Degradation of the polycyclic aromatic hydrocarbon (PAH) fluorene is retarded in a Scots pine ectomycorrhizosphere

Polycyclic aromatic hydrocarbons (PAHs) are an important class of persistent organic pollutants (POPs) in the environment and accumulate in forest soils. These soils are often dominated by ectomycorrhizal (EcM) roots, but little is known about how EcM fungi degrade PAHs, or the overall effect of field colonized EcM roots on the fate of PAHs. The ability of eight EcM fungi to degrade PAHs in liquid culture spiked with ¹⁴C labelled PAHs was investigated. Microcosms were used to determine the impact of naturally colonized mycorrhizal pine seedlings on PAH mineralization and volatilization. Only two EcM fungi (Thelephora terrestris and Laccaria laccata) degraded at least one PAH and none were able to mineralize the PAHs in pure culture. Where degradation occurred, the compounds were only mono-oxygenated. EcM pine seedlings did not alter naphthalene mineralization or volatilization but retarded fluorene mineralization by 35% compared with unplanted, ectomycorrhizosphere soil inoculated, microcosms. The EcM fungi possessed limited PAH degrading abilities, which may explain why EcM dominated microcosms retarded fluorene mineralization. This observation is considered in relation to the 'Gadgil-effect', where retarded litter decomposition has been observed in the presence of EcM roots. © New Phytologist (2004).

High-affinity phosphate/arsenate transport in white lupin (Lupinus albus) is relatively insensitive to phosphate status

The development of proteoid roots under phosphorus deficiency by white lupin (Lupinus albus) may result in increased arsenate uptake, as arsenate is a phosphate analogue. This, together with its high biomass production, rapid growth and ability to survive in soils with low phosphate and nitrogen contents, low pH and high metal contents make them an interesting species to investigate with respect to revegetation, and possibly also for long-term phytoremediation, of arsenic contaminated soils. Kinetic parameters for arsenate uptake for P-deficient and P-sufficient plants, as well as for proteoid and nonproteoid roots were obtained. Down-regulation of arsenate uptake by phosphate, as well as phosphate/arsenate competition for P-deficient and P-sufficient plants was studied. Arsenate uptake was reduced by phosphate, but small differences were found between P-deficient and P-sufficient plants. Arsenate uptake by proteoid roots was higher than for nonproteoid roots of P-deficient plants, with higher V_max and similar K_m values. Down-regulation of the high affinity phosphate/arsenate uptake system by phosphate does take place but seems to be slower than in other plants. This study suggests that the low sensitivity of the phosphate/arsenate uptake system to regulation by phosphate may be related to the adaptations of white lupin to low P available environments. Such adaptation are absent in plants unable to develop proteoid roots.

Arbuscular mycorrhizal fungi confer enhanced arsenate resistance on Holcus lanatus

The role of arbuscular mycorrhizal fungi (AMF) in arsenate resistance in arbuscular mycorrhizal associations is investigated here for two Glomus spp. isolated from the arsenate-resistant grass Holcus lanatus. Glomus mosseae and Glomus caledonium were isolated from H. lanatus growing on an arsenic-contaminated mine-spoil soil. The arsenate resistance of spores was compared with nonmine isolates using a germination assay. Short-term arsenate influx into roots and long-term plant accumulation of arsenic by plants were also investigated in uninfected arsenate resistant and nonresistant plants and in plants infected with mine and nonmine AMF. Mine AMF isolates were arsenate resistant compared with nonmine isolates. Resistant and nonresistant G. mosseae both suppressed high-affinity arsenate/phosphate transport into the roots of both resistant and nonresistant H. lanatus. Resistant AMF colonization of resistant H. lanatus growing in contaminated mine spoil reduced arsenate uptake by the host. We conclude that AMF have evolved arsenate resistance, and conferred enhanced resistance on H. lanatus.

