Differential gene expressions in arbuscular mycorrhizal-colonized tomato grown under heavy metal stress

Insight into the role of grafting and arbuscular mycorrhiza on cadmium stress tolerance in tomato

Physiological, biochemical, metabolite changes, and gene expression analysis of greenhouse tomato (Solanum lycopersicum L.) were investigated in two grafting combinations (self-grafted 'Ikram' and 'Ikram' grafted onto interspecific hybrid rootstock `Maxifort'), with and without arbuscular mycorrhizal (AM), exposed to 0 and 25 μM Cd. Tomato plants responded to moderate Cadmium (Cd) concentration by decreasing yield and crop growth parameters due to the accumulation of Cd in leaf tissue, inhibition of the PS II activity, reduced nutrients translocation, and also to the oxidative stress as evidenced by enhanced hydrogen peroxide (H2O2) generation, ion leakage, and lipid peroxidation. AM inoculation significantly enhanced the metal concentration in shoots and reduced growth and yield. The Ikram/Maxifort combination induced higher antioxidant enzymes, higher accumulation of proline and reduction of lipid peroxidation products. This suggests that the use of Maxifort rootstock in tomato has a high reactive oxygen species scavenging activity since lower H2O2 concentrations were observed in the presence of Cd. The higher crop performance of Ikram/Maxifort in comparison to Ikram/Ikram combination was also due to the improved nutritional status (higher P, K, Ca, Fe, Mn, and Zn) and increased availability of metabolites involved in cadmium tolerance (phytochelatin PC2, fructans, and inulins). The up-regulation of LeNRAMP3 gene in leaf of Ikram/Maxifort could explain the better nutritional status of interspecific grafting combination (higher Fe, Mn, and Zn).

Evaluation of heavy metals, cytotoxicity, and antioxidant activity of tomatoes grown in toxic muddy soils

This research studies tomatoes grown in polluted soils to ascertain their phytochemical and nutritive features. Pulp and seeds from tomatoes grown in muddy soils were analyzed for their antioxidant power and their toxicity because of the possibility that heavy metals were present in the soils. An antioxidant assay on methanol extracts was made by using DDPH, while an ABTS [2,2'-Azino-bis-(3-ethylbenzthiazoline-6-sulfonic acid)] assay was used to evaluate the antioxidant activity of lipophilic fractions. Results of the antioxidant assay showed that the tomatoes maintained a high level of antioxidant activity especially in the lipophilic fractions which contain the most representative compounds. Cytotoxic activity was performed on HeLa, PDAC, and A375 cell lines by [3-(4,5-dimethylthiazol-2-yl)-2,5-phenyl-2H-tetrazolium bromide] (MTT) assay. Results showed that neither the seeds, nor the pulp, of the extracts was cytotoxic. The presence of heavy metals was evaluated by using spectroscopy of atomic absorption with a graphite oven. Test results show the absence of heavy metals and these results have an interesting scientific role because they provide useful information for promoting food safety.

The effects of arbuscular mycorrhizal fungi on sex-specific responses to Pb pollution in Populus cathayana

Using fast-growing trees to remediate soils polluted by heavy metals (HMs) has received increasingly more attention, especially for recalcitrant Pb, as one of the most seriously toxic HMs. However, little is known about the responses of plants to a diffused level of Pb pollution, and a more combined phytoremediation technique is needed to explore. In this study, an arbuscular mycorrhizal fungus (AMF), i.e., Funneliformis mosseae, isolated from Populus euphratica distributed in a tailing of Pb/Zn ore, was introduced to investigate its effects on sex-specific responses of P. cathayana in morphology, physiology, and Pb phytoremediation capacity, when exposed to a diffused level of Pb pollution (100mg Pb(2+) kg(-1) dry soil). Symbiosis with exotic AMF did not significantly affect growth of both sexes and biomass allocation. However, when inoculated with AMF, both sexes absorbed more P, but not N in the roots, especially when exposed to the exogenous addition of Pb. The improvement of nutrient status under such conditions might be associated with a further increase in activity of antioxidant enzymes (particularly for superoxide dismutase (SOD) and catalase (CAT)), and the mitigation of oxidation stress induced by excessive reactive oxygen species (ROS). We also observed that exotic AMF could promote the uptake and accumulation of Pb in roots of females, but not in that of males. Therefore, under this diffused pollution level, the infected females might be more suitable for remediation of this metal than infected males, due to the higher capacity of HM accumulation without obvious negative effects on growth and physiological traits. Moreover, field surveys are needed to testify our experimental results, due to diversity of soil microbial community and complexities of their interaction.

Glomus mosseae enhances root growth and Cu and Pb acquisition of upland rice (Oryza sativa L.) in contaminated soils

A pot culture experiment was carried out to investigate the roles of Glomus mosseae in Cu and Pb acquisition by upland rice (Oryza sativa L.) and the interactions between Cu and Pb. The soil was treated with three Cu levels (0, 100 and 200 mg kg(-1)) and three Pb levels (0, 300, and 600 mg kg(-1)). All treatments were designed with (+M) or without (-M) G. mosseae inoculation in a randomized block design. The addition of Cu and Pb significantly decreased root mycorrhizal colonization. Compared with -M, +M significantly increased root biomass in almost all treatments, and also significantly increased shoot biomass in the Pb(0)Cu(200), Pb(300)Cu(0), and all Pb(600) treatments. AM fungi enhanced plant Cu acquisition, but decreased plant Cu concentrations with all Cu plus Pb treatments, except for shoot in the Cu(200)Pb(600) treatment. Irrespective of Cu and Pb levels, +M plants had higher Pb uptakes than -M plants, but had lower root Pb and higher shoot Pb concentrations than those of -M plants. Another interpretation for the higher shoot Pb concentration in +M plants relied on Cu-Pb interactions. The study provided further evidences for the protective effects of AM fungi on upland rice against Cu and Pb contamination, and uncovered the phenomenon that Cu addition could promote Pb uptake and Pb partitioning to shoot. The possible mechanisms by which AM fungi can alleviate the toxicity induced by Cu and Pb are also discussed.

