Rhizosphere Oxidation in Rice: An Analysis of Intervarietal Differences in Oxygen Flux from the Roots

The polarographic technique (Armstrong 1967) has been found suitable for the detection of significant intervarietal differences in oxygen diffusion from rice roots. All the evidence obtained supports the thesis that intervarietal differences in oxygen diffusion rate occur and the varieties tested may be arranged in order of increasing oxygen flux as follows N 36 < N 22 < N 29 < N 32 < Yubae. The lowest oxygen flux rates are found in those varieties (N 22 and N 36) susceptible to Akagare disease of rice while the flux rates of varieties resistant to Akagare (N 29 and N 32) are higher. On the basis of these results one would also expect Yubae to be resistant to Akagare. The results are discussed with reference to previous work which has shown the oxidising activity and carbohydrate exudation properties of rice roots to be similarly correlated with resistance or susceptibility to Akagare. Attention is drawn to what it is felt are the shortcomings in existing techniques for measuring the oxidising activity of roots. It is suggested that with certain provisions the polarographic technique may he found useful in any effort to build up high oxidising power into the new plant type proposed for the tropics.     

ELEMENTAL ANALYSIS OF DEPOSITS ON THE ROOTS OF SPARTINA ALTERNIFLORA LOISEL.

Spartina alterniflora, salt marsh cordgrass, is the dominant angiosperm of a large majority of regularly flooded marshes of the Atlantic and Gulf coasts of the United States. In Louisiana, this species often occurs in two distinct zones: a more productive streamside site (adjacent to tidal creeks), and a less productive and sparsely populated inland area. Reddish-brown deposits are present on the roots of streamside Spartina and visually absent from the roots of inland plants. A study of streamside roots using scanning electron microscope and energy dispersive X-ray microanalysis demonstrated that the coatings are restricted to the outer cell wall of the epidermis and are composed primarily of iron. Roots of inland plants have minor iron deposits. Citrate-dithionite extraction of the coatings and subsequent atomic absorption spectrophotometric analysis confirmed these deposits to be iron, although some manganese was present. Approximately 50 times more iron was found on streamside roots compared to roots from inland plants. These results indicate a better developed oxidized rhizosphere associated with streamside Spartina roots than the inland and, hence, the potential for a more favorable environment in which nutrient uptake may proceed.     

Investigations of intercalation in inorganic solids with layered structures: iron-57 mössbauer spectroscopy studies of size fractionated and iron-exchanged montmorillonite clays

⁵⁷Fe mössbauer spectroscopy has been used to examine some naturally occurring layer silicates in which cations located in exchange sites in the interlayer regions can be replaced by other species. The ⁵⁷Fe Mössbauer spectra recorded from differing size fractions of two types of non-exchanged and sodium-exchanged montmorillonite clays were found to be independent of the fraction size. The spectra have been interpreted in terms of the occupation by iron(III) of a heterogeneity of similar sites within the montmorillonite lattice. No justification has been obtained for computer analysis of the data in terms of more than one characterisable lattice site and no evidence has been found for the association of any iron oxyhydroxide impurity with the montmorillonite fractions.The ⁵⁷Fe Mössbauer parameters recorded from iron(III)-exchanged montmorillonite, in which iron(III) species are intercalated within the layers, show that the process is best performed at fairly low pH using low concentrations of iron(III). Failure to control such conditions can result in the formation of iron oxyhydroxides or hydrolysed iron(III) species. The preparation of iron(II)-exchanged montmorillonite was accompanied by partial oxidation of the iron(II) to iron(III).     

A theoretical study of the<Emphasis Type="Italic"> cis</Emphasis>-dihydroxylation mechanism in naphthalene 1,2-dioxygenase

The catalytic mechanism of naphthalene 1,2-dioxygenase has been investigated by means of hybrid density functional theory. This Rieske-type enzyme, which contains an active site hosting a mononuclear non-heme iron(II) complex, uses dioxygen and two electrons provided by NADH to carry out the cis-dihydroxylation of naphthalene. Since a (hydro)peroxo-iron(III) moiety has been proposed to be involved in the catalytic cycle, it was probed whether and how this species is capable of cis-dihydroxylation of the aromatic substrate. Different oxidation and protonation states of the Fe–O₂ complex were studied on the basis of the crystal structure of the enzyme with oxygen bound side-on to iron. It was found that feasible reaction pathways require a protonated peroxo ligand, Fe^III–OOH; the deprotonated species, the peroxo-iron(III) complex, was found to be inert toward naphthalene. Among the different chemical patterns which have been explored, the most accessible one involves an epoxide intermediate, which may subsequently evolve toward an arene cation, and finally to the cis-diol. The possibility that an iron(V)-oxo species is formed prior to substrate hydroxylation was also examined, but found to implicate a rather high energy barrier. In contrast, a reasonably low barrier might lead to a high-valent iron-oxo species [i.e. iron(IV)-oxo] if a second external electron is supplied to the mononuclear iron center before dioxygenation.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00775-004-0537-0     

