C
photosynthesis exhibits efficient CO
assimilation in ambient air by concentrating CO
around ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) through a metabolic pathway called the C
cycle. ...It has been suggested that cyclic electron flow (CEF) around PSI mediated by chloroplast NADH dehydrogenase-like complex (NDH), an alternative pathway of photosynthetic electron transport (PET), plays a crucial role in C
photosynthesis, although the contribution of NDH-mediated CEF is small in C
photosynthesis. Here, we generated NDH-suppressed transformants of a C
plant, Flaveria bidentis, and showed that the NDH-suppressed plants grow poorly, especially under low-light conditions. CO
assimilation rates were consistently decreased in the NDH-suppressed plants under low and medium light intensities. Measurements of non-photochemical quenching (NPQ) of Chl fluorescence, the oxidation state of the reaction center of PSI (P700) and the electrochromic shift (ECS) of pigment absorbance indicated that proton translocation across the thylakoid membrane is impaired in the NDH-suppressed plants. Since proton translocation across the thylakoid membrane induces ATP production, these results suggest that NDH-mediated CEF plays a role in the supply of ATP which is required for C
photosynthesis. Such a role is more crucial when the light that is available for photosynthesis is limited and the energy production by PET becomes rate-determining for C
photosynthesis. Our results demonstrate that the physiological contribution of NDH-mediated CEF is greater in C
photosynthesis than in C
photosynthesis, suggesting that the mechanism of PET in C
photosynthesis has changed from that in C
photosynthesis accompanying the changes in the mechanism of CO
assimilation.
By concentrating CO2, C4 photosynthesis can suppress photorespiration and achieve high photosynthetic efficiency, especially under conditions of high light, high temperature, and drought. To ...concentrate CO2, extra ATP is required, which would also require a change in photosynthetic electron transport in C4 photosynthesis from that in C3 photosynthesis. Several analyses have shown that the accumulation of the components of cyclic electron flow (CEF) around photosystem I, which generates the proton gradient across thylakoid membranes (DeltapH) and functions in ATP production without producing NADPH, is increased in various NAD-malic enzyme and NADP-malic enzyme C4 plants, suggesting that CEF may be enhanced to satisfy the increased need for ATP in C4 photosynthesis. However, in C4 plants, the accumulation patterns of the components of two partially redundant pathways of CEF, NAD(P)H dehydrogenase-like complex and PROTON GRADIENT REGULATION5-PGR5-like1 complex, are not identical, suggesting that these pathways may play different roles in C4 photosynthesis. Accompanying the increase in the amount of NDH, the expression of some genes which encode proteins involved in the assembly of NDH is also increased at the mRNA level in various C4 plants, suggesting that this increase is needed to increase the accumulation of NDH. To better understand the relation between CEF and C4 photosynthesis, a reverse genetic approach to generate C4 transformants with respect to CEF will be necessary.
By concentrating CO sub(2), C sub(4) photosynthesis can suppress photorespiration and achieve high photosynthetic efficiency, especially under conditions of high light, high temperature, and drought. ...To concentrate CO sub(2), extra ATP is required, which would also require a change in photosynthetic electron transport in C sub(4) photosynthesis from that in C sub(3) photosynthesis. Several analyses have shown that the accumulation of the components of cyclic electron flow (CEF) around photosystem I, which generates the proton gradient across thylakoid membranes ( Delta pH) and functions in ATP production without producing NADPH, is increased in various NAD-malic enzyme and NADP-malic enzyme C sub(4) plants, suggesting that CEF may be enhanced to satisfy the increased need for ATP in C sub(4) photosynthesis. However, in C sub(4) plants, the accumulation patterns of the components of two partially redundant pathways of CEF, NAD(P)H dehydrogenase-like complex and PROTON GRADIENT REGULATION5-PGR5-like1 complex, are not identical, suggesting that these pathways may play different roles in C sub(4) photosynthesis. Accompanying the increase in the amount of NDH, the expression of some genes which encode proteins involved in the assembly of NDH is also increased at the mRNA level in various C sub(4) plants, suggesting that this increase is needed to increase the accumulation of NDH. To better understand the relation between CEF and C sub(4) photosynthesis, a reverse genetic approach to generate C sub(4) transformants with respect to CEF will be necessary.
By concentrating CO.sub.2, C.sub.4 photosynthesis can suppress photorespiration and achieve high photosynthetic efficiency, especially under conditions of high light, high temperature, and drought. ...To concentrate CO.sub.2, extra ATP is required, which would also require a change in photosynthetic electron transport in C.sub.4 photosynthesis from that in C.sub.3 photosynthesis. Several analyses have shown that the accumulation of the components of cyclic electron flow (CEF) around photosystem I, which generates the proton gradient across thylakoid membranes (DELTApH) and functions in ATP production without producing NADPH, is increased in various NAD-malic enzyme and NADP-malic enzyme C.sub.4 plants, suggesting that CEF may be enhanced to satisfy the increased need for ATP in C.sub.4 photosynthesis. However, in C.sub.4 plants, the accumulation patterns of the components of two partially redundant pathways of CEF, NAD(P)H dehydrogenase-like complex and PROTON GRADIENT REGULATION5-PGR5-like1 complex, are not identical, suggesting that these pathways may play different roles in C.sub.4 photosynthesis. Accompanying the increase in the amount of NDH, the expression of some genes which encode proteins involved in the assembly of NDH is also increased at the mRNA level in various C.sub.4 plants, suggesting that this increase is needed to increase the accumulation of NDH. To better understand the relation between CEF and C.sub.4 photosynthesis, a reverse genetic approach to generate C.sub.4 transformants with respect to CEF will be necessary.