Arbuscular mycorrhizal fungi play a role in protecting roots of Sophora viciifolia Hance. from Pb damage associated with increased phytochelatin synthase gene expression

Understanding the influence of arbuscular mycorrhizal (AM) fungi on the expressions of the dominant plant-related genes under heavy metal (HM) stress is important for developing strategies to reclaim polluted sites. In this study, we cloned full-length cDNAs of phytochelatin synthase gene (PCS1) and Actin of Sophora viciifolia Hance., a predominant plant in Qiandongshan lead and zinc mine, by rapid amplification of cDNA ends. Consequently, we studied the response of SvPCS1 to Funneliformis mosseae inoculation under lead stress (0, 50, and 200 μM Pb(NO3)2) at different durations (1, 3, and 7 days) using quantitative reverse-transcription polymerase chain-reaction (qRT-PCR) technique. The Pb concentrations and chlorophyll fluorescence parameters were also measured to assay Pb toxicity to Sophora viciifolia. We found that Pb concentrations in roots increased with increasing Pb application and the durations; the F v /F m , F v /F o , qP, and Y(II) decreased; NPQ rose with increasing Pb concentrations; mycorrhizal symbiosis alleviated the Pb toxicity to plants; and SvPCS1 was constitutively expressed in the roots. It was also found that F. mosseae inoculation could promote the expression of SvPCS1 with the concentration ≤ 200 μM at the exposure time shorter than 7 days.

Arbuscular mycorrhizal fungal phylogenetic groups differ in affecting host plants along heavy metal levels

Arbuscular mycorrhizal fungi (AMF) are important components of soil microbial communities, and play important role in plant growth. However, the effects of AMF phylogenetic groups (Glomeraceae and non-Glomeraceae) on host plant under various heavy metal levels are not clear. Here we conducted a meta-analysis to compare symbiotic relationship between AMF phylogenetic groups (Glomeraceae and non-Glomeraceae) and host plant functional groups (herbs vs. trees, and non-legumes vs. legumes) at three heavy metal levels. In the meta-analysis, we calculate the effect size (ln(RR)) by taking the natural logarithm of the response ratio of inoculated to non-inoculated shoot biomass from each study. We found that the effect size of Glomeraceae increased, but the effect size of non-Glomeraceae decreased under high level of heavy metal compared to low level. According to the effect size, both Glomeraceae and non-Glomeraceae promoted host plant growth, but had different effects under various heavy metal levels. Glomeraceae provided more benefit to host plants than non-Glomeraceae did under heavy metal condition, while non-Glomeraceae provided more benefit to host plants than Glomeraceae did under no heavy metal. AMF phylogenetic groups also differed in promoting plant functional groups under various heavy metal levels. Interacting with Glomeraceae, herbs and legumes grew better than trees and non-legumes did under high heavy metal level, while trees and legumes grew better than herbs and non-legumes did under medium heavy metal level. Interacting with non-Glomeraceae, herbs and legumes grew better than trees and non-legumes did under no heavy metal. We suggested that the combination of legume with Glomeraceae could be a useful way in the remediation of heavy metal polluted environment.

Approach to engineer tomato by expression of AtHMA4 to enhance Zn in the aerial parts

The aim of this work was to assess the potential for using AtHMA4 to engineer enhanced efficiency of Zn translocation to shoots, and to increase the Zn concentration in aerial tissues of tomato. AtHMA4, a P1B-ATPase, encodes a Zn export protein known to be involved in the control of Zn root-to-shoot translocation. In this work, 35S::AtHMA4 was expressed in tomato (Lycopersicon esculentum var. Beta). Wild-type and transgenic plants were tested for Zn and Cd tolerance; Zn, Fe and Cd accumulation patterns, and for the expression of endogenous Zn/Fe-homeostasis genes. At 10μM Zn exposure, a higher Zn concentration was observed in leaves of AtHMA4-expressing lines compared to wild-type, which is promising in terms of Zn biofortification. AtHMA4 also transports Cd and at 0.25μM Cd the transgenic plants showed similar levels of this element in leaves to wild-type but lower levels in roots, therefore indicating a reduction of Cd uptake due to AtHMA4 expression. Expression of this transgene AtHMA4 also resulted in distinct changes in Fe accumulation in Zn-exposed plants, and Fe/Zn-accumulation in Cd-exposed plants, even though Fe is not a substrate for AtHMA4. Analysis of the transcript abundance of key Zn/Fe-homeostasis genes showed that the pattern was distinct for transgenic and wild-type plants. The reduction of Fe accumulation observed in AtHMA4-transformants was accompanied by up-regulation of Fe-deficiency marker genes (LeFER, LeFRO1, LeIRT1), whereas down-regulation was detected in plants with the status of Fe-sufficiency. Furthermore, results strongly suggest the importance of the up-regulation of LeCHLN in the roots of AtHMA4-expressing plants for efficient translocation of Zn to the shoots. Thus, the modifications of Zn/Fe/Cd translocation to aerial plant parts due to AtHMA4 expression are closely related to the alteration of the endogenous Zn-Fe-Cd cross-homeostasis network of tomato.

Contribution of arbuscular mycorrhizal fungi to the development of maize (Zea mays L.) grown in three types of coal mine spoils

Coal mine spoils are usually unfavorable for plant growth and have different properties according to dumping years, weathering degree, and the occurrence of spontaneous combustion. The establishment of plant cover in mine spoils can be facilitated by arbuscular mycorrhizal fungi (AMF). A greenhouse pot experiment was conducted to evaluate the importance of AMF in plant adaptation to different mine spoils and the potential role of AMF for revegetation practices. We investigated the effects of Glomus aggregatum, Rhizophagus intraradices (syn. Glomus intraradices), and Funneliformis mosseae (syn. Glomus mosseae) on the growth, nutritional status, and metal uptake of maize (Zea mays L.) grown in recent discharged (S1), weathered (S2), and spontaneous combusted (S3) coal mine spoils. Symbiotic associations were successfully established between AMF and maize in three substrates. Mycorrhizal colonization effectively promoted plant growth by significantly increasing the uptake of nitrogen (N), phosphorus (P), and potassium (K), adjusting C:N:P stoichiometry and alleviating toxic effects of heavy metals. G. aggregatum, R. intraradices, and F. mosseae exhibited different mycorrhizal effects in response to mine spoil types. F. mosseae was the most effective in the development of maize in S1 and may be the most appropriate for revegetation of this substrate, while R. intraradices played the most beneficial role in S2 and S3. Our results suggest that inoculation with AMF can enhance plant adaptation to different types of coal mine spoils and play a positive role in the revegetation of coal mine spoil banks.

