Incorporation of 3H-Gibberellin A20 in roots of G2 peas

Following application of ³H-Gibberellin A₂₀ (GA₂₀) to roots of G2 pea seedlings and homogenization of the roots, about 3% of the radioactivity in the tissue could be precipitated from a 30,000 × g supernatant with trichloroacetic acid (TCA) (soluble fraction) while about 5% of the radioactivity pelleted at 30,000 × g (particulate fraction). The radioactivity in the particulate fraction was soluble in sodium dodecyl sulfate (SDS), but was not dialyzable and was insoluble in ethanol. Electrophoresis of the soluble fraction gave only one band of radioactivity, while that of the particulate fraction gave multiple bands. Acid hydrolysis of the soluble fraction released radioactivity that ran coincident with acid-treated GA₂₀ on silicic-acid column chromatography. The particulate fraction gave numerous radioactive peaks following acid hydrolysis, two of which were coincident with GA₂₀ and GA₂₉ (hydroxylation product of GA₂₀) on silicic acid chromatography. Treatment of the particulate and soluble fractions with RNase, DNase, and proteases showed a significant solubilization of radioactivity only with the proteases, suggesting that the GA is bound to a proteinaceous macromolecule. Complete proteolytic hydrolyis followed by thin layer chromatography showed 65% of the radioactivity from the soluble fraction running separately from free GAs or the individual amino acids; the particulate fraction gave mainly (60%) free GAs on enzymatic hydrolysis and much smaller amounts (17%) in a position separate from that of the GAs or amino acids. Binding of ³H-GA to protease-sensitive material was obtained with biologically active ³H-GA₂₀ and ³H-GA₁.     

The impact of aphicide drenches on Micromus tasmaniae (Walker) (Neuroptera: Hemerobiidae) and the implications for pest control in lettuce crops

Abstract  The recent arrival of lettuce aphid ( Nasonovia ribis-nigri (Mosley) ) in Australia has resulted in a pesticide-based protection program based upon seedling drenches of imidacloprid being promoted by many advisory agencies and accepted by growers as the only option available. This has caused concern about potential for incompatibility with existing integrated pest management programs for other pests in lettuce. Two neonicotinoid insecticides, imidacloprid (Confidor 200SC) and thiamethoxam (Actara), were applied to lettuce seedlings by drenching. A model aphid ( Macrosiphum euphorbiae (Thomas) ), used because N. ribis-nigri was not present in mainland Australia at that time, was periodically released onto the seedlings over 10 weeks. The effects of imidacloprid and thiamethoxam on larvae of predatory brown lacewings ( Micromus tasmaniae (Walker) ) which fed on the aphids were measured over 10 weeks by bioassay. Imidacloprid applied at a rate of 11 mL active ingredient (ai) per 1000 seedlings and thiamethoxam applied at 0.5 g ai per 1000 seedlings were highly toxic to M. tasmaniae that consumed aphids from the seedlings for up to 4 weeks after application. A 1/10 rate of imidacloprid (1.1 mL ai per 1000 seedlings) caused moderate toxicity for 3 weeks, and was then harmless to M. tasmaniae . Thiamethoxam and the high rate of imidacloprid caused almost complete mortality of aphids for about 6 weeks after application, and the low rate of imidacloprid displayed similarly high activity for about 3 weeks.     