Arsenic uptake and metabolism in arsenic resistant and nonresistant plant species

Elevation of arsenic levels in soils causes considerable concern with respect to plant uptake and subsequent entry into wildlife and human food chains, Arsenic speciation in the environment is complex, existing in both inorganic and organic forms, with interconversion between species regulated by biotic and abiotic processes. To understand and manage the risks posed by soil arsenic it is essential to know how arsenic is taken up by the roots and metabolized within plants. Some plant species exhibit phenotypic variation in response to arsenic species, which helps us to understand the toxicity of arsenic and the way in which plants have evolved arsenic resistances. This knowledge, for example, could be used produce plant cultivars that are more arsenic resistant or that have reduced arsenic uptake. This review synthesizes current knowledge on arsenic uptake, metabolism and toxicity for arsenic resistant and nonresistant plants, including the recently discovered phenomenon of arsenic hyperaccumulation in certain fern species. The reasons why plants accumulate and metabolize arsenic are considered in an evolutionary context. © New Phytologist.

Sensitivity to Cd or Zn of host and symbiont of ectomycorrhizal Pinus sylvestris L. (Scots pine) seedlings

Pinus sylvestris seedlings infected with either the ectomycorrhizal (ECM) fungus Paxillus involutus or Suillus variegatus were exposed to a range of Cd or Zn concentrations. This was done to investigate the relationship between the sensitivity of ECM fungi and their host plants over a wide range of concentrations. P involutus ameliorated the toxicity of Cd and Zn to P. sylvestris with respect to root length, despite significant inhibition of ECM infection levels by Cd (Cd EC₅₀ [effective concentration which inhibits ECM infection by 50%] values were: P. involutus 3.7 μg g^-1 Cd; S. variegatus 2.3 μg g^-1 Cd). ECM infection by P. involutus also decreased Cd and Zn transport to the plant shoots at potentially toxic concentrations and also influenced the proportion of Zn transported to the roots and shoots, with a higher proportion retained in the roots of the seedlings. ECM infection did increase host biomass production, but this was not affected by the presence of Cd or Zn. Root and shoot biomass production by P. sylvestris, in both the presence and absence of ECM fungi, was unaffected by Cd and Zn at all concentrations tested.

Toxic effects of cadmium and zinc on ectomycorrhizal colonization of scots pine (Pinus sylvestris L.) from soil inoculum

Scots pine seedlings colonized by ectomycorrhizal (ECM) fungi from natural soil inoculum were exposed to a range of Cd or Zn concentrations to investigate the effects of metals on ECM fungi-Scots pine associations in a realistic soil environment. Experiments focused on the relationship between the sensitivity of ECM fungi and their host plants, the influence of metals on ECM community dynamics on Scots pine roots, and the effects of metal exposure on ECM colonization from soil-borne propagules. Ectomycorrhizal colonization was inhibited by Cd and Zn, with a decrease in the proportion of ECM-colonized root tips. Shoot and root biomass, total root length, and total root-tip density, however, were unaffected by Cd or Zn. A decrease in the diversity of ECM morphotypes also occurred, which could have a negative effect on tree vigor. Overall, colonization by ECM fungi was more sensitive than seedling growth to Cd and Zn, and this could have serious implications for successful tree establishment on metal-contaminated soils.

Genetic diversity of root-associated fungal endophytes from Calluna vulgaris at contrasting field sites

A total of 107 putative ericoid mycorrhizal endophytes were isolated from hair roots of Calluna vulgaris from two abandoned arsenic/copper mine sites and a natural heathland site in southwest England. The endophytes were initially grouped as 14 RFLP types, based on the results of ITS-RFLP analysis using the restriction endonucleases Hinf I, Rsa I and Hae III. ITS sequences were obtained for representative isolates from each RFLP type and compared phylogenetically with sequences for known ericoid mycorrhizal endophytes and selected ascomycetes. The majority of endophyte isolates (62-92%) from each site were identified as Hymenoscyphus ericae, but a number of other less common mycorrhizal RFLP types were also identified, all of which appear to have strong affinities with the order Leotiales. None of the less common RFLP types was isolated from C. vulgaris at more than one field site. Neighbour-joining analysis indicated similarities between the endophytes from C. vulgaris and mycorrhizal endophytes isolated from other Ericaceae and Epacridaceae hosts in North America and Australia.

Assessment of lux-marked Pseudomonas fluorescens for reporting on organic carbon compounds

Carbon-flow from plant roots to the rhizosphere provides a major source of nutrients for the soil microbial population. However, quantification of carbon-flow is problematic due to its complex composition. This study investigated the potential of lux-marked Pseudomonas fluorescens to discriminate between forms of carbon present in the rhizosphere by measuring the light response to a range of carbon compounds. Results indicate that bioluminescence of short-term carbon-starved P. fluorescens is dependent upon the source and concentration of carbon. This system, therefore, has the potential to both quantify and qualify organic acids present in rhizodeposits.