Epigenetic control of heavy metal stress response in mycorrhizal versus non-mycorrhizal poplar plants

It was previously shown that arbuscular mycorrhizal fungi (AMF) exert a significant improvement of growth in a tolerant white poplar (Populus alba L.) clone (AL35) grown on Cu- and Zn-polluted soil via foliar alterations in the levels of defence/stress-related transcripts and molecules. However, nothing is known about the epigenetic changes which occur during tolerance acquisition in response to heavy metals (HMs) in the same mycorrhizal vs. non-mycorrhizal poplar plants. In order to analyse the epigenome in leaves of AL35 plants inoculated or not with AMF and grown in a greenhouse on multimetal polluted or unpolluted soil, the Methylation Sensitive Amplification Polymorphism (MSAP) approach was adopted to detect cytosine DNA methylation. Modest changes in cytosine methylation patterns were detected at first sampling (4 months from planting), whereas extensive alterations (hypomethylation) occurred at second sampling (after 6 months) in mycorrhizal plants grown in the presence of HMs. The sequencing of MSAP fragments led to the identification of genes belonging to several Gene Ontology categories. Seven MSAP fragments, selected on the basis of DNA methylation status in treated vs control AL35 leaves at the end of the experiment, were analysed for their transcript levels by means of qRT-PCR. Gene expression varied in treated samples relative to controls in response to HMs and/or AMF inoculation; in particular, transcripts of genes involved in RNA processing, cell wall and amino acid metabolism were upregulated in the presence of AMF with or without HMs.

Biotrophic transportome in mutualistic plant-fungal interactions

Understanding the mechanisms that underlie nutrient use efficiency and carbon allocation along with mycorrhizal interactions is critical for managing croplands and forests soundly. Indeed, nutrient availability, uptake and exchange in biotrophic interactions drive plant growth and modulate biomass allocation. These parameters are crucial for plant yield, a major issue in the context of high biomass production. Transport processes across the polarized membrane interfaces are of major importance in the functioning of the established mycorrhizal association as the symbiotic relationship is based on a 'fair trade' between the fungus and the host plant. Nutrient and/or metabolite uptake and exchanges, at biotrophic interfaces, are controlled by membrane transporters whose regulation patterns are essential for determining the outcome of plant-fungus interactions and adapting to changes in soil nutrient quantity and/or quality. In the present review, we summarize the current state of the art regarding transport systems in the two major forms of mycorrhiza, namely ecto- and arbuscular mycorrhiza.

The role of arbuscular mycorrhizas in decreasing aluminium phytotoxicity in acidic soils: a review

Soil acidity is an impediment to agricultural production on a significant portion of arable land worldwide. Low productivity of these soils is mainly due to nutrient limitation and the presence of high levels of aluminium (Al), which causes deleterious effects on plant physiology and growth. In response to acidic soil stress, plants have evolved various mechanisms to tolerate high concentrations of Al in the soil solution. These strategies for Al detoxification include mechanisms that reduce the activity of Al3+ and its toxicity, either externally through exudation of Al-chelating compounds such as organic acids into the rhizosphere or internally through the accumulation of Al-organic acid complexes sequestered within plant cells. Additionally, root colonization by symbiotic arbuscular mycorrhizal (AM) fungi increases plant resistance to acidity and phytotoxic levels of Al in the soil environment. In this review, the role of the AM symbiosis in increasing the Al resistance of plants in natural and agricultural ecosystems under phytotoxic conditions of Al is discussed. Mechanisms of Al resistance induced by AM fungi in host plants and variation in resistance among AM fungi that contribute to detoxifying Al in the rhizosphere environment are considered with respect to altering Al bioavailability.

Transcript analysis of stress defence genes in a white poplar clone inoculated with the arbuscular mycorrhizal fungus Glomus mosseae and grown on a polluted soil

In this study we investigated if the symbiosis with the arbuscular mycorrhizal fungus Glomus mosseae, which contributes to alleviate heavy metal stress in plants, may affect the transcription of genes involved in the stress defence in the white poplar clone 'AL35' grown on a multimetal (Cu and Zn) contaminated soil. The results obtained showed that the symbiosis with G. mosseae reduced transcript abundance of genes involved in antioxidant defence in leaves and roots of 'AL35' plants grown on the heavy metal-polluted soil. Moreover, the interaction between this poplar clone and the arbuscular mycorrhizal fungus induced the gene coding for phytochelatin synthase in leaves, whereas the expression of genes involved in heavy metal homeostasis did not change in roots. The present results suggest that, in presence of high levels of heavy metals, inoculation with G. mosseae may confer to 'AL35' a more efficient control of the oxidant level. Moreover, in mycorrhizal plants heavy metal chelation pathways appear involved in the defence strategies in leaves, whereas in roots they do not seem to contribute to increase the plant tolerance of heavy metals.

Iron uptake and homeostasis related genes in potato cultivated in vitro under iron deficiency and overload

Potato is one of the most important staple food in the world because it is a good source of vitamin C, vitamin B6 but also an interesting source of minerals including mainly potassium, but also magnesium, phosphorus, manganese, zinc and iron to a lesser extent. The lack of iron constitutes the main form of micronutrient deficiency in the world, namely iron deficiency anemia, which strongly affects pregnant women and children from developing countries. Iron biofortification of major staple food such as potato is thus a crucial issue for populations from these countries. To better understand mechanisms leading to iron accumulation in potato, we followed in an in vitro culture experiment, by qPCR, in the cultivar Désirée, the influence of media iron content on the expression of genes related to iron uptake, transport and homeostasis. As expected, plantlets grown in a low iron medium (1 mg L(-1) FeNaEDTA) displayed a decreased iron content, a strong induction of iron deficiency-related genes and a decreased expression of ferritins. Inversely, plantlets grown in a high iron medium (120 mg L(-1) FeNaEDTA) strongly accumulated iron in roots; however, no significant change in the expression of our set of genes was observed compared to control (40 mg L(-1) FeNaEDTA).