Lateral root formation in pine seedlings

The role of assimilates in lateral root development was studied in Pinus pinea seedlings grown in a nutrient solution. Seedlings were treated with ¹⁴CO₂ for 2 h following removal of the tap root tip at various times prior to the application of ¹⁴CO₂ or removal of a different number of cotyledons at one time. In seedlings with intact root systems most of the radioactivity accumulated in the lower section of the root containing the tap root apex. When the tap root tip was removed, the pattern of radioactivity accumulation along the root was affected by the presence and the stage of lateral root development. Removing the tap root tip of young seedlings (with no lateral roots) resulted in an almost equal distribution of radioactivity along the root. About 50% of the total radioactivity was found in the section showing the highest lateral root growth. Removing the tap root tip of mature seedlings (with lateral roots in the upper section) resulted in an immediate increase in the radioactivity accumulation in the upper section. When lateral roots appeared in the middle section, the pattern of radioactivity distribution was similar to that found in root decapitated young seedlings. Removal of cotyledons of mature seedlings somewhat increased the transport of radioactivity to the lower root section at the expense of the radioactivity in the lateral roots of the upper section. The present study suggests that competition within the root system between the tap root apex and the lateral roots may play an important role in determining the morphology of the root system.     

The Binding of Indole-3-acetic Acid and 3-Methyleneoxindole to Plant Macromolecules

Homogenates of pea (Pisum sativum L., var. Alaska) seedlings exposed to ¹⁴C-indole-3-acetic acid or ¹⁴C-3-methyleneoxindole, an oxidation product of indole-3-acetic acid, were extracted with phenol. In both cases 90% of the bound radioactivity was found associated with the protein fraction and 10% with the water-soluble, ethanol-insoluble fraction. The binding of radioactivity from ¹⁴C-indole-3-acetic acid is greatly reduced by the addition of unlabeled 3-methyleneoxindole as well as by chlorogenic acid, an inhibitor of the oxidation of indole-3-acetic acid to 3-methyleneoxindole. Chlorogenic acid does not inhibit the binding of ¹⁴C-3-methyleneoxindole. The labeled protein and water-soluble, ethanol-insoluble fractions of the phenol extract were treated with an excess of 2-mercaptoethanol. Independently of whether the seedlings had been exposed to ¹⁴C-indole-3-acetic acid or ¹⁴C-3-methyleneoxindole, the radioactivity was recovered from both fractions in the form of a 2-mercaptoethanol-3-methyleneoxindole adduct. These findings indicate that 3-methyleneoxindole is an intermediate in the binding of indole-3-acetic acid to macromolecules.     

Studies on the release of exogenous and endogenous GABA and glutamate from rat brain synaptosomes

The aim of the present study was to determine whether exogenous radioactive GABA and glutamate previously taken up by rat brain synaptosomes are released preferentially with respect to the endogenous unlabeled amino acids. Preferential release was monitored by comparing the specific radioactivity of the amino acids released to that present in synaptosomes at the beginning and at the end of the release period. The GABA released spontaneously or by depolarizing the synaptosomes with high K⁺ in the presence of Ca²⁺ had the same specific radio-activity as that present in synaptosomes before or after superfusion. Depolarization with veratridine or superfusion with OH-GABA caused a moderate increase (15–20%) in the specific radioactivity of the GABA released and a corresponding slight decrease in that of superfused synaptosomes. In conditions causing a supraadditive release of exogenous and endogenous GABA (see ref. 13), the specific radioactivity of the GABA released was increased 20–30%. The GABA with higher-than-average specific radioactivity is probably representative of the cytoplasmic pool of this amino acid. The glutamate released spontaneously had a specific radioactivity lower than that present in synaptosomes at the start of superfusion, and also the specific radioactivity in superfused synaptosomes was lower than at the start of superfusion. The glutamate released by aspartate (by heteroexchange), by veratridine, or by high K⁺ had a specific radioactivity higher than that of the amino acid released spontaneously, similar to that present in synaptosomes at the start of superfusion, and higher than that found in superfused synaptosomes. These findings suggest that exogenous radioactive glutamate is released preferentially with respect to the endogenous amino acid and to the glutamate synthesized from glucose during the superfusion period.     

