Metabolism and subcellular distribution of anthracene in tall fescue (Festuca arundinacea Schreb.)

Aim

Understanding the uptake processes of organic contaminants by plants is essential when assessing crop contamination and subsequent human exposure. Unfortunately, limited information is available on plant metabolism and subcellular distribution of polycyclic aromatic hydrocarbons (PAHs), a group of highly toxic organic contaminants with carcinogenic, mutagenic, and teratogenic properties. Thus we seek to investigate the metabolism and intracellular distribution of PAH in tall fescue (Festuca arundinacea Schreb.).

Methods

This work was conducted utilizing greenhouse hydroponical experiments. Seedlings of tall fescue were firstly cultured in half-strength Hoagland solution with anthracene (ANT). Then treated plants were removed from the ANT-spiked solution, and transplanted into solutions free of ANT. After 0–16 days, the seedlings were sampled and prepared for ANT analysis. Seedlings were washed using Milli-Q water and then separated into different parts to measure ANT distributions at the subcellular level.

Results

ANT concentrations and dissipation amounts in root and shoot of tall fescue decreased in 0–16 days after transplantation from an ANT-spiked culture solution to a solution free of PAH, revealing ANT metabolism in the plant. The accumulation of the examined primary metabolites of ANT, i.e., anthrone and anthraquinone, against time also supported plant metabolism of ANT. About 10 % of ANT resided in the plants at 8–16 days due to the formation of ANT-bound residues with plant tissues. ANT concentrations in the cell walls and organelles of plant roots and shoots decreased significantly during the 16-day period. Cell walls and organelles were the dominant storage compartments for ANT and anthraquinone in plants at 16 days, whereas the distribution of anthrone at 16 days in root cells was ordered as cell organelles >cell soluble fraction >cell wall and in shoot cells as cell soluble fraction >> cell organelle ? cell wall. Although the organelle content is smaller, the concentrations of ANT and metabolites in the organelle fraction were much higher than those in cell walls.

Conclusions

This was a primary investigation into the metabolism and intracellular distribution of PAH in plant. We demonstrated the metabolism of ANT in tall fescue based on the observed reduced concentrations, dissipated amounts, and detected metabolites. ANT and its metabolites were distributed into the cell water-soluble fraction, cell walls, and organelles. Results of this work will enhance the understanding of PAH transfer and transformation in plants and will be valuable for risk assessments of plant contamination at polluted sites.     

Partitioning of polycyclic aromatic hydrocarbons between plant roots and water

Partition of phenanthrene between water and roots was determined for 13 plant species using a batch equilibration technique. Partition coefficients (K _rt) from 734 to 2,564 L/kg were measured. A simple model to estimate the partition of organic contaminants between roots and water was developed based on the composition of plant roots and the 1-octanol/water partitioning coefficient. The estimates were close to the observed results, with differences of < 14%. The partition coefficients of phenanthrene by root cell walls were 13–84% greater than sorption by the corresponding roots. The cell wall fraction—the dominant fraction of root organic components—was identified as the primary domain for partition of phenanthrene. The measured hydroponic uptake of phenanthrene into roots was always less than phenanthrene partition by plant roots. A modified sorption model containing a quasi-equilibrium factor (α_pt) could reasonably predict hydroponic uptake by plant roots. The results obtained from this study provide insights into partition of highly lipophilic organic chemicals in roots, and provide convenient methods to estimate this partition as well as uptake of such chemicals in root–water systems.     

植物对水中菲和芘的吸收

以菲和芘为多环芳烃（PAHs）代表物,采用水培体系研究了黑麦草（Lolium multiflorum Lam）对水中PAHs的吸收作用,重点研究了植物吸收菲和芘的时间动态.水中菲和芘起始浓度分别为1.00mg/L和0.12mg/L.0～288h内,黑麦草根和茎叶中菲和芘含量均先快速增加而后降低,积累量不断增大,植物根系和茎叶富集系数则先快速升高而后趋于稳定.茎叶中菲和芘含量、茎叶对菲和芘的富集系数比根低1～3数量级,积累量也明显小于根系.黑麦草根系对水中芘有更强的富集能力,其根系富集系数比菲大85%～179%;而其茎叶对菲的富集作用则略强.菲和芘在植物体内有明显的传导作用.0～288h,传导系数（TF）先显著升高而后趋于恒定;但实验条件下,菲和芘的TF值均很小,分别不高于0.031和0.009,且芘的TF值明显小于菲,表明供试植物对芘的传导能力更弱.     

