Amino Acids In Xylem Sap Research Articles

CO2 Fixation in Alfalfa and Birdsfoot Trefoil Root Nodules and Partitioning of 14C to the Plant1

Nonphotosynthetic CO2 fixation by root nodules of legumes may provide carbon skeletons for assimilation of symbiotically fixed N2. However, the products formed by this process and their physiological significance in perennial legumes is poorly understood. Therefore, the objectives of this study were to determine if nonphotosynthetic CO2 fixation by root nodules contributes carbon for the assimilation of fixed N2 in alfalfa (Medicago sativa L.) and birdsfoot trefoil (Lotus corniculatus L.) and if assimilation products are partitioned to different plant organs. Effective alfalfa nodules excised from or attached to roots had apparent 14CO2 fixation rates of 50 to 80 μg CO2 kg‐1 s‐1 (dry weight) at 0.0012 to 0.0038 mole fraction CO2. Nodule CO2 fixation rates increased six‐ to seven‐fold as ambient CO2 was raised from 0.0038 to 0.0663 mole fraction. These rates were double to triple those of comparably treated nodules of ineffective alfalfa and those of birdsfoot trefoil. Respiration rates of nodules (3 to 4 mg CO2 kg‐1 s‐1) were 10 to 100‐fold higher than 14CO2 fixation rates of nodules. This finding suggests that attached nodules normally function at saturating CO2 levels and may fix up to 1600 μg CO2 kg‐1 s‐1 when concurrent 14C export to roots and shoots is accounted for. Pulse chase experiments with 14CO2 combined with nodule and xylem sap analysis demonstrated the initial products of root and nodule CO2 fixation were organic acids. However, the export of fixed 14C from effective nodules was primarily in the form of amino acids. In contrast, nodule and/or root fixed 14C in ineffectively nodulated alfalfa and denodulated effective alfalfa and birdsfoot trefoil was transported primarily as organic acids. Aspartate, asparagine, alanine, glutamate, and glutamine were the most heavily labeled compounds in the amino acid fraction of both effective alfalfa and birdsfoot trefoil nodules exposed to 14CO2. By contrast, asparate, asparagine, and glutamine were the predominantly labeled amino acids in xylem sap collected from nodulated effective roots exposed to 14CO2. The occurrence of nodule CO2 fixation in alfalfa and birdsfoot trefoil and the export of fixed carbon as asparagine and aspartate to roots and shoots is consistent with a role for CO2 fixation by nodules in providing carbon skeletons for assimilation and transport of symbiotically fixed N2.

4-Methyleneglutamine in Peanut Plants: Dynamics of Formation, Levels, and Turnover in Relation to Other Free Amino Acids

Neither 4-methyleneglutamine nor 4-methyleneglutamic acid were found in free or bound form in ungerminated peanut seeds (Arachis hypogaea L.). Both, however, were formed soon after germination; whereas, 4-methyleneglutamic acid appeared slightly before 4-methyleneglutamine, the former remained at a low concentration while the level of 4-methyleneglutamine rose rapidly between 2 and 10 days of germination and declined slowly thereafter. Free proline and glutamine followed a pattern similar to 4-methyleneglutamine; on the other hand, asparagine increased for at least 20 days but other free amino acids remained at relatively low, constant levels. In mature peanut plants, 4-methyleneglutamine occurred in all parts except developing pods, was virtually the only free amino acid in xylem sap, and constituted about 70% of the total soluble nitrogen of sap. In contrast, 4-methyleneglutamic acid was found only in leaves and stems in highly variable amounts.Levels of 4-methyleneglutamine were largely independent of the processes of nodulation and nitrogen fixation. This amide was degraded much more slowly in nitrogen-deficient plants than were any of the other free amino acids and did not become labeled when a photosynthesizing plant was exposed to (14)CO(2) for 24 hours. The results indicate that 4-methyleneglutamine is the principle carrier of nitrogen in the xylem of peanut plants but its carbon skeleton appears to turn over very slowly.

Nitrogen redistribution during grain growth in wheat (Triticum aestivum L.)