Perspectives of plant-associated microbes in heavy metal phytoremediation

"Phytoremediation" know-how to do-how is rapidly expanding and is being commercialized by harnessing the phyto-microbial diversity. This technology employs biodiversity to remove/contain pollutants from the air, soil and water. In recent years, there has been a considerable knowledge explosion in understanding plant-microbes-heavy metals interactions. Novel applications of plant-associated microbes have opened up promising areas of research in the field of phytoremediation technology. Various metabolites (e.g., 1-aminocyclopropane-1-carboxylic acid deaminase, indole-3-acetic acid, siderophores, organic acids, etc.) produced by plant-associated microbes (e.g., plant growth promoting bacteria, mycorrhizae) have been proposed to be involved in many biogeochemical processes operating in the rhizosphere. The salient functions include nutrient acquisition, cell elongation, metal detoxification and alleviation of biotic/abiotic stress in plants. Rhizosphere microbes accelerate metal mobility, or immobilization. Plants and associated microbes release inorganic and organic compounds possessing acidifying, chelating and/or reductive power. These functions are implicated to play an essential role in plant metal uptake. Overall the plant-associated beneficial microbes enhance the efficiency of phytoremediation process directly by altering the metal accumulation in plant tissues and indirectly by promoting the shoot and root biomass production. The present work aims to provide a comprehensive review of some of the promising processes mediated by plant-associated microbes and to illustrate how such processes influence heavy metal uptake through various biogeochemical processes including translocation, transformation, chelation, immobilization, solubilization, precipitation, volatilization and complexation of heavy metals ultimately facilitating phytoremediation.

Growth, photosynthesis and antioxidant responses of endophyte infected and non-infected rice under lead stress conditions

An endophytic fungus was tested in rice (Oryza sativa L.) exposed to four levels of lead (Pb) stress (0, 50, 100 and 200 μM) to assess effects on plant growth, photosynthesis and antioxidant enzyme activity. Under Pb stress conditions, endophyte-infected seedlings had greater shoot length but lower root length compared to non-infected controls, and endophyte-infected seedlings had greater dry weight in the 50 and 100 μM Pb treatments. Under Pb stress conditions, chlorophyll and carotenoid levels were significantly higher in the endophyte-infected seedlings. Net photosynthetic rate, transpiration rate and water use efficiency were significantly higher in endophyte-infected seedlings in the 50 and 100 μM Pb treatments. In addition, chlorophyll fluorescence parameters Fv/Fm and Fv/Fo were higher in the infected seedlings compared to the non-infected seedlings under Pb stress. Malondialdehyde accumulation was induced by Pb stress, and it was present in higher concentration in non-infected seedlings under higher concentrations of Pb (100 and 200 μM). Antioxidant activity was either higher or unchanged in the infected seedlings due to responses to the different Pb concentrations. These results suggest that the endophytic fungus improved rice growth under moderate Pb levels by enhancing photosynthesis and antioxidant activity relative to non-infected rice.

Molecular Mechanism of Heavy Metal Toxicity and Tolerance in Plants: Central Role of Glutathione in Detoxification of Reactive Oxygen Species and Methylglyoxal and in Heavy Metal Chelation

Heavy metal (HM) toxicity is one of the major abiotic stresses leading to hazardous effects in plants. A common consequence of HM toxicity is the excessive accumulation of reactive oxygen species (ROS) and methylglyoxal (MG), both of which can cause peroxidation of lipids, oxidation of protein, inactivation of enzymes, DNA damage and/or interact with other vital constituents of plant cells. Higher plants have evolved a sophisticated antioxidant defense system and a glyoxalase system to scavenge ROS and MG. In addition, HMs that enter the cell may be sequestered by amino acids, organic acids, glutathione (GSH), or by specific metal-binding ligands. Being a central molecule of both the antioxidant defense system and the glyoxalase system, GSH is involved in both direct and indirect control of ROS and MG and their reaction products in plant cells, thus protecting the plant from HM-induced oxidative damage. Recent plant molecular studies have shown that GSH by itself and its metabolizing enzymes—notably glutathione S-transferase, glutathione peroxidase, dehydroascorbate reductase, glutathione reductase, glyoxalase I and glyoxalase II—act additively and coordinately for efficient protection against ROS- and MG-induced damage in addition to detoxification, complexation, chelation and compartmentation of HMs. The aim of this review is to integrate a recent understanding of physiological and biochemical mechanisms of HM-induced plant stress response and tolerance based on the findings of current plant molecular biology research.

Influence of cadmium stress and arbuscular mycorrhizal fungi on nodule senescence in Cajanus cajan (L.) Millsp

Cadmium (Cd) causes oxidative damage and affects nodulation and nitrogen fixation process of legumes. Arbuscular mycorrhizal (AM) fungi have been demonstrated to alleviate heavy metal stress of plants. The present study was conducted to assess role of AM in alleviating negative effects of Cd on nodule senescence in Cajanus cajan genotypes differing in their metal tolerance. Fifteen day-old plants were subjected to Cd treatments--25 mg and 50 mg Cd per kg dry soil and were grown with and without Glomus mosseae. Cd treatments led to a decline in mycorrhizal infection (MI), nodule number and dry weights which was accompanied by reductions in leghemoglobin content, nitrogenase activity, organic acid contents. Cd supply caused a marked decrease in nitrogen (N), phosphorus (P), and iron (Fe) contents. Conversely, Cd increased membrane permeability, thiobarbituric acid reactive substances (TBARS), hydrogen peroxide (H2O2), and Cd contents in nodules. AM inoculations were beneficial in reducing the above mentioned harmful effects of Cd and significantly improved nodule functioning. Activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) increased markedly in nodules of mycorrhizal-stressed plants. The negative effects of Cd were genotype and concentration dependent.

Influence of arbuscular mycorrhizal fungi and copper on growth, accumulation of osmolyte, mineral nutrition and antioxidant enzyme activity of pepper (Capsicum annuum L.)

The effect of arbuscular mycorrhizal (AM) fungi inoculation on pepper (Capsicum annuum L. cv. Zhongjiao 105) plant growth and on some physiological parameters in response to increasing soil Cu concentrations was studied. Treatments consisted of inoculation or not with Glomus mosseae and the addition of Cu to soil at the concentrations of 0 (control), 2 (low), 4 (medium), and 8 (high) mM CuSO(4). AM fungal inoculation decreased Cu concentrations in plant organs and promoted biomass yields as well as the contents of chlorophyll, soluble sugar, total protein, and the concentrations of P, K, Ca, and Mg. Plants grown in high Cu concentration exhibited a Cu-induced proline accumulation and also an increase in total free amino acid contents; however, both were lower in mycorrhizal pepper. Cu-induced oxidative stress by increasing lipid peroxidation rates and the activity of superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase, and AM symbiosis enhanced these antioxidant enzyme activities and decreased oxidative damage to lipids. In conclusion G. mosseae was able to maintain an efficient symbiosis with pepper plants in contaminated Cu soils, improving plant growth under these conditions, which is likely to be due to reduced Cu accumulation in plant tissues, reduced oxidative stress and damage to lipids, or enhanced antioxidant capacity.