Cross-colonization of scots pine (Pinus sylvestris) seedlings by the ectomycorrhizal fungus Paxillus involutus in the presence of inhibitory levels of Cd and Zn

The effects of Cd and Zn on cross-colonization by Paxillus involutus of Scots pine seedlings was examined by using pairs of ectomycorrhizal (ECM) and non-mycorrhizal (NM) seedlings grown in the same vessel. This was done to assess, first, the ability of P. involutus to colonize NM Scots pine seedlings by growth from colonized roots of other Scots pine seedlings in the presence of Cd or Zn, and, second whether ECM colonization of Scots pine by P. involutus provided a competitive advantage over NM seedlings. Ectomycorrhizal colonization of Scots pine was shown to be more sensitive than Scots pine itself to Cd and Zn, but prior colonization did provide a competitive advantage with respect to biomass production. This beneficial effect over NM seedlings was, however, equal in the control, Cd and Zn treatments, and was due simply to growth stimulation in the presence of ECM colonization. Cross-colonization from an ECM to a NM seedling was reduced but not prevented by Cd and Zn. Cd had a more negative effect on cross-colonization than on initial colonization of seedlings, whereas Zn had an equally inhibitory effect on both parameters. These results have important implications for plant establishment on metal-contaminated sites. If cross-colonization between plants is reduced by toxic metals, plant establishment on contaminated sites might be retarded.

Response of soil microbial biomass to 1,2-dichlorobenzene addition in the presence of plant residues

The impact of 1,2-dichlorobenzene on soil microbial biomass in the presence and absence of fresh plant residues (roots) was investigated by assaying total vital bacterial counts, vital fungel hyphal length, total culturable bacterial counts, and culturable fluorescent pseudomonads. Diversity of the fluorescent pseudomonads was investigated using fatty acid methyl ester (FAME) characterization in conjunction with metabolic profiling of the sampled culturable community (Biolog). Mineralization of [¹⁴C]1,2- dichlorobenzene was also assayed. Addition of fresh roots stimulated 1,2- dichlorobenzene mineralization by over 100%, with nearly 20% of the label mineralized in root-amended treatments by the termination of the experiment. Presence of roots also buffered any impacts of 1,2-dichlorobenzene on microbial numbers. In the absence of roots, 1,2-dichlorobenzene greatly stimulated total culturable bacteria and culturable pseudomonads in a concentration-dependent manner. 1,2-Dichlorobenzene, up to concentrations of 50 μg/g soil dry weight had little or no deleterious effects on microbial counts. The phenotypic diversity of the fluorescent pseudomonad population was unaffected by the treatments, even though fluorescent pseudomonad numbers were greatly stimulated by both roots and 1,2-dichlorobenzene. The presence of roots had no detectable impact on the bacterial community composition. No phenotypic shifts in the natural population were required to benefit from the presence of roots and 1,2-dichlorobenzene. The metabolic capacity of the culturable bacterial community was altered in the presence of roots but not in the presence of 1,2-dichlorobenzene. It is argued that the increased microbial biomass and shifts in metabolic capacity of the microbial biomass are responsible for enhanced degradation of 1,2-dichlorobenzene in the presence of decaying plant roots.

Biomass allocation, phosphorus nutrition and vesicular-arbuscular mycorrhizal infection in clones of Yorkshire Fog, Holcus lanatus L. (Poaceae) that differ in their phosphate uptake kinetics and tolerance to arsenate

Biomass and phosphorus allocation were determined in arsenate tolerant and non-tolerant clones of the grass Holcus lanatus L. in both solution culture and in soil. Arsenate is a phosphate analogue and is taken up by the phosphate uptake system. Tolerance to arsenate in this grass is achieved by suppression of arsenate (and phosphate) influx. When clones differing in their arsenate tolerance were grown in solution culture with a range of phosphate levels, a tolerant clone did not fare as well as a non-tolerant at low levels of phosphate nutrition in that it had reduced shoot biomass production, increased biomass allocation to the roots and lower shoot phosphorus concentration. At a higher level of phosphate nutrition there was little or no difference in these parameters, suggesting that differences at lower levels of phosphate nutrition were due solely to differences in the rates of phosphate accumulation. In experiments in sterile soil (potting compost) the situation was more complicated with tolerant plants having lower growth rates but higher phosphorus concentrations. The gene for arsenate tolerance is polymorphic in arsenate uncontaminated populations. When phosphorus concentration of tolerant phenotypes was determined in one such population, again tolerants had a higher phosphorus status than non-tolerants. Tolerants also had higher rates of vesicular-arbuscular mycorrhizal (VAM) infection. The ecological implications of these results are that it appears that suppression of the high affinity uptake system, is at least in part, compensated by increased mycorrhizal infection. © 1994 Kluwer Academic Publishers.

