Metals (Fe, Mn, Zn) in the root plaque of submerged aquatic plants collected in situ: Relations with metal concentrations in the adjacent sediments and in the root tissue

We have investigated the extent of iron oxyhydroxide deposition on the roots of two common freshwater species, Vallisneria americana Michx. and Heteranthera dubia (Jacq.) MacM., collected from different sites in the St. Lawrence River, Québec, Canada, and have related metal concentrations in the root plaques both to the geochemical conditions prevailing in the host sediments (pH; metal partitioning) and to the metal concentrations within the plant root tissue. Possible effects of root plaque on sediment geochemistry are also discussed.At those sites where the two submerged plants co-existed, the amounts of Fe deposited on their respective root surfaces were positively correlated, indicating that sediment geochemistry (pH; concentration of labile metal) exerted a more important influence on plaque formation than did inter-species differences (root physiology, morphology). Iron and Mn concentrations in the root plaque were positively correlated with each other, and with the readily extractable fractions (F1, 172) of these metals in the adjacent sediments. In contrast, Zn concentrations in the root plaque of V. americana were not related to Zn concentrations in the sediments — the dominant geochemical process at the root surface is Fe deposition, such that the quantities of Zn deposited on the roots are determined not by Zn geochemistry per se but rather by the amount of Fe deposition. Indeed the Zn/Fe ratios in the root plaque were related to the Zn/Fe ratios in the surrounding sediments (NH₂OHHCl extract).On a concentration basis (g/g), more Fe, Mn and Zn was found outside the root, in the iron plaque, than inside the root tissues. For all 3 metals, significant relationships were observed between the metal concentrations in the plaque and those inside the roots. For Zn, however, the best statistical relationship was not with [Zn]_plaque, but rather with the [Zn]/[Fe] ratio in the plaque. It is hypothesized that the Zn/Fe ratio in the root plaque reflects the free Zn²⁺ concentration adjacent to the root surface, and that this in turn affects Zn uptake by the plant root. For a given value of Zn in the sediments or in the root plaque, the Zn content of the root is inversely related to the concentration of Fe oxyhydroxides, implying that Fe plays a protective role in regulating Zn bioavailability.     

Trace metals in mangrove seedlings: role of iron plaque formation

Metal-rich mineral deposits on the roots of aquatic plants, denominated iron plaques, may moderate the uptake of essential, but potentially toxic metals by roots. We investigated the iron plaque formation on the fine, nutritive roots of mangrove seedlings growing in contrasting environments (oxidizing sand flat sediments and reducing mangrove forest sediments) in southeast Brazil. The results indicate that Avicennia schaueriana, Laguncularia racemosa, and Rhizophora mangle seedlings developed an efficient exclusion of Fe, Mn, and Zn through iron plaque formation. This process seems to be influenced substantially by species-specific responses to environmental conditions. While Fe and Zn translocation to leaves appear to be suppressed by accumulation within root tissues, this did not appear to occur for Mn, suggesting that Mn trapping in rhizosphere sediments and iron plaque formation are the main mechanisms responsible for the Mn exclusion from the organism level. In addition to factors well recognized as affecting mangrove seedling development (e.g., salinity stress and nutrient availability), the mediation of trace metal uptake by iron plaque formation possibly contribute to determine the seedling adaptability to waterlogged conditions.     

Sulphide intrusion in eelgrass (Zostera marina L.)

Sudden events of seagrass die‐off have been suggested to be induced by invasion of the phytotoxin sulphide under environmental stress generating low oxygen supply in seagrass tissues. Laboratory experiments were conducted with eelgrass (Zostera marina L.) to measure intra‐plant changes in oxygen and sulphide by means of microelectrodes at different oxygen concentrations in the water column. The objectives were to examine whether sulphide intrusion into seagrass tissues can be induced, to determine the role of plant oxygen status for sulphide intrusion and to determine how fast internal sulphide pools are depleted after internal oxygen supplies have been restored. Under conditions with oxygen partial pressures (pO₂) above 7.4 kPa (> 35% of air saturation) within eelgrass rhizomes or meristematic tissues no intrusion of sulphide occurred in spite of high sediment concentrations of gaseous sulphide (> 1000 µm ). Lack of sulphide intrusion at high internal pO₂ suggested that oxygen release from the roots ensured complete re‐oxidation of sulphide in the rhizosphere. Under oxygen stress, however, the experiments clearly demonstrated intrusion of sulphide in eelgrass rhizomes and meristematic tissues. Rates of sulphide intrusion were controlled by internal pO₂, which in turn was controlled by water column oxygen concentrations. Maximum internal sulphide content reached 325 µm which by far exceeded the 1–10 µm known to inhibit mitochondrial activity in eukaryotic cells. Sulphide and low levels of oxygen could coexist in the eelgrass tissues reflecting fast internal transport of sulphide and slow rates of sulphide re‐oxidation. Upon re‐establishment of high internal oxygen concentrations the depletion of the sulphide pool was slow (half‐life = 20–30 min) indicating, that sulphide re‐oxidation within the eelgrass tissue was not bacterially or enzymatically facilitated but occurred by simple chemical oxidation. The results of this study are consistent with the proposed detrimental role of sulphide intrusion in events of sudden seagrass die‐off.     

