Mn accumulation in a submerged plant Egeria densa (Hydrocharitaceae) is mediated by epiphytic bacteria

Many aquatic plants act as biosorbents, removing and recovering metals from the environment. To assess the biosorbent activity of Egeria densa, a submerged freshwater macrophyte, plants were collected monthly from a circular drainage area in Lake Biwa basin and the Mn concentrations of the plants were analysed. Mn concentrations in these plants were generally above those of terrestrial hyperaccumulators, and were markedly higher in spring and summer than in autumn. Mn concentrations were much lower in plants incubated in hydroponic medium at various pH levels with and without Mn supplementation than in field‐collected plants. The precipitation of Mn oxides on the leaves was determined by variable pressure scanning electron microscopy‐energy dispersive X‐ray analysis and Leucoberbelin blue staining. Several strains of epiphytic bacteria were isolated from the field‐collected E. densa plants, with many of these strains, including those of the genera Acidovorax, Comamonas, Pseudomonas and Rhizobium, found to have Mn‐oxidizing activity. High Mn concentrations in E. densa were mediated by the production of biogenic Mn oxide in biofilms on leaf surfaces. These findings provide new insights into plant epidermal bacterial flora that affect metal accumulation in plants and suggest that these aquatic plants may have use in Mn phytomining.     

Identification of myosin in a flowering plant, Egeria densa.

A myosin-like protein was extracted and partially purified from a flowering plant, Egeria densa. It had no p-nitrophenyl phosphatase activity, but exhibited EDTA(K+)-ATPase [EC 3.6.1.3] activity at high ionic strength. Its molecular weight as estimated by gel filtration was 4-5 X 10(5). The presence of a heavy chain (MW = about 1.8 X 10(5)) was indicated by SDS-gel electrophoresis. Egeria myosin aggregated in an environment of low ionic strength and formed bipolar filaments. It bound with skeletal muscle F-actin with a periodicity of 40 nm.     

CO2-concentrating mechanisms in Egeria densa,a submersed aquatic plant

Egeria densa is an aquatic higher plant which has developed different mechanisms to deal with photosynthesis under conditions of low CO₂ availability. On the one hand it shows leaf pH-polarity, which has been proposed to be used for bicarbonate utilization. In this way, at high light intensities and low dissolved carbon concentration, this species generates a low pH at the adaxial leaf surface. This acidification shifts the equilibrium HCO₃^–/CO₂ towards CO₂, which enters the cell by passive diffusion. By this means, E. densa increases the concentration of CO₂ available for photosynthesis inside the cells, when this gas is limiting. On the other hand, under stress conditions resulting from high temperature and high light intensities, it shows a biochemical adaptation with the induction of a C₄-like mechanism but without Kranz anatomy. Transfer from low to high temperature and light conditions induces increased levels of phospho enol pyruvate carboxylase (PEPC, EC 4.1.1.31) and NADP-malic enzyme (NADP-ME, EC 1.1.1.40), both key enzymes participating in the Hatch-Slack cycle in plants with C₄ metabolism. Moreover, one PEPC isoform, whose synthesis is induced by high temperature and light, is phosphorylated in the light, and changes in kinetic and regulatory properties are correlated with changes in the phosphorylation state of this enzyme. In the present review, we describe these two processes in this submersed angiosperm that appear to help it perform photosynthesis under conditions of extreme temperatures and high light intensities.     

A dynamic model for photosynthesis by an aquatic plant, Egeria densa

Abstract This paper describes a dynamic model for photosynthesis by an aquatic plant, Egeria densa. The model takes into account an HCO^?₃ pump, high diffusion resistances and PEP carboxylase, and develops a set of differential equations to form the time-dependent solutions for photosynthesis. The predicted changes in pH, [CO₂]_aq and total inorganic carbon are compared with experimental data and the model is found to describe the data. The model is then used to examine the effect of O₂ on photosynthesis under these conditions, and shows that the increase in internal CO₂ concentration due to the recycling of photorespiratory CO₂ directly stimulates gross CO₂ fixation and can more than compensate for the O₂ inhibition of gross photosynthesis. The importance of the HCO^?₃ pump in O₂ inhibition is also examined. The CO₂ compensation point (where inorganic carbon influx and efflux are equal) is examined and the importance of the HCO^?₃ pump and PEP carboxylase in reducing the compensation concentration is discussed. The model was developed in order to study the photosynthesis of an aquatic weed, which will be reported in a later paper.     

