Quantitative genetics of preference and performance on chickpeas in the noctuid moth, Helicoverpa armigera

If a novel, resistant host-plant genotype arises in the environment, insect populations utilising that host must be able to overcome that resistance in order that they can maintain their ability to feed on that host. The ability to evolve resistance to host-plant defences depends upon additive genetic variation in larval performance and adult host-choice preference. To investigate the potential of a generalist herbivore to respond to a novel resistant host, we estimated the heritability of larval performance in the noctuid moth, Helicoverpa armigera, on a resistant and a susceptible variety of the chickpea, Cicer arietinum, at two different life stages. Heritability estimates were higher for neonates than for third-instar larvae, suggesting that their ability to establish on plants could be key to the evolution of resistance in this species; however, further information regarding the nature of selection in the field would be required to confirm this prediction. There was no genetic correlation between larval performance and oviposition preference, indicating that female moths do not choose the most suitable plant for their offspring. We also found significant genotype by environment interactions for neonates (but not third-instar larvae), suggesting that the larval response to different plant genotypes is stage-specific in this species.

Genomic insights into the origin of parasitism in the emerging plant pathogen Bursaphelenchus xylophilus.

Bursaphelenchus xylophilus is the nematode responsible for a devastating epidemic of pine wilt disease in Asia and Europe, and represents a recent, independent origin of plant parasitism in nematodes, ecologically and taxonomically distinct from other nematodes for which genomic data is available. As well as being an important pathogen, the B. xylophilus genome thus provides a unique opportunity to study the evolution and mechanism of plant parasitism. Here, we present a high-quality draft genome sequence from an inbred line of B. xylophilus, and use this to investigate the biological basis of its complex ecology which combines fungal feeding, plant parasitic and insect-associated stages. We focus particularly on putative parasitism genes as well as those linked to other key biological processes and demonstrate that B. xylophilus is well endowed with RNA interference effectors, peptidergic neurotransmitters (including the first description of ins genes in a parasite) stress response and developmental genes and has a contracted set of chemosensory receptors. B. xylophilus has the largest number of digestive proteases known for any nematode and displays expanded families of lysosome pathway genes, ABC transporters and cytochrome P450 pathway genes. This expansion in digestive and detoxification proteins may reflect the unusual diversity in foods it exploits and environments it encounters during its life cycle. In addition, B. xylophilus possesses a unique complement of plant cell wall modifying proteins acquired by horizontal gene transfer, underscoring the impact of this process on the evolution of plant parasitism by nematodes. Together with the lack of proteins homologous to effectors from other plant parasitic nematodes, this confirms the distinctive molecular basis of plant parasitism in the Bursaphelenchus lineage. The genome sequence of B. xylophilus adds to the diversity of genomic data for nematodes, and will be an important resource in understanding the biology of this unusual parasite.

How does global change affect the strength of trophic interactions?

Recent research has generally shown that a small change in the number of species in a food web can have consequences both for community structure and ecosystem processes. However 'change' is not limited to just the number of species in a community, but might include an alteration to such properties as precipitation, nutrient cycling and temperature, all of which are correlated with productivity. Here we argue that predicted scenarios of global change will result in increased plant productivity. We model three scenarios of change using simple Lotka-Volterra dynamics, which explore how a global change in productivity might affect the strength of local species interactions and detail the consequences for community and ecosystem level stability. Our results indicate that (i) at local scales the average population size of consumers may decline because of poor quality food resources, (ii) that the strength of species interactions at equilibrium may become weaker because of reduced population size, and (iii) that species populations may become more variable and may take longer to recover from environmental or anthropogenic disturbances. At local scales interaction strengths encompass such properties as feeding rates and assimilation efficiencies, and encapsulate functionatty important information with regard to ecosystem processes. Interaction strengths represent the pathways and transfer of energy through an ecosystem. We examine how such local patterns might be affected given various scenarios of 'global change' and discuss the consequences for community stability and ecosystem functioning. (C) 2004 Elsevier GmbH. All rights reserved.