A critical review of labelling techniques used to quantify rhizosphere carbon-flow

The rhizosphere is a major sink for photo-assimilated carbon and quantifying inputs into this sink is one of the main goals of rhizosphere biology as organic carbon lost from plant roots supports a higher microbial population in the rhizosphere compared to bulk soil. Two fundamentally different¹⁴CO₂ labelling strategies have been developed to estimate carbon fluxes through the rhizosphere - continuous feeding of shoots with labelled carbon dioxide and pulse-chase experiments. The biological interpretation that can be placed on the results of labelling experiments is greatly biased by the technique used. It is the purpose of this paper to assess the advantages, disadvantages and the biological interpretation of both continuous and pulse labelling and to consider how to partition carbon fluxes within the rhizosphere. © 1994 Kluwer Academic Publishers.

Suppression of the high affinity phosphate uptake system:A mechanism of arsenate tolerance in Holcus lanatus L.

In Holcus lanatus L. phosphate and arsenate are taken up by the same transport system. Short-term uptake kinetics of the high affinity arsenate transport system were determined in excised roots of arsenate-tolerant and non-tolerant genotypes. In tolerant plants the V_max of ion uptake in plants grown in phosphate-free media was decreased compared to non-tolerant plants, and the affinity of the uptake system was lower than in the non-tolerant plants. Both the reduction in V_max and the increase in K_m led to reduced arsenate influx into tolerant roots. When the two genotypes were grown in nutrient solution containing high levels of phosphate, there was little change in the uptake kinetics in tolerant plants. In non-tolerant plants, however, there was a marked decrease in the V_max to the level of the tolerant plants but with little change in the K_m. This suggests that the low rate of arsenate uptake over a wide range of differing root phosphate status is due to loss of induction of the synthesis of the arsenate (phosphate) carrier. © 1992 Oxford University Press.

A novel method of quantifying root exudation in the presence of soil microflora

A microcosm is described in which root exudation may be estimated in the presence of microorganisms. Ryegrass seedlings are grown in microcosms in which roots were spatially separated from a microbial inoculant by a Millipore membrane. Seedlings grown in the microcosms were labelled with [¹⁴C]-CO₂, and the fate of the label within the plant and rhizosphere was determined. Inoculation of the microcosms with Cladosporium resinae increased net fixation of the [¹⁴C] label compared to plants grown under sterile conditions. Inoculation also increased root exudation. The use of the microcosm was illustrated and its applications discussed. © 1991 Kluwer Academic Publishers.

The effect of soil pH on rhizosphere carbon flow of Lolium perenne

Perennial rye-grass plants were grown at 15°C in microcosms containing soil sampled from field plots that had been maintained at constant pH for the last 30 years. Six soil pH values were tested in the experiment, with pH ranging from 4.3-6.5. After 3 weeks growth in the microcosms, plant shoots were exposed to a pulse of ¹⁴C-CO₂. The fate of this label was determined by monitoring ¹⁴C-CO₂ respired by the plant roots/soil and by the shoots. The ¹⁴C remaining in plant roots and shoots was determined when the plants were harvested 7 days after receiving the pulse label. The amount of ¹⁴C (expressed as a percentage of the total ¹⁴C fixed by the plant) lost from the plant roots increased from 12.3 to 30.6% with increasing soil pH from 4.3 to 6. Although a greater percentage of the fixed ¹⁴C was respired by the root/soil as soil pH increased, plant biomass was greater with increasing soil pH. Possible reasons for observed changes in the pattern of ¹⁴C distribution are discussed and, it is suggested that changes in the soil microbial biomass and in plant nitrogen nutrition may, in particular be key factors which led to increased loss of carbon from plant roots with increasing soil pH. © 1990 Kluwer Academic Publishers.