Arabidopsis ubiquitin ligases ATL31 and homologue ATL6 control the carbon/nitrogen nutrient and pathogen responses. A mutant with the loss-of-function of both atl31 and atl6 developed light ...intensity-dependent pale-green true leaves, whereas the single knockout mutants did not. Plastid ultrastructure and Blue Native-PAGE analyses revealed that pale-green leaves contain abnormal plastid structure with highly reduced levels of thylakoid proteins. In contrast, the pale-green leaves of the atl31/atl6 mutant showed normal Fv/Fm. In the pale-green leaves of the atl31/atl6, the expression of HEMA1, which encodes the key enzyme for 5-aminolevulinic acid synthesis, the rate-limiting step in chlorophyll biosynthesis, was markedly down-regulated. The expression of key transcription factor GLK1, which directly promotes HEMA1 transcription, was also significantly decreased in atl31/atl6 mutant. Finally, application of 5-aminolevulinic acid to the atl31/atl6 mutants resulted in recovery to a green phenotype. Taken together, these findings indicate that the 5-aminolevulinic acid biosynthesis step was inhibited through the down-regulation of chlorophyll biosynthesis-related genes in the pale-green leaves of atl31/atl6 mutant.
PsbO protein is an extrinsic subunit of photosystem II (PSII) and has been proposed to play a central role in stabilization of the catalytic manganese cluster. Arabidopsis thaliana has two psbO genes ...that express two PsbO proteins; PsbO1 and PsbO2. We reported previously that a mutant plant that lacked PsbO1 (psbo1) showed considerable growth retardation despite the presence of PsbO2 Murakami, R., Ifuku, K., Takabayashi, A., Shikanai, T., Endo, T., and Sato, F. (2002) FEBS Lett523, 138-142. In the present study, we characterized the functional differences between PsbO1 and PsbO2. We found that PsbO1 is the major isoform in the wild-type, and the amount of PsbO2 in psbo1 was significantly less than the total amount of PsbO in the wild-type. The amount of PsbO as well as the efficiency of PSII in psbo1 increased as the plants grew; howeVER, it neVER reached the total PsbO level observed in the wild-type, suggesting that the poor activity of PSII in psbo1 was caused by a shortage of PsbO. In addition, an in vitro reconstitution experiment using recombinant PsbOs and urea-washed PSII particles showed that oxygen evolution was better recoVERed by PsbO1 than by PsbO2. Further analysis using chimeric and mutated PsbOs suggested that the amino acid changes Val186-->Ser, Leu246-->Ile, and Val204-->Ile could explain the functional difference between the two PsbOs. Therefore we concluded that both the lower expression level and the inferior functionality of PsbO2 are responsible for the phenotype observed in psbo1.
is a little-known green alga, with a unique evolutionary position and distinctive photosynthetic features. Here, we present the complete chloroplast genome sequence of
.
The chloroplast NAD(P)H dehydrogenase (NDH) complex, which reduces plastoquinones in thylakoid membranes, is involved in PSI cyclic electron flow and chlororespiration. In addition to land plants, ...the NDH complex is conserved in cyanobacteria. In this study, we identified a novel NDH-related gene of Arabidopsis, NDH-dependent cyclic electron flow 5 (NDF5, At1g55370). Post-illumination increases in chlorophyll fluorescence were absent in ndf5 mutant plants, which indicated that NDF5 is essential for NDH activity. Sequence analysis did not reveal any known functional motifs in NDF5, but there was some homology in amino acid sequence between NDF5 and NDF2, a known NDH subunit. NDF5 and NDF2 homologs were present in higher plants, but not cyanobacteria. A single homolog, which had similarity to both NDF5 and NDF2, was identified in the moss Physcomitrella patens. Immunoblot analysis showed that NDF5 localizes to membrane fractions of chloroplasts. The stability of NdhH, a subunit of the NDH complex, as well as NDF5 and NDF2, was decreased in ndf5, ndf2 and double ndf2/ndf5 mutants, resulting in a loss of NDH activity in these mutants. These results indicated that both NDF5 and NDF2 have essential functions in the stabilization of the NDH complex. We propose that NDF5 and NDF2 were acquired by land plants during evolution, and that in higher plants both NDF5 and NDF2 are critical to regulate NDH activity and each other's protein stability, as well as the stability of additional NDH subunits.
A 33-kDa protein component of the oxygen-evolving complex in photosystem II is essential for photosynthesis, and it has been believed that mutants with deletion of this 33-kDa protein are not found ...in higher plants. We report here the first isolation of an
Arabidopsis thaliana mutant with a defect in one of the genes for the 33-kDa proteins,
psbO, and an intact gene (
psbO2). This mutant showed considerable growth retardation, suggesting that there is a functional difference between
psbO and
psbO2.
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