Violets of the section Melanium, their colonization by arbuscular mycorrhizal fungi and their occurrence on heavy metal heaps

Violets of the sections Melanium were examined for their colonization by arbuscular mycorrhizal fungi (AMF). Heartsease (Viola tricolor) from several heavy metal soils was AMF-positive at many sites but not at extreme biomes. The zinc violets Viola lutea ssp. westfalica (blue zinc violet) and ssp. calaminaria (yellow zinc violet) were always AMF-positive on heavy metal soils as their natural habitats. As shown for the blue form, zinc violets germinate independently of AMF and can be grown in non-polluted garden soils. Thus the zinc violets are obligatorily neither mycotrophs nor metalophytes. The alpine V. lutea, likely ancestor of the zinc violets, was at best poorly colonized by AMF. As determined by atomic absorption spectrometry, the contents of Zn and Pb were lower in AMF colonized plants than in the heavy metal soils from where the samples had been taken. AMF might prevent the uptake of toxic levels of heavy metals into the plant organs. Dithizone staining indicated a differential deposition of heavy metals in tissues of heartsease. Leaf hairs were particularly rich in heavy metals, indicating that part of the excess of heavy metals is sequestered into these cells.

A tomato stem cell extract, containing antioxidant compounds and metal chelating factors, protects skin cells from heavy metal-induced damages

Heavy metals can cause several genotoxic effects on cells, including oxidative stress, DNA sequence breakage and protein modification. Among the body organs, skin is certainly the most exposed to heavy metal stress and thus the most damaged by the toxic effects that these chemicals cause. Moreover, heavy metals, in particular nickel, can induce the over-expression of collagenases (enzymes responsible for collagen degradation), leading to weakening of the skin extracellular matrix. Plants have evolved sophisticated mechanisms to protect their cells from heavy metal toxicity, including the synthesis of metal chelating proteins and peptides, such as metallothioneins and phytochelatins (PC), which capture the metals and prevent the damages on the cellular structures. To protect human skin cells from heavy metal toxicity, we developed a new cosmetic active ingredient from Lycopersicon esculentum (tomato) cultured stem cells. This product, besides its high content of antioxidant compounds, contained PC, effective in the protection of skin cells towards heavy metal toxicity. We have demonstrated that this new product preserves nuclear DNA integrity from heavy metal damages, by inducing genes responsible for DNA repair and protection, and neutralizes the effect of heavy metals on collagen degradation, by inhibiting collagenase expression and inducing the synthesis of new collagen.

Arbuscular mycorrhizal symbiosis elicits shoot proteome changes that are modified during cadmium stress alleviation in Medicago truncatula

BACKGROUND

Arbuscular mycorrhizal (AM) fungi, which engage a mutualistic symbiosis with the roots of most plant species, have received much attention for their ability to alleviate heavy metal stress in plants, including cadmium (Cd). While the molecular bases of Cd tolerance displayed by mycorrhizal plants have been extensively analysed in roots, very little is known regarding the mechanisms by which legume aboveground organs can escape metal toxicity upon AM symbiosis. As a model system to address this question, we used Glomus irregulare-colonised Medicago truncatula plants, which were previously shown to accumulate and tolerate heavy metal in their shoots when grown in a substrate spiked with 2 mg Cd kg(-1).

RESULTS

The measurement of three indicators for metal phytoextraction showed that shoots of mycorrhizal M. truncatula plants have a capacity for extracting Cd that is not related to an increase in root-to-shoot translocation rate, but to a high level of allocation plasticity. When analysing the photosynthetic performance in metal-treated mycorrhizal plants relative to those only Cd-supplied, it turned out that the presence of G. irregulare partially alleviated the negative effects of Cd on photosynthesis. To test the mechanisms by which shoots of Cd-treated mycorrhizal plants avoid metal toxicity, we performed a 2-DE/MALDI/TOF-based comparative proteomic analysis of the M. truncatula shoot responses upon mycorrhization and Cd exposure. Whereas the metal-responsive shoot proteins currently identified in non-mycorrhizal M. truncatula indicated that Cd impaired CO2 assimilation, the mycorrhiza-responsive shoot proteome was characterised by an increase in photosynthesis-related proteins coupled to a reduction in glugoneogenesis/glycolysis and antioxidant processes. By contrast, Cd was found to trigger the opposite response coupled the up-accumulation of molecular chaperones in shoot of mycorrhizal plants relative to those metal-free.

CONCLUSION

Besides drawing a first picture of shoot proteome modifications upon AM symbiosis and/or heavy metal stress in legume plants, the current work argues for allocation plasticity as the main driving force for Cd extraction in aboveground tissues of M. truncatula upon mycorrhization. Additionally, according to the retrieved proteomic data, we propose that shoots of mycorrhizal legume plants escape Cd toxicity through a metabolic shift implying the glycolysis-mediated mobilization of defence mechanisms at the expense of the photosynthesis-dependent symbiotic sucrose sink.

Hyperaccumulators, arbuscular mycorrhizal fungi and stress of heavy metals

Use of plants, with hyperaccumulating ability or in association with soil microbes including the symbiotic fungi, arbuscular mycorrhiza (AM), are among the most common biological methods of treating heavy metals in soil. Both hyperaccumulating plants and AM fungi have some unique abilities, which make them suitable to treat heavy metals. Hyperaccumulator plants have some genes, being expressed at the time of heavy metal pollution, and can accordingly localize high concentration of heavy metals to their tissues, without showing the toxicity symptoms. A key solution to the issue of heavy metal pollution may be the proper integration of hyperaccumulator plants and AM fungi. The interactions between the soil microbes and the host plant can also be important for the treatment of soils polluted with heavy metals.