The effect of spring water on the growth of a submerged macrophyte Egeria densa

We elucidated the effect of spring water on the growth of Egeria densa Planch., a widespread submerged macrophyte in Japan. We observed the longitudinal distributions of physical (water temperature, particle diameter of the bed sediment, sediment layer thickness, etc.), chemical [pH, dissolved oxygen (DO), dissolved inorganic carbon (DIC), phosphate (PO₄-P), total nitrogen (TN), and total phosphorus (TP) content of the sediment, etc.], and biological (species composition, biomass, and growth rate) factors related to E. densa in the Kurohashi River, a spring-fed stream flowing into the Lake Biwa. It was found that E. densa growth rate from summer to autumn was negatively correlated to pH and DO, which implies that the low pH spring water increases the growth rate of the species. The growth rate was also positively correlated to the free carbon dioxide (CO₂) concentration (r = 0.67, p = 0.02). These results indicate that the low pH spring water increases E. densa growth rate by affecting free CO₂ concentration in water.     

Oxygen transport in the submerged freshwater macrophyte Egeria densa planch. I. Oxygen production,storage and release

Oxygen exchange in the Egeria densa Planch. (Hydrocharitaceae) lacunar system was investigated using Clark-type oxygen electrodes to monitor root and shoot oxygen release into a bicompartment apparatus. An average root O₂ loss rate of 2.97±0.21 (s.e., n = 50) μl O₂ h⁻¹ cm⁻² root surface area was found, which was of a similar magnitude to values reported by other authors.Root O₂ loss was 4–5 times higher in the light than in the dark, and was approximately 60% higher with still water around the shoot than with a slow, continuous water flow across the shoot surface. Furthermore, both these root O₂ loss rates were reduced when the lower portion of the stem was darkened to simulate self-shading. Under illumination, the boundary layer and stem wall resistances caused thet stem lacunar oxygen partial pressure to be up to 5 kPa higher than in the water.Measurements of lacunar dimensions in Egeria allowed calculation of the oxygen gradient required to drive the observed transport rate. Simple gas-phase diffusion down a gradient of 5.49 × 10⁻² cm³ O₂ cm⁻³ m⁻¹ is sufficient to account for the oxygen transport rates in the experimental material.The ecological significance of the results is discussed, and a schematic model describing the diffusive transport of oxygen in the Egeria lacunar system is developed.     

Best of both worlds: The thermal physiology of Hydrellia egeriae,a biological control agent for the submerged aquatic weed,Egeria densa in South Africa

BioControl - The submerged aquatic weed, Egeria densa Planch. (Hydrocharitaceae) or Brazilian waterweed, is a secondary invader of eutrophic freshwater systems in South Africa, following the...     

Biological control of phytoplankton by the subtropical submerged macrophytes Egeria densa and Potamogeton illinoensis: a mesocosm study

1. In temperate regions, submerged macrophytes can hamper phytoplankton blooms. Such an effect could arise directly, for instance via allelopathy, or indirectly, via competition for nutrients or the positive interaction between submerged macrophytes and zooplankton grazing. However, there is some evidence that the positive interaction between submerged macrophytes and zooplankton grazing is less marked in warmer regions, where the interaction is less well studied, and that negative effects of higher water plants on phytoplankton biomass are weaker. 2. We carried out two consecutive mesocosm experiments in Uruguay (subtropical South America) to study the effects of two common submerged macrophytes from this region (Egeria densa and Potamogeton illinoensis) on phytoplankton biomass, in the absence of zooplankton grazing. We compared phytoplankton development between different macrophyte treatments (no macrophytes, artificial macrophytes, real Egeria and real Potamogeton). We used artificial macrophytes to differentiate between physical effects (i.e. shading, sedimentation and competition with periphyton) and biological effects (i.e. nutrient competition and allelopathy). 3. In Experiment 1, we found no evidence for physical effects of macrophytes on phytoplankton biomass, but both macrophyte species seemed to exert strong biological effects on phytoplankton biomass. Only Egeria affected phytoplankton community structure, particularly tempering the dominance of Scenedesmus. Nutrient addition assays revealed that only Egeria suppressed phytoplankton through nutrient competition. 4. We performed a second mesocosm experiment with the same design, but applying saturating nutrient conditions as a way of excluding the effects of competition for nutrients. This experiment showed that both macrophytes were still able to suppress phytoplankton through biological mechanisms, providing evidence for allelopathic effects. Our results indicate that both common macrophytes are able to keep phytoplankton biomass low, even in the absence of zooplankton grazing.     

