NADP-MALATE DEHYDROGENASE IN THE C-4 PLANT FLAVERIA-BIDENTIS - COSENSE SUPPRESSION OF ACTIVITY IN MESOPHYLL AND BUNDLE-SHEATH CELLS AND CONSEQUENCES FOR PHOTOSYNTHESIS

Flaveria bidentis, a C-4 dicot, was transformed with sorghum (a monoco t) cDNA clones encoding NADP-malate dehydrogenase (NADP-MDH; EC 1.1.1. 82) driven by the cauliflower mosaic virus 35S promoter. Although thes e constructs were designed for overexpression, many transformants cont ained between 5 and 50% of normal NADP-MDH activity, presumably by cos ense suppression of the native gene. The activities of a range of othe r photosynthetic enzymes were unaffected. Rates of photosynthesis in p lants 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 N ADP-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 C-4 photosynthesis in F. bid entis. Cosense suppression of gene expression was similarly effective in both the mesophyll and bundle-sheath cells. Photosynthesis of plant s with very low activity of NADP-MDH in the bundle-sheath cells was on ly slightly inhibited, suggesting that the presence of the enzyme in t his compartment is not essential for supporting maximum rates of photo synthesis.