Factors affecting iron plaque on the roots ofPhragmites australis (Cav.) Trin. ex Steudel

Phragmites australis (the common reed) was collected at six sites in southern Québec and Ontario, Canada, in order to study the accumulation of iron plaque on the roots. The deposition of iron oxides on roots ofP. australis did not correlate directly with soil measurements; however, the amounts of iron-bound-to-carbonates fraction of the soil/sediment, responsible for the iron plaque accumulation, correlated with the % of water, % of organic matter, % of clay and pH of the substrate. Plants located very near flowing water accumulated more iron plaque on the roots than plants in other habitats through the summer; it is hypothesized that carbonates associated with iron come from the flowing water. In wetlands or sites near flowing water, most root iron was found on the surface, as iron plaque, while there was more iron inside the root in dry environments. Radial oxygen loss from the roots is probably the most important source of oxygen for the oxidation of iron.     

Lupinus species differ in their requirements for iron

The effect of iron supply on the growth and nodulation ofLupinus angustifolius L. (Gungurru),Lupinus luteus L. (R-1171) andLupinus pilosus Murr. (P20957) was studied in acid solutions. Plants of the three species were grown together in the same solution and inoculated withBradyrhizobium (Lupinus) WU 425. Plants were then grown with or without applied NH₄NO₃. The lupin species differed greatly in their sensitivity to low iron concentrations in solution withL. pilosus being most tolerant andL. luteus most sensitive.L. pilosus had the highest iron concentration in tissues and had a higher ratio of iron concentration in the youngest fully expanded leaf blades (YEB) to that in roots than the other two species.L. luteus had higher iron concentrations in roots but lower iron concentration in YEB and shoots than didL. angustifolius. The requirements of internal iron for the maximal chlorophyll synthesis in YEB were 65 μg g^-1 forL. angustifolius andL. luteus, and 52 μg g^-1 forL. pilosus. In contrast to effects on growth, the three species had similar external iron requirements for nodule formation in roots and for maximal nitrogen concentrations in shoots. The results indicate that iron tolerant lupin species require lower internal and external iron supply and have a greater ability than sensitive species to translocate iron from roots to shoots.     

Culture of Acidiphilium cryptum BV1 with halotolerant Alicyclobacillus-like spp.: effects on cell growth and iron oxidation

Acidiphilium cryptum was co-isolated with two Alicyclobacillus-like species from a terrestrial saline mine site. Co-cultivation of A. cryptum BV1 with the Alicyclobacillus-like species under saline conditions resulted in faster cell growth and increased iron oxidation compared with the pure mono-cultures. The rates of iron oxidation relative to cell numbers increased three fold.     

The alternative respiratory pathway mediates carboxylate synthesis in white lupin cluster roots under phosphorus deprivation

Plant adaptations associated with a high efficiency of phosphorus (P) acquisition can be used to increase productivity and sustainability in a world with a growing population and decreasing rock phosphate reserves. White lupin (Lupinus albus) produces cluster roots that release carboxylates to efficiently mobilize P from P‐sorbing soils. It has been hypothesized that an increase in the activity of the alternative oxidase (AOX) would allow for the mitochondrial oxidation of NAD(P)H produced during citrate synthesis in cluster roots at a developmental stage when there is a low demand for ATP. We used the oxygen‐isotope fractionation technique to study the in vivo respiratory activities of the cytochrome oxidase pathway (COP) and the alternative oxidase pathway (AOP) in different root sections of white lupins grown hydroponically with and without P. In parallel, AOX protein levels and internal carboxylate concentrations were determined in cluster and non‐cluster roots. Higher in vivo AOP activity was measured in cluster roots when malate and citrate concentrations were also high, thus confirming our hypothesis. AOX protein levels were not always correlated with in vivo AOP activity, suggesting post‐translational regulation of AOX.     

The acquisition of phosphorus byLupinus albus L.

