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PubMed1982

Relative Content of NO(3) and Reduced N in Xylem Exudate as an Indicator of Root Reduction of Concurrently Absorbed NO(3).

It is unclear if the relative content of NO(3) (-) and reduced N in xylem exudate provides an accurate estimate of the percentage reduction of concurrently absorbed NO(3) (-) in the root. Experiments were conducted to determine whether NO(3) (-) and reduced N in xylem exudate of vegetative, nonnodulated soybean plants (Glycine max [L.] Merr., ;Ransom') originated from exogenous recently absorbed (15)NO(3) (-) or from endogenous (14)N pools. Plants either were decapitated and exposed to (15)NO(3) (-) solutions for 2 hours or were decapitated for the final 20 minutes of a 50-minute exposure to (15)NO(3) (-) in the dark and in the light. Considerable amounts of (14)NO(3) (-) and reduced (14)N were transported into the xylem, but almost all of the (15)N was present as (15)NO(3) (-). Dissimilar changes in transport of (14)NO(3) (-), reduced (14)N and (15)NO(3) (-) during the 2 hours of sap collection resulted in large variability over time in the percentage of total N in the exudate which was reduced N. Over a 20-minute period the rate of (15)N transport into the xylem of decapitated plants was only 21 to 36% of the (15)N delivered to the shoot of intact plants. Based on the proportion of total (15)N which was found as reduced (15)N in exudate and in intact plants in the dark, it was estimated that 5 to 17% of concurrently absorbed (15)NO(3) (-) was reduced in the root. This was much less than the 38 to 59% which would have been predicted from the relative content of total NO(3) (-) and total reduced N in the xylem exudate.

Journal Article↗

PubMed2001

Allelochemicals in wheat (Triticum aestivum L.): cultivar difference in the exudation of phenolic acids.

Analysis by GC-MS/MS showed that a worldwide collection of 58 wheat accessions differed significantly in the amounts of 7 known phenolic acids exuded by the living roots of 17-day-old wheat seedlings. The quantities of exuded allelochemicals varied with the specific compound and ranged from 2.3 to 18.6, from 0.6 to 17.5, from 0.1 to 4.9, from 0.0 to 52.7, from 0.33 to 12.7, from 1.5 to 20.5, and from 1.6 to 23.4 microg/L of water/agar for p-hydroxybenzoic, vanillic, cis-p-coumaric, syringic, cis-ferulic, trans-p-coumaric, and trans-ferulic acids, respectively. The concentrations of p-hydroxybenzoic and vanillic acids exuded by wheat seedlings were normally distributed in the 58 accessions. The level of each phenolic acid in root exudates did not correlate well to that previously observed in wheat. In comparison with weakly allelopathic accessions, strongly allelopathic accessions exuded larger quantities of allelochemicals into the growth medium. The chemical basis for wheat seedling allelopathy is an area for further investigation.

Carboxylic Acids↗

PubMed2006

Extracellular proteins in pea root tip and border cell exudates.

Newly generated plant tissue is inherently sensitive to infection. Yet, when pea (Pisum sativum) roots are inoculated with the pea pathogen, Nectria haematococca, most newly generated root tips remain uninfected even though most roots develop lesions just behind the tip in the region of elongation. The resistance mechanism is unknown but is correlated spatially with the presence of border cells on the cap periphery. Previously, an array of >100 extracellular proteins was found to be released while border cell separation proceeds. Here we report that protein secretion from pea root caps is induced in correlation with border cell separation. When this root cap secretome was proteolytically degraded during inoculation of pea roots with N. haematococca, the percentage of infected root tips increased from 4% +/- 3% to 100%. In control experiments, protease treatment of conidia or roots had no effect on growth and development of the fungus or the plant. A complex of >100 extracellular proteins was confirmed, by multidimensional protein identification technology, to comprise the root cap secretome. In addition to defense-related and signaling enzymes known to be present in the plant apoplast were ribosomal proteins, 14-3-3 proteins, and others typically associated with intracellular localization but recently shown to be extracellular components of microbial biofilms. We conclude that the root cap, long known to release a high molecular weight polysaccharide mucilage and thousands of living cells into the incipient rhizosphere, also secretes a complex mixture of proteins that appear to function in protection of the root tip from infection.

14-3-3 Proteins↗

PubMed2001

Differential RNA expression of alpha-expansin gene family members in the parasitic angiosperm Triphysaria versicolor (Scrophulariaceae).