Possible interactions between environmental factors in determining species optima

Questions: 1. Indicator values, such as those of Ellenberg, for different environmental factors are seen as independent. We tested for the presence of interactions between environmental factors ( soil moisture and reaction) to see if this assumption is simplistic. 2. How close are Ellenberg indicator values (IVs) related to the observed optima of species response curves in an area peripheral to those where they have been previously employed and 3. Can the inclusion of bryophytes add to the utility of IVs?

Location: South Uist, Outer Hebrides, Scotland, UK.

Methods: Two grids (ca. 2000 m x 2000 m) were sampled at 50-m intervals across the transition from machair to upland communities covering an orthogonal gradient of both soil pH ( reaction) and soil moisture content. Percentage cover data for vascular plants, bryophytes and lichens were recorded, along with pH and moisture content of the underlying sand/soil/peat. Reaction optima, derived from species response curves calculated using HOF models, were compared between wet and dry sites, and moisture optima between acidic and basic samples. Optima for the whole data set were compared to Ellenberg IVs to assess their performance in this area, with and without the inclusion of bryophytes.

Results: A number of species showed substantially different pH optima at high and low soil moisture contents (18% of those tested) and different soil moisture optima at high and low pH (49%). For a number of species the IVs were poor predictors of their actual distribution across the sampled area. Bryophytes were poor at explaining local variation in the environmental factors and also their inclusion with vascular plants negatively affected the strength of relationships.

Conclusions: A substantial number of species showed an interaction between soil moisture and reaction in determining their optima on the two respective gradients. It should be borne in mind that IVs such as Ellenberg's may not be independent of one another.

Rice-arsenate interactions in hydroponics:whole genome transcriptional analysis

Rice (Oryza sativa) varieties that are arsenate-tolerant (Bala) and -sensitive (Azucena) were used to conduct a transcriptome analysis of the response of rice seedlings to sodium arsenate (AsV) in hydroponic solution. RNA extracted from the roots of three replicate experiments of plants grown for 1 week in phosphate-free nutrient with or without 13.3 muM AsV was used to challenge the Affymetrix (52K) GeneChip Rice Genome array. A total of 576 probe sets were significantly up-regulated at least 2-fold in both varieties, whereas 622 were down-regulated. Ontological classification is presented. As expected, a large number of transcription factors, stress proteins, and transporters demonstrated differential expression. Striking is the lack of response of classic oxidative stress-responsive genes or phytochelatin synthases/synthatases. However, the large number of responses from genes involved in glutathione synthesis, metabolism, and transport suggests that glutathione conjugation and arsenate methylation may be important biochemical responses to arsenate challenge. In this report, no attempt is made to dissect differences in the response of the tolerant and sensitive variety, but analysis in a companion article will link gene expression to the known tolerance loci available in the BalaxAzucena mapping population.

Rice-arsenate interactions in hydroponics: a three-gene model for tolerance

In this study, the genetic mapping of the tolerance of root growth to 13.3 muM arsenate [As(V)] using the BalaxAzucena population is improved, and candidate genes for further study are identified. A remarkable three-gene model of tolerance is advanced, which appears to involve epistatic interaction between three major genes, two on chromosome 6 and one on chromosome 10. Any combination of two of these genes inherited from the tolerant parent leads to the plant having tolerance. Lists of potential positional candidate genes are presented. These are then refined using whole genome transcriptomics data and bioinformatics. Physiological evidence is also provided that genes related to phosphate transport are unlikely to be behind the genetic loci conferring tolerance. These results offer testable hypotheses for genes related to As(V) tolerance that might offer strategies for mitigating arsenic (As) accumulation in consumed rice.