Summary It has been demonstrated by an agar film technique thatL. albus can cause the breakdown of colloids of iron/silicate, iron/phosphate, aluminium/silicate and aluminium phosphate and destabilise suspensions of manganese dioxide, calcium mono-hydrogen phosphate and ferric hydroxide. Dissolution of these compounds was most marked in areas adjacent to proteoid roots (dense clusters of secondary laterals of limited growth which develop on lateral roots) and parts of the tap root. Soil associated with these regions of the root system contained more reductants and chelating agents than the bulk soil. Soil from around the roots ofL. albus exhibited much greater reducing and chelating activity than that associated with the roots of rape and buckwheat.     

THE COMPARATIVE AND DEVELOPMENTAL MORPHOLOGY OF THE ROOT SYSTEM OF SELAGINELLA SELAGINOIDES (L.) LINK

All of the roots of Selaginella selaginoides are attached laterally to the base of the shoot, which has monopolar growth as is characteristic of Selaginella. The first three roots are produced by meristematic activity in the cortex of the hypocotyl as in several other species of Selaginella. The fourth root is produced in the same way as the first three, except that not all of the cortical cells which become meristematic mature into root tissue. Some of the meristematic tissue remains undifferentiated and continues to produce additional roots. Potentially an unlimited number of roots could be produced, but no plant was found to have more than eight. There is some secondary growth in the cortex of the basal swelling on the hypocotyl, but no secondary vascular tissue is produced and no cambium of any sort is ever organized. On the basis of comparisons with other living species of Selaginella. the centralized root system of S. selaginoides is interpreted as having been modified from a noncentralized type of root system by the persistence of the juvenile mode of root production.     

Fe reduction in cell walls of soybean roots

Reduction of Fe^IIIEDTA by excised roots of soybean seedlings (Glycine max L.) is stimulated by l-malate in the bathing solution. Reduction occurs much more rapidly with roots of seedlings grown in the absence of iron than with roots of seedlings grown with iron. Cell-wall preparations from these roots catalyze reduction of Fe^IIIEDTA by NADH. They also contain NAD⁺-dependent l-malate dehydrogenase. Enzymic activity of the cell-wall preparations is not affected by previous iron nutrition of the plants, but the amount of l-malate in the roots is increased when seedlings have been deprived of iron. We propose that reduction of iron before absorption by soybean roots occurs in the cell-wall space, with l-malate secreted from the roots serving as the source of electrons. Part of the iron reductase activity of the cell walls can be solubilized by extraction with 1 molar NaCl. The enzyme has been partially purified.     

Effect of lectins from <Emphasis Type="Italic">Azospirillum brasilense</Emphasis> to peroxidase and oxalate oxidase activity regulation in wheat roots

Lectins were extracted from the surface of nitrogen-fixing soil bacteria Azospirillum brasilense Sp7 and from its mutant A. brasilense Sp7.2.3 defective in lectin activity. The ability of lectins to stimulate the rapid formation of hydrogen peroxide related to increase of oxalate oxidase and peroxidase activity in the roots of wheat seedlings has been demonstrated. The most rapid induced pathway of hydrogen peroxide formation in the roots of wheat seedlings was the oxalic acid oxidation by oxalate oxidase which is the effect of lectin in under 10 min in a concentration of 10 μg/ml. The obtained results show that lectins from Azospirillum are capable of inducing the adaptation processes in the roots of wheat seedlings.     

Rhizosphere Oxidation in Rice and other Species: A Mathematical Model Based on the Oxygen Flux Component

A mathematical model is presented from which one can predict the likely dimensions of oxidised rhizospheres due to oxygen diffusing from roots into anaerobic media such as wet soil. The analysis applies Fick's law of diffusion to the diffusion of oxygen from a cylindrical object (the root) into a sink (the soil) which is absorbing oxygen at a constant rate M. Solution of the final equation gives the dimensions of the oxygenated rhizosphere, i.e. the distance from the root at which the oxygen concentration becomes zero. The results obtained supoprt the view that oxygen diffusing from roots will produce a rhizosphere ‘sheath’ charged with oxygen. Furthermore, some of the predicted limits for this oxygen sheath correspond with those of ferric iron sheaths around roots in waterlogged soils. Calculations also show that oxygenated rhizospheres around the roots of plants very tolerant of reducing conditions. e.g. Menyanthes trifoliata, Oryza sativa and Eriophorum angustifolium, should be from two to three times as broad as for Molinia coerulea which is a species intolerant of strong reducing conditions. Consequently if oxidation reactions are not instantaneous but occur only gradually as quantities of reduced products enter the oxygenated zone, then the plants with the larger rhizospheres will be significantly better protected from absorbing large amounts of reduced products than will those plants with smaller oxygenated zones. This may in part explain the reason for intervarietal defferences in physiological disease resistance in rice. Finally the observation is made that small lateral roots can be expected to oxygenate a zone nearly as large as, or probably larger, than the primary root from which they have arisen.     