Colonization on Root Surface by a Phenanthrene-Degrading Endophytic Bacterium and Its Application for Reducing Plant Phenanthrene Contamination

A phenanthrene-degrading endophytic bacterium, Pn2, was isolated from Alopecurus aequalis Sobol grown in soils contaminated with polycyclic aromatic hydrocarbons (PAHs). Based on morphology, physiological characteristics and the 16S rRNA gene sequence, it was identified as Massilia sp. Strain Pn2 could degrade more than 95% of the phenanthrene (150 mg·L⁻¹) in a minimal salts medium (MSM) within 48 hours at an initial pH of 7.0 and a temperature of 30°C. Pn2 could grow well on the MSM plates with a series of other PAHs, including naphthalene, acenaphthene, anthracene and pyrene, and degrade them to different degrees. Pn2 could also colonize the root surface of ryegrass (Lolium multiflorum Lam), invade its internal root tissues and translocate into the plant shoot. When treated with the endophyte Pn2 under hydroponic growth conditions with 2 mg·L⁻¹ of phenanthrene in the Hoagland solution, the phenanthrene concentrations in ryegrass roots and shoots were reduced by 54% and 57%, respectively, compared with the endophyte-free treatment. Strain Pn2 could be a novel and useful bacterial resource for eliminating plant PAH contamination in polluted environments by degrading the PAHs inside plants. Furthermore, we provide new perspectives on the control of the plant uptake of PAHs via endophytic bacteria.     

Comparison of cadmium subcellular distribution in different organs of two water spinach (Ipomoea aquatica Forsk.) cultivars

Aims

Mechanisms of low cadmium (Cd) accumulations in cultivars of water spinach are poorly investigated. We aimed to improve understanding of the subcellular biochemical properties of the mechanisms involved.

Methods

A pot experiment was conducted to investigate the subcellular distributions of Cd in lateral and main roots, stems, and young and old leaves of a high-Cd (T308) and a low-Cd cultivar (QLQ).

Results

The ratio of main root:lateral roots Cd concentration in QLQ was lower (0.34–0.35) than that in T308 (0.39–0.55). The ratio of stem:main root Cd concentration in QLQ was much lower (0.60–0.73) than that in T308 (1.19–1.58). QLQ has higher capacity to sequester Cd in cell wall fractions of main and lateral roots than T308.

Conclusions

The difference in shoot Cd concentration between QLQ and T308 is attributable to the difference in Cd translocation from lateral to main roots and from roots to the stem. Fixation of large amounts of Cd in old leaves is beneficial to protect young leaves from Cd toxicity. Cadmium immobilization by the cell wall is important in Cd detoxification, especially in main and lateral roots of QLQ and the shoot of T308.     

Indirect effects of polycyclic aromatic hydrocarbon contamination on microbial communities in legume and grass rhizospheres

Background and aims

Biodegradation of polycyclic aromatic hydrocarbons (PAHs) is accelerated in the presence of plants, due to the stimulation of rhizosphere microbes by plant exudates (nonspecific enhancement). However, plants may also recruit specific microbial groups in response to PAH stress (specific enhancement). In this study, plant effects on the development of rhizosphere microbial communities in heterogeneously contaminated soils were assessed for three grasses (ryegrass, red fescue and Yorkshire fog) and four legumes (white clover, chickpea, subterranean clover and red lentil).

Methods

Plants were cultivated using a split-root model with their roots divided between two independent pots containing either uncontaminated soil or PAH-contaminated soil (pyrene or phenanthrene). Microbial community development in the two halves of the rhizosphere was assessed by T-RFLP (bacterial and fungal community) or DGGE (bacterial community), and by 16S rRNA gene tag-pyrosequencing.

Results

In legume rhizospheres, the microbial community structure in the uncontaminated part of the split-root model was significantly influenced by the presence of PAH-contamination in the other part of the root system (indirect effect), but this effect was not seen for grasses. In the contaminated rhizospheres, Verrucomicrobia and Actinobacteria showed increased populations, and there was a dramatic increase in Denitratisoma numbers, suggesting that this genus may be important in rhizoremediation processes.