Haustoria are parasitic plant specific organs that locate, attach to, and invade host plant tissues. Parasitic species of the Scrophulariaceae develop haustoria on their roots in response to chemical signals released by host plant roots. Haustorium development was induced in vitro in roots of the parasitic Scrophulariaceae Triphysaria versicolor by treating them with exudates obtained from maize roots, the chemical 2,6-dimethoxybenzoquinone (DMBQ) or the cytokinin 6-benzylaminopurine (BAP). Morphological responses of T. versicolor roots to these haustoria inducing factors (HIFs) included localized swelling and epidermal hair proliferation near the root tips. These responses were not observed when roots of the non-parasitic Scrophulariaceae Lindenbergia muraria were similarly treated. Because expansin proteins are closely associated with plant cell wall expansion and growth, we examined the expression of expansin genes in response to HIFs. We isolated cDNAs homologous to transcripts encoding three distinct alpha-expansin proteins in T. versicolor. Northern-blot analyses indicated that these transcripts were differentially abundant in different tissues. Steady-state levels of two expansin transcripts increased in T. versicolor roots exposed to BAP, but not DMBQ or maize root exudates. Expansin transcript abundance also increased in L. muraria in response to BAP treatment. These results suggest that the expansins examined fulfill functions distinct from haustorium development.

Adenine↗

PubMed2000

Regulation of sulfur nutrition in wild-type and transgenic poplar over-expressing gamma-glutamylcysteine synthetase in the cytosol as affected by atmospheric H2S.

This study with poplar (Populus tremula x Populus alba) cuttings was aimed to test the hypothesis that sulfate uptake is regulated by demand-driven control and that this regulation is mediated by phloem-transported glutathione as a shoot-to-root signal. Therefore, sulfur nutrition was investigated at (a) enhanced sulfate demand in transgenic poplar over-expressing gamma-glutamylcysteine (gamma-EC) synthetase in the cytosol and (b) reduced sulfate demand during short-term exposure to H2S. H(2)S taken up by the leaves increased cysteine, gamma-EC, and glutathione concentrations in leaves, xylem sap, phloem exudate, and roots, both in wild-type and transgenic poplar. The observed reduced xylem loading of sulfate after H2S exposure of wild-type poplar could well be explained by a higher glutathione concentration in the phloem. In transgenic poplar increased concentrations of glutathione and gamma-EC were found not only in leaves, xylem sap, and roots but also in phloem exudate irrespective of H(2)S exposure. Despite enhanced phloem allocation of glutathione and its accumulation in the roots, sulfate uptake was strongly enhanced. This finding is contradictory to the hypothesis that glutathione allocated in the phloem reduces sulfate uptake and its transport to the shoot. Correlation analysis provided circumstantial evidence that the sulfate to glutathione ratio in the phloem may control sulfate uptake and loading into the xylem, both when the sulfate demand of the shoot is increased and when it is reduced.

Biological Transport↗

PubMed2006

Rates of root and organism growth, soil conditions, and temporal and spatial development of the rhizosphere.

BACKGROUND: Roots growing in soil encounter physical, chemical and biological environments that influence their rhizospheres and affect plant growth. Exudates from roots can stimulate or inhibit soil organisms that may release nutrients, infect the root, or modify plant growth via signals. These rhizosphere processes are poorly understood in field conditions. SCOPE AND AIMS: We characterize roots and their rhizospheres and rates of growth in units of distance and time so that interactions with soil organisms can be better understood in field conditions. We review: (1) distances between components of the soil, including dead roots remnant from previous plants, and the distances between new roots, their rhizospheres and soil components; (2) characteristic times (distance(2)/diffusivity) for solutes to travel distances between roots and responsive soil organisms; (3) rates of movement and growth of soil organisms; (4) rates of extension of roots, and how these relate to the rates of anatomical and biochemical ageing of root tissues and the development of the rhizosphere within the soil profile; and (5) numbers of micro-organisms in the rhizosphere and the dependence on the site of attachment to the growing tip. We consider temporal and spatial variation within the rhizosphere to understand the distribution of bacteria and fungi on roots in hard, unploughed soil, and the activities of organisms in the overlapping rhizospheres of living and dead roots clustered in gaps in most field soils. CONCLUSIONS: Rhizosphere distances, characteristic times for solute diffusion, and rates of root and organism growth must be considered to understand rhizosphere development. Many values used in our analysis were estimates. The paucity of reliable data underlines the rudimentary state of our knowledge of root-organism interactions in the field.

Diffusion↗

PubMed1982

Penetration of soybean root systems by abscisic Acid isomers.

