Arabidopsis Thylakoid Formation 1 Is a Critical Regulator for Dynamics of PSII-LHCII Complexes in Leaf Senescence and Excess Light

It is well known that disassembly of PSIILHCII complexes is required for chlorophyll degradation and D1 repair. We discovered that a pH-dependent Lhcb-interacting protein THF1 plays a dual role in regulation of PSIILHCII complexes in excess light and leaf senescence.In higher plants, photosystem II (PSII) is a large pigmentprotein supramolecular complex composed of the PSII core complex and the plant-specific peripheral light-harvesting complexes (LHCII). PSIILHCII complexes are highly dynamic in their quantity and macro-organization to various environmental conditions. In this study, we reported a critical factor, the Arabidopsis Thylakoid Formation 1 (THF1) protein, which controls PSIILHCII dynamics during dark-induced senescence and light acclimation. Loss-of-function mutations in THF1 lead to a stay-green phenotype in pathogen-infected and senescent leaves. Both LHCII and PSII core subunits are retained in dark-induced senescent leaves of thf1, indicative of the presence of PSIILHCII complexes. Blue native (BN)-polyacrylamide gel electrophoresis (PAGE) and immunoblot analysis showed that, in dark- and high-light-treated thf1 leaves, a type of PSIILHCII megacomplex is selectively retained while the stability of PSIILHCII supercomplexes significantly decreased, suggesting a dual role of THF1 in dynamics of PSIILHCII complexes. We showed further that THF1 interacts with Lhcb proteins in a pH-dependent manner and that the stay-green phenotype of thf1 relies on the presence of LHCII complexes. Taken together, the data suggest that THF1 is required for dynamics of PSIILHCII supramolecular organization in higher plants.