NADP-malate dehydrogenase in the C4 plant Flaveria bidentis. Cosense suppression of activity in mesophyll and bundle-sheath cells and consequences for photosynthesis.

摘要

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 concentrations, but were unaffected at low light or at CO2 concentrations below about 150 鎙 litre-1. The large decrease in maximum activity of NADP-MDH was accompanied by anincrease 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 demonstrated 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.