The mechanistic basis of interactions between mycorrhizal associations and toxic metal cations

Mycorrhizal associations, including ericoid, arbuscular and ecto-mycorrhizas, are found colonising highly metal contaminated soils. How do mycorrhizal fungi achieve metal resistance, and does this metal resistance confer enhanced metal resistance to plant symbionts? These are the questions explored in this review by considering the mechanistic basis of mycorrhizal adaptation to metal cations. Recent molecular and physiological studies are discussed. The review reappraises what constitutes metal resistance in the context of mycorrhizal associations and sets out the constitutive and adaptive mechanisms available for mycorrhizas to adapt to contaminated sites. The only direct evidence of mycorrhizal adaptation to metal cation pollutants is the exudation of organic acids to alter pollutant availability in the rhizosphere. This is not to say that other mechanism of adaptation do not exist, but conclusive evidence of adaptive mechanisms of tolerance are lacking. For constitutive mechanisms of resistance, there is much more evidence, and mycorrhizas possess the same constitutive mechanisms for dealing with metal contaminants as other organisms. Rhizosphere chemistry is critical to understanding the interactions of mycorrhizas with polluted soils. Soil pH, mineral weathering, pollutant precipitation with plant excreted organic acids all may have a key role in constitutive and adaptive tolerance of mycorrhizal associations present on contaminated sites. The responses of mycorrhizal fungi to toxic metal cations are diverse. This, linked to the fact that mycorrhizal diversity is normally high, even on highly contaminated sites, suggests that this diversity may have a significant role in colonisation of contaminated sites by mycorrhizas. That is, the environment selects for the fungal community that can best cope with the environment, so having diverse physiological attributes will enable colonisation of a wide range of metal contaminated micro-habitats.

Mechanisms of arsenic hyperaccumulation in Pteris vittata. Uptake kinetics, interactions with phosphate, and arsenic speciation

The mechanisms of arsenic (As) hyperaccumulation in Pteris vittata, the first identified As hyperaccumulator, are unknown. We investigated the interactions of arsenate and phosphate on the uptake and distribution of As and phosphorus (P), and As speciation in P. vittata. In an 18-d hydroponic experiment with varying concentrations of arsenate and phosphate, P. vittata accumulated As in the fronds up to 27,000 mg As kg(-1) dry weight, and the frond As to root As concentration ratio varied between 1.3 and 6.7. Increasing phosphate supply decreased As uptake markedly, with the effect being greater on root As concentration than on shoot As concentration. Increasing arsenate supply decreased the P concentration in the roots, but not in the fronds. Presence of phosphate in the uptake solution decreased arsenate influx markedly, whereas P starvation for 8 d increased the maximum net influx by 2.5-fold. The rate of arsenite uptake was 10% of that for arsenate in the absence of phosphate. Neither P starvation nor the presence of phosphate affected arsenite uptake. Within 8 h, 50% to 78% of the As taken up was distributed to the fronds, with a higher translocation efficiency for arsenite than for arsenate. In fronds, 49% to 94% of the As was extracted with a phosphate buffer (pH 5.6). Speciation analysis using high-performance liquid chromatography-inductively coupled plasma mass spectroscopy showed that >85% of the extracted As was in the form of arsenite, and the remaining mostly as arsenate. We conclude that arsenate is taken up by P. vittata via the phosphate transporters, reduced to arsenite, and sequestered in the fronds primarily as As(III).

Choosing and using diversity indices: Insights for ecological applications from the German Biodiversity Exploratories