Conclusion

Our results show that Trifolium and other legumes respond to PAH-contamination stress in a systemic manner, to influence the microbial diversity in their rhizospheres.     

Utilizing pyrene-degrading endophytic bacteria to reduce the risk of plant pyrene contamination

Aims

Endophytic bacteria are ubiquitous in plants, but little information is available on the influence of endophytic bacteria on the uptake and metabolism of PAH by plants. Thus, we seek to investigate whether the colonization of a target plant by a PAH-degrading endophytic bacterium would improve the PAH metabolism of the plant and reduce the risk of plant PAH contamination.

Methods

A pyrene-degrading endophyte was isolated from PAH-contaminated plants using enrichment culture. After root inoculation with the isolated bacterium, greenhouse container experiments were conducted. Pyrene residues in soil and plant samples were analyzed by HPLC.

Results

A pyrene-degrading endophytic bacterium, Staphylococcus sp. BJ06, was isolated from Alopecurus aequalis and could degrade 56.0 % of pyrene (50 mg?·?L^?1) within 15 days. BJ06 grew and degraded pyrene efficiently under environmental conditions. The bacterium significantly promoted ryegrass growth and pyrene removal from contaminated soil in container experiments. The pyrene concentrations in ryegrass roots and shoots in endophyte-inoculated planted soil were reduced by 31.01 % and 44.22 %, respectively, compared with endophyte-free planted soil.

Conclusions

We have provided new perspectives on the regulation and control of plant uptake of organic contaminants with endophytic bacteria. The results of this study will be valuable to risk assessments of plant PAH contamination.     

Use of a gnotobiotic plant assay for assessing root colonization and mineral phosphate solubilization by <Emphasis Type="Italic">Paraburkholderia bryophila</Emphasis> Ha185 in association with perennial ryegrass (<Emphasis Type="Italic">Lolium perenne</Emphasis> L.)

Aims

The mechanisms by which rhizosphere bacteria increase the availability of mineral P precipitates for plant use are understudied. However, Paraburkholderia bryophila Ha185 is known to solubilize inorganic phosphate in vitro via a novel process. Therefore, this study aimed to demonstrate P solubilization by Ha185 in association with roots of perennial ryegrass (Lolium perenne L.).

Methods

We developed a gnotobiotic plant assay to assess P solubilization by Ha185 on ryegrass roots under various nutrient conditions. A green fluorescent protein (GFP)-tagged derivative of Ha185 was used in conjunction with fluorescent microscopy and confocal microscopy to visualize colonization of ryegrass roots.

Results

Ha185 solubilized mineral P (hydroxyapatite) in association with ryegrass roots and increased ryegrass growth by 20% under P-limited conditions. The GFP-tagged Ha185 strain colonized the rhizoplane and penetrated the primary root of ryegrass, possibly through “crack entry” at the point of lateral root emergence, but also by entering the epidermal cells via root hairs.

Conclusions

Ha185 supported ryegrass growth under P-limited conditions, indicating this strain may improve availability of soil P for uptake by ryegrass. Tools developed in this study have broad application in the study of rhizobacteria-plant interactions.

     

Role of roots in cadmium accumulation of two water spinach cultivars: reciprocal grafting and histochemical experiments

Background and Aims

Cultivars of water spinach (Ipomoea aquatica Forsk.) differ widely in their shoot cadmium (Cd) concentration. Previously, we suggested that low-Cd cultivars are better able to retain Cd in their roots and thus prevent root-to-shoot Cd translocation. In this study, we explored the roles of roots and shoots in Cd accumulation in a high-Cd (T308) and low-Cd cultivar (QLQ).

Methods

We used reciprocal grafting to determine the importance of roots and shoots in Cd accumulation, and a dithizone histochemical method to investigate Cd distribution in the roots.

Results

The T308 scion with QLQ rootstock accumulated less Cd than the shoot of non-grafted T308. The QLQ scion with T308 rootstock showed a significantly higher Cd concentration than that in the shoot of non-grafted QLQ. Cadmium induced thicker phellem formation in the main roots of QLQ than in those of T308 and only QLQ showed thickening of the outer cortex cell walls in lateral roots.