Albumin stimulates the release of arachidonic acid from phosphatidylcholine in hamster lungs

¹⁴C-Arachidonic acid injected into the pulmonary circulation of isolated hamster lungs was effectively incorporated into lung lipids. Once retained the radiolabel was relatively stable but the release of radioactivity increased up to 10-fold when bovine serum albumin (1 %) was added to the perfusate. This efflux of radioactivity was not blocked by quinacrine, a phospholipase A₂ inhibitor. In albumin experiments the released ¹⁴C-araehidonate griginated mainly from the phospholipid fraction in which phosphatidylcholine was the main source of the released radioactivity.Pulmonary infusion of albumin had no significant effect on the amount of ¹⁴C-arachidonic acid in the neutral lipid or free fatty acid fractions of perfused lungs. In experiments with albumin about 80 % of the released radioactivity co-chromatographed with unlabelled arachidonic acid whereas in the absence of albumin only about 20 % of the released radioactivity was unmetabolized arachidonic acid. This study indicates that albumin stimulates the release of arachidonic acid from isolated hamster lungs and that the release is increased mainly from the phosphatidyl choline fraction.     

Effects of temperature on the degradation and biliary secretion of asialoorosomucoid by the perfused rat liver: evidence for two intracellular pathways

We have utilized the in situ perfused rat liver under nonrecirculating conditions to examine the effect of temperature on the metabolism and biliary secretion of [125I]-asialoorosomucid (ASOR). In this manner we were able to follow the fate of a single round of internalized ligand. In control livers perfused at 37 degrees C, approximately 50% of [125I]-ASOR injected into the portal vein was extracted on first pass. Five minutes after the injection, radioactivity, which had been extracted initially, began to appear in the hepatic venous effluent. Within 25 min, 50% of the initially extracted radioactivity was released into the perfusion medium; the bulk of this radioactivity (greater than 95%) was soluble in trichloroacetic acid. In livers perfused at temperatures slightly less than 37 degrees C (30-35 degrees C), first-pass extraction of [125I]-ASOR was similar to that observed at 37 degrees C. However, a severalfold decrease in the rate of release of radioactivity from the liver into the perfusion medium was noted at the lower perfusion temperatures; whereas greater than 50% of the initially extracted radioactivity was released within 30 min from livers perfused at 37 degrees C, only 5% was released at 30 degrees C. At the lower perfusion temperature, a larger proportion of the released radioactivity was acid precipitable (24% vs. 5%). Some radioactivity also was recovered in the bile; of the total amount of radioactivity released from the liver in 30 min at 37 degrees C, approximately 5% was directed into the bile. At lower temperatures of perfusion, a greater fraction of the radioactivity that was released from the liver was directed into the bile (20% at 30 degrees C vs. 5% at 37 degrees C). The data imply that the endosomal pathway to the lysosome is highly sensitive to slight reductions in temperature while the transcytotic route into bile is less sensitive. Lower temperatures might prolong the residence time of ASOR in the prelysosomal endosomal compartments, and thereby increase the likelihood that undegraded ligand will be returned to the blood or be missorted into bile.     

Effetto dell'Irraggiamento X sulla Biosintesi degli Amminoacidi in Piante di Orzo

Abstract

The effects of irradiation on aminoacids biosynthesis in barley. — The effect of radiations on the biosynthesis of aminoacids in barley has been studied. Seeds have been irradiated with X-tays of 7.5 and seedlings with 7.5, 15 and 30 Kr. After 6–7 days of growth the control and irradiated seedlings, and the seedlings from control and irradiated seeds, have been supplied with 10–100 μC of C¹⁴O₂ in a closed chamber for 44–42 hours. The material was extracted with hot ethanol (85%–80% and 40%). Total radioactivity and the radioactivity of the basic fraction, eluted with HCl from Dowex 50, X-8′ (200–400 mesh, H^?) have been measured. It was observed that radiations increase the radioactivity of the aminoacid fraction, both in leaves and roots. A correlation between the decrease of fresh weight and increase of aminoacids radioactivity was shown. A possible explanation might be visualized in terms of the inhibition of growth and protein synthesis by radiations.     