The penetration of soybean (Glycine max L. cv. Ransom) root systems by exogenously applied isomers of abscisic acid was monitored by measuring the concentration of the chemical in the xylem exudate of root systems exposed to a three bar hydrostatic pressure difference. The cis-trans isomer penetrated more readily than the trans-trans isomer; however, up to 6 hours was needed to reach steady-state values. Exogenous abscisic acid also decreased volume flux through the root system and increased total carbon dioxide efflux from the vessel containing the root system.

Journal Article↗

PubMed2005

A role for cyclic hydroxamates in aluminium resistance in maize?

Hydroxamate siderophores have been found to alleviate Al toxicity in bacteria. In Poaceae plants cyclic hydroxamates, like DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) and its derivatives have mostly been studied in relation to either defence against insects or allelopathy. In this study the influence of Al on concentrations of these benzoxazinoids (Bx) in root tips, whole roots and root xylem exudates of Zea mays L. varieties differing in Al resistance was analyzed by HPLC-MS. Aluminium resistant maize variety Sikuani maintained considerably higher Bx levels in root tips than the Al sensitive variety Bakero. In vitro binding of Al to DIMBOA was shown by fluorescence quenching. Addition of DIMBOA to Al-containing nutrient solution protected the sensitive maize against Al toxicity as shown by bioassays using callose and haematoxylin staining of root tips as stress indicators. This is the first study showing that Bx can detoxify Al in solution. Tissue analysis data provide first, circumstantial, support for a role of Bx in defence against Al toxicity also in planta.

Aluminum↗

PubMed2007

Effects of tree root-derived substrates and inorganic nutrients on pyrene mineralization in rhizosphere and bulk soil.

This study investigated the effects of organic and inorganic nutrients on the microbial degradation of the common soil contaminant pyrene. The material used in this investigation was collected from potted trees that had been growing for over a year in a soil artificially contaminated with polycyclic aromatic hydrocarbons. Soil was removed from the nonroot (bulk) and root (rhizosphere) zones of these pots and used in mineralization studies that tracked microbial degradation of 14C-pyrene. The factors influencing degradation in these zones were then tested by amendment with essential inorganic nutrients or with root-derived materials. As expected, pyrene mineralization was greater in soil removed from the rhizosphere than in bulk soil. The rate of mineralization in rhizosphere soil was inhibited by inorganic nutrient amendment, whereas nutrients stimulated mineralization in the bulk soil. Pyrene mineralization in bulk soil was also increased by the addition of root extracts intended to mimic exudation by living roots. However, amendment with excised fine roots that were allowed to decay over time in soil initially inhibited mineralization. With time, the rate of mineralization increased, eventually exceeding that of unamended bulk soil. Combined, the initial inhibition and subsequent stimulation produced a zero net impact of decaying fine roots on bulk soil mineralization. Our results, in conjunction with known temporal patterns of fine root dynamics in natural systems, support the idea that seasonal variations in nutrient and substrate availability may influence the long-term effect of plants on organic degradation in soil, possibly reducing or negating the beneficial effects of vegetation that are often observed in short-term studies.

Plant Roots↗

PubMed1975

General model for osmotic and pressure-induced flow in plant roots.

This paper presents a general model to describe coupled solute and water flow through plant roots when they are subjected to osmotic or hydrostatic pressure gradients, or both simultaneously. The model is based on well-established membrane transport equations derived from irreversible thermodynamic considerations. A variety of experimentally observed phenomena such as changes in root resistance with flow rate, apparent negative resistance effects, xylem sap dilution, and apparent non-osmotic water transport can be adequately explained with this model. The model also predicts that an exuding excised root system will be very insensitive to changes in the hydraulic conductivity coefficient. Previous work with a flat membrane of unit surface area and uniform properties is confirmed for a cylindrical coordinate system of nonuniform characteristics.

Journal Article↗

PubMed1966

Day-night Differences in the Accumulation and Translocation of Ions by Tobacco Plants.

More K(42), Rb(86), and Br(82) was accumulated by intact tobacco plants and root systems from detopped plants during the night than during the day when studies for both periods were made in dark rooms controlled at 18 degrees . More, however, was translocated to shoots during the day than during the night in intact plants. This latter effect parallels the periodicity of exudation in root systems from detopped plants.

Journal Article↗

PubMedDate not supplied

Effects of flavonoids released naturally from bean (Phaseolus vulgaris) on nodD-regulated gene transcription in Rhizobium leguminosarum bv. phaseoli.

