The role of chloroplast electron transport and metabolites in modulating Rubisco activity in tobacco. Insights from transgenic plants with reduced amounts of cytochrome b/f complex or glyceraldehyde 3-phosphate dehydrogenase.
Leaf metabolites, adenylates, and Rubisco activation were studied in two transgenic tobacco (Nicotiana tabacum L. cv W38) types. Plants with reduced amounts of cytochrome b/f complex (anti-b/f) have impaired electron transport and a low transthylakoid pH gradient that restrict ATP and NADPH synthesis. Plants with reduced glyceraldehyde 3-phosphate dehydrogenase (anti-GAPDH) have a decreased capacity to use ATP and NADPH in carbon assimilation. The activation of the chloroplast NADP-malate dehydrogenase decreased in anti-b/f plants, indicating a low NADPH/NADP(+) ratio. The whole-leaf ATP/ADP in anti-b/f plants was similar to wild type, while it increased in anti-GAPDH plants. In both plant types, the CO(2) assimilation rates decreased with decreasing ribulose 1, 5-bisphosphate concentrations. In anti-b/f plants, CO(2) assimilation was further compromised by reduced carbamylation of Rubisco, whereas in anti-GAPDH plants the carbamylation remained high even at subsaturating ribulose 1,5-bisphosphate concentrations. We propose that the low carbamylation in anti-b/f plants is due to reduced activity of Rubisco activase. The results suggest that light modulation of activase is not directly mediated via the electron transport rate or stromal ATP/ADP, but some other manifestation of the balance between electron transport and the consumption of its products. Possibilities include the transthylakoid pH gradient and the reduction state of the acceptor side of photosystem I and/or the degree of reduction of the thioredoxin pathway.
Ruuska
S A
SA
Molecular Plant Physiology, Research School of Biological Sciences, The Australian National University, G.P.O. Box 475, Canberra, Australian Capital Territory 2601, Australia.
Andrews
T J
TJ
Badger
M R
MR
Price
G D
GD
von Caemmerer
S
S
D000249:Adenosine Monophosphate / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism; D002736:Chloroplasts / Q000201:enzymology / Q000378:metabolism*; D003573:Cytochrome b Group / Q000378:metabolism*; D045346:Cytochrome b6f Complex; D004579:Electron Transport; D004789:Enzyme Activation; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases / Q000378:metabolism*; D008291:Malate Dehydrogenase / Q000378:metabolism; D050538:Malate Dehydrogenase (NADP+); D010446:Peptide Fragments / Q000378:metabolism*; D030821:Plants, Genetically Modified; D010947:Plants, Toxic; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism*; D014026:Nicotiana / Q000201:enzymology* / Q000378:metabolism
D003573:Cytochrome b Group; D010446:Peptide Fragments; C068142:glyceraldehyde 3-phosphate dehydrogenase (304-313); D002245:Carbon Dioxide; D000249:Adenosine Monophosphate; D045346:Cytochrome b6f Complex; D008291:Malate Dehydrogenase; D050538:Malate Dehydrogenase (NADP+); D005987:Glyceraldehyde-3-Phosphate Dehydrogenases; D012273:Ribulose-Bisphosphate Carboxylase