Improved tolerance of maize (Zea mays L.) to heavy metals by colonization of a dark septate endophyte (DSE) Exophiala pisciphila

Dark septate endophytes (DSE) are ubiquitous and abundant in stressful environments including heavy metal (HM) stress. However, our knowledge about the roles of DSE in improving HM tolerance of their host plants is poor. In this study, maize (Zea mays L.) was inoculated with a HM tolerant DSE strain Exophiala pisciphila H93 in lead (Pb), zinc (Zn), and cadmium (Cd) contaminated soils. E. pisciphila H93 successfully colonized and formed typical DSE structures in the inoculated maize roots. Colonization of E. pisciphila H93 alleviated the deleterious effects of excessive HM supplements and promoted the growth of maize (roots and shoots) under HM stress conditions, though it significantly decreased the biomass of inoculated maize under no HM stress. Further analysis showed that the colonization of E. pisciphila H93 improved the tolerance of maize to HM by restricting the translocation of HM ions from roots to shoots. This study demonstrated that under higher HM stress, such a mutual symbiosis between E. pisciphila and its host (maize) may be an efficient strategy to survive in the stressful environments.

Effect of Fusarium oxysporum f. sp. lycopersici on the soil-to-root translocation of heavy metals in tomato plants susceptible and resistant to the fungus

The purpose of this work was to gain an insight on the potential role of the phytopathogenic fungus Fusarium oxysporum f. sp. lycopersici in the translocation of metals and metalloids from soil to plant roots in tomato (Lycopersicum esculentum). Two varieties of tomato (one susceptible and another resistant to infection by Fusarium oxysporum f. sp. lycopersici) were challenged with the fungus for different periods of time, and several elements (V, Cr, Mn, Co, Cu, Zn, As, Se, Mo, Ag, Cd, Pb) were determined in roots and in soil substrate. Additionally, phenolic plant products were also analyzed for the evaluation of the plant response to biotic stress. In order to obtain representative results for plants cultivated in noncontaminated environments, the infected and control plants were grown in commercial soil with natural, relatively low metal concentrations, partly associated with humic substances. Using such an experimental design, a specific role of the fungus could be observed, while possible effects of plant exposure to elevated concentrations of heavy metals were avoided. In the infected plants of two varieties, the root concentrations of several metals/metalloids were increased compared to control plants; however, the results obtained for elements and for phenolic compounds were significantly different in the two plant varieties. It is proposed that both Lycopersicum esculentum colonization by Fusarium oxysporum f. sp. lycopersici and the increase of metal bioavailability due to fungus-assisted solubilization of soil humic substances contribute to element traffic from soil to roots in tomato plant.

Arbuscular mycorrhizal fungi restore normal growth in a white poplar clone grown on heavy metal-contaminated soil, and this is associated with upregulation of foliar metallothionein and polyamine biosynthetic gene expression

BACKGROUND AND AIMS

It is increasingly evident that plant tolerance to stress is improved by mycorrhiza. Thus, suitable plant-fungus combinations may also contribute to the success of phytoremediation of heavy metal (HM)-polluted soil. Metallothioneins (MTs) and polyamines (PAs) are implicated in the response to HM stress in several plant species, but whether the response is modulated by arbuscular mycorrhizal fungi (AMF) remains to be clarified. The aim of the present study was to check whether colonization by AMF could modify growth, metal uptake/translocation, and MT and PA gene expression levels in white poplar cuttings grown on HM-contaminated soil, and to compare this with plants grown on non-contaminated soil.

METHODS

In this greenhouse study, plants of a Populus alba clone were pre-inoculated, or not, with either Glomus mosseae or G. intraradices and then grown in pots containing either soil collected from a multimetal- (Cu and Zn) polluted site or non-polluted soil. The expression of MT and PA biosynthetic genes was analysed in leaves using quantitative reverse transcription-PCR. Free and conjugated foliar PA concentrations were determined in parallel.

RESULTS

On polluted soil, AMF restored plant biomass despite higher Cu and Zn accumulation in plant organs, especially roots. Inoculation with the AMF caused an overall induction of PaMT1, PaMT2, PaMT3, PaSPDS1, PaSPDS2 and PaADC gene expression, together with increased free and conjugated PA levels, in plants grown on polluted soil, but not in those grown on non-polluted soil.

CONCLUSIONS

Mycorrhizal plants of P. alba clone AL35 exhibit increased capacity for stabilization of soil HMs, together with improved growth. Their enhanced stress tolerance may derive from the transcriptional upregulation of several stress-related genes, and the protective role of PAs.

Arbuscular mycorrhiza alters metal uptake and the physiological response of Coffea arabica seedlings to increasing Zn and Cu concentrations in soil

Studies on mycorrhizal symbiosis effects on metal accumulation and plant tolerance are not common in perennial crops under metal stress. The objective of this study was to evaluate the influence of mycorrhization on coffee seedlings under Cu and Zn stress. Copper (Cu) and zinc (Zn) uptake and some biochemical and physiological traits were studied in thirty-week old Coffea arabica seedlings, in response to the inoculation with arbuscular mycorrhizal fungi (AMF) and to increasing concentrations of Cu or Zn in soil. The experiments were conducted under greenhouse conditions in a 2×4 factorial design (inoculation or not with AMF and 0, 50, 150 and 450mgkg(-1) Cu or 0, 100, 300 and 900mgkg(-1) Zn). Non-mycorrhizal plants maintained a hampered and slow growth even in a soil with appropriate phosphorus (P) levels for this crop. As metal levels increased in soil, a greater proportion of the total absorbed metals were retained by roots. Foliar Cu concentrations increased only in non-mycorrhizal plants, reaching a maximum concentration of 30mgkg(-1) at the highest Cu in soil. Mycorrhization prevented the accumulation of Cu in leaves, and mycorrhizal plants showed higher Cu contents in stems, which indicated a differential Cu distribution in AMF-associated or non-associated plants. Zn distribution and concentrations in different plant organs followed a similar pattern independently of mycorrhization. In mycorrhizal plants, only the highest metal concentrations caused a reduction in biomass, leading to significant changes in some biochemical indicators, such as malondialdehyde, proline and amino acid contents in leaves and also in foliar free amino acid composition. Marked differences in these physiological traits were also found due to mycorrhization. In conclusion, AMF protected coffee seedlings against metal toxicity.