Nine flavonoid aglycones released from black bean (Phaseolus vulgaris 'PI165426CS') seeds and roots induced nodC::lacZ transcription in Rhizobium leguminosarum bv. phaseoli strains containing extra cloned copies of the regulatory genes nodD1, nodD2, or nodD3 from that biovar. Individual flavonoids generally induced highest levels of nodC::lacZ transcription (Imax) with extra copies of nodD2, and the concentration required for half-maximum induction (I50) was lowest with extra copies of nodD1 genes. One apparently unique feature of R. l. bv. phaseoli is that naturally released flavonoids with very diverse structures induce nod genes. For all three nodD genes, two compounds exuded from roots, genistein and naringenin, produced much higher levels of nodC::lacZ transcription than other flavonoids, but this fact was not explained by increased transcription of the nodD genes themselves. The remaining seven flavonoid aglycones showed reproducibly different capacities to induce nodC::lacZ transcription, but all were considerably less powerful inducers than genistein and naringenin in strains with extra copies of each of the nodD genes. Tests with glycosides of the nod-gene inducers showed that glycosides, which are normally released by bean, had lower I50 values than the corresponding aglycones with all nodD genes. Additive interactions observed between the strong nod-gene inducer genistein and the weak inducer eriodictyol remain to be explained at the molecular level.

Bacterial Proteins↗

PubMed2007

Physiological girdling of pine trees via phloem chilling: proof of concept.

Quantifying below-ground carbon (C) allocation is particularly difficult as methods usually disturb the root-mycorrhizal-soil continuum. We reduced C allocation below ground of loblolly pine trees by: (1) physically girdling trees and (2) physiologically girdling pine trees by chilling the phloem. Chilling reduced cambium temperatures by approximately 18 degrees C. Both methods rapidly reduced soil CO2 efflux, and after approximately 10 days decreased net photosynthesis (P(n)), the latter indicating feedback inhibition. Chilling decreased soil-soluble C, indicating that decreased soil CO2 efflux may have been mediated by a decrease in root C exudation that was rapidly respired by microbes. These effects were only observed in late summer/early autumn when above-ground growth was minimal, and not in the spring when above-ground growth was rapid. All of the effects were rapidly reversed when chilling was ceased. In fertilized plots, both chilling and physical girdling methods reduced soil CO2 efflux by approximately 8%. Physical girdling reduced soil CO2 efflux by 26% in non-fertilized plots. This work demonstrates that phloem chilling provides a non-destructive alternative to reducing the movement of recent photosynthate below the point of chilling to estimate C allocation below ground on large trees.

Carbon Dioxide↗

PubMed2002

Host recognition by pathogenic fungi through plant flavonoids.

A common characteristic among fungal pathogens of plants is that each specializes on a narrow range of specific plants as hosts. One adaptation to a specific host plant is the recognition of the host's chemicals which can be used to trigger genes or developmental pathways needed for pathogenesis. The production of characteristic flavonoids by plants, particularly those exuded from roots by legumes, appear to be used as signals for various microbes, including symbionts as well as pathogens. Nectria haematococca MPVI (anamorph: Fusarium solani) is a soil-borne pathogen of garden pea (Pisum sativum) which serves as a useful model in studying host flavonoid recognition. This fungus displays flavonoid induction of specific pathogenicity genes as well as stimulation of development needed for pathogenesis. Here, we summarize the study of flavonoid-inducible signal pathways which regulate these trait, through identification of transcription factors and regulatory components which control these responses. The characterization of the components a pathogen uses to specifically recognize its host provides insights into the host adaptation process at the molecular level.

Adaptation, Physiological↗

PubMed2004

Xylem sap protein composition is conserved among different plant species.

Xylem sap from broccoli (Brassica oleracea L. cv. Calabrais), rape (Brassica napus L. cv. Drakkar), pumpkin (Cucurbita maxima Duch. cv. gelber Zentner) and cucumber (Cucumis sativus L. cv. Hoffmanns Giganta) was collected by root pressure exudation from the surface of cut stems of healthy, adult plants. Total protein concentrations were in the range of 100 microg ml(-1). One-dimensional gel electrophoresis (SDS-PAGE) resulted in 10-20 visible protein bands in a molecular mass range from 10 to 100 kDa. The main bands were cut out, digested with trypsin, and analysed using tandem mass spectrometry. Fifty bands resulted in amino acid sequence information that was used to perform database similarity searches. Sequences from 30 bands showed high homology to proteins present in databases. Among them, we found mostly peroxidases, but could also identify the lectin-like xylem protein XSP30, a glycine-rich protein, serine proteases, an aspartyl protease family protein, chitinases, and a lipid transfer protein-like polypeptide. Sequence analysis predicted apoplastic secretion signals for all database entries similar to the partial xylem protein sequences. This and the lack of cross-reactivity with phloem protein-specific antibodies suggest that the proteins really originate from the xylem and do not result from phloem contamination. Most of the highly similar proteins probably function in repair and defence reactions. Some of the most abundant proteins (peroxidases, chitinases, serine proteases) were present in xylem exudate of all species analysed, often in more than one band. This indicates an important basic role of these proteins in maintaining xylem function.