A flexible sand coating (Conniflex) for the protection of conifer seedlings against damage by the pine weevil Hylobius abietis

1 A new method for the physical protection of conifer seedlings against feeding damage by Hylobius abietis (L.), is described and evaluated in field trials in Swedish forest plantations.
2 The lower 60% of the stem of the seedling is protected by the Conniflex coating, consisting of fine sand (grain size = 0.2 mm) embedded in an acrylate dispersion that remains flexible after drying.
3 Seedlings are treated in the nursery by a large-scale application procedure involving four steps: (i) spraying the seedlings with water; (ii) application of fixative to the lower sections of the stems, (iii) application of fine sand to the fixative; and (iv) drying of the fixative.
4 A field experiment over three seasons demonstrated a significant increase in survival for coated seedlings compared with untreated seedlings. The survival rate increased from 29% to 97% for Scots pine and from 26% to 86% for Norway spruce. Coating the lower 30% of the stem (instead of 60%) provided inferior protection, resulting in only 64% survival in spruce.
5 Field trials in 11 commercial plantation areas indicated that the Conniflex sand coating was as effective in protecting seedlings as treatment with the insecticide imidacloprid.
6 The new method of coating conifer seedlings with fine sand provides an effective and environmentally sound alternative to insecticide treatment.     

Asparate transcarbamylase activity in etiolated cowpea hypocotyls treated with 2,4-dichlorophenoxyacetic Acid

Treating etiolated cowpea (Vigna unguiculata) seedlings with 2,4-dichlorophenoxyacetic acid resulted in 2.5-, 3.9-, and 6.5-fold increases in DNA, soluble protein, and RNA, respectively, over untreated controls 84 hours after treatment. Aspartate transcarbamylase activity increased within 12 hours after treatment, and by 84 hours it was almost 12-fold greater than that in the untreated controls. During that time, activity in untreated controls dropped 60%. The assay used ¹⁴C-aspartate, which was then separated from the ¹⁴C-ureidosuccinate product by Dowex 50 (H⁺ form) column chromatography. Thin layer chromatography of the reaction product indicated that most of the carbamyl-phosphate-dependent radioactivity co-chromatographed with ureidosuccinate. The reaction has a pH optimum near 10.0 and is inhibited by uridine 5′-phosphate and succinate. The data suggest that aspartate transcarbamylase is important in pyrimidine biosynthesis in 2,4-dichlorophenoxyacetic acid-treated seedlings.     

Phenylalanine ammonia-lyase-induced freezing tolerance in jack pine (Pinus banksiana) seedlings treated with low, ambient levels of ultraviolet‐B radiation

Phenylalanine ammonia-lyase (PAL) induction in UVB-exposed plants leads to an increased synthesis of UV-absorbing phenols. As phenols, including anthocycanins, are linked to many protective mechanisms in plants, we tested the hypothesis that UVB-induced phenol accumulation, mediated by PAL, may confer freezing tolerance in jack pine ( Pinus banksiana Lamb) seedlings. The hypothesis was tested by applying UVB in the presence and absence of the PAL-inhibitor, 2-aminoindan-2-phosphonic acid (AIP). Jack pine seedlings were grown for 3 weeks with and without 10 µ M aqueous AIP. Each treatment was then divided into two groups. One group received near-ambient UVB (5.5 kJ m⁻² day⁻¹of biologically effective radiation) for up to 30 h. A second, control group of seedlings received no UVB. Anthocyanin concentration declined by > 99% in PAL-inhibited seedlings and other methanol-extractable UV-absorbing phenols declined by > 48%, relative to the controls. A 20-h exposure to UVB increased seedling freezing (−15°C) tolerance in the absence of the PAL-inhibitor, as shown by a 30% reduction in membrane injury, determined by electrolyte leakage measurements. In PAL-inhibited seedlings, by contrast, the same UVB pre-treatment increased freezing injury by 48%. A longer (30 h) UVB exposure was damaging to both AIP-treated and untreated seedlings. Root feeding with 10 µ M AIP during a 3-week exposure of older (6-month-old) seedlings similarly reduced phenol accumulation in UVB-exposed seedlings. The decline in phenol production in PAL-inhibited seedlings correlated with increased freezing injury. These results suggest a role for ambient UVB in seedling frost hardiness, mediated by a PAL-induced production of phenolic compounds.     