Conclusions

Shoot Cd accumulation was primarily determined by root-to-shoot Cd translocation, not root Cd uptake. The thicker phellem and outer cortex cell walls in QLQ than in T308 may be one reason why QLQ roots were able to retain more Cd, and thus reducing Cd translocation to shoots.     

Capacity of biological soil crusts colonized by the lichen <Emphasis Type="Italic">Diploschistes</Emphasis> to metabolize simple phenols

Background and aims

Soil compaction strongly affects water uptake by roots. The aim of the work was to examine soil—plant interactions with focus on the impact of distribution of compacted soil layers on growth and water uptake by wheat roots.

Methods

The growth-chamber experiment was conducted on wheat growth in soil with compacted soil layers. The system for maintaining constant soil water potential and measurement of daily water uptake from variously compacted soil layers was used.

Results

Layered soil compaction differentiated vertical root distribution to higher extent for root length than root mass. The propagation rate of a water extraction front was the highest through layers of moderately compacted soil. The root water uptake rate was on average 67 % higher from moderately than heavily compacted soil layers. Correlations between water uptake and the length of thick roots were increasing with increasing level of soil compaction.

Conclusions

The study shows that root amount, water uptake, propagation of water extraction and shoot growth strongly depend on the existence of compacted layers within soil profile. The negative effects of heavily compacted subsoil layer on water uptake were partly compensated by increased uptake from looser top soil layers and significant contribution of thicker roots in water uptake.     

Isolated cell walls exhibit cation binding properties distinct from those of plant roots

Aims

The principal contributor to the cation binding properties of roots is currently considered to be the cell wall or, alternatively, the plasma membrane. The aim of this study was to highlight their respective contributions in the binding properties.

Methods

Cell walls of a dicotyledon (Solanum lycopersicum L.) and monocotyledon (Triticum aestivum L.) were isolated from roots and their binding properties were compared to those of their respective roots. Cell wall and root binding capacities were evaluated by potentiometric titrations and cation exchange capacity measurements, while their biochemical composition was analyzed by ¹³C-NMR spectroscopy.

Results

The lower binding capacity of isolated cell walls compared to roots revealed that cell plasma membranes had a higher binding site density than cell walls. The significant decrease in some NMR signals, i.e. carbonyl C, N alkyl/methoxyl C and alkyl C regions, suggested that carboxyl, amine and phosphate binding sites, borne by proteins and phospholipid plasma membranes, contribute to the binding capacity.

Conclusions

Cell walls and plasma membranes were found to be jointly involved in root binding properties and their respective contributions seemed vary between plants.     

Accumulation mechanisms and subcellular distribution of Cu in maize grown on soil treated with [S, S]-ethylenediamine disuccinic acid

Aims

Many studies have proved that EDTA (ethylenediaminetetraacetic acid), EDDS ([S, S’]-ethylenediamine disuccinic acid), and other chelating agents significantly enhance phyto-extraction of copper (Cu) from soil. However, some key factors, such as changes in membrane permeability of root cells and subcellular distribution of Cu and Cu-EDDS complex in leaves and roots, remain unresolved.

Methods

A pot-culture experiment was conducted using soil artificially contaminated with Cu to different degrees to compare its effect on the above factors and the relationship between them in maize (Zea mays L.).

Results

Treatment with 0.5–6.0?mmol?kg^?1 (soil) EDDS increased membrane permeability in root cells significantly (p?<?0.05). Chelated Cu accounted for 14.6%–17.4% of the total Cu content of roots and 77.7%–78.8% of that of leaves and was distributed mainly in cell walls in both.

Conclusions

EDDS increases Cu accumulation in shoots mainly by increasing the content of soluble Cu in soil and membrane permeability of root cells. Cu in soil may be absorbed through the apoplastic pathway into the root xylem, translocated to the shoots, and accumulated there as a Cu-EDDS complex.     

Neutron imaging reveals internal plant water dynamics

Background and aims

Knowledge of plant water fluxes is critical for assessing mechanistic processes linked to biogeochemical cycles, yet resolving root water transport dynamics has been a particularly daunting task. Our objectives were to demonstrate the ability to non-invasively monitor individual root functionality and water fluxes within Zea mays L. (maize) and Panicum virgatum L. (switchgrass) seedlings using neutron imaging.

