High light-induced phosphorylation of Lhcb6 (CP24) enhances non-photochemical quenching dependent on the PSI-PSII megacomplexes

Plants have evolved diverse photoprotective mechanisms to cope with fluctuating light environments, among which non-photochemical quenching (NPQ) plays a central role in dissipating excess excitation energy and preventing photosystem II (PSII) damage. While the molecular basis of NPQ is well established in model species such as Arabidopsis thaliana, its regulation in non-model plants remains less understood, offering untapped potential for discovering novel photoprotective strategies. Here, we report a previously unrecognized mechanism in angiosperms Berteroa incana: high-light (HL)-induced phosphorylation of the minor antenna protein CP24 (Lhcb6), specifically at Thr178, serves as a critical regulator of NPQ, a close relative of Arabidopsis thaliana with exceptional phototolerance. Unlike LHCII phosphorylation mediated by STN7 kinases, CP24 phosphorylation is governed by a distinct kinase system and is selectively dephosphorylated by the phosphatase rPBCP. Using biochemical and functional analyses, we demonstrate that phosphorylated CP24 (P-CP24) predominantly resides within the PSII-PSI megacomplex (mc1), where it facilitates efficient energy transfer from CP47 to PSI. This mechanism drives a previously unreported Zeaxanthin- and Lutein-independent fluorescence quenching pathway under HL conditions in terrestrial plants. Our results suggest that P-CP24 enhances PSII photoprotection by dynamically redirecting excess excitation energy away from PSII reaction centers, thereby reducing reactive oxygen species (ROS) production, maintaining PSII activity, and optimizing energy utilization under stress.