Relationship between intracellular pH and N metabolism in maize (Zea mays L.) roots

In vivo ³¹P nuclear magnetic resonance (NMR) was used to characterize the effect of the N form (NO₃ vs. NH₄) and the external pH (4, 6, and 8), on the intracellular pH of root tips (0–5 mm) and root segments (5–30 mm). Ammonium-grown root tips were the most sensitive to changes in the external pH. In vivo ¹⁵N NMR was used to characterize the pathway of primary ammonium assimilation in the ammonium-grown roots and to compare the activity of the apical and more-basal root parts. The kinetics of ¹⁵NH₄ ⁺ incorporation showed that primary assimilation in both root tips and root segments followed the glutamine synthetase (GS) pathway. In agreement with the reported gradient of GS along the seminal root of maize, incorporation of label into glutamine amide was more rapid in tips than in segments. It is suggested that this higher GS activity increases the endogenous proton production and thus contributes to the greater dependence of the cytoplasmic pH on the external pH in the ammonium-treated root tips.     

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.     

Root iron plaque formation and characteristics under N2 flushing and its effects on translocation of Zn and Cd in paddy rice seedlings (Oryza sativa)

Background and Aims

Anoxic conditions are seldom considered in root iron plaque induction of wetland plants in hydroponic experiments, but such conditions are essential for root iron plaque formation in the field. Although ferrous ion availability and root radial oxygen loss capacity are generally taken into account, neglect of anoxic conditions in root iron plaque formation might lead to an under- or over-estimate of their functional effects, such as blocking toxic metal uptake. This study hypothesized that anoxic conditions would influence root iron plaque formation characteristics and translocation of Zn and Cd by rice seedlings.

Methods

A hydroponic culture was used to grow rice seedlings and a non-disruptive approach for blocking air exchange between the atmosphere and the induction solution matrix was applied for root iron plaque formation, namely flushing the headspace of the induction solution with N₂ during root iron plaque induction. Zn and Cd were spiked into the solution after root iron plaque formation, and translocation of both metals was determined.

Key Results

Blocking air exchange between the atmosphere and the nutrient solution by N₂ flushing increased root plaque Fe content by between 11 and 77 % (average 31 %). The N₂ flushing treatment generated root iron plaques with a smoother surface than the non-N₂ flushing treatment, as observed by scanning electron microscopy, but Fe oxyhydroxides coating the rice seedling roots were amorphous. The root iron plaques sequestrated Zn and Cd and the N₂ flushing enhanced this effect by approx. 17 % for Zn and 71 % for Cd, calculated by both single and combined additions of Zn and Cd.

Conclusions

Blocking of oxygen intrusion into the nutrient solution via N₂ flushing enhanced root iron plaque formation and increased Cd and Zn sequestration in the iron plaques of rice seedlings. This study suggests that hydroponic studies that do not consider redox potential in the induction matrices might lead to an under-estimate of metal sequestration by root iron plaques of wetland plants.     