In vitro biosynthesis of 1,4-β-galactan attached to rhamnogalacturonan I

 The biosynthesis of galactan was investigated using microsomal membranes isolated from suspension-cultured cells of potato (Solanum tuberosum L. var. AZY). Incubation of the microsomal membranes in the presence of UDP-[¹⁴C]galactose resulted in a radioactive product insoluble in 70% methanol. The product released only [¹⁴C]galactose upon acid hydrolysis. Treatment of the product with Aspergillus niger endo-1,4-β-galactanase released 65–70% of the radioactivity to a 70%-methanol-soluble fraction. To a minor extent, [¹⁴C]galactose was also incorporated into proteins, however these galactoproteins were not a substrate for Aspergillus niger endo-1,4-β-galactanase. Thus, the majority of the ¹⁴C-labelled product was 1,4-β-galactan. Compounds released by the endo-1,4-β-galactanase treatment were mainly [¹⁴C]galactose and [¹⁴C]galactobiose, indicating that the synthesized 1,4-β-galactan was longer than a trimer. In vitro synthesis of 1,4-β-galactan was most active with 6-d-old cells, which are in the middle of the linear growth phase. The optimal synthesis occurred at pH 6.0 in the presence of 7.5 mM Mn²⁺. Aspergillus aculeatus rhamnogalacturonase A digested at least 50% of the labelled product to smaller fragments of approx. 14 kDa, suggesting that the synthesized [¹⁴C]galactan was attached to the endogenous rhamnogalacturonan I. When rhamnogalacturonase A digests of the labelled product were subsequently treated with endo-1,4-β-galactanase, radioactivity was not only found as [¹⁴C]galactose or [¹⁴C]galactobiose but also as larger fragments. The larger fragments were likely the [¹⁴C]galactose or [¹⁴C]galactobiose still attached to the rhamnogalacturonan backbone since treatment with β-galactosidase together with endo-1,4-β-galactanase digested all radioactivity to the fraction eluting as [¹⁴C]galactose. The data indicate that the majority of the [¹⁴C]galactan was attached directly to the rhamnose residues in rhamnogalacturonan I. Thus, isolated microsomal membranes contain enzyme activities to both initiate and elongate 1,4-β-galactan sidechains in the endogenous pectic rhamnogalacturonan I. Received: 24 June 1999 / Accepted: 30 August 1999     

Biosynthesis of γ-aminobutyric acid from spermine in maize seedlings

The administration of labelled spermine [tetramethylene-1,4-¹⁴C] to Zea mays shoots resulted in the formation of radioactive γ-aminobutyric acid (GABA). A chemical degradation of radioactive GABA suggested that its radioactivity was located on C-1 and C-4, indicating that GABA is a product of spermine metabolism in maize seedlings.     

Size of sampling unit strongly influences detection of seedling limitation in a wet tropical forest

Seedling limitation could structure communities, but often is evaluated with sampling units that are orders of magnitude smaller than mature plants. We censused seedlings for 5.5 years in five 1 × 200-m transects in a wet Neotropical forest. For 106 common species (≥ 10 seedlings in a transect), we calculated prevalence (occurrence of ≥ 1 newly emerged seedlings per sampling unit) at 1 m² and at 1 m × mature crown diameter units by aggregating adjacent quadrats. For most species, prevalence was 2–25% at 1 m², but 20–92% at mature crown scales. Increased prevalence arose from broadly distributed seedlings within transects, with unoccupied segments generally shorter than crown diameters. At the landscape scale, 69% of 301 species were locally rare (< 10 seedlings) and only 16% were represented in all transects (maximally separated by 2.4 km). Nonetheless, for more common species, much lower estimates of seedling limitation at mature crown scales suggest weaker influence of seedling limitation on community dynamics than previously assumed.     

