Grana-Localized Proteins, RIQ1 and RIQ2, Affect the Organization of Light-Harvesting Complex II and Grana Stacking in Arabidopsis.

Grana are stacked thylakoid membrane structures in land plants that contain PSII and light-harvesting complex II proteins (LHCIIs). We isolated two mutants, () and , in which stacking of grana was enhanced. The () mutant was previously shown to lack grana structure. In , the grana were enlarged with more stacking, and in , the thylakoids were abnormally stacked and aggregated. Despite having different phenotypes in thylakoid structure, , , and showed a similar defect in the level of nonphotochemical quenching of chlorophyll fluorescence (NPQ). In double mutants, NPQ induction was more severely affected than in either single mutant. In mutants, state transitions were inhibited and the PSII antennae were smaller than in wild-type plants. The defects did not affect NPQ induction in the chlorophyll -less mutant. and are paralogous and encode uncharacterized grana thylakoid proteins, but despite the high level of identity of the sequence, the functions of RIQ1 and RIQ2 were not redundant. RIQ1 is required for RIQ2 accumulation, and the wild-type level of RIQ2 did not complement the NPQ and thylakoid phenotypes in We propose that RIQ proteins link the grana structure and organization of LHCIIs.