Water Transfer in an Alfalfa/Maize Association : Survival of Maize during Drought

We investigated the possibility of interspecific water transfer in an alfalfa (Medicago sativa L.) and maize (Zea mays L.) association. An alfalfa plant was grown through two vertically stacked plastic tubes. A 5 centimeter air gap between tubes was bridged by alfalfa roots. Five-week old maize plants with roots confined to the top tube were not watered, while associated alfalfa roots had free access to water in the bottom tube (the −/+ treatment). Additional treatments included: top and bottom tubes watered (+/+), top and bottom tubes droughted (−/−), and top tube droughted after removal of alfalfa root bridges and routine removal of alfalfa tillers (−^*). Predawn leaf water potential of maize in the −/+ treatment fell to −1.5 megapascals 13 days after the start of drought; thereafter, predawn and midday potentials were maintained near −1.9 megapascals. Leaf water potentials of maize in the −/− and −^* treatments declined steadily; all plants in these treatments were completely desiccated before day 50. High levels of tritium activity were detected in water extracted from both alfalfa and maize leaves after ³H₂O was injected into the bottom −/+ tube at day 70 or later. Maize in the −/+ treatment was able to survive an otherwise lethal period of drought by utilizing water lost by alfalfa roots.     

P-Nuclear Magnetic Resonance Determination of Phosphate Compartmentation in Leaves of Reproductive Soybeans (Glycine max L.) as Affected by Phosphate Nutrition

Most leaf phosphorus is remobilized to the seed during reproductive development in soybean. We determined, using ³¹P-NMR, the effect phosphorus remobilization has on vacuolar inorganic phosphate pool size in soybean (Glycine max [L.] Merr.) leaves with respect to phosphorus nutrition and plant development. Phosphate compartmentation between cytoplasmic and vacuolar pools was observed and followed in intact tissue grown hydroponically, at the R2, R4, and R6 growth stages. As phosphorus in the nutrient solution decreased from 0.45 to 0.05 millimolar, the vacuolar phosphate peak became less prominent relative to cytoplasmic phosphate and hexose monophosphate peaks. At a nutrient phosphate concentration of 0.05 millimolar, the vacuolar phosphate peak was not detectable. At higher levels of nutrient phosphate, as plants progressed from the R2 to the R6 growth stage, the vacuolar phosphate peak was the first to disappear, suggesting that storage phosphate was remobilized to a greater extent than metabolic phosphate. Under suboptimal phosphate nutrition (≤ 0.20 millimolar), the hexose monophosphate and cytoplasmic phosphate peaks declined earlier in reproductive development than when phosphate was present in optimal amounts. Under low phosphate concentrations (0.05 millimolar) cytoplasmic phosphate was greatly reduced. Carbon metabolism was coincidently disrupted under low phosphate nutrition as shown by the appearance of large, prominent starch grains in the leaves. Cytoplasmic phosphate, and leaf carbon metabolism dependent on it, are buffered by vacuolar phosphate until late stages of reproductive growth.     

P Nutrition during Seed Development : Leaf Senescence, Pod Retention, and Seed Weight of Soybean

Soybean (Glycine max [L.] Merrill) leaf senescence, which may partially result from mineral redistribution, appears to limit grain yield. Two experiments were designed to test the effects of supplemental inorganic phosphate (Pi), K, malate, and methionine (Met) infusions on senescence and yield. A novel stem infusion technique using pediatric intravenous kits was developed to supply these nutrients throughout seed growth. An average of 48.4 milliliters per plant was successfully infused into lower stem internodes during a 4 to 6 week period. Senescence was unaffected by K or malate infusions, but was delayed by Pi infusions (up to 8 days) and by increased nutrient solution Pi levels (up to 21 days) in separate experiments. Treatments which delayed senescence also improved yield as much as 3-fold, due primarily to increased pod retention and secondarily to increased seed size. Met infusions further increased pod retention at the lower, infused nodes, and thus increased total plant yield also. The influence of higher Pi levels during reproductive growth on soybean pod retention and yield may have been the result of sustained sucrose export due to altered C partitioning in leaves. The role of Met in improving yield was not clear. However, these results clearly demonstrate the importance of adequate Pi for delaying senescence and improving pod retention and yield.     

Comparison of the independent solvent structures of dimeric alpha-chymotrypsin with themselves and with gamma-chymotrypsin

The solvent structure of alpha-chymotrypsin has been determined in the restrained least squares refinement (1.67-A resolution) of the dimeric molecule (Blevins, R. A., and Tulinsky, A. (1985) J. Biol. Chem. 260, 4264-4275). A total of 247 water molecules reduced the R-factor by 0.039 to 0.179. The average occupancy of solvent is 0.77 and the average isotropic thermal parameter is 22 A2. About 80% of the solvent is around the surface, 10% is in the dimer interface, and 10% is interior. There are 49 pairs of water molecules related by 2-fold noncrystallographic symmetry (within 1.0 A) and 199 waters that can potentially hydrogen bond with protein or themselves. The specificity sites contain 5 water molecules, 2 of which are displaced by substrate binding. The remainder probably aid in identifying and positioning the latter for catalysis. Four of these waters also occur in gamma-chymotrypsin. Considering the water structure in the dimer interface region of alpha-chymotrypsin with that of gamma-chymotrypsin reveals that about two-thirds of the solvent in this region is lost on dimerization. Last, 4 of the water molecules of alpha-chymotrypsin have been identified to be sulfate ions from a difference map based on crystals with selenate exchanged mother liquor.     

