荞麦根吸收铝和分泌草酸的部位

荞麦根在Al诱导下分泌草酸。从根尖开始依次划分为 0～ 0 .5、0 .5～ 1.0、1.0～ 1.5、1.5～ 2 .0cm区段 ,在 0 .5～ 1.0cm区Al的吸收最多 ,草酸的分泌部位主要在 0～ 0 .5cm区 ,显示Al诱导下 ,Al的吸收和草酸的分泌不在同一部位     

Oxalic Acid Metabolism in Begonia semperflorens

The primary accumulation of oxalate in the early seedling stagesaccompanies the synthesis of both protein and reserve carbohydrate.A further accumulation accompanies the normal growth of theyoung expanding leaf. Free oxalic acid accounts for most ofthe total oxalate content at the various stages. The form in which nitrogen is supplied to the young seedling(ammonium nitrogen or nitrate nitrogen) markedly affects bothgrowth and the amount of oxalate formed. More oxalate and poorergrowth is found in plants grown on ammonium as a sole sourceof nitrogen. In the mature leaf the oxalate content remains constant undera wide variety of different conditions. No enzyme systems could be detected which might be active inthe formation of oxalate. It is concluded that oxalic acid is only formed during rapidgrowth in Begonia, and that once formed it takes little furtherpart in metabolism.     

The Metabolism of Aluminum Citrate and Biosynthesis of Oxalic Acid in <Emphasis Type="Italic">Pseudomonas fluorescens</Emphasis>

¹³CNMR and ¹HNMR studies revealed that aluminum citrate (Al-citrate) was metabolized intracellularly and that oxalic acid was an important product in the Al-stressed cells. This dicarboxylic acid was produced via the oxidation of glyoxylate, a precursor generated through the cleavage of isocitrate. In the control cells, citrate was biotransformed essentially with the aid of regular tricarboxylic cycle (TCA) enzymes. However, these control cells were able neither to uptake nor to metabolize Al-citrate. Al-stressed cells obtained at 38–40 h of growth showed maximal Al-citrate uptake and biotransforming activities. At least a fourfold increase in the activity of the enzyme isocitrate lyase (ICL, E. C. 4.1.3.1) has been observed in the Al-stressed cells compared with the control cells. The transport of Al-citrate was sensitive to p-dinitrophenol and sodium azide, but not to dicyclohexylcarbodiimide. Experiments with the dye 9-aminoacridine revealed that the translocation of Al-citrate led to an increase in intracellular pH. Thus, it appears that after the uptake of Al-citrate, this complex is metabolized intracellularly. Received: 13 August 2002 / Accepted: 4 September 2002     

Aluminum Resistance in the Arabidopsis Mutant alr-104 Is Caused by an Aluminum-Induced Increase in Rhizosphere pH

A mechanism that confers increased Al resistance in the Arabidopsis thaliana mutant alr-104 was investigated. A modified vibrating microelectrode system was used to measure H⁺ fluxes generated along the surface of small Arabidopsis roots. In the absence of Al, no differences in root H⁺ fluxes between wild type and alr-104 were detected. However, Al exposure induced a 2-fold increase in net H⁺ influx in alr-104 localized to the root tip. The increased flux raised the root surface pH of alr-104 by 0.15 unit. A root growth assay was used to assess the Al resistance of alr-104 and wild type in a strongly pH-buffered nutrient solution. Increasing the nutrient solution pH from 4.4 to 4.5 significantly increased Al resistance in wild type, which is consistent with the idea that the increased net H⁺ influx can account for greater Al resistance in alr-104. Differences in Al resistance between wild type and alr-104 disappeared when roots were grown in pH-buffered medium, suggesting that Al resistance in alr-104 is mediated only by pH changes in the rhizosphere. This mutant provides the first evidence, to our knowledge, for an Al-resistance mechanism based on an Al-induced increase in root surface pH.     

