PLANT PHYSIOLOGY , Vol 113, Issue 4 1153-1165, Copyright © 1997 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
NADP-Malate Dehydrogenase in the C4 Plant Flaveria bidentis (Cosense Suppression of Activity in Mesophyll and Bundle-Sheath Cells and Consequences for Photosynthesis)
S. J. Trevanion, R. T. Furbank and A. R. Ashton
Cooperative Research Centre for Plant Science, c/o Research School of Biological Sciences, Australian National University, P.O. Box 475, Canberra, ACT 2601, Australia (S.J.T., R.T.F., A.R.A.)
Flaveria bidentis, a C4 dicot, was transformed with sorghum (a monocot)
cDNA clones encoding NADP-malate dehydrogenase (NADP-MDH; EC 1.1.1.82)
driven by the cauliflower mosaic virus 35S promoter. Although these
constructs were designed for over-expression, many transformants contained
between 5 and 50% of normal NADP-MDH activity, presumably by cosense
suppression of the native gene. The activities of a range of other
photosynthetic enzymes were unaffected. Rates of photosynthesis in plants
with less than about 10% of normal activity were reduced at high light and
at high [CO2], but were unaffected at low light or at [CO2] below about 150
[mu]L L-1. The large decrease in maximum activity of NADP-MDH was
accompanied by an increase in the activation state of the enzyme. However,
the activation state was unaffected in plants with 50% of normal activity.
Metabolic flux control analysis of plants with a range of activities
demonstrates that this enzyme is not important in regulating the
steady-state flux through C4 photosynthesis in F. bidentis. Cosense
suppression of gene expression was similarly effective in both the
mesophyll and bundle-sheath cells. Photosynthesis of plants with very low
activity of NADP-MDH in the bundle-sheath cells was only slightly
inhibited, suggesting that the presence of the enzyme in this compartment
is not essential for supporting maximum rates of photosynthesis.
