MALATE-DEHYDROGENASE SYSTEM IN WOLFFIA-ARRHIZA - ITS CHARACTERIZATIONAND ROLE IN PLANT ADAPTATION TO LIGHT AND DARKNESS

The total activity, subcellular localization, and isoenzyme contents o f the malate dehydrogenase system (NAD- and NADP-dependent malate dehy drogenases, and NAD- and NADP-dependent malic enzymes) were studied in Wolffia arrhiza (L.) Hork. ex Wimmer plants grown in the light or in darkness with or without malate added to the nutrient solution. The NA D-malate oxidoreductase activity in the plant homogenate was several t imes higher than the activities of the other enzymes. Light enhanced t he activity of NADP-dependent malate dehydrogenases and reduced the ac tivities of NAD-MDG and NAD-ME. The NADP-dependent enzyme activities p revailed in the chloroplasts, whereas the mitochondrial and cytosol fr actions were enriched with the NAD-dependent enzymes. Several NAD-MDG and NAD-ME isoenzymes related to different subcellular fractions were identified by electrophoresis. In the light, the metabolic flow of mal ate was diverted to chloroplastic NADP-MDG and NADP-ME. In darkness, a sharp rise in NAD-MDG activity due to the synthesis of additional iso enzymes was noted. In the case of heterotrophic growth in darkness, on e of the Wolffia adaptive responses to high malate content was the enh ancement of chlorophyll synthesis to the level typical for illuminated (150 mu E/(m(2) s)) plants. The adaptive role of alterations in the r ates of metabolic pathways that utilize malate via chloroplastic and m itochondrial malate dehydrogenase systems is discussed.