Cell wall-associated malate dehydrogenase activity from maize roots

Isolated cell walls from maize (Zea mays L.) roots exhibited ionically and covalently bound NAD-specific malate dehydrogenase activity. The enzyme catalyses a rapid reduction of oxaloacetate and much slower oxidation of malate. The kinetic and regulatory properties of the cell wall enzyme solubilized with 1 M NaCl were different from those published for soluble, mitochondrial or plasma membrane malate dehydrogenase with respect to their ATP, Pi, and pH dependence. Isoelectric focusing of ionically-bound proteins and specific staining for malate dehydrogenase revealed characteristic isoforms present in cell wall isolate, different from those present in plasma membranes and crude homogenate. Much greater activity of cell wall-associated malate dehydrogenase was detected in the intensively growing lateral roots compared to primary root with decreased growth rates. Presence of Zn2+ and Cu2+ in the assay medium inhibited the activity of the wall-associated malate dehydrogenase. Exposure of maize plants to excess concentrations of Zn2+ and Cu2+ in the hydroponic solution inhibited lateral root growth, decreased malate dehydrogenase activity and changed isoform profiles. The results presented show that cell wall malate dehydrogenase is truly a wall-bound enzyme, and not an artefact of cytoplasmic contamination, involved in the developmental processes, and detoxification of heavy metals.

► This is the first biochemical characterization of cell wall bound malate dehydrogenase.
► NAD-malate dehydrogenase is ionically and covalently attached to cell walls.
► Biochemical properties and isoforms are different from other cell compartments MDH.
► Roles in root growth and stress response in apoplastic compartment are proposed.