Absorption and distribution of high specific radioactivity 2-C-abscisic Acid in cotton seedlings

High specific radioactivity (26.3 mc/mmole) racemic 2-¹⁴C-abscisic acid was synthesized. An aliquot of abscisic acid, 1.2 × 10⁻⁴m in aqueous methanolic solution, was applied to the surface of either a cotyledon or the first true leaf of 8- to 32-day-old cotton seedlings (Gossypium hirsutum L.). After various intervals (6-192 hours), the seedlings were processed for autoradiography, counting, and identification of the radioactivity. After 6 hours, radioactivity was observed moving basipetally out of the treated leaf toward the roots. Four days later, radioactivity could be detected throughout the whole seedling. After 8 days, 10% of the recovered radioactivity was found in the roots, and 80% remained in the treated leaf blade. Neither leaf type nor age had any effect on the abscisic acid movement or pattern of distribution. Isolated radioactivity from the roots was identified as abscisic acid, based on comparison with an authentic standard by thin layer chromatography, gas-liquid chromatography, or gas-liquid chromatography-mass spectrometry.     

Evidence for a convalent intermediate between alpha-glucosidase and glucose

A stable enzyme-glucose intermediate has been obtained in the short-term reaction between α-methyl-

-glucosidase and α-methyl-

-¹⁴C-glucopyranoside. A rapid-flow technique was employed in which phenol was used to terminate the reaction and to trap the product. It is believed that a covalent linkage is involved because (a) continued washing of the denatured protein failed to remove the radioactivity and (b) the radioactivity was retained by a tryptic peptide isolated by gel filtration. Treatment of the labeled protein with 2 $\text{N}$ HCl at room temperature released over 80% of the radioactivity as a compound with the same chromatographic mobility as glucose. No radioactive product was formed when bovine serum albumin replaced the enzyme, nor when glucosylamine, a potent glucosidase inhibitor, was present with the enzyme.     

The lateral transport of IAA in intact coleoptiles of Avena sativa L. and Zea mays L. during geotropic stimulation

Summary Movement of IAA was studied in excised coleoptile apices and whole seedlings of Zea mays L. and Avena sativa L. during geotropic stimulation. A micropipette technique permitted the application of [5-³H]IAA at predetermined points on the coleoptiles with minimal tissue damage.When [5-³H]IAA was applied to the upper side of a horizontal excised Zea coleoptile, about 60% of the recoverable radioactivity had moved into the lower half after 2 h. In contrast, when application was made to the lower side of a horizontal excised coleoptile, only 4% of the radioactivity migrated to the upper half. There was, thus, a net downward movement of 56%. Similar patterns of distribution were found for radioactivity in both the tissue and the basal receiver blocks. In horizontal shoot tissues of intact Zea seedlings a net downward movement of about 30% of the recoverable radioactivity occurred after 1 h of geotropic stimulation. Comparable experiments with Avena indicated a net downward movement of 6–12% in excised apices of coleoptiles and in the intact shoot. In both Zea and Avena chromatographic analyses of tissue and receiver blocks indicated that the movement of radioactivity reflected that of IAA.In Zea coleoptiles, the lateral migration of radioactivity after 2 h was 3 to 4 times greater in the apical tissues than in the basal tissues. A significant net downward movement of radioactivity was detected after 10 min of geotropic stimulation in the extreme apex of Zea coleoptiles but not in the more basal regions.These experiments show that downward lateral transport of IAA occurs in intact shoots of Zea and Avena seedlings upon geotropic stimulation. Lateral transport of IAA had previously been demonstrated only in sub-apical segments of Zea coleoptiles.     