Transfer cell formation in sugar beet roots induced by latent Fe deficiency

Leaves of Fe deficient sugar beets precultured in complete nutrient solution with Fe(III)EDTA remained green during the first 6 days of –Fe treatment when grown in a small nutrient solution volume (0.5 L/plant). After 3 days of –Fe treatment, roots placed in agar showed enhanced H⁺ release and ferric reduction at the tips of young laterals where short root hairs and transfer cells had developed. However, the H⁺ release was too weak to cause a pH decrease of the bulk nutrient solution. Nevertheless, the Fe stress response reactions probably lead to mobilization of Fe from the apoplasmic pool so that chlorosis development was prevented. Slight chlorosis symptoms appeared only after 4 more days of Fe deficiency and the pH of the bulk nutrient solution decreased to pH 4.5 simultaneously with renewed transfer cell formation and subsequent rapid regreening. In the 10 times higher volume of 5 L-Fe solution/plant, laterals with root hairs and transfer cells also showed localized acidification of the agar system. However, the protons released were so diluted that no pH decrease of the bulk solution was measurable. Instead, the leaves showed continuously increasing chlorosis with degenerated chloroplast ultrastructure. It is concluded that root hairs and transfer cells are not only formed under severe chlorosis but, instead, they seem to be an integral part of the adaptive response to latent Fe deficiency.     

Influence of external zinc and phosphorus supply on Cd uptake by rice (Oryza sativa L.) seedlings with root surface iron plaque

Rice seedlings were grown in hydroponic culture to determine the effects of external Zn and P supply on plant uptake of Cd in the presence or absence of iron plaque on the root surfaces. Iron plaque was induced by supplying 50 mg l⁻¹ Fe²⁺ in the nutrient solution for 2 day. Then 43-day-old seedlings were exposed to 10 μmol l⁻¹ Cd together with 10 μmol l⁻¹ Zn or without Zn (Zn–Cd experiment), or to 10 μmol l⁻¹ Cd with 1.0 mmol l⁻¹ P or without P (P–Cd experiment) for another 2 day. The seedlings were then harvested and the concentrations of Fe, Zn, P and Cd in dithionite–citrate–bicarbonate (DCB) extracts and in roots and shoots were determined. The dry weights of roots and shoots of seedlings treated with 50 mg l⁻¹ Fe were significantly lower than when no Fe was supplied. Adsorption of Cd, Zn and P on the iron plaque increased when Fe was supplied but Cd concentrations in DCB extracts were unaffected by external Zn or P supply levels. Cd concentrations in shoots and roots were lower when Fe was supplied. Zn additions decreased Cd concentrations in roots but increased Cd concentrations in shoots, whereas P additions significantly increased shoot and root Cd concentrations and this effect diminished when Fe was supplied. The percentage of Cd in DCB extracts was significantly lower than in roots or shoots, accounting for up to 1.8–3.8% of the plant total Cd, while root and shoot Cd were within the ranges 57–76% and 21–40% respectively in the two experiments. Thus, the main barrier to Cd uptake seemed to be the root tissue and the contribution of iron plaque on root surfaces to plant Cd uptake was minor. The changes in plant Cd uptake were not due to Zn or P additions altering Cd adsorption on iron plaque, but more likely because Zn or P interfered with Cd uptake by the roots and translocation to the shoots.     

Role of iron plaque in Cd uptake by and translocation within rice (Oryza sativa L.) seedlings grown in solution culture

A hydroponics culture experiment was conducted to investigate the effect of iron plaque on Cd uptake by and translocation within rice seedlings grown under controlled growth chamber conditions. Rice seedlings were pre-cultivated for 43 days and then transferred to nutrient solution containing six levels of Fe (0, 10, 30, 50, 80 and 100 mg L⁻¹) for 6 days to induce different amounts of iron plaque on the root surfaces. Seedlings were then exposed to solution containing three levels of Cd (0, 0.1 and 1.0 mg L⁻¹) for 4 days. In order to differentiate the uptake capability of Cd by roots with or without iron plaque, root tips (white root part without iron plaque) and middle root parts (with iron plaque) of pre-cultivated seedlings treated with 0, 30 and 50 mg L⁻¹ Fe were exposed to ¹⁰⁹Cd for 24 h. Reddish iron plaque gradually became visible on the surface of rice roots but the visual symptoms of the iron plaque on the roots differed among treatments. In general, the reddish color of the iron plaque became darker with increasing Fe supply, and the iron plaque was more homogeneously distributed all along the roots. The Fe concentrations increased significantly with increasing Fe supply regardless of Cd additions. The Cd concentrations in dithionite–citrate–bicarbonate (DCB)-extracts and in shoots and roots were significantly affected by Cd and Fe supply in the nutrient solution. The Cd concentrations increased significantly with increasing Cd supply in the solution and were undetectable when no Cd was added. The Cd concentrations in DCB-extracts with Fe supplied tended to be higher than that at Fe₀ at Cd_0.1, and at Cd_1.0, DCB-Cd with Fe supplied was significantly lower. Cd concentrations in roots and shoots decreased with increasing Fe supply at both Cd additions. The proportion of Cd in DCB-extracts was significantly lower than in roots or shoots. Compared to the control seedlings without Fe supply, the radioactivity of ¹⁰⁹Cd in shoots of seedlings treated with Fe decreased when root tips were exposed to ¹⁰⁹Cd and did not change significantly when middle parts of roots were exposed. Our results suggest that root tissue rather than iron plaque on the root surface is a barrier to Cd uptake and translocation within rice plants, and the uptake and translocation of Cd appear to be related to Fe nutritional levels in the plants.     