Prediction of Egeria najas and Egeria densa occurrence in a large subtropical reservoir (Itaipu Reservoir,Brazil-Paraguay)

Incidence data of two native submerged aquatic macrophytes (Egeria najas Planch. and Egeria densa Planch.) were obtained in eight arms of a large (1350 km²) subtropical reservoir (Itaipu Binacional Reservoir, Brazil-Paraguay). Environmental variables were measured simultaneously. Two large-scale surveys in the same localities identified by a global positioning system were carried out in April 1999 (n = 235) and January 2001 (n = 230). Logistic regressions were used to test the effect of environmental variables on the likelihood of E. najas and E. densa presence or absence. The two species were found under different environmental conditions: conductivity, light attenuation coefficient (k) and fetch were, in this order, the most important environmental variables in predicting the probability of occurrence of E. najas, whereas light attenuation coefficient was the main predictor of the probability of occurrence of E. densa. Thus, both species were negatively affected by the light attenuation coefficient. However, this effect was stronger in E. densa. The small area occupied by these species may be accounted for by the permanent high turbidity of Itaipu Reservoir. Additionally, the dominance of E. najas over of E. densa can be explained by the probably higher light requirements of E. densa. In other reservoirs worldwide, with higher water transparency, the opposite is frequently true. Between 1999 and 2001, an episodic water-level drawdown (5 m) caused the disappearance of submerged vegetation. After water-level normalization, previous vegetation presence (in 1999) was an important predictor of the probability of occurrence of E. najas in 2001.     

A two-component high-affinity nitrate uptake system in barley

The analysis of genome databases for many different plants has identified a group of genes that are related to one part of a two-component nitrate transport system found in algae. Earlier work using mutants and heterologous expression has shown that a high-affinity nitrate transport system from the unicellular green algae, Chlamydomonas reinhardtii required two gene products for function. One gene encoded a typical carrier-type structure with 12 putative trans-membrane (TM) domains and the other gene, nar2 encoded a much smaller protein that had only one TM domain. As both gene families occur in plants we investigated whether this transport model has more general relevance among plants. The screening for nitrate transporter activity was greatly helped by a novel assay using (15)N-enriched nitrate uptake into Xenopus oocytes expressing the proteins. This assay enables many oocytes to be rapidly screened for nitrate transport activity. The functional activity of a barley nitrate transporter, HvNRT2.1, in oocytes required co-injection of a second mRNA. Although three very closely related nar2-like genes were cloned from barley, only one of these was able to give functional nitrate transport when co-injected into oocytes. The nitrate transport performed by this two-gene system was inhibited at more acidic external pH and by acidification of the cytoplasm. This specific requirement for two-gene products to give nitrate transport function has important implications for attempts to genetically manipulate this fundamental process in plants.     

In vivo phosphorylation of phosphoenolpyruvate carboxylase in Egeria densa, a submersed aquatic species

In vivo phosphorylation of PEPC in Egeria densa was studied using plants at high temperature and in light, and plants kept at low temperature and in light. The isoform induced by high temperature and light was more phosphorylated in the light. Changes in kinetic and regulatory properties correlated with changes in the phosphorylation state of PEPC.     

Application of environmental DNA methods for the detection and abundance estimation of invasive aquatic plant Egeria densa in lotic habitats

Limnology - Estimating the presence and abundance of non-native species in the early stage of invasion is important to prevent further spread of non-native species in aquatic systems. Environmental...     