Microbial Processes Associated with Roots of Bulbous Rush Coated with Iron Plaques

Bulbous rush (Juncus bulbosus) is a pioneer species in acidic, iron-rich, coal mining lakes in the eastern part of Germany. Juncus roots are coated with iron plaques, and it has been suggested that microbial processes under the iron plaques might be supportive for Juncus plant growth. The objectives of this work were to enumerate the microbes involved in the turnover of iron and organic root exudates in the rhizoplane, to investigate the effect of oxygen and pH on the utilization of these exudates by the rhizobacteria, and to study the ability of the root-colonizing microbiota to reduce sulfate. Enumeration studies done at pH 3 demonstrated that 10⁶ Fe(III) reducers and 10⁷ Fe(II) oxidizers g (fresh wt root)^–1 were associated with Juncus roots. When roots were incubated in goethite-containing medium without and with supplemental glucose, Fe(II) was formed at rates approximating 1.1 mmol g (fresh wt root) ^–1 d^–1 and 3.6 mmol g (fresh wt root)^–1 d^–1 under anoxic conditions, respectively. These results suggest that a rapid microbially mediated cycling of iron occurs in the rhizosphere of Juncus roots under changing redox conditions. Most-probable-number estimates of aerobes and anaerobes capable of consuming root exudates at pH 3 were similar in the rhizosphere sediment and in Juncus roots, but numbers of aerobes were significantly higher than those of anaerobes. At pH 3, supplemental organic exudates were primarily subject to aerobic oxidation to CO₂ and not subject to fermentation. However, at pH 4.5, root exudates were also rapidly utilized under anoxic conditions. Root-associated sulfate reduction was not observed at pH 3 to 4.5 but was observed at pH 4.9. The pH increased during all root-incubation studies both under oxic and anoxic conditions. Thus, as result of the microbial turnover of organic root exudates, pH and CO₂ levels might be elevated at the root surface and favor Juncus plants to colonize acidic habitats.     

SPATIAL AND TEMPORAL DEVELOPMENT OF IRON(III) REDUCTASE ACTIVITY IN ROOT SYSTEMS OF PISUM SATIVUM (FABACEAE) CHALLENGED WITH IRON-DEFICIENCY STRESS

The development of plasma membrane-associated iron(III) reductase activity was characterized in root systems of Pisum sativum during the first 2 wk of growth, as plants were challenged with iron-deficiency stress. Plants of a parental genotype (cv. Sparkle) and a functional iron-deficiency mutant genotype (E107) were grown hydroponically with or without supplemental iron. Iron(III) reductase activity was visualized by placing the roots in an agarose matrix containing 0.2 idm Fe(III)-ethylenediaminetetraacetic acid and 0.3 mM Na₂-bathophenanthrolinedisulfonic acid (BPDS). Red staining patterns, resulting from the formation of Fe(II)-BPDS, were used to identify iron(III)-reducing regions. Iron(III) reduction was extensive on roots of E107 as early as d 7, but not until d 11 for -Fe-treated Sparkle. Roots of +Fe-treated Sparkle showed limited regions of reductase activity throughout the period of study. For secondary lateral roots, iron(III) reduction was found for all growth types except + Fe-treated Sparkle. Treating Sparkle plants alternately to a cycle of iron deficiency, iron sufficiency, and iron deficiency revealed that reductase activity at a given root zone could be alternatively present, absent, and again present. Our results suggest that for Pisum roots grown under the present conditions, iron-deficiency stress induces the activation of iron(III) reductase capacity within 2 d.     

Iron Deficiency Decreases Suberization in Bean Roots through a Decrease in Suberin-Specific Peroxidase Activity

