THE ISOLATION AND CHARACTERIZATION OF A MUTANT OF THE C-4 PLANT AMARANTHUS-EDULIS DEFICIENT IN NAD-MALIC ENZYME-ACTIVITY

A mutant of Amaranthus edulis (Speg.) lacking activity of the C-4 leaf form of NAD-malic enzyme (ME; EC 1.1.1.39) has been isolated. Homozyg ous mutant (5% wild-type ME activity) and heterozygous (50% wild-type ME activity) F-2 plants were shown to contain both the alpha and beta NAD-ME subunits in similar amounts to those detected in the wild-type leaves. The rate of photosynthetic CO2 assimilation was reduced in the homozygous mutant to 5% of that observed for the wild-type leaves. Ot her C-4 enzymes were not downregulated in the mutant plants. There was little difference in photosynthetic rate of the heterozygous plants c ompared to the wild-type, suggesting that NAD-NE exerts little control over the rate of C-4 photosynthesis, and that in the wild-type the en zyme has a very low control coefficient. The activity loss in the hete rozygote may therefore be compensated by regulatory mechanisms that in crease the activity of the enzyme in vivo. Data for bundle-sheath stra nds indicated that although the homozygous mutants were able to oxidis e malate via the Krebs cycle, they were unable to convert malate to py ruvate and alanine via NAD-ME.