitro and physically interact with it in vivo . Phosphorylation of OsPsbP1 appears to directly influence the dissociation of the OEC from the PSII core monomer, thereby providing an explanation for why loss of MPH2 interferes with the proper disassembly of PSII monomers throughout the PSII repair. Our results support a model in which phosphorylation of lumenal OsPsbP1 by OsTLP16.5 facilitates PSII repair, thereby maintaining photosynthetic efficiency and agronomic performance in rice.