Localization of 12-hydroxyeicosatetraenoic acid in endothelial cells

Bovine aortic endothelial cells take up 12-hydroxyeicosatetraenoic acid (12-HETE), a lipoxygenase product formed from arachidonic acid. The uptake of [3H]12-HETE reached a maximum in 2 to 4 h. At this time, from 75 to 80% of the incorporated radioactivity was contained in phospholipids, about 85% of the esterified radioactivity remained in the form of 12-HETE, and at least 90% of the phospholipid radioactivity was present in the sn-2-position. Subcellular fractionation on Percoll and sucrose gradients demonstrated that 65 to 74% of the radioactivity was present in membranes enriched in NADPH-cytochrome c reductase and UDP-galactosyl transferase. The specific radioactivity relative to protein of these intracellular membranes was 2.9-times higher than in a plasma membrane fraction enriched in 5'-nucleotidase. A similar intracellular localization was observed when [3H]5-HETE or [3H]arachidonic acid were taken up. The 12-HETE was contained primarily in the choline glycerophospholipids of the microsomal membranes. After incorporation, [3H]12-HETE was removed from the cell lipids much more rapidly than [3H]arachidonic acid, and 80% of the radioactivity released into the medium during the first hour remained as 12-HETE. Because it accumulates in microsomal membranes, 12-HETE uptake may perturb certain intracellular processes and thereby lead to endothelial dysfunction. The relatively rapid removal of the newly incorporated 12-HETE may be an important protective mechanism that prevents excessive accumulation and more extensive endothelial damage.     

Over-expression of STP13, a hexose transporter, improves plant growth and nitrogen use in Arabidopsis thaliana seedlings

In Arabidopsis thaliana , the regulation of hexose levels by the large monosaccharide transporter (MST) gene family influences many aspects of plant growth. The cloning and transgenic expression of one family member (STP13) enabled the manipulation of carbon (C) and nitrogen (N) metabolism in Arabidopsis . Transgenic seedlings constitutively over-expressing STP13 (STP13OX) had increased rates of glucose uptake, higher endogenous sucrose levels and accumulated more total C and biomass per plant when grown on soil-less media supplemented with 55 m m glucose and sufficient N (9 m m nitrate). Furthermore, STP13OX seedlings acquired 90% more total N than the Col-0 seedlings, and had higher levels of expression of the nitrate transporter NRT2.2 . In addition, STP13OX seedlings were larger and had higher biomass than Col-0 seedlings when grown under a limiting N condition (3 m m nitrate). Transgene analysis of STP13 reveals that its gene product is localized to the plasma membrane (PM) in tobacco BY-2 suspension cells, that it encodes a functional MST in planta , and that the STP13 promoter directs GUS expression to the vasculature and to leaf mesophyll cells. This work highlights the link between C and N metabolism, demonstrating that a plant's N use may be improved by increasing the availability of C.     

The Fate of l-Phenylalanine Fed to Germinating Pea Seeds, Pisum sativum (L.) var. Alaska, during Imbibition

Radioactive l-phenylalanine-l-¹⁴C or -U-¹⁴C was fed to pea seeds during imbibition. More than 95% was imbibed. Less than 1% of the radioactivity was respired as CO₂. Of the radioactivity taken into the embryos, 80% was still in the cotyledons by 3 days. About half of this was unchanged phenylalanine: 5% free, 10 to 20% in soluble proteins, 1 to 6% in cell wall proteins, and 14% released by mild acid hydrolysis. No other radioactive amino acid was found. About 0.3% of the radioactivity was identified as free caffeic, ferulic, and coumaric acids or their glycosides, and a further 5% was released by mild acid hydrolysis into a phenolic acid fraction. About half of the radioactivity in the cotyledons was lost in the fractionation procedures.