The isolation and characterisation of a mutant of the C₄ plant Amaranthus edulis deficient in NAD-malic enzyme activity

Abstract.

A mutant of Amaranthus edulis (Speg.) lacking activity of the C₄ leaf form of NAD-malic enzyme (ME; EC 1.1.1.39) has been isolated. Homozygous mutant (5% wild-type ME activity) and heterozygous (50% wild-type ME activity) F₂ plants were shown to contain both the α and β 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. Other C₄ enzymes were not down-regulated in the mutant plants. There was little difference in photosynthetic rate of the heterozygous plants compared to the wild-type, suggesting that NAD-ME exerts little control over the rate of C₄ photosynthesis, and that in the wild-type the enzyme has a very low control coefficient. The activity loss in the heterozygote may therefore be compensated by regulatory mechanisms that increase the activity of the enzyme in vivo. Data for bundle-sheath strands indicated that although the homozygous mutants were able to oxidise malate via the Krebs cycle, they were unable to convert malate to pyruvate and alanine via NAD-ME.