Topology of the plastid Ndh complex and its NDH-F subunit in thylakoid membranes

On the Edge of Dispensability, the Chloroplast ndh Genes

The polypeptides encoded by the chloroplast ndh genes and some nuclear genes form the thylakoid NADH dehydrogenase (Ndh) complex, homologous to the mitochondrial complex I. Except for Charophyceae (algae related to higher plants) and a few Prasinophyceae, all eukaryotic algae lack ndh genes. Among vascular plants, the ndh genes are absent in epiphytic and in some species scattered among different genera, families, and orders. The recent identification of many plants lacking plastid ndh genes allows comparison on phylogenetic trees and functional investigations of the ndh genes. The ndh genes protect Angiosperms under various terrestrial stresses, maintaining efficient photosynthesis. On the edge of dispensability, ndh genes provide a test for the natural selection of photosynthesis-related genes in evolution. Variable evolutionary environments place Angiosperms without ndh genes at risk of extinction and, probably, most extant ones may have lost ndh genes recently. Therefore, they are evolutionary endpoints in phylogenetic trees. The low number of sequenced plastid DNA and the long lifespan of some Gymnosperms lacking ndh genes challenge models about the role of ndh genes protecting against stress and promoting leaf senescence. Additional DNA sequencing in Gymnosperms and investigations into the molecular mechanisms of their response to stress will provide a unified model of the evolutionary and functional consequences of the lack of ndh genes.

The rise of the photosynthetic rate when light intensity increases is delayed in ndh gene-defective tobacco at high but not at low CO2 concentrations

The 11 plastid ndh genes have hovered frequently on the edge of dispensability, being absent in the plastid DNA of many algae and certain higher plants. We have compared the photosynthetic activity of tobacco (Nicotiana tabacum, cv. Petit Havana) with five transgenic lines (ΔndhF, pr-ΔndhF, T181D, T181A, and ndhF FC) and found that photosynthetic performance is impaired in transgenic ndhF-defective tobacco plants at rapidly fluctuating light intensities and higher than ambient CO2 concentrations. In contrast to wild type and ndhF FC, which reach the maximum photosynthetic rate in less than 1 min when light intensity suddenly increases, ndh defective plants (ΔndhF and T181A) show up to a 5 min delay in reaching the maximum photosynthetic rate at CO2 concentrations higher than the ambient 360 ppm. Net photosynthesis was determined at different CO2 concentrations when sequences of 130, 870, 61, 870, and 130 μmol m(-2) s(-1) PAR sudden light changes were applied to leaves and photosynthetic efficiency and entropy production (Sg) were determined as indicators of photosynthesis performance. The two ndh-defective plants, ΔndhF and T181A, had lower photosynthetic efficiency and higher Sg than wt, ndhF FC and T181D tobacco plants, containing full functional ndh genes, at CO2 concentrations above 400 ppm. We propose that the Ndh complex improves cyclic electron transport by adjusting the redox level of transporters during the low light intensity stage. In ndhF-defective strains, the supply of electrons through the Ndh complex fails, transporters remain over-oxidized (specially at high CO2 concentrations) and the rate of cyclic electron transport is low, impairing the ATP level required to rapidly reach high CO2 fixation rates in the following high light phase. Hence, ndh genes could be dispensable at low but not at high atmospheric concentrations of CO2.

Plastid ndh genes in plant evolution

The plastid ndh genes encode components of the thylakoid Ndh complex which purportedly acts as an electron feeding valve to adjust the redox level of the cyclic photosynthetic electron transporters. During the process of evolution from endosymbiosis to modern chloroplast, most cyanobacterial genes were lost or transferred to nucleus. Eleven ndh genes are among the 150-200 genes remaining in higher plant chloroplast DNA, out of some 3000 genes in the original prokaryotic Cyanobacteria in which homologues to ndh genes encode components of the respiratory Complex I and probably other complexes. The ndh genes are absent in all sequenced plastid DNAs of algae except for the Charophyceae and some Prasinophyceae. With the possible exclusion of some Conifers and Gnetales, the plastid DNA of all photosynthetic land plants contains the ndh genes, whereas they are absent in epiphytic plants that have also lost genes for the photosynthetic machinery. Therefore, the functional role of the ndh genes seems closely related to the land adaptation of photosynthesis. Transcripts of several plastid genes require C to U editing. The ndh genes concentrate about 50% of the editing sites of angiosperm plastid transcripts. Editing sites may be remnants from an ancestor in which a number of T to C inactivating mutations took place in the ndh genes which, during evolution, are being corrected back to T. The comparison of homologous editing sites in the mRNAs of angiosperm ndh genes provides a tool to investigate selective and permissive environmental conditions of past evolutionary events.

Functional characterization of the thylakoid Ndh complex phosphorylation by site-directed mutations in the ndhF gene

To investigate the phosphorylation of the NDH-F subunit of the thylakoid Ndh complex, we constructed three site-directed mutant transgenic tobaccos (Nicotiana tabacum) (T181A, T181S and T181D) in which the (541)ACT(543) triplet encoding the Thr-181 has been substituted by GCT, TCT or GAT encoding alanine, serine and aspartic acid, respectively. Western blots with phospho-threonine antibody detected the 73 kD NDH-F phosphorylated polypeptide in control but not in mutant tobaccos. Differences in Ndh activity, chlorophyll fluorescence and photosynthesis among mutants and control plant demonstrate the key role of the phosphorylation of conserved Thr-181 in the activity and function of the Ndh complex. The substitution of aspartic acid for threonine in T181D mimics the presumable activation effects of the threonine phosphorylation in Ndh activity, post-illumination increase of chlorophyll fluorescence and photosynthesis rapid responses to changing light intensities. A tentative role of the phosphorylation-activated Ndh complex is suggested to poise the redox level and, consequently, optimizing the rate of cyclic electron transport under field conditions.

Inactivation of a plastid evolutionary conserved gene affects PSII electron transport, life span and fitness of tobacco plants

Chloroplasts contain a plastoquinone-NADH-oxidoreductase (Ndh) complex involved in protection against stress and the maintenance of cyclic electron flow. Inactivation of the Ndh complex delays the development of leaf senescence symptoms. Chlorophyll a fluorescence measurements, blue native gel electrophoresis, immunodetection and other techniques were employed to study tobacco (Nicotiana tabacum) Ndh-defective mutants (DeltandhF). The DeltandhF mutants compared with wild-type plants presented: (i) higher photosystem II : photosystem I (PSII : PSI) ratios; (ii) similar or higher levels of ascorbate, carotenoids, thylakoid peroxidase and superoxide dismutase, yield (Phi(PSII)) and maximal photochemical efficiency of PSII levels (F(v)/F(m)) than wild-type plant leaves of the same age; (iii) lower values of nonphotochemical quenching yield (Phi(NPQ)), but not at very high light intensities or during induced leaf senescence; (iv) a similar decrease of antioxidants during senescence; (v) no significant differences in the total foliar area and apical growth rate; and (vi) a production of viable seeds significantly higher than wild-type plants. These results suggest that the Ndh complex is involved in one of the redundant mechanisms that play a safety role in photosynthesis under stress, which has been conserved during evolution, but that its deletion increases fitness when plants are grown under favourable controlled conditions.

Active NDH-1 complexes from the cyanobacterium Synechocystis sp. strain PCC 6803

We identified eight bands by staining native gels for NADPH-nitroblue tetrazolium oxidoreductase activity after electrophoresis of n-dodecyl-beta-d-maltoside-treated membranes of Synechocystis sp. strain PCC 6803. Among them, bands A, C, D and E were attributed to the activity of NADPH dehydrogenase (NDH-1). Band A is a highly active supercomplex of NDH-1 (about 1,000 kDa) that was absent in the DeltandhD1/D2 mutant and was suppressed under low CO(2). Band C was induced under low CO(2) or in the DeltandhD1/D2 mutant and was converted to bands D and E. Bands A and C appear to be an NDH-1L dimer and NDH-1M, respectively, with subunits essential for the activity.

Multiple phosphorylation sites in the beta subunit of thylakoid ATP synthase

Proteomic analyses of the beta subunit of the plastid ATP synthase of barley (Hordeum vulgare L.) revealed that mature protein was not carboxy terminus processed and suggested the correction of the 274 codon (GAT to AAT) in the data bank that was confirmed by DNA sequencing. Six isoforms of the ATP synthase beta subunit with pI ranging from 4.95 to 5.14 were resolved by two-dimensional electrophoresis (2-DE). Mass spectrometry analyses indicated that the six isoforms differ in their phosphorylation degree, which was confirmed by the disappearance of more acidic forms after incubation with the protein phosphatase calcineurin. Six Ser and/or Thr were detected as phosphorylated, among them the conserved Thr-179 that is also phosphorylated in the beta subunit of human mitochondria. The results are discussed in relation with the proposed regulation of the ATP synthase by phosphorylation and 14-3-3 proteins.

Chloroplasts regulate leaf senescence: delayed senescence in transgenic ndhF-defective tobacco

Mitochondrial involvement has not been identified in the programmed cell death (PCD) of leaf senescence which suggests that processes such as those involving reactive oxygen species (ROS) are controlled by chloroplasts. We report that transgenic tobacco (DeltandhF), with the plastid ndhF gene knocked-out, shows low levels of the plastid Ndh complex, homologous to mitochondrial complex I, and more than a 30-day-delay in leaf senescence with respect to wt. The comparison of activities and protein levels and analyses of genetic and phenotypic traits of wtxDeltandhF crosses indicate that regulatory roles of mitochondria in animal PCD are assumed by chloroplasts in leaf senescence. The Ndh complex would increase the reduction level of electron transporters and the generation of ROS. Chloroplastic control of leaf senescence provides a nonclassical model of PCD and reveals an unexpected role of the plastid ndh genes that are present in most higher plants.