Biodiversity, a multidimensional property of natural systems, is difficult to quantify partly because of the multitude of indices proposed for this purpose. Indices aim to describe general properties of communities that allow us to compare different regions, taxa, and trophic levels. Therefore, they are of fundamental importance for environmental monitoring and conservation, although there is no consensus about which indices are more appropriate and informative. We tested several common diversity indices in a range of simple to complex statistical analyses in order to determine whether some were better suited for certain analyses than others. We used data collected around the focal plant Plantago lanceolata on 60 temperate grassland plots embedded in an agricultural landscape to explore relationships between the common diversity indices of species richness (S), Shannon's diversity (H'), Simpson's diversity (D1), Simpson's dominance (D2), Simpson's evenness (E), and Berger–Parker dominance (BP). We calculated each of these indices for herbaceous plants, arbuscular mycorrhizal fungi, aboveground arthropods, belowground insect larvae, and P. lanceolata molecular and chemical diversity. Including these trait-based measures of diversity allowed us to test whether or not they behaved similarly to the better studied species diversity. We used path analysis to determine whether compound indices detected more relationships between diversities of different organisms and traits than more basic indices. In the path models, more paths were significant when using H', even though all models except that with E were equally reliable. This demonstrates that while common diversity indices may appear interchangeable in simple analyses, when considering complex interactions, the choice of index can profoundly alter the interpretation of results. Data mining in order to identify the index producing the most significant results should be avoided, but simultaneously considering analyses using multiple indices can provide greater insight into the interactions in a system.

A simple inhibition coefficient for quantifying potency of biocontrol agents against plant-pathogenic fungi

Microbial interactions depend on a range of biotic and environmental variables, and are both dynamic and unpredictable. For some purposes, and under defined conditions, it is nevertheless imperative to evaluate the inhibitory efficacy of microbes, such as those with potential as biocontrol agents. We selected six, phylogenetically diverse microbes to determine their ability to inhibit the ascomycete Fusarium
coeruleum, a soil-dwelling pathogen of potato tubers that causes the storage disease dry rot. Interaction assays, where colony development was quantified (for both fungal pathogen and potential control agents), were therefore carried out on solid media. The key parameters that contributed to, and were indicative of, inhibitory efficacy were identified as: fungal growth-rates (i) prior to contact with the biocontrol
agent and (ii) if/once contact with the biocontrol agent was established (i.e. in the zone of mixed
culture), and (iii) the ultimate distance traveled by the fungal mycelium. It was clear that there was no correlation between zones of fungal inhibition and the overall reduction in the extent of fungal colony development. An inhibition coefficient was devised which incorporated the potential contributions of distal inhibition of fungal growth-rate; prevention of mycelium development in the vicinity of the biocontrol
agent; and ability to inhibit plant-pathogen growth-rate in the zone of mixed culture (in a ratio of 2:2:1). The values derived were 84.2 for Bacillus subtilis (QST 713), 74.0 for Bacillus sp. (JC12GB42), 30.7 for Pichia anomala (J121), 19.3 for Pantoea agglomerans (JC12GB34), 13.9 for Pantoea sp. (S09:T:12), and
21.9 (indicating a promotion of fungal growth) for bacterial strain (JC12GB54). This inhibition coefficient, with a theoretical maximum of 100, was consistent with the extent of F. coeruleum-colony development (i.e. area, in cm2) and assays of these biocontrol agents carried out previously against Fusarium
spp., and other fungi. These findings are discussed in relation to the dynamics and inherent complexity of natural ecosystems, and the need to adapt models for use under specific sets of conditions.

Conserved histidine of metal transporter AtNRAMP1 is crucial for optimal plant growth under manganese deficiency at chilling temperatures

Manganese (Mn) is an essential nutrient required for plant growth, in particular in the process of photosynthesis. Plant performance is influenced by various environmental stresses including contrasting temperatures, light or nutrient deficiencies. The molecular responses of plants exposed to such stress factors in combination are largely unknown. 

Screening of 108 Arabidopsis thaliana (Arabidopsis) accessions for reduced photosynthetic performance at chilling temperatures was performed and one accession (Hog) was isolated. Using genetic and molecular approaches, the molecular basis of this particular response to temperature (GxE interaction) was identified. 