湿地植物根表的铁锰氧化物膜

湿地植物根系具有泌氧能力 ,使其根表及根际微环境呈氧化状态。因而 ,土壤溶液中一些还原性物质被氧化 ,如 Fe2 ,Mn2 ,形成的氧化物呈红色或红棕色胶膜状包裹在根表 ,称为铁锰氧化物膜。铁锰氧化物膜及其根际微环境是湿地植物根系吸收养分和污染物的门户 ,势必会影响这些物质的吸收。主要综述了铁锰氧化物膜的形成和组成 ,以及根表形成的氧化物膜的生态效应 ,也就是氧化物胶膜对植物根系吸收外部介质中的养分及污染物质——重金属离子的影响     

Morphological synergism in root hair length,density, initiation and geometry for phosphorus acquisition in Arabidopsis thaliana: A modeling approach

Low phosphorus availability regulates root hair growth in Arabidopsis by (1) increasing root hair length, (2) increasing root hair density, (3) decreasing the distance between the root tip and the point at which root hairs begin to emerge, and (4) increasing the number of epidermal cell files that bear hairs (trichoblasts). The coordinated regulation of these traits by phosphorus availability prompted us to speculate that they are synergistic, that is, that they have greater adaptive value in combination than they do in isolation. In this study, we explored this concept using a geometric model to evaluate the effect of varying root hair length (short, medium, and long), density (0, 24, 48, 72, 96, and 120 root hairs per mm of root length), tip to first root hair distance (0.5, 1, 2, and 4 mm), and number of trichoblast files (8 vs. 12) on phosphorus acquisition efficiency (PAE) in Arabidopsis. SimRoot, a dynamic three-dimensional geometric model of root growth and architecture, was used to simulate the growth of Arabidopsis roots with contrasting root hair parameters at three values of phosphorus diffusion coefficient (D _e=1×10^–7, 1×10^–8, and 1×10^–9 cm² s^–1) over time (20, 40, and 60 h). Depzone, a program that dynamically models nutrient diffusion to roots, was employed to estimate PAE and competition among root hairs. As D _e decreased from 1×10^–7 to 1×10^–9 cm² s^–1, roots with longer root hairs and higher root hair densities had greater PAE than those with shorter and less dense root hairs. At D _e=1×10^–9 cm² s^–1, the PAE of root hairs at any given density was in the order of long hairs > medium length hairs > short hairs, and the maximum PAE occurred at density = 96 hairs mm^–1 for both long and medium length hairs. This was due to greater competition among root hairs when they were short and dense. Competition over time decreased differences in PAE due to density, but the effect of length was maintained, as there was less competition among long hairs than short hairs. At high D _e(1×10^–7 cm² s^–1), competition among root hairs was greatest among long hairs and lowest among short hairs, and competition increased with increasing root hair densities. This led to a decrease in PAE as root hair length and density increased. PAE was also affected by the tip to first root hair distance. At low D _e values, decreasing tip to first root hair distance increased PAE of long hairs more than that of short hairs, whereas at high D _e values, decreasing tip to first root hair distance increased PAE of root hairs at low density but decreased PAE of long hairs at very high density. Our models confirmed the benefits of increasing root hair density by increasing the number of trichoblast files rather than decreasing the trichoblast length. The combined effects of all four root hair traits on phosphorus acquisition was 371% greater than their additive effects, demonstrating substantial morphological synergy. In conclusion, our data support the hypothesis that the responses of root hairs to low phosphorus availability are synergistic, which may account for their coordinated regulation.     