Hydrogenase-3 Contributes to Anaerobic Acid Resistance of Escherichia coli

Background

Hydrogen production by fermenting bacteria such as Escherichia coli offers a potential source of hydrogen biofuel. Because H₂ production involves consumption of 2H⁺, hydrogenase expression is likely to involve pH response and regulation. Hydrogenase consumption of protons in E. coli has been implicated in acid resistance, the ability to survive exposure to acid levels (pH 2–2.5) that are three pH units lower than the pH limit of growth (pH 5–6). Enhanced survival in acid enables a larger infective inoculum to pass through the stomach and colonize the intestine. Most acid resistance mechanisms have been defined using aerobic cultures, but the use of anaerobic cultures will reveal novel acid resistance mechanisms.

Methods and Principal Findings

We analyzed the pH regulation of bacterial hydrogenases in live cultures of E. coli K-12 W3110. During anaerobic growth in the range of pH 5 to 6.5, E. coli expresses three hydrogenase isoenzymes that reversibly oxidize H₂ to 2H⁺. Anoxic conditions were used to determine which of the hydrogenase complexes contribute to acid resistance, measured as the survival of cultures grown at pH 5.5 without aeration and exposed for 2 hours at pH 2 or at pH 2.5. Survival of all strains in extreme acid was significantly lower in low oxygen than for aerated cultures. Deletion of hyc (Hyd-3) decreased anoxic acid survival 3-fold at pH 2.5, and 20-fold at pH 2, but had no effect on acid survival with aeration. Deletion of hyb (Hyd-2) did not significantly affect acid survival. The pH-dependence of H₂ production and consumption was tested using a H₂-specific Clark-type electrode. Hyd-3-dependent H₂ production was increased 70-fold from pH 6.5 to 5.5, whereas Hyd-2-dependent H₂ consumption was maximal at alkaline pH. H₂ production, was unaffected by a shift in external or internal pH. H₂ production was associated with hycE expression levels as a function of external pH.

Conclusions

Anaerobic growing cultures of E. coli generate H₂ via Hyd-3 at low external pH, and consume H₂ via Hyd-2 at high external pH. Hyd-3 proton conversion to H₂ is required for acid resistance in anaerobic cultures of E. coli.     

Aluminum-Resistant Arabidopsis Mutants That Exhibit Altered Patterns of Aluminum Accumulation and Organic Acid Release from Roots

Al-resistant (alr) mutants of Arabidopsis thaliana were isolated and characterized to gain a better understanding of the genetic and physiological mechanisms of Al resistance. alr mutants were identified on the basis of enhanced root growth in the presence of levels of Al that strongly inhibited root growth in wild-type seedlings. Genetic analysis of the alr mutants showed that Al resistance was semidominant, and chromosome mapping of the mutants with microsatellite and random amplified polymorphic DNA markers indicated that the mutants mapped to two sites in the Arabidopsis genome: one locus on chromosome 1 (alr-108, alr-128, alr-131, and alr-139) and another on chromosome 4 (alr-104). Al accumulation in roots of mutant seedlings was studied by staining with the fluorescent Al-indicator dye morin and quantified via inductively coupled argon plasma mass spectrometry. It was found that the alr mutants accumulated lower levels of Al in the root tips compared with wild type. The possibility that the mutants released Al-chelating organic acids was examined. The mutants that mapped together on chromosome 1 released greater amounts of citrate or malate (as well as pyruvate) compared with wild type, suggesting that Al exclusion from roots of these alr mutants results from enhanced organic acid exudation. Roots of alr-104, on the other hand, did not exhibit increased release of malate or citrate, but did alkalinize the rhizosphere to a greater extent than wild-type roots. A detailed examination of Al resistance in this mutant is described in an accompanying paper (J. Degenhardt, P.B. Larsen, S.H. Howell, L.V. Kochian [1998] Plant Physiol 117: 19–27).     

Metabolic Conversion of l-Ascorbic Acid to Oxalic Acid in Oxalate-accumulating Plants

l-Ascorbic acid-1-(14)C and its oxidation product, dehydro-l-ascorbic acid, produced labeled oxalic acid in oxalate-accumulating plants such as spinach seedlings (Spinacia oleracea) and the detached leaves of woodsorrel (Oxalis stricta and O. oregana), shamrock (Oxalis adenopylla), and begonia (Begonia evansiana). In O. oregana, conversion occurred equally well in the presence or absence of light. This relationship between l-ascorbic acid metabolism and oxalic acid formation must be given careful consideration in attempts to explain oxalic accumulation in plants.     