Hog showed an induction of a severe leaf chlorosis and impaired growth after transfer to lower temperatures. We demonstrated that this response was dependent on the nutrient content of the soil. Genetic mapping and complementation identified NRAMP1 as the causal gene. Chlorotic phenotype was associated with a histidine to tyrosine (H239Y) substitution in the allele of Hog NRAMP1. This led to lethality when Hog seedlings were directly grown at 4 degrees C. 

Chemical complementation and hydroponic culture experiments showed that Mn deficiency was the major cause of this GxE interaction. For the first time, the NRAMP-specific highly conserved histidine was shown to be crucial for plant performance.

Plant community assembly at small scales: spatial versus environmental factors in a central European grassland

Dispersal limitation and environmental conditions are crucial drivers of plant species distribution and establishment. As these factors operate at different spatial scales, we asked: Do the environmental factors known to determine community assembly at broad scales operate at fine scales (few meters)? How much do these factors account for community variation at fine scales? In which way do biotic and abiotic interactions drive changes in species composition? We surveyed the plant community within a dry grassland along a very steep gradient of soil characteristics like pH and nutrients. We used a spatially explicit sampling design, based on three replicated macroplots of 15x15, 12x12 and 12x12 meters in extent. Soil samples were taken to quantify several soil properties (carbon, nitrogen, plant available phosphorus, pH, water content and dehydrogenase activity as a proxy for overall microbial activity). We performed variance partitioning to assess the effect of these variables on plant composition and statistically controlled for spatial autocorrelation via eigenvector mapping. We also applied null model analysis to test for non-random patterns in species co-occurrence using randomization schemes that account for patterns expected under species interactions. At a fine spatial scale, environmental factors explained 18% of variation when controlling for spatial autocorrelation in the distribution of plant species, whereas purely spatial processes accounted for 14% variation. Null model analysis showed that species spatially segregated in a non-random way and these spatial patterns could be due to a combination of environmental filtering and biotic interactions. Our grassland study suggests that environmental factors found to be directly relevant in broad scale studies are present also at small scales, but are supplemented by spatial processes and more direct interactions like competition.

Interactions between ectomycorrhizal fungi and soil saprotrophs:Implications for decomposition of organic matter in soils and degradation of organic pollutants in the rhizosphere

Ectomycorrhizal fungi and saprotrophic microorganisms coexist and interact in the mycorrhizosphere. We review what is known regarding these interactions and how they may influence processes such as ectomycorrhiza formation, mycelial growth, and the dynamics of carbon movement to and within the rhizosphere. Particular emphasis is placed on the potential importance of interactions in decomposition of soil organic matter and degradation of persistant organic pollutants in soil. While our knowledge is currently fairly limited, it seems likely that interactions have profound effects on mycorrhizosphere processes. More extensive research is warranted to provide novel insights into mycorrhizosphere ecology and to explore the potential for manipulating the ectomycorrhizosphere environment for biotechnological purposes.

Toxic interactions of metal ions (Cd2+, Pb2+, Zn2+ and Sb3- on in vitro biomass production of ectomycorrhizal fungi

A number of ectomycorrhizal (ECM) fungi, from sites uncontaminated by toxic metals, were investigated to determine their sensitivity to Cd^2-, Pb²⁺, Zn²⁺ and Sb^3-, measured as an inhibition of fungal biomass production. Isolates were grown in liquid media amended with the metals, individually (over a range of concentrations) and in combination (at single concentrations) to determine any significant interactions between the metals. Significant interspecific variation in sensitivity to Cd²⁺ and Zn²⁺ was recorded, while Pb²⁺ and Sb^3- individually had little effect. The presence of Pb²⁺ and Sb^3- in the media did however, ameliorate Cd²⁺ and Zn²⁺ toxicity in some circumstances. Interactions between Cd²⁺ and Zn²⁺ were investigated further over a range of concentrations. Zn²⁺ was found to significantly ameliorate the toxicity of Cd²⁺ to three of the four isolates tested. The influence of Zn²⁺ varied between ECM species and with the concentrations of metals tested.