Methods

Seedlings were propagated for 1–3 weeks in aluminum chambers containing sand. Pulses of water or deuterium oxide were then tracked through the root systems by collecting consecutive radiographs during exposure to a cold-neutron source. Water flux was manipulated by cycling on a growth lamp to alter foliar demand for water.

Results

Neutron radiography readily illuminated root structure, root growth, and relative plant and soil water content. After irrigation there was rapid root water uptake from the newly wetted soil, followed by hydraulic redistribution of water through the root system to roots terminating in dry soil. Water flux within individual roots responded differentially to foliar illumination based on supply and demand of water within the root system.

Conclusions

Sub-millimeter scale image resolution revealed timing and magnitudes of root water uptake, redistribution within the roots, and root-shoot hydraulic linkages—relationships not well characterized by other techniques.     

抑制剂和安全剂对高羊茅根中酶活性和菲代谢的影响

以高羊茅(Festuca arundinacea)为供试植物,利用水培体系研究了抑制剂和安全剂对植物根中过氧化物酶(POD)和多酚氧化酶(PPO)活性以及菲代谢的影响。供试安全剂为浓度0.3%的NaCl,抑制剂为浓度2.00 mg/L的Vc。结果表明,2.00 mg/L的Vc作用下,1—16d,高羊茅根的菲含量显著高于对照处理,而供试安全剂对植物根中菲含量的影响不显著。抑制剂作用下植物根部的PPO和POD活性显著降低;16d,抑制剂作用下的植物根部PPO和POD活性为对照组的1/6和1/9,表现出强抑制效应。而安全剂作用下植物根部PPO和POD活性则略高于对照组,但差异不显著（P＜0.05）。植物体内酶的初始活性是影响植物代谢PAHs菲的关键因素。抑制剂主要通过调节酶活性来影响根系代谢菲,其对植物根中PPO和POD活性的抑制效率与根部菲代谢抑制效率显著正相关。     

Copper Uptake by Ryegrass Seedlings; Contribution of Cell Wall Adsorption

Net copper uptake by cellulose discs, isolated root cell walls,and by live and dead roots of whole ryegrass seedlings, werestudied using ⁶⁴Cu as a tracer. Uptake by cellulose discs stoppedafter around 10 h while uptake by isolated root cell walls continuedfor up to 50 h. An initial fast phase of uptake consisting predominantlyof cell wall adsorption was similar in live and dead tissuefor up to 19 h. A slower phase of uptake continued for up to50 h, greater in live than in dead tissue, the slower phaseof uptake in live tissue consisting of both a living and a deadcomponent. Based on these results, an alternative to the desorptionmethod for estimating the apoplastic contribution to total copperuptake is presented. Time-course studies with seedlings givena variety of growing solution/uptake solution regimes, and therelationship between copper uptake and external copper concentration,for short (4.8 h) and long (42.4 h) term uptakes, suggest thatdiffering contributions of cell wall adsorption and symplasmicabsorption may be responsible for differing effects of externalcopper concentration on uptake being expressed by the same tissue.Water flux had little effect on total uptake of copper althougha possible effect on absorption could not be ruled out. Key words: Copper uptake, cell wall adsorption, ryegrass seedlings     

Spatial root distribution of plants growing in vertical media for use in living walls

Background and Aims

For plants growing in living walls, the growth potential is correlated to the roots ability to utilize resources in all parts of the growing medium and thereby to the spatial root distribution. The aim of the study was to test how spatial root distribution was affected by growing medium, planting position and competition from other plants.

Methods

Five species (Campanula poscharskyana cv. ‘Stella’, Fragaria vesca cv. ‘Småland’, Geranium sanguineum cv. ‘Max Frei’, Sesleria heufleriana and Veronica officinalis cv. ‘Allgrün’) were grown in three growing media (coir and two of rockwool) in transparent boxes under greenhouse conditions. Root frequency was registered and the activity of individual root systems was studied via ¹⁵N uptake and plant dry weight was measured.

Results

Plants in coir had stronger root growth in all parts of the medium than plants in rockwool. Upwards root growth was limited for plants in the middle or lower parts of the medium and ¹⁵N measurements confirmed that only plants in the bottom of the box had active roots in the bottom of the medium. The species differed in root architecture and spatial root distribution.

Conclusions

The choice of growing medium, plant species and planting position is important for a living wall as it affects the spatial root growth of the plants.     

