title: NADP-Malate Dehydrogenase in the C4 Plant Flaveria bidentis (Cosense Suppression of Activity in Mesophyll and Bundle-Sheath Cells and Consequences for Photosynthesis).
abstract: 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.
authors.affiliation: 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.).
authors.forename: S. J.
authors.initials: SJ
authors.lastname: Trevanion
authors.forename: R. T.
authors.initials: RT
authors.lastname: Furbank
authors.forename: A. R.
authors.initials: AR
authors.lastname: Ashton