Metallomics: lessons for metalliferous soil remediation

The term metallomics has been established for the investigation of transcriptome, proteome, and metabolome changes induced by metals. The mechanisms allowing the organisms to cope with metals in the environment, metal resistance factors, will in turn change biogeochemical cycles of metals in soil, coupling the metal pool with the root system of plants. This makes microorganisms key players in introducing metals into food webs, as well as for bioremediation strategies. Research on physiological and metabolic responses of microorganisms on metal stress in soil is thus essential for the selection of optimized consortia applicable in bioremediation strategies such as bioaugmentation or microbially enhanced phytoextraction. The results of metallomics studies will help to develop applications including identification of biomarkers for ecotoxicological studies, bioleaching, in situ soil regeneration, and microbially assisted phytoremediation of contaminated land. This review will therefore focus on the molecular understanding of metal resistance in bacteria and fungi, as can be derived from metallomics studies.

GintABC1 encodes a putative ABC transporter of the MRP subfamily induced by Cu, Cd, and oxidative stress in Glomus intraradices

A full-length cDNA sequence putatively encoding an ATP-binding cassette (ABC) transporter (GintABC1) was isolated from the extraradical mycelia of the arbuscular mycorrhizal fungus Glomus intraradices. Bioinformatic analysis of the sequence indicated that GintABC1 encodes a 1513 amino acid polypeptide, containing two six-transmembrane clusters (TMD) intercalated with sequences characteristics of the nucleotide binding domains (NBD) and an extra N-terminus extension (TMD0). GintABC1 presents a predicted TMD0-(TMD-NBD)(2) topology, typical of the multidrug resistance-associated protein subfamily of ABC transporters. Gene expression analyses revealed no difference in the expression levels of GintABC1 in the extra- vs the intraradical mycelia. GintABC1 was up-regulated by Cd and Cu, but not by Zn, suggesting that this transporter might be involved in Cu and Cd detoxification. Paraquat, an oxidative agent, also induced the transcription of GintABC1. These data suggest that redox changes may be involved in the transcriptional regulation of GintABC1 by Cd and Cu.

Beneficial role of plant growth promoting bacteria and arbuscular mycorrhizal fungi on plant responses to heavy metal stress

Heavy metal pollution is a major worldwide environmental concern that has recently motivated researchers to develop a variety of novel approaches towards its cleanup. As an alternative to traditional physical and chemical methods of environmental cleanup, scientists have developed phytoremediation approaches that include the use of plants to remove or render harmless a range of compounds. Both plant growth promoting bacteria (PGPB) and arbuscular mycorrhizal fungi (AMF) can be used to facilitate the process of phytoremediation and the growth of plants in metal-contaminated soils. This review focuses on the recent literature dealing with the effects of plant growth-promoting bacteria and AM fungi on the response of plants to heavy metal stress and points the way to strategies that may facilitate the practical realization of this technology.

Zn uptake, physiological response and stress attenuation in mycorrhizal jack bean growing in soil with increasing Zn concentrations

The influence of arbuscular mycorrhizal fungi (AMF) inoculation on Canavalia ensiformis growth, nutrient and Zn uptake, and on some physiological parameters in response to increasing soil Zn concentrations was studied. Treatments were applied in seven replicates in a 2 x 4 factorial design, consisting of the inoculation or not with the AMF Glomus etunicatum, and the addition of Zn to soil at the concentrations of 0, 100, 300 and 900 mg kg(-1). AMF inoculation enhanced the accumulation of Zn in tissues and promoted biomass yields and root nodulation. Mycorrhizal plants exhibited relative tolerance to Zn up to 300 mg kg(-1) without exhibiting visual symptoms of toxicity, in contrast to non-mycorrhizal plants which exhibited a significant growth reduction at the same soil Zn concentration. The highest concentration of Zn added to soil was highly toxic to the plants. Leaves of plants grown in high Zn concentration exhibited a Zn-induced proline accumulation and also an increase in soluble amino acid contents; however proline contents were lower in mycorrhizal jack beans. Plants in association or not with the AMF exhibited marked differences in the foliar soluble amino acid profile and composition in response to Zn addition to soil. In general, Zn induced oxidative stress which could be verified by increased lipid peroxidation rates and changes in catalase, ascorbate peroxidase, glutathione reductase and superoxide dismutase activities. In summary, G. etunicatum was able to maintain an efficient symbiosis with jack bean plants in moderately contaminated Zn-soils, improving plant performance under those conditions, which is likely to be due to a combination of physiological and nutritional changes caused by the intimate relation between fungus and plant. The enhanced Zn uptake by AMF inoculated jack bean plants might be of interest for phytoremediation purposes.

Comparative transcriptome analysis of arsenate and arsenite stresses in rice seedlings

The effect of arsenic (As) exposure on genome-wide expression was examined in rice (Oryza sativa L., ssp. Indica). A group of defense and stress-responsive genes, transporters, heat-shock proteins, metallothioneins, sulfate-metabolizing proteins, and regulatory genes showed differential expression in rice seedlings challenged with arsenate (AsV) and arsenite (AsIII). AsV stress led to upregulation or downregulation of an additional set of genes in comparison to AsIII. Differential expression of several genes that showed the highest contrast in a microarray analysis was validated by following the quantitative changes in the levels of individual transcripts following challenge with AsV, AsIII, Cd, Cr, and Pb. Most of the selected genes responded to challenge by heavy metals such as arsenic. However, expression of one of the cytochrome P450 genes (Os01g43740) in rice root was induced by AsV but not by other heavy metals. Similarly, one glutaredoxin (Os01g26912) is expressed specifically in the AsIII-treated shoot.

The Thlaspi caerulescens NRAMP homologue TcNRAMP3 is capable of divalent cation transport

The NRAMP gene family encodes integral membrane protein and mediates the transport of Fe, however, its function in transport of toxic metal ions is not very clear in plants. TcNRAMP3 was isolated from Thlaspi caerulescens, and encoded a metal transporter member of the NRAMP family. TcNRAMP3 was predominantly expressed in roots of T. caerulescens by semi-quantitative RT-PCR. The expression of TcNRAMP3 was induced by iron starvation and by the heavy metals Cd and Ni in roots. TcNRAMP3 was able to rescue growth of an iron uptake fet3fet4 mutant yeast strain, suggesting a possible role in iron transport. Expression of TcNRAMP3 in yeast increased Cd sensitivity and Cd content, while it enhanced the Ni resistance and reduced Ni accumulation, indicating that TcNRAMP3 could accumulate Cd and exclude Ni in yeast. Furthermore, overexpression of TcNRAMP3 in tobacco resulted in slight Cd sensitivity of root growth and did not influence Ni resistance. These results suggested that TcNRAMP3 played a role in metal cation homeostasis in plant.

