(1988). Inhibition of legume nodule formation and N2 fixation by nitrate. Critical Reviews in Plant Sciences: Vol. 7, No. 1, pp. 1-23.

Inhibition of legume nodule formation and N2 fixation by nitrate

Changes in apoplastic carbohydrate concentrations and activities of carbohydrate-degrading enzymes were determined in crown tissues of oat (Avena sativa L., cv Wintok) during cold hardening. During second-phase hardening (−3°C for 3 d) levels of fructan, sucrose, glucose, and fructose in the apoplast increased significantly above that in nonhardened and first-phase-hardened plants. The extent of the increase in apoplastic fructan during second-phase hardening varied with the degree of fructan polymerization (DP) (e.g. DP3 and DP4 increased to a greater extent than DP7 and DP > 7). Activities of invertase and fructan exohydrolase in the crown apoplast increased approximately 4-fold over nonhardened and first-phase-hardened plants. Apoplastic fluid extracted from nonhardened, first-phase-hardened, and second-phase-hardened crown tissues had low levels, of symplastic contamination, as determined by malate dehydrogenase activity. The significance of these results in relation to increases in freezing tolerance from second-phase hardening is discussed.

Apoplastic Sugars, Fructans, Fructan Exohydrolase, and Invertase in Winter Oat: Responses to Second-Phase Cold Hardening

Role and regulation of L-glutamate dehydrogenase activity in higher plants

Carbon and nitrogen metabolism in legume root nodules

Proteins in extracts from cotyledons, hypocotyls, and roots of 5-d-old, dark-grown soybean (Glycine max L. Merr. cv Williams) seedlings were separated by polyacrylamide gel electrophoresis. Three isoforms of glutamate dehydrogenase (GDH) were resolved and visualized in gels stained for GDH activity. Two isoforms with high electrophoretic mobility, GDH1 and GDH2, were in protein extracts from cotyledons and a third isoform with the lowest electrophoretic mobility, GDH3, was identified in protein extracts from root and hypocotyls. Subcellular fractionation of dark-grown soybean tissues demonstrated that GDH3 was associated with intact mitochondria. GDH3 was purified to homogeneity, as determined by native and sodium dodecyl sulfate-polyacrylamide gels. The isoenzyme was composed of a single 42-kD subunit. The pH optima for the reductive amination and the oxidative deamination reactions were 8.0 and 9.3, respectively. At any given pH, GDH activity was 12- to 50-fold higher in the direction of reductive amination than in the direction of the oxidative deamination reaction. GDH3 had a cofactor preference for NAD(H) over NADP(H). The apparent Michaelis constant values for [alpha]-ketoglutarate, ammonium, and NADH at pH 8.0 were 3.6, 35.5, and 0.07 mM, respectively. The apparent Michaelis constant values for glutamate and NAD were 15.8 and 0.10 mM at pH 9.3, respectively. To our knowledge, this is the first biochemical and physical characterization of a purified mitochondrial NAD(H)-dependent GDH isoenzyme from soybean.

Purification of Mitochondrial Glutamate Dehydrogenase from Dark-Grown Soybean Seedlings.

The effect of nitrogen addition on N2-fixation and on glutamate dehydrogenase and glutamate synthase activities. in nodules and roots of soybeans inoculated with various strains of Rhizobium japonicum

Pisum seedling and Pastinaca storage roots contained high glutanrate dehydrogenase (GDH) activity in areas of reported rapid growth and high phytoctrome content. A similar distribution was observed for malate dehydrogenase. Freeze-thawings of mitochondrial preparations from Pisum roots always resulted in increases of GDH specific activity; however, the observed increases were much larger with basal than apical sections. Both intact and freeze-thawed mitochondrial preparations from seedling roots exhibited increases in GDH activity with time after isolation. In intact mitochondrial preparations from roots of etiolated seedlings, an increase in malate dehydrogenase activity was observed similar to that of GDH activity; however, no increased malate dehydrogenase activity was noted in preparations from light-grown seedlings.



Illuminating Pisum seedlings with far-red light slowly increased GDH activity in roots over a period of two weeks. Since these observed increases were not due to direct exposure of roots to light, other factors were likely involved.

Glutamate Dehydrogenase Activity in Roots: Distribution in a Seedling and Storage Root, and the Effects of Red and Far‐red Illuminations

Lemna perpusilla Torr, strain 6746 clones were maintained under conditions of continuous illumination with various concentrations of sucrose, glucose or fructose added to the growth, medium. After two weeks of growth, plants were harvested and either assayed for total glutamate dehydrogenase activity or fractionated into one chloroplast-rich and one mitochondria-rich preparation and then assayed for glutamate dehydrogenase activity. In all assays for glutamate dehydrogenase it was necessary to add bovine serum albumin to the extraction medium in order to obtain sufficient enzyme activity for accurate and reproducible results. The presence of sucrose in the growth medium reduced glutamate dehydrogenase activity in all studies. When samples containing intact organelles were assayed, sucrose inhibition of activity appeared to occur primarily in the chloroplast fraction. Glucose, on the other hand, increased glutamate dehydrogenase activity in the chloroplast-rich fractions. Upon freeze-thawing differences between the various treatments were less obvious. The results from these studies indicate possible differences in sugar uptake and/or utilization in Lemna perpusilla.

Stanley H. Duke

Papers

The effect of nitrogen addition on N2-fixation and on glutamate dehydrogenase and glutamate synthase activities. in nodules and roots of soybeans inoculated with various strains of Rhizobium japonicum

Glutamate Dehydrogenase Activity in Roots: Distribution in a Seedling and Storage Root, and the Effects of Red and Far‐red Illuminations

Glutamate Dehydrogenase Activity in Lemna perpusilla 6746: The Effects of Sucrose, Glucose and Fructose Addition to Growth Media