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Plant Physiol, February 2000, Vol. 122, pp. 491-504
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
Dehydrogenase1
Sari A.
Ruuska,2
T. John
Andrews,
Murray R.
Badger,
G.
Dean
Price, and
Susanne
von Caemmerer*
Molecular Plant Physiology (S.A.R., T.J.A., M.R.B., G.D.P., S.v.C.)
and Photobioenergetics (S.A.R., S.v.C.) Groups, Research School of
Biological Sciences, The Australian National University, G.P.O. Box
475, Canberra, Australian Capital Territory 2601, Australia.
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
CO2 assimilation rates decreased with decreasing ribulose
1,5-bisphosphate concentrations. In anti-b/f plants, CO2 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.
1
This work was supported in part by an Australian
National University Doctoral Scholarship, an Overseas Postgraduate
Research Scholarship, and a grant from the Academy of Finland to
S.A.R.
2
Present address: Department of Botany and Plant
Pathology, Michigan State University, East Lansing, MI 48824.
*
Corresponding author; e-mail susanne{at}rsbs.anu.edu.au; fax
61-2-6249-5075.
© 2000 American Society of Plant Physiologists
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