Amino Acid Sequence↗

PubMed2003

Herman Jan Phaff: professor, mentor, friend and colleague.

Herman Jan Phaff, the father of yeast ecology, was born in the Netherlands in 1913. In his early years, he spent much time in his family's winery, which sparked his interest in microbes. Trained in the famous Delft tradition, Phaff discovered many unrecognized ecological niches of yeast, such as shellfish, rabbit stomach, frass of bark beetles, tree exudates, cactus roots, Capri figs, sewage, Drosophila flies and shrimp. He is also remembered for his pioneering work on the coevolution of yeasts, insects and plants as well as for his work on yeast beta-glucanase, which resulted in major advances in the understanding of the nature of the yeast cell wall. Phaff's legacy includes research on pectin degradation by fungal enzymes and the application of molecular approaches to yeast systematics. He discovered and described many yeasts, such as the yeast named in his honor, Phaffia rhodozyma, which led to the establishment of a very important industrial fermentation process yielding high concentrations of the pigment astaxanthin, used throughout the world to provide a natural source of this important carotenoid.

History, 20th Century↗

PubMed2002

Reversed-phase liquid chromatographic determination of phytometallophores from Strategy II Fe-uptake species by 9-fluorenylmethyl chloroformate fluorescence.

An HPLC method to quantitate phytometallophores (phytosiderophores) exuded from roots of barley (Hordeum vulgare L.) growing in nutrient solution culture was developed. 9-Fluorenylmethyl chloroformate (FMOC) derivatives of phytometallophores were separated on a C18 reverse-phase column using a sodium acetate (pH 7.2) and acetonitrile-methanol gradient over 20 min followed by fluorescence detection. Detection limits ranged from 15 to 370 pmol depending on the particular phytometallophore. The effectiveness of this method was demonstrated using the response of barley seedlings to Fe-sufficient and Fe-deficient nutrient solution conditions. Phytometallophores collected in root washings of Fe-deficient barley seedlings increased with plant age while phytometallophore release from Fe-adequate roots was negligible.

Chromatography, High Pressure Liquid↗

PubMed1998

cis-isomers of cytokinins predominate in chickpea seeds throughout their development

Trans-isomers of cytokinins (CK) are thought to predominate and have greater biological activity than corresponding cis-isomers in higher plants. However, this study demonstrates a system within which the predominant CK are cis-isomers. CK were measured at four developmental stages in developing chickpea (Cicer arietinum L. cultivar Kaniva) seeds by gas chromatography-mass spectrometry. Concentrations were highest at an early endospermic fluid stage and fell considerably when the cotyledons expanded. The cis-isomers of zeatin nucleotide ([9R-MP]Z), zeatin riboside ([9R]Z), and zeatin (Z) were present in greater concentrations than those of corresponding trans-isomers: (trans)[9R-MP]Z, (trans)[9R]Z, (trans)Z, or dihydrozeatin riboside. Dihydrozeatin, dihydrozeatin nucleotide, and the isopentenyl-type CK concentrations were either low or not detectable. Root xylem exudates also contained predominantly cis-isomers of [9R-MP]Z and [9R]Z. Identities of (cis)[9R]Z and (cis)Z were confirmed by comparison of ion ratios and retention indices, and a full spectrum was obtained for (cis)[9R]Z. Tissues were extracted under conditions that minimized the possibility of RNase hydrolysis of tRNA following tissue disruption, being a significant source of the cis-CK. Since no isomerization of (trans)[2H]CK internal standards occurred, it is unlikely that the cis-CK resulted from enzymic or nonenzymic isomerization during extraction. Although quantities of total CK varied, similar CK profiles were found among three different chickpea cultivars and between adequately watered and water-stressed plants. Developing chickpea seeds will be a useful system for investigating the activity of cis-CK or determining the origin and metabolism of free CK.

Journal Article↗