Root phosphate exudation and pH shift in the rhizosphere are not responsible for aluminum resistance in rice

A series of hydroponic experiments and an agar culture experiment were carried out to investigate aluminum (Al) accumulation and translocation in two rice (Oryza sativa L.) cultivars (Kasalath and Koshihikari) that differ in Al resistance. Al-resistance mechanisms, including Pi exudation under Al stress and pH shifts in the rhizosphere, were also studied. Al content in rice shoots was 41 mg kg⁻¹ on average and did not differ between the two cultivars, which demonstrated that the rice cultivars were not Al accumulators. The majority of Al (95–97%) accumulated in roots. Al content in roots in the resistant cultivar (Koshihikari) was lower than that in the sensitive cultivar (Kasalath), which indicated that Al-exclusion mechanisms were mainly acting in rice. However, the rate of Pi exudation from the whole root or root tips was very low in both cultivars and was not significantly influenced by Al exposure, and thus seemed not to be the main Al-resistance mechanism. On the other hand, experiments with pH-buffered solution and color changes following culture in agar medium containing bromocresol purple revealed that the Al-induced pH increase could not explain the high Al resistance of rice. In addition, the Al content in shoots of Koshihikari was lower after the formation of iron plaque on the root surface, whereas that of Kasalath was not lower. These results suggested that rice roots cell wall components or root surfaces such as iron plaque, rather than pH changes and/or root exudates including organic acids and phosphate, play important roles in Al resistance in rice.     

Calcium influx at the tip of growing root-hair cells of Arabidopsis thaliana

The role of extracellular Ca²⁺ in root-hair tip growth has been investigated in Arabidopsis thaliana (L.) Heynh. Root-hair length was found to be dependent on the concentration of Ca²⁺ in the growth medium, with maximum length achieved at [Ca²⁺] of 0.3–3.0 mM. Using a non-intrusive calcium-specific vibrating microelectrode, an extracellular Ca²⁺ gradient was detected at the tips of individual growing root-hair cells. The direction of the gradient indicated a net influx of Ca²⁺ into root-hair cells. No gradient was detected near the sides of the root hairs or at the tips of non-growing root hairs. When root hairs were exposed to the Ca²⁺-channel blocker nifedipine, tip growth stopped and the extracellular Ca²⁺ gradient was abolished. These results indicate that Ca²⁺ influx through plasma-membrane Ca²⁺ channels is required for normal root-hair tip growth.Abbreviation APW artificial pond water We thank L.F. Jaffe, W. Kuhtreiber and A. Miller of the National Vibrating Probe Facility, Marine Biological Laboratory, Woods Hole, Mass., USA for their technical assistance and helpful discussions. We also thank Liam Dolan, Martin Steer, and Susan Ford for helpful discussions. This research was supported by National Science Foundation grant PCM-9004568.     

Correlation between infection by Rhizobium leguminosarum and lectin on the surface of Pisum sativum L. roots

The lectin on the surface of 4- and 5-dold pea roots was located by the use of indirect immunofluorescence. Specific antibodies raised in rabbits against pea seed isolectin 2, which crossreact with root lectins, were used as primary immunoglobulins and were visualized with fluorescein- or tetramethylrhodamine-isothiocyanate-labeled goat antirabbit immunoglobulin G. Lectin was observed on the tips of newly formed, growing root hairs and on epidermal cells located just below the young hairs. On both types of cells, lectin was concentrated in dense small patches rather than uniformly distributed. Lectin-positive young hairs were grouped opposite the (proto)xylematic poles. Older but still-elongating root hairs presented only traces of lectin or none at all. A similar pattern of distribution was found in different pea cultivars, as well as in a supernodulating and a non-nodulating pea mutant. Growth in a nitrate concentration which inhibits nodulation did not affect lectin distribution on the surface of pea roots of this age. We tested whether or not the root zones where lectin was observed were susceptible to infection by Rhizobium leguminosarum. When low inoculum doses (consisting of less than 10⁶ bacteria·ml^-1) were placed next to lectin-positive epidermal cells and on newly formed root hairs, nodules on the primary roots were formed in 73% and 90% of the plants, respectively. Only a few plants showed primary root nodulation when the inoculum was placed on the root zone where lectin was scarce or absent. These results show that lectin is present at those sites on the pea root that are susceptible to infection by the bacterial symbiont.Abbreviations FITC fluorescein isothiocyanate - TRIC tetramethylrhodamine isothiocyanate     

Seasonal variation in rates of heterotrophic nitrogen fixation (acetylene reduction) in Zostera noltii meadows and uncolonised sediments of the Bassin d'Arcachon, south-west France