The technique of EDTA-enhanced phloem exudation (King and Zeevaart, 1974: Plant Physiol. 53, 96-103) was evaluated with respect to the collection and identification of amino acids exported from senescing wheat leaves. Whilst the characteristics of the exudate collected conform with many of the accepted properties of phloem exudate, unexpectedly high molar proportions of phenylalanine and tyrosine were observed. By comparing exudation into a range chelator solutions with exudation into water, the increased exudation of phenylalanine and tyrosine relative to the other amino acids occurring when ethylene-diaminetetracetic acid was used, was considered to an artefact.In plants thought to be relying heavily on mobilisation of protein reserves to satisfy the nitrogen requirements of the grain, the major amino acids present in flag-leaf phloem exudate were glutamate, aspartate, serine, alanine and glycine. Only small proportions of amides were present until late in senescence when glutamine became the major amino acid in phloem exudate (25 molar-%). Glutamine was always the major amino acid in xylem sap (50 molar-%).The activities of glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 1.4.7.1), glutamate dehydrogenase (EC 1.4.1.3) and asparagine synthetase (EC 5.3.5.4) were measured in the flag leaf throughout the grain-filling period. Glutamine synthetase and glutamate-synthase activities declined during this period. Glutamate-dehydrogenase activity was markedly unchanged despite variation in the number of multiple forms visualised after gel electrophoresis. The activity of the enzyme reached a peak only very late in the course of senescence of the flag leaf. No asparagine-synthetase activity could be detected in the flag leaf during the grain-filling period.

Xylem translocation of amino acids from roots to shoots in cottonwood plants

Xylem sap produced by root pressure and collected from topped cottonwood (Populusdeltoides Bartr.) contained sugars, organic acids, amino acids, protein, and inorganic nutrients. Amino acids and NO3N were present at the highest concentrations. Total amino acids in xylem sap ranged from 3 to 10 mol m−3 and NO3N ranged from 7 to 10 mol m−3. When lower leaves were photosynthetically labeled with 14CO2, activity was rapidly translocated to roots and recycled to shoots. Within 6 h after labeling leaves, xylem exudate contained between 1 and 2 × 106 disintegrations min−1 cm−3. Most of the activity (50 to 70%) was in the amino acid fraction. Although nine amino acids contained some activity, more than 90% of the 14C was found in five common transport and storage amino acids: asparagine, glutamine, serine, alanine, and γ-aminobutyric acid. Glutamine contained 60 to 70% of the activity in the amino acid fraction 2 h after topping. Sugars, translocated from lower leaves to roots via the ploem, are converted to amino acids in roots and then translocated to the developing shoot via the xylem.

Pseudomonas solanacearum: virulence of biochemical mutants.

The virulence of tryptophan auxotrophs (Trp−) of Pseudomonas solanacearum K60 was significantly lower than that of the wild type when inoculated on tobacco (cult. "Bottom Special") and tomato (cult. "Bonny Best"). These auxotrophs persisted at the site of inoculation for several weeks, but did not cause systemic infection. Auxotrophs for methionine (Met−) and leucine (Leu−) biosynthesis, however, were virulent on tobacco and weakly virulent on tomato. In tobacco leaves infiltrated with suspensions of Trp−, Met−, and Leu− mutant strains and strain K60, the population doubling times of each isolate were 16.0, 4.3, 4.2, and 2.3 h respectively. The amino acid requirement of each auxotroph was determined in vitro and compared with levels of these amino acids in xylem sap collected from decapitated plants. Tobacco sap contained sufficient concentrations of required amino acids to support 0, 53, and 54% of maximum growth in vitro of the Trp−, Met−, and Leu− mutant strains, respectively. Tryptophan, methionine, and leucine levels in tomato sap supported 0, 3, and 13% maximum growth of the respective strains in vitro. The relative concentrations of methionine and leucine in xylem sap were sufficiently different to explain the susceptibility of tobacco and resistance of tomato to these auxotrophs. The virulence of tryptophan auxotrophs was restored by genetic transformation to prototrophy and by supplying tryptophan to inoculated tobacco seedlings.