Modelling the uptake of nitrate by a growing plant with an adjustable root nitrate uptake capacity

A new model is presented to predict the plant uptake of nitrate supplied by diffusion and mass flow to its roots. Plant growth, root-shoot ratio and the plant's nitrate uptake capacity are all set dependent on the plant's N nutrition state. By thoroughly integrating processes occurring in both plant and soil, the model enables to control the relative importance of both under a wide range of different nutritional scenarios.Soil parameters D₀ diffusion coefficient in water (m² day^-1) - D_e diffusion coefficient in soil (m² day^-1) - C nitrate concentration in soil (mol m^-3) - f tortuosity (-) - volumetric moisture content (-) - R radial distance from root axis (m) Plant parameters b₁, b₂ parameters of biomass partitioning Equation (10) - IR interroot distance (m) - K_mU Michaelis-Menten constant of the uptake system (mol m^-3) - K_mNRA Michaelis-Menten constant of nitrogen reduction system (mol g^-1) - k₁, k₂, k₃ parameters of growth model Equation (9) - L_v Root length density (m m^-3) - NO_{3 set} ^- Set point of the cytoplasmatic nitrate pool (mol g^-1 dw) - NO_{3 c} ^- cytoplasmatic nitrate concentration (mol g^-1 dw) - NO_{3 v} ^- vacuolar nitrate concentration (mol g^-1 dw) - NRA_max maximum nitrate reductase activity (mol g^-1 dw day^-1) - N_re reduced nitrogen content (mol) - N_remax maximum reduced N concentration in the plant (mol g^-1 dw) - P partitioning coefficient of nitrate between cyplasm and vacuole - R(1) root radius (m) - RGR relative growth rate (day^-1) - U uptake rate (mol day^-1 m^-2) - U_max maximum uptake rate (Eq. 6) (day^-1 m^-2) - Vo water flux at root surface (m day^-1) - W_r root dry weight (g) - W_sh shoot dry weight (g) - X model parameter: number of root compartments - Y model parameter: number of nodes     

Nitrate reductase activity is required for nitrate uptake into fungal but not plant cells

The ability to transport net nitrate was conferred upon transformant cells of the non-nitrate-assimilating yeast Pichia pastoris after the introduction of two genes, one encoding nitrate reductase and the other nitrate transport. It was observed that cells of this lower eukaryote transformed with the nitrate transporter gene alone failed to display net nitrate transport despite having the ability to produce the protein. In addition, loss-of-function nitrate reductase mutants isolated from several nitrate-assimilating fungi appeared to be unable to accumulate nitrate. Uptake assays using the tracer (13)NO(3)(-) showed that nitrate influx is negligible in cells of a nitrate reductase null mutant. In parallel studies using a higher eukaryotic plant, Arabidopsis thaliana, loss-of-function nitrate reductase strains homozygous for both NIA1 insertion and NIA2 deletion were found to have no detectable nitrate reductase mRNA or nitrate reductase activity but retained the ability to transport nitrate. The reasons for these fundamental differences in nitrate transport into the cells of representative members of these two eukaryotic kingdoms are discussed.     

Characterization of a two-component high-affinity nitrate uptake system in Arabidopsis. Physiology and protein-protein interaction

The identification of a family of NAR2-type genes in higher plants showed that there was a homolog in Arabidopsis (Arabidopsis thaliana), AtNAR2.1. These genes encode part of a two-component nitrate high-affinity transport system (HATS). As the Arabidopsis NRT2 gene family of nitrate transporters has been characterized, we tested the idea that AtNAR2.1 and AtNRT2.1 are partners in a two-component HATS. Results using the yeast split-ubiquitin system and Xenopus oocyte expression showed that the two proteins interacted to give a functional HATS. The growth and nitrogen (N) physiology of two Arabidopsis gene knockout mutants, atnrt2.1-1 and atnar2.1-1, one for each partner protein, were compared. Both types of plants had lost HATS activity at 0.2 mm nitrate, but the effect was more severe in atnar2.1-1 plants. The relationship between plant N status and nitrate transporter expression revealed a pattern that was characteristic of N deficiency that was again stronger in atnar2.1-1. Plants resulting from a cross between both mutants (atnrt2.1-1 x atnar2.1-1) showed a phenotype like that of the atnar2.1-1 mutant when grown in 0.5 mm nitrate. Lateral root assays also revealed growth differences between the two mutants, confirming that atnar2.1-1 had a stronger phenotype. To show that the impaired HATS did not result from the decreased expression of AtNRT2.1, we tested if constitutive root expression of a tobacco (Nicotiana plumbaginifolia) gene, NpNRT2.1, previously been shown to complement atnrt2.1-1, can restore HATS to the atnar2.1-1 mutant. These plants did not recover wild-type nitrate HATS. Taken together, these results show that AtNAR2.1 is essential for HATS of nitrate in Arabidopsis.     