Nitrogen fixation (acetylene reduction) rates were measured over an annual cycle in meadows of the seagrass Z. noltii and uncolonised sediments of the Bassin d'Arcachon, south-west France, using both slurry and whole core techniques. Measured rates using the slurry technique in Z. noltii colonised sediments were consistently higher than those determined in isolated cores. This was probably due to the release of labile organic carbon sources during preparation of the slurries. Thus, in colonised sediments the whole core technique may provide a more accurate estimate of in situ activity. Acetylene reduction rates measured by the whole core technique in colonised sediments were 1.8 to 4-fold greater, dependent upon the season, in the light compared with those measured in the dark, indicating that organic carbon released by the plant roots during photosynthesis was an important factor regulating nitrogen fixation. In contrast acetylene reduction rates in uncolonised sediments were independent of light.Addition of sodium molybdate, a specific inhibitor of sulphate reduction inhibited acetylene reduction activity in Z. noltii colonised sediments by > 80% as measured by both slurry and whole core techniques irrespective of the light regime, throughout the year inferring that sulphate reducing bacteria (SRB) were the dominant component of the nitrogen fixing microflora. A mutualistic relationship between Z. noltii and nitrogen fixing SRB in the rhizosphere, based on the exchange of organic carbon and fixed nitrogen is proposed. In uncolonised sediments sodium molybdate initially severely inhibited acetylene reduction rates, but the level of this inhibition declined over the course of the year. These data indicate that the nitrogen fixing SRB associated with the Zostera roots and rhizomes were progressively replaced by an aerobic population of nitrogen fixers associated with the decomposition of this recalcitrant high C:N ratio organic matter.Acetylene and sulphate reduction rates in the seagrass beds showed distinct summer maxima which correlated with a reduced availability of NH ₄ ⁺ in the sediment and the growth cycle of Z. noltii in the Bassin. Overall, these data indicate that acetylene reduction (nitrogen fixation) activity in the rhizosphere of Z. noltii was regulated both by release of organic carbon from the plant roots and maintenance of low ammonium concentrations in the root zone due to efficient ammonium assimilation.Nitrogen fixation rates determined from acetylene reduction rates measured by the whole core technique ranged from 0.1 to 7.3 mg N m^–2 d^–1 in the Z. noltii beds and between 0.02 and 3.7 mg N m^–2 d^–1 in uncolonised sediments, dependent upon the season. Nitrogen fixation in the rhizosphere of Z. noltii was calculated to contribute between 0.4 and 1.1 g N m^–2 y^–1 or between 6.3 and 12% of the annual fixed nitrogen requirement of the plants. Heterotrophic nitrogen fixation therefore represents a substantial local input of fixed nitrogen to the sediments of this shallow coastal lagoon and contributes to the overall productivity of Z. noltii in this ecosystem.     

Nitrogen sink strength of ectomycorrhizal morphotypes of <Emphasis Type="Italic">Quercus douglasii</Emphasis>, <Emphasis Type="Italic">Q. garryana</Emphasis>, and <Emphasis Type="Italic">Q. agrifolia</Emphasis> seedlings grown in a northern California oak woodland

Little information is known on what the magnitude of nitrogen (N) processed by ectomycorrhizal (ECM) fungal species in the field. In a common garden experiment performed in a northern California oak woodland, we investigated transfer of nitrogen applied as ¹⁵NH₄ or ¹⁵NO₃ from leaves to ectomycorrhizal roots of three oak species, Quercus agrifolia, Q. douglasii, and Q. garryana. Oak seedlings formed five common ectomycorrhizal morphotypes on root tips. Mycorrhizal tips were more enriched in ¹⁵N than fine roots. N transfer was greater to the less common morphotypes than to the more common types. ¹⁵N transfer from leaves to roots was greater when , not , was supplied. ¹⁵N transfer to roots was greater in seedlings of Q. agrifolia than in Q. douglasii and Q. garryana. Differential N transfer to ectomycorrhizal root tips suggests that ectomycorrhizal morphotypes can influence flows of N from leaves to roots and that mycorrhizal diversity may influence the total N requirement of plants.     