Soybean leaf urease: Comparison with seed urease

Soybeans, Glycine max (L.) Merr., from ureides for transport of nitrogen from the root nodule to the shoot. The most direct routes for ureide utilization include the degradation of ureide-derived urea to NH₃ and CO₂. Ureolytic activity was found in leaf disks of soybean and exhbited optimal activity at pH 7 in the presence of a high concentration of urea (250 m M ). In vitro studies showed neither urea amidolyase nor urea dehydrogenase activity in soybean leaves and the ureolytic activity was characterized as urease. Several biochemical properties of soybean leaf urease were determined and compared to seed urease properties. Soybean leaf urease differed from that of seed in five ways: pH optima (5.25 and 8.75), apparent K_m (0.8 m M ), no inhibition by hydroxyurea, faster electrophoretic mobility and no cross-reactivity with soybean seed urease antibodies. The data suggest that urease is the primary urea metabolizing enzyme present in soybean leaves. The properties of soybean leaf urease support the conclusion that a unique isozyme of urease is present in leaf tissue.     

NMR View: A computer program for the visualization and analysis of NMR data

Summary NMR View is a computer program designed for the visualization and analysis of NMR data. It allows the user to interact with a practically unlimited number of 2D, 3D and 4D NMR data files. Any number of spectral windows can be displayed on the screen in any size and location. Automatic peak picking and facilitated peak analysis features are included to aid in the assignment of complex NMR spectra. NMR View provides structure analysis features and data transfer to and from structure generation programs, allowing for a tight coupling between spectral analysis and structure generation. Visual correlation between structures and spectra can be done with the Molecular Data Viewer, a molecular graphics program with bidirectional communication to NMR View. The user interface can be customized and a command language is provided to allow for the automation of various tasks.Inquiries concerning the availability of NMR View and the Molecular Data Viewer should be sent via email to johnsonb@merck.com or to Bruce A. Johnson, Merck Research Laboratories, RY80Y-103, P.O. Box 2000, Rahway, NJ 07065, U.S.A.     

Purification and properties of soybean nodule xanthine dehydrogenase

Xanthine dehydrogenase (EC 1.2.1.37), an essential enzyme for ureide metabolism was purified from the cytosol fraction of soybean nodules. The purified xanthine dehydrogenase was shown to be homogeneous by electrophoresis and a pI of 4.7 was determined by isoelectric focusing. The enzyme had a molecular weight of 285,000 and two subunits of molecular weight 141,000 each. The holoenzyme contained 1.7 (±0.7) mol Mo and 8.1 (±2.0) mol Fe/mol enzyme and the enzyme also contained FMN and is thus a molybdoironflavoprotein. Soybean xanthine dehydrogenase is the second enzyme in plants demonstrated to contain Mo and the first xanthine-oxidizing enzyme reported to contain FMN, rather than FAD as the flavin cofactor.     

Gibberellins and the Legume-Rhizobium Symbiosis : I. Endogenous Gibberellins of Lima Bean (Phaseolus lunatus L.) Stems and Nodules

The content of gibberellin-like substances in nodules formed by Bradyrhizobium species strain 127E14 on roots of lima bean (Phaseolus lunatus L.) has been previously found to be relatively high. The objectives of the present study were to purify and identify the endogenous gibberellins from the stems and nodules of lima bean. By sequential silica gel partition column chromatography, C₁₈ reverse-phase high performance liquid chromatography, and combined gas chromatography-mass spectrometry, the gibberellins A₁, A₃, A₁₉, A₂₀, A₂₉, and A₄₄ were identified from root nodules. Gibberellins A₁, A₃, A₁₉, A₂₀, and A₄₄ were also identified from lima bean stem tissue. These data provide the first mass spectral-based evidence that gibberellins are present in leguminous root nodules. The presence of the gibberellins identified indicates that the early 13-hydroxylation gibberellin biosynthetic pathway predominates in stem and nodule tissue. However, it is not known if the gibberellins within the nodules are produced in situ, or if they are imported from some remote host plant tissue.     

Enzymes of amide and ureide biogenesis in developing soybean nodules

Amide and ureide biogenic enzymes were measured in the plant fraction of soybean (Glycine max) nodules during the period 11 to 23 days after inoculation with Rhizobium japonicum (USDA 3I1b142). Enzymes involved in the initial assimilation of ammonia, i.e. glutamine synthetase, glutamate synthase, and aspartate aminotransferase, showed substantial increases in their specific activities over the time course. These increases paralleled the induction of nitrogenase activity in the bacteroid and leghemoglobin synthesis in the plant fraction. The specific activity of asparagine synthetase, however, showed a rapid decline after an initial increase in specific activity. Following the initial increases in the ammonia assimilatory enzymes, there was an increase in the activity of 5-phosphoribosylpyrophosphate amidotransferase, the enzyme which catalyzes the first committed step of de novo purine biosynthesis. This was followed by a dramatic increase in the purine oxidative enzymes, xanthine dehydrogenase and uricase. Smaller increases were observed in the activities of enzymes associated with the supply of metabolites to the purine biosynthetic pathway: phosphoglycerate dehydrogenase, serine hydroxymethylase, and methylene tetrahydrofolate dehydrogenase.