The suberin content of young root parts of iron-deficient and iron-sufficient Phaseolus vulgaris L. cv Prélude was determined. The aliphatic components that could be released from suberin-enriched fractions by LiAID₄ depolymerization were identified by gas chromatography-mass spectrometry. In the normal roots, the major aliphatic components were ω-hydroxy acids and dicarboxylic acids in which saturated C₁₆ and monounsaturated C₁₈ were the dominant homologues. Iron-deficient bean roots contained only 11% of the aliphatic components of suberin found in control roots although the relative composition of the constituents was not significantly affected by iron deficiency. Analysis of the aromatic components of the suberin polymer that could be released by alkaline nitrobenzene oxidation of bean root samples showed a 95% decrease in p-hydroxybenzaldehyde, vanillin, and syringaldehyde under iron-deficient conditions. The inhibition of suberin synthesis in bean roots was not due to a decrease in Fe-dependent ω-hydroxylase activity since normal ω-hydroxylation could be demonstrated, both in vitro with microsomal preparations and in situ by labeling of ω-hydroxy and dicarboxylic acids with [¹⁴C]acetate. The level of the isozyme of peroxidase that is specifically associated with suberization was suppressed by iron deficiency to 25% of that found in control roots. None of the other extracted isozymes of peroxidase was affected by the iron nutritional status. The activity of the suberin-associated peroxidase was restored within 3 to 4 days after application of iron to the growth medium. The results suggest that, in bean roots, iron deficiency causes inhibition of suberization by causing a decrease in the level of isoperoxidase activity which is required for polymerization of the aromatic domains of suberin, while the ability to synthesize the aliphatic components of the suberin polymer is not impaired.     

Genetic evidence that induction of root Fe(III) chelate reductase activity is necessary for iron uptake under iron deficiency†

Reduction of Fe(III) to Fe(II) by Fe(III) chelate reductase is thought to be an obligatory step in iron uptake as well as the primary factor in making iron available for absorption by all plants except grasses. Fe(III) chelate reductase has also been suggested to play a more general role in the regulation of cation absorption. In order to experimentally address the importance of Fe(III) chelate reductase activity in the mineral nutrition of plants, three Arabidopsis thaliana mutants (frd1-1, frd1-2 and frd1-3), that do not show induction of Fe(III) chelate reductase activity under iron-deficient growth conditions, have been isolated and characterized. These mutants are still capable of acidifying the rhizosphere under iron-deficiency and accumulate more Zn and Mn in their shoots relative to wild-type plants regardless of iron status. frd1 mutants do not translocate radiolabeled iron to the shoots when roots are presented with a tightly chelated form of Fe(III). These results: (1) confirm that iron must be reduced before it can be transported, (2) show that Fe(III) reduction can be uncoupled from proton release, the other major iron-deficiency response, and (3) demonstrate that Fe(III) chelate reductase activity per se is not necessarily responsible for accumulation of cations previously observed in pea and tomato mutants with constitutively high levels of Fe(III) chelate reductase activity.     

The formation of iron plaques on roots and rhizomes of the seagrass Cymodocea serrulata (R. Brown) Ascherson with implications for sulphide intrusion

Despite the fact that iron plaque formation is ubiquitous in aquatic macrophytes and has been known for several decades, there are few reports of plaque occurrence in seagrasses to date. Herein we present the first microscopical observation and chemical quantification of iron (Fe) plaques on the shoots, rhizomes and roots of the seagrass Cymodocea serrulata (R. Brown) Ascherson collected from intertidal seagrass beds in Thailand. Plaques were observed on shoot bases, rhizomes and roots with the highest concentrations of iron in the plaques from the roots, reaching an average of 509 μmol gDW⁻¹. Interestingly, the most negative stable sulphur isotope (δ³⁴S) values, indicating H₂S intrusion into the plants occurred in the sampling site with the most intense root oxidizing capacity, as indicated by a greater Fe plaque formation. These apparently contradictory findings may be attributed to oxidizing capacity of root tips and root hairs sufficient to promote Fe(III) deposition in the rhizosphere, preceding deposition of plaques on the roots. While this rhizosphere oxidation may result in a more efficient sulphide detoxification during the day photosynthetic phase, root tips and hairs may serve as vulnerable sites for sulphide intrusion at night. The presence of Fe plaque on C. serrulata roots and rhizomes reveals the complexity of seagrass–sediment interactions and deserves further attention to understand if this is a local phenomenon or a newly discovered adaptive mechanism in seagrasses.     

Microbial degradation of the morphine alkaloids: identification of morphinone as an intermediate in the metabolism of morphine by Pseudomonas putida M10

A strain of Pseudomonas putida was isolated by selective enrichment with morphine that was capable of utilising morphine as a primary source of carbon and energy for growth. Experiments with whole cells showed that both morphine and codeine, but not thebaine, could be utilised. A novel NADP-dependent dehydrogenase, morphine dehydrogenase, was purified from crude cell extracts and was shown to be capable of oxidising morphine and codeine to morphinone and codeinone, respectively. This NADP-dependent morphine dehydrogenase was not observed in any other species of pseudomonads examined and was quite distinct from the -hydroxysteroid dehydrogenase found in Pseudomonas testosteroni, which had previously been shown to have activity against morphine.