Root exudates as mediators of mineral acquisition in low-nutrient environments

Plant developmental processes are controlled by internal signals that depend on the adequate supply of mineral nutrients by soil to roots. Thus, the availability of nutrient elements can be a major constraint to plant growth in many environments of the world, especially the tropics where soils are extremely low in nutrients. Plants take up most mineral nutrients through the rhizosphere where micro-organisms interact with plant products in root exudates. Plant root exudates consist of a complex mixture of organic acid anions, phytosiderophores, sugars, vitamins, amino acids, purines, nucleosides, inorganic ions (e.g. HCO₃ ^–, OH^–, H⁺), gaseous molecules (CO₂, H₂), enzymes and root border cells which have major direct or indirect effects on the acquisition of mineral nutrients required for plant growth. Phenolics and aldonic acids exuded directly by roots of N₂-fixing legumes serve as major signals to Rhizobiaceae bacteria which form root nodules where N₂ is reduced to ammonia. Some of the same compounds affect development of mycorrhizal fungi that are crucial for phosphate uptake. Plants growing in low-nutrient environments also employ root exudates in ways other than as symbiotic signals to soil microbes involved in nutrient procurement. Extracellular enzymes release P from organic compounds, and several types of molecules increase iron availability through chelation. Organic acids from root exudates can solubilize unavailable soil Ca, Fe and Al phosphates. Plants growing on nitrate generally maintain electronic neutrality by releasing an excess of anions, including hydroxyl ions. Legumes, which can grow well without nitrate through the benefits of N₂ reduction in the root nodules, must release a net excess of protons. These protons can markedly lower rhizosphere pH and decrease the availability of some mineral nutrients as well as the effective functioning of some soil bacteria, such as the rhizobial bacteria themselves. Thus, environments which are naturally very acidic can pose a challenge to nutrient acquisition by plant roots, and threaten the survival of many beneficial microbes including the roots themselves. A few plants such as Rooibos tea (Aspalathus linearis L.) actively modify their rhizosphere pH by extruding OH^– and HCO₃ ^– to facilitate growth in low pH soils (pH 3 – 5). Our current understanding of how plants use root exudates to modify rhizosphere pH and the potential benefits associated with such processes are assessed in this review.     

Resistance ofUrtica dioica to mycorrhizal colonization: a possible involvement ofUrtica dioica agglutinin

Roots of stinging nettle (Urtica dioica L.) were sampled at different sites around Basel (Switzerland) and examined under the microscope. They were completely devoid of mycorrhizal structures. Similarly, stinging nettle plants grown in the greenhouse in the presence of the arbuscular mycorrhizal fungusGlomus mosseae did not show any signs of mycorrhiza formation. Spread ofG. mosseae through the rhizosphere of stinging nettle plants was inhibited, and application of extracts of stinging nettle roots and rhizomes to hyphal tips ofG. mosseae reduced hyphal growth.Urtica dioica agglutinin, an antifungal protein present in the rhizomes of stinging nettle, inhibited hyphal growth in a similar way as the crude root extract. The possibility thatUrtica dioica agglutinin is at least partially responsible for the inability of stinging nettle to form the arbuscular mycorrhizal symbiosis withG. mosseae is discussed.     

Cutting out the middle clam: lucinid endosymbiotic bacteria are also associated with seagrass roots worldwide

Seagrasses and lucinid bivalves inhabit highly reduced sediments with elevated sulphide concentrations. Lucinids house symbiotic bacteria (Ca. Thiodiazotropha) capable of oxidising sediment sulphide, and their presence in sediments has been proposed to promote seagrass growth by decreasing otherwise phytotoxic sulphide levels. However, vast and productive seagrass meadows are present in ecosystems where lucinids do not occur. Hence, we hypothesised that seagrasses themselves host these sulphur-oxidising Ca. Thiodiazotropha that could aid their survival when lucinids are absent. We analysed newly generated and publicly available 16S rRNA gene sequences from seagrass roots and sediments across 14 seagrass species and 10 countries and found that persistent and colonising seagrasses across the world harbour sulphur-oxidising Ca. Thiodiazotropha, regardless of the presence of lucinids. We used fluorescence in situ hybridisation to visually confirm the presence of Ca. Thiodiazotropha on roots of Halophila ovalis, a colonising seagrass species with wide geographical, water depth range, and sedimentary sulphide concentrations. We provide the first evidence that Ca. Thiodiazotropha are commonly present on seagrass roots, providing another mechanism for seagrasses to alleviate sulphide stress globally.Subject terms: Microbial ecology, Plant ecology, Soil microbiology