Accumulation of phenanthrene by roots of intact wheat (<Emphasis Type="Italic">Triticum acstivnm</Emphasis>L.) seedlings: passive or active uptake?

Background  

Polycyclic aromatic hydrocarbons (PAHs) are of particular concern due to their hydrophobic, recalcitrant, persistent, potentially carcinogenic, mutagenic and toxic properties, and their ubiquitous occurrence in the environment. Most of the PAHs in the environment are present in surface soil. Plants grown in PAH-contaminated soils or water can become contaminated with PAHs because of their uptake. Therefore, they may threaten human and animal health. However, the mechanism for PAHs uptake by crop roots is little understood. It is important to understand exactly how PAHs are transported into the plant root system and into the human food chain, since it is beneficial in governing crop contamination by PAHs, remedying soils or waters polluted by PAHs with plants, and modeling potential uptake for risk assessment.     

Net root growth and nutrient acquisition in response to predicted climate change in two contrasting heathland species

Background and aims

Accurate predictions of nutrient acquisition by plant roots and mycorrhizas are critical in modelling plant responses to climate change.

Methods

We conducted a field experiment with the aim to investigate root nutrient uptake in a future climate and studied root production by ingrowth cores, mycorrhizal colonization, and fine root N and P uptake by root assay of Deschampsia flexuosa and Calluna vulgaris.

Results

Net root growth increased under elevated CO₂, warming and drought, with additive effects among the factors. Arbuscular mycorrhizal colonization increased in response to elevated CO₂, while ericoid mycorrhizal colonization was unchanged. The uptake of N and P was not increased proportionally with root growth after 5 years of treatment.

Conclusions

While aboveground biomass was unchanged, the root growth was increased under elevated CO₂. The results suggest that plant production may be limited by N (but not P) when exposed to elevated CO₂. The species-specific response to the treatments suggests different sensitivity to global change factors, which could result in changed plant competitive interactions and belowground nutrient pool sizes in response to future climate change.     

Nature and nurture: the importance of seed phosphorus content

Background

Low phytoavailability of phosphorus (P) limits crop production worldwide. Increasing seed P content can improve plant establishment and increase yields. This is thought to be a consequence of faster initial root growth, which gives seedlings earlier access to growth-limiting resources, such as water and mineral elements. It can be calculated that seed P reserves can sustain maximal growth of cereal seedlings for several weeks after germination, until the plant has three or more leaves and an extensive root system.

Case study

In this issue of Plant and Soil, Muhammad Nadeem and colleagues report (1) that measurable P uptake by roots of maize seedlings begins about 5 d after germination, (2) that the commencement of root P uptake is coincident with the transition from carbon heterotrophy to carbon autotrophy, and (3) that neither the timing nor the rate of uptake of exogenous P by the developing root system is influenced by initial seed P content.

Hypothesis

Here it is hypothesised that the delay in P acquisition by roots of maize seedlings might be explained if the expression of genes encoding phosphate transporters is not upregulated either (1) because the plant has sufficient P for growth or (2) because a systemic signal from the shoot, which relies on photosynthesis or phloem development, is not produced, translocated or perceived.     

Slow decomposition and limited nitrogen release by lower order roots in eight Chinese temperate and subtropical trees

Background and aims

Roots of the lowest branch orders have the highest mortality rate, and may contribute predominately to plant carbon (C) and nutrient transfer into the soil. Yet patterns and controlling factors of the decomposition of these roots are poorly understood.

Methods

We conducted a two-year field litterbag study on different root orders and leaf litter in four temperate and four subtropical tree species.

Results

Five species showed slower decay rates in lower- (order 1–2) than higher-order (order 3–5) roots, and all species showed slower decay rates in lower-order roots than leaf litter. These patterns were strongly related to higher acid-insoluble fraction in lower- than higher-order roots, and in roots than in leaf litter, but were unrelated to initial N concentration. Litter N was predominantly in recalcitrant forms and limited amount of N was released during the study period;only 12 % of root N and 26 % of leaf litter N was released in 2 years.

Conclusions

We conclude that the slow decomposition of lower-order roots may be a common phenomenon and is mainly driven by their high acid-insoluble fraction. Moreover, litter N, especially root N, is retained during decomposition and may not be available for immediate plant uptake.