32P uptake in three submerged aquatic plant species

Summary Highest uptake of³²P by young shoots of three plant species was observed and lowest by old ones. The uptake of³²P was highest inHydrilla shoots, followed byVallisneria andPotamogeton.Kinetin (0.23 mM) pretreatment (24 h) increased the uptake of³²P, while 0.69 mM ethrel or 0.075 mM ABA decreased it in all species.³²P was transported to the largest extent to the young shoots of the submerged plants and to the smallest extent to the old ones by kinetin pretreatment. Kinetin enhanced the uptake of³²P most inHydrilla shoots, followed byVallisneria andPotamogeton. Ethrel diminished³²P uptake most inPotamogeton shoots and to the smallest extent inHydrilla, while ABA lowered it most inHydrilla shoots and to the smallest extent inPotamogeton. Kinetin, ethrel and ABA can modify the uptake of³²P of these aquatic plants.     

高等植物根细胞高亲和性吸收钾的机制

K^ 是高等植物所必需的大量元素,它在植物的膨压调节、电荷平衡、叶片运动和蛋白质合成中都具有重要的作用。高等植物根细胞吸收K^ 通过高亲和性K^ 吸收系统和低亲和性K^ 吸收系统和低亲和性K^ 吸收系统两条途径。高新和性K^ 吸收系统,是在微摩尔浓度的外界K^ 水平时起作用,K^ 的吸收必须消耗能量。近年来,随着分子生物学技术和电生理技术的飞速发展,植物根细胞吸收K^ 的机制取得了较大进展。本文对高等植物根细胞高亲和性吸收K^ 的机制的研究进展进行了综述。     

Oxygen inhibition of nitrate uptake is a general regulatory mechanism in nitrate respiration

It has recently been demonstrated that oxygen inhibits nitrate uptake by denitrifying Pseudomonas aeruginosa. The purpose of the present investigation was to determine if this novel mechanism of regulation is universal for the regulation of nitrate respiration in other widely divergent species of bacteria. Nitrate transport by whole cell suspensions was completely and reversibly inhibited in 11 out of 12 species tested, whereas nitrate reduction by cell-free extracts was not affected by oxygen or was only partially inhibited in some cases. These results indicate that oxygen inhibition of nitrate uptake is a general regulatory phenomenon.     

Arabidopsis nitrate transporter NRT1.9 is important in phloem nitrate transport

This study of the Arabidopsis thaliana nitrate transporter NRT1.9 reveals an important function for a NRT1 family member in phloem nitrate transport. Functional analysis in Xenopus laevis oocytes showed that NRT1.9 is a low-affinity nitrate transporter. Green fluorescent protein and β-glucuronidase reporter analyses indicated that NRT1.9 is a plasma membrane transporter expressed in the companion cells of root phloem. In nrt1.9 mutants, nitrate content in root phloem exudates was decreased, and downward nitrate transport was reduced, suggesting that NRT1.9 may facilitate loading of nitrate into the root phloem and enhance downward nitrate transport in roots. Under high nitrate conditions, the nrt1.9 mutant showed enhanced root-to-shoot nitrate transport and plant growth. We conclude that phloem nitrate transport is facilitated by expression of NRT1.9 in root companion cells. In addition, enhanced root-to-shoot xylem transport of nitrate in nrt1.9 mutants points to a negative correlation between xylem and phloem nitrate transport.     

The uptake,translocation and release of phosphorus by Elodea densa

Short-term (16 h) laboratory studies of ³²P uptake by Elodea densa rooted in sediment demonstrated both foliar and root uptake, and that translocation occurred acropetally and basipetally. Root absorption is projected to provide 83–85% of total phosphorus uptake during 12–16 h photoperiod days. Measured foliar uptake and excretion rates suggest that there would be no net leakage of phosphorus into the water by undamaged actively-growing E. densa. Foliar uptake decreased and root uptake increased in the dark relative to rates under light.