Submitted to: Alfalfa Improvement Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 7/16/1996
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Malic acid, derived from malate dehydrogenase (MDH), is critical to plant function playing a role in oxidation of fatty acids, generation of energy through the tricarboxylic acid cycle, C4 photosynthesis, crassulacean acid metabolism, and nitrogen fixation. Since it has many functional roles, the control of malic acid is affected by numerous isozymes of MDH. Because of our interest in nitrogen fixation and nitrogen assimilation, we thought it important to isolate the genes controlling the various isoforms of MDH in root nodules. Using heterologous MDH probes from other plants we isolated plastid, mitochondrial, and glyoxysomal MDHs, pMDH, mMDH, and gMDH, respectively. Complementation of an Escherichia coli MDH- mutant allowed us to isolate cytosolic MDH (cMDH) and nodule enhanced MDH (nMDH). The similarity between the various MDHs from alfalfa is no greater than 65%. RNA blots showed that mMDH, gMDH, and cMDH are expressed relatively uniformly in root, leaves, stems, cotyledons, and root nodules. By comparison the pMDH and nMDH are most highly expressed in leaves and root nodules, respectively. Developmental studies showed that nMDH expression is associated with the formation of effective nodules and the activity of bacterial nitrogenase. Ineffective nodules had only basal levels of nMDH expression. To further study the role of MDH in nodules, alfalfa nodule enhanced and cytosolic MDH protein were purified from the complemented E. coli MDH- mutants and antibodies were immunologically distinct, reflecting their differences in deduced amino acid sequence. Two bands of cytosic MDH polypeptide, of near equal intensity, were found in all organs of alfalfa suggesting that there are at least three isoforms of cMDH. A single polypeptide band was detected for nMDH.