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Abstract
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.
Highlights
Some 30 years after their discovery (Ohyama et al, 1986; Shinozaki et al, 1986), the functional role of the 11 plastidencoded ndh genes is still a mystery
TOBACCO LINES WITH PARTIAL RECOVERY OF ndhF GENE COPIES In addition to previously described wt, ndhF FC, ndhF, T181A, and T181D tobacco plants (Martín et al, 2009), we assayed partially reverted phenotypes of ndhF that we have found among descendants of the ndh-deficient ndhF tobacco transgenic, as identified by the increase of the 515 bp PCRamplified band (Figure 1A, lane pr- ndhF) characteristic (Martín et al, 2004; Zapata et al, 2005) of the non-transformed plastid DNA of wt (Figure 1A, lane wt)
We are not yet able to control its emergence or its inheritance, the finding of pr- ndhF phenotype provides an additional retromutant control that confirms the involvement of ndh genes in photosynthesis and other processes
Summary
Some 30 years after their discovery (Ohyama et al, 1986; Shinozaki et al, 1986), the functional role of the 11 plastidencoded ndh genes (which are homologous to genes encoding components of Complex I of the mitochondrial respiratory chain) is still a mystery. Most photosynthetic land plants contain the ndh genes, which are absent in parasitic non-photosynthetic species of the genera Cuscuta, Epiphagus, Orobanche and the Orchidaceae family. This suggests that the thylakoid Ndh complex, encoded by the 11 plastid ndh genes and an as yet unknown number of nuclear genes, has a role in land plant photosynthesis. The 11 ndh genes account for about 50% of all C to U editing sites identified in the transcripts of plastid genes (Tillich et al, 2005), which again suggests that dispensable ndh genes in the ancestors of extant plants accumulated inactivating mutations (Martín and Sabater, 2010). Tottering on the edge of dispensability, the ndh genes provide an excellent opportunity to test the natural selection of photosynthesis-related genes during plant evolution
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