NAD malic enzyme in leaves with C 4-pathway photosynthesis and its role in C 4 acid decarboxylation

Abstract

Decarboxylating NAD-malate dehydrogenase (NAD-malic enzyme, NAD-ME, EC 1.1.1.39) has been investigated under a long-term drought during pre-anthesis, anthesis and seed-formation phases of ontogenesis of a NAD-ME type C4 plant Amaranthus cruentus L. using cytosol, chloroplast and mitochondrial fractions of mesophyll (M) and bundle sheath (BS) cells. We detected several molecular forms of NAD-ME with different subcellular localization patterns in the studied phases of amaranth ontogenesis. However, no enzyme activity was observed experimentally in chloroplasts of M and BS cells. In the pre-anthesis phase NAD-ME isoform with molecular weight of ∼115 kDa was found in cytosol of M and BS cells of control and drought-exposed plants. One of NAD-ME isoforms with molecular weight of 110 kDa was located in mitochondria of BS cells of control and drought-exposed plants, and a new isoform of ∼121 kDa was formed in mitochondria of BS cells under the influence of drought. After resuming watering this isoform (∼121 kDa) disappeared again. Approximately 90.6% and 9.4% of the total NAD-ME activity were localized in mitochondrial stroma and cytosol of BS cells, respectively, while in mesophyll cells 100% activity was found in cytosol fractions. The reaction catalyzed by NAD-ME follows Michaelis–Menten equation. NAD+, l-malate and Mn2+ activate this enzyme in mitochondria. Appearance of the ∼121 kDa isoform of NAD-ME in the mitochondrial fraction of BS cells under drought and its disappearance after resuming watering could be attributed to one of the protection functions of plants.