Benzothiadiazole-Mediated Induced Resistance to Fusarium oxysporum f. sp. radicis-lycopersici in Tomato

Benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), a synthetic chemical, was applied as a foliar spray to tomato (Lycopersicon esculentum) plants and evaluated for its potential to confer increased resistance against the soil-borne pathogen Fusarium oxysporum f. sp. radicis-lycopersici (FORL). In nontreated tomato plants all root tissues were massively colonized by FORL hyphae. Pathogen ingress toward the vascular stele was accompanied by severe host cell alterations, including cell wall breakdown. In BTH-treated plants striking differences in the rate and extent of fungal colonization were observed. Pathogen growth was restricted to the epidermis and the outer cortex, and fungal ingress was apparently halted by the formation of callose-enriched wall appositions at sites of fungal penetration. In addition, aggregated deposits, which frequently established close contact with the invading hyphae, accumulated in densely colonized epidermal cells and filled most intercellular spaces. Upon incubation of sections with gold-complexed laccase for localization of phenolic-like compounds, a slight deposition of gold particles was observed over both the host cell walls and the wall appositions. Labeling was also detected over the walls of fungal cells showing signs of obvious alteration ranging from cytoplasm disorganization to protoplasm retraction. We provide evidence that foliar applications of BTH sensitize susceptible tomato plants to react more rapidly and more efficiently to FORL attack through the formation of protective layers at sites of potential fungal entry.     

Heritable Transmission of Stress Resistance by High Dietary Glucose in Caenorhabditis elegans

Glucose is a major energy source and is a key regulator of metabolism but excessive dietary glucose is linked to several disorders including type 2 diabetes, obesity and cardiac dysfunction. Dietary intake greatly influences organismal survival but whether the effects of nutritional status are transmitted to the offspring is an unresolved question. Here we show that exposing Caenorhabditis elegans to high glucose concentrations in the parental generation leads to opposing negative effects on fecundity, while having protective effects against cellular stress in the descendent progeny. The transgenerational inheritance of glucose-mediated phenotypes is dependent on the insulin/IGF-like signalling pathway and components of the histone H3 lysine 4 trimethylase complex are essential for transmission of inherited phenotypes. Thus dietary over-consumption phenotypes are heritable with profound effects on the health and survival of descendants.     

Further Studies on Oxalic Acid Biosynthesis in Oxalate-accumulating Plants

l-Ascorbic acid functions as a precursor of oxalic acid in several oxalate-accumulating plants. The present study extends this observation to include Rumex crispus L. (curly dock), Amaranthus retroflexus L. (red root pigweed), Chenopodium album L. (lamb's-quarters), Beta vulgaris L. (sugar beet), Halogeton glomeratus M. Bieb. (halogeton), and Rheum rhabarbarum L. (rhubarb). Several species with low oxalate content are also examined.     

Antioxidant Systems in Sunflower as Affected by Oxalic Acid

Changes in antioxidant systems in sunflower (Helianthus annuus L.) inbred lines were studied in relation to different concentrations (0, 2, 3 and 4 mM) of oxalic acid. A great variability of the total superoxide dismutase (SOD) activity was observed in different genotypes after application of oxalic acid. Genotypes showing no change in SOD activity, as well as those with decreased and increased activities, have been found. The highest guaiacol peroxidase (GPX) activity was recorded in the leaves of Ha-26A genotype where the activity of SOD was low. In genotypes with increased GPX activity the malonyldialdehyde content was low. The increase in reduced glutathione content occured in almost all of the genotypes studied with the increase in oxalate concentration. Some genotypes such as PR-ST-3B, showed distinctive and combined activity of several antioxidant systems. Also, obtained results for the content of total phenols support the idea of using soluble phenols as molecular markers and confirm the hypothesis of the defensive role of these compounds in plants. This revised version was published online in July 2006 with corrections to the Cover Date.     

