Novel Nucleus-Encoded Chloroplast Protein, PIFI, Is Involved in NAD(P)H Dehydrogenase Complex-Mediated Chlororespiratory Electron Transport in Arabidopsis

A transient rise in chlorophyll fluorescence after turning off actinic light reflects nonphotochemical reduction of the plastoquinone (PQ) pool. This process is dependent on the activity of the chloroplast NAD(P)H dehydrogenase (NDH) complex, which mediates electron flow from stromal reductants to the PQ pool. In this study, we characterized an Arabidopsis (Arabidopsis thaliana) T-DNA insertion mutant pifi (for postillumination chlorophyll fluorescence increase), which possesses an intact NDH complex, but lacks the NDH-dependent chlorophyll fluorescence increase after turning off actinic light. The nuclear gene PIFI (At3g15840) containing the T-DNA insertion encodes a chloroplast-targeted protein localized in the stroma and is annotated as a protein of unknown function. The pifi mutant exhibited a lower capacity for nonphotochemical quenching, but similar CO₂ assimilation rates, photosystem II (PSII) quantum efficiencies (ΦPSII), and reduction levels of the primary electron acceptor of PSII (1 - qL) as compared with the wild type. The pifi mutant grows normally under optimal conditions, but exhibits greater sensitivity to photoinhibition and long-term mild heat stress than wild-type plants, which is consistent with lower capacity of nonphotochemical quenching. We conclude that PIFI is a novel component essential for NDH-mediated nonphotochemical reduction of the PQ pool in chlororespiratory electron transport.