Abstract
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 CO(2) assimilation rates, photosystem II (PSII) quantum efficiencies (PhiPSII), 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.
Highlights
A transient rise in chlorophyll fluorescence after turning off actinic light reflects nonphotochemical reduction of the plastoquinone (PQ) pool
Chlororespiration in higher plants is estimated to be very low in comparison with photosynthetic activity, it could function as an additional protective mechanism and it might be involved in reoxidation of stromal reductants for various metabolic pathways in the chloroplast
A BLAST search revealed that homologs of the PIFI gene are only found in higher plants and not in algae, such as Chlamydomonas reinhardtii, which does not have the chloroplast NAD(P)H dehydrogenase (NDH) complex (Maul et al, 2002) or cyanobacteria from which the chloroplast NDH complex is thought to originate
Summary
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. We characterized an Arabidopsis T-DNA insertion mutant, pifi (for postillumination chlorophyll fluorescence increase), that apparently lacks NDHdependent nonphotochemical reduction of the PQ pool, but possesses an intact NDH complex. By comparing the Chl fluorescence parameters of the pifi mutant with wild type and their photosynthetic performance under stressful conditions, we conclude that PIFI is a novel component of the chlororespiratory electron transport pathways and can help to protect plants from stressful environmental conditions (such as high light and mild heat stress)
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