An ATP and Oxalate Generating Variant Tricarboxylic Acid Cycle Counters Aluminum Toxicity in Pseudomonas fluorescens

Although the tricarboxylic acid (TCA) cycle is essential in almost all aerobic organisms, its precise modulation and integration in global cellular metabolism is not fully understood. Here, we report on an alternative TCA cycle uniquely aimed at generating ATP and oxalate, two metabolites critical for the survival of Pseudomonas fluorescens. The upregulation of isocitrate lyase (ICL) and acylating glyoxylate dehydrogenase (AGODH) led to the enhanced synthesis of oxalate, a dicarboxylic acid involved in the immobilization of aluminum (Al). The increased activity of succinyl-CoA synthetase (SCS) and oxalate CoA-transferase (OCT) in the Al-stressed cells afforded an effective route to ATP synthesis from oxalyl-CoA via substrate level phosphorylation. This modified TCA cycle with diminished efficacy in NADH production and decreased CO₂-evolving capacity, orchestrates the synthesis of oxalate, NADPH, and ATP, ingredients pivotal to the survival of P. fluorescens in an Al environment. The channeling of succinyl-CoA towards ATP formation may be an important function of the TCA cycle during anaerobiosis, Fe starvation and O₂-limited conditions.     

Ion Fluxes and Short-Circuit Current in Internally Perfused Cells of Valonia ventricosa

Ion transport in the giant celled marine alga, Valonia ventricosa, was studied during internal perfusion and short-circuiting of the vacuole potential. The perfusing and bathing solutions were similar to natural Valonia sap and contained the following concentrations of major ions: Na 51, K 618, and Cl 652 mM. The average short-circuit current (I _sc) was 97 pEq/cm² sec (inward positive current), and the average open-circuit potential difference (PD) was 74 mv (vacuole positive to external solution). Perfused and short-circuited cells showed a small net influx of Na (2.0 pEq/cm² sec) and large net influxes of K (80 pEq/cm² sec) and Cl (50 pEq/cm² sec). Unidirectional K influx was proportional to I _sc, but more than one-half of the I _sc remained unaccounted for. Both the I _sc and PD were partly light-dependent, declining rapidly during the first 1–2 min of darkness. Ouabain (5 x 10^-4 M) had little effect on the influx of Na or K and had no effect on I _inf or PD. Fluid was absorbed at a rate of about 93 pliter/cm² sec. Reversing the direction of fluid movement by adding mannitol to the outside solution had little effect on ion movements. The ionic and electrical properties of normal and perfused cells of Valonia are compared.     

Nematode Community Structure in Forest Woodlots. II. Ordination of Nematode Communities

Intersite relationships among nematode communities of 18 Indiana mixed hardwood stands of varying composition, soils, physiography and past management practices were determined by community ordination techniques. All sites were sampled in April, July and October of 1968 and 1969, and ordinations were based on the number of individuals of each nematode species at each site at each sampling period. The resulting groupings correlated well with groupings based on forest types and successional stages of the tree communities at the sites, and also with groupings based on well-defined soil types. Results were similar to those obtained previously with a resemblance equation which used qualitative data only; but the present study provided more information on species associations and relationships and ecological distance between sites.     

Cyanide Resistance in Achromobacter II. Mechanism of Cyanide Resistance.

Oka, Tetuo (University of Tokyo, Tokyo, Japan), and Kei Arima. Cyanide resistance in Achromobacter. II. Mechanism of cyanide resistance. J. Bacteriol. 90:744-747. 1965.-Photochemical data showed that the only oxidase found in the cyanide-sensitive cells of Achromobacter was cytochrome o, and that cyanide-resistant cells contained at least two oxidases. The oxidase responsible for cyanide resistance was a pigment the CO compound of which had its absorption band at a wavelength longer than 580 mmu. In addition, kinetic data suggested that there were two oxidases having different affinities for cyanide. From the data presented, resistance to cyanide in Achromobacter strain D was attributed to the induced formation of cytochrome a(2), which has a very low affinity for cyanide. Several characteristics of cytochrome a(2) as a cytochrome oxidase are summarized.