Nicotiana tabacum NADP-malic enzyme: Cloning, characterization and analysis of biological role

Abstract

NADP-malic enzyme (NADP-ME) catalyzes the oxidative decarboxylation of l-malate, producing pyruvate, CO2 and NADPH. The photosynthetic role of this enzyme in C4 and Crassulacean acid metabolism (CAM) plants has been well established; however, the biological role of several non-photosynthetic isoforms described in C3, C4 and CAM plants is still speculative. In this study, the characterization of the NADP-ME isoforms from Nicotiana tabacum was performed. Three different nadp-me transcripts were identified in this C3 plant, two of which encode for putative cytosolic isoforms (DQ923118 and EH663836), while the third encodes for a plastidic counterpart (DQ923119). Although the three transcripts are expressed in vegetative as well as in reproductive tissues, they display different levels of expression. With regards to enzyme activity, root is the tissue that displays the highest NADP-ME activity. Recombinant NADP-MEs encoded by DQ923118 and DQ923119 were expressed in Escherichia coli and their kinetic parameters and response to different metabolic effectors were analyzed. Studies carried out with crude extracts and with the recombinant proteins indicate that the cytosolic and plastidic isoforms aggregate as tetramers of subunits of 65 and 63 kDa, respectively. Real-time reverse transcription-PCR studies show that the three nadp-me tobacco transcripts respond differently to several biotic and abiotic stress stimuli. Finally, the physiological role of each isoform is discussed in terms of the occurrence, kinetic properties and response to stress. The structure of the NADP-ME family in tobacco is compared with those of other C3 species. © The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved.