Submitted to: North American Conference on Symbiotic Nitrogen Fixation
Publication Type: Abstract Only
Publication Acceptance Date: 9/27/1995
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Organic acids derived from the degradation of photosynthate and from root nodule nonautotrophic CO2 fixation are the primary source of energy for nitrogen fixation by symbiotic bacteroids. The plant enzyme malate dehydrogenase (MDH) is thought to play a critical role in the synthesis of organic acids in nodules. In order to more fully understand how organic acid metabolism is controlled in alfalfa nodules, we thought it important to characterize MDH gene expression. cDNAs for mitochondrial and glyoxysomal MDH were isolated from a nodule cDNA library, while a plastid MDH cDNA was isolated from a leaf cDNA library. In addition, cytosolic and nodule-enhanced MDH cDNAs were isolated by complementation of an E. coli Mdh- mutant. The deduced amino acid sequences of the alfalfa glyoxysomal and mitochondrial MDHs are very similar to those previously reported from melon, and the chloroplast MDH is very similar to chloroplast MDHs from maize, sorghum, and ice plant. RNA blots show that the glyoxysomal and mitochondrial MDHs are expressed rather uniformly in all tissues. The plastid MDH shows preferential expression in green tissues. These three MDH forms do not show high level induction in root nodules as compared to other tissues. While this MDH appears to be more closely related to mitochondrial and glyoxysomal forms, its transit peptide does not resemble mitochondrial or glyoxysomal targeting peptides. Preliminary Northern analysis indicated that this MDH is more highly expressed in nodule, than other alfalfa tissue. Analysis of Southern blots revealed that the nodule- enhanced MDH is encoded by a locus distinct from that of the previously characterized mitochondrial or glyoxysomal MDHs.