Tolerance and induction of tolerance to Ni of arbuscular mycorrhizal fungi from New Caledonian ultramafic soils

The influence of Ni on arbuscular mycorrhizal fungi (AMF) has not been studied yet. We tested the tolerance to Ni of five AMF isolates from New Caledonian ultramafic soils. Spore germination indicated that these isolates were clearly more tolerant to Ni than three other isolates from non-ultramafic soils. They were able to germinate at 30 microg g(-1) Ni, whereas spores of the non-ultramafic isolates were totally inhibited at 15 microg g(-1) Ni. Among the ultramafic isolates, two were obtained from roots of Ni-hyperaccumulating plants. Their tolerance to Ni was clearly higher than all the other isolates. The proportion of germinated spores of the different isolates in contact with ultramafic soils showed the same tendencies as those observed with Ni solutions. Tolerance to Ni increased when spores were produced from mycorrhiza on plants grown on sand containing 20 microg g(-1) Ni, in comparison with those produced on sand without Ni. These results indicate that the tolerance to Ni of AMF spores can be induced by the presence of this metal in the substrate.

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.

Expression of the fluorescence markers DsRed and GFP fused to a nuclear localization signal in the arbuscular mycorrhizal fungus Glomus intraradices

Here, arbuscular mycorrhizal (AM) fungi were monitored in vivo introducing the fluorescent reporters DsRed and GFP (green fluorescent protein) in Glomus intraradices using a biolistic approach and Agrobacterium tumefaciens-mediated transformation. Both reporter genes were fused to the nuclear localization signal of the Aspergillus nidulans transcription factor StuA to target fluorescence to nuclei. Expression of DsRed was driven by two Glomus mosseae promoters highly expressed during early symbiosis, GmPMA1 and GmFOX2, while expression of GFP was driven by the A. nidulans gpd promoter. All promoters worked in G. intraradices as well as in A. nidulans. Red and green fluorescence was localized to nuclei of G. intraradices spores and hyphae 3 d after bombardment. However, expression was transient. The efficiency of the Agrobacterium-mediated transformation was very low. These results indicate that the biolistic method allows the expression of foreign DNA into G. intraradices with high frequency, but it is insufficient to render stable transformants. DsRed vs GFP is a more appropriate living reporter to be used in G. intraradices because of the lower autofluorescence in the red channel but targeted to the nucleus both marker genes can be visualized. This is the first report of fluorescent marker expression in an AM fungus driven by arbuscular mycorrhizal promoters.

GintMT1 encodes a functional metallothionein in Glomus intraradices that responds to oxidative stress

A full-length metallothionein (MT) gene (GintMT1) was isolated from Glomus intraradices extraradical mycelium. This is the first MT gene reported in the genus Glomus, third in the Glomeromycota. Functional analysis of GintMT1 in a MT-defective Saccharomyces cerevisiae strain indicates that it encodes a functional MT. Gene expression analyses revealed that the transcript levels of GintMT1 were elevated in mycelia treated with 5 mM Cu or paraquat but inhibited in mycelia treated with 50 microM Cu or 450 microM Cd. The elevated expression of GintMT1 in the 5 mM Cu-treated mycelia together with the ability of GintMT1 to provide tolerance to a Cu-sensitive yeast suggests that GintMT1 might afford protection against Cu. Induction of GintMT1 expression by paraquat and 5 mM Cu, treatments that also produced an oxidative damage to the fungal membranes, suggests that GintMT1 may play a role in the regulation of the redox status of the extraradical mycelium of G. intraradices.

Identification of new potential regulators of the Medicago truncatula-Sinorhizobium meliloti symbiosis using a large-scale suppression subtractive hybridization approach

We set up a large-scale suppression subtractive hybridization (SSH) approach to identify Medicago truncatula genes differentially expressed at different stages of the symbiotic interaction with Sinorhizobium meliloti, with a particular interest for regulatory genes. We constructed 7 SSH libraries covering successive stages from Nod factor signal transduction to S. meliloti infection, nodule organogenesis, and functioning. Over 26,000 clones were differentially screened by two rounds of macroarray hybridizations. In all, 3,340 clones, corresponding to genes whose expression was potentially affected, were selected, sequenced, and ordered into 2,107 tentative gene clusters, including 767 MtS clusters corresponding to new M. truncatula genes. In total, 52 genes encoding potential regulatory proteins, including transcription factors (TFs) and other elements of signal transduction cascades, were identified. The expression pattern of some of them was analyzed by quantitative reverse-transcription polymerase chain reaction in wild-type and in Nod- M. truncatula mutants blocked before or after S. meliloti infection. Three genes, coding for TFs of the bHLH and WRKY families and a C2H2 zinc-finger protein, respectively, were found to be upregulated, following S. meliloti inoculation, in the infection-defective mutant lin, whereas the bHLH gene also was expressed in the root-hair-curling mutant hcl. The potential role of these genes in early symbiotic steps is discussed.

Transcriptional changes in two types of pre-mycorrhizal roots and in ectomycorrhizas of oak microcuttings inoculated with Piloderma croceum

The formation of the ectomycorrhiza implies an alteration in gene expression of both the plant and fungal partners, a process which starts before the formation of any symbiotic interface. However, little is known on the regulation pattern occurring in different parts of the root system. Our experimental system consisting of a micropropagated oak with a hierarchical root system was shown to exhibit symbiosis functional traits prior to any mycorrhizal tissue differentiation after the inoculation with the basidiomycete Piloderma croceum. Using a cDNA array, the plant gene regulation was analyzed in the pre-mycorrhizal phase. Seventy-five transcripts showed differential expression in pre-mycorrhizal lateral and principal roots, and both root types exhibited different sets of responsive genes. For transcripts selected according to a statistical analysis, the alteration in gene expression was confirmed by RT-PCR and quantitative real-time PCR. Genes regulated in pre-mycorrhizal lateral roots displayed an almost identical expression in mycorrhizas. In contrast, genes regulated in pre-mycorrhizal principal roots were often regulated differently in ectomycorrhizas. Down-regulation affected most of the regulated genes involved in metabolism, whereas most of the regulated genes related to cell rescue functions, water regulation and defence response were up-regulated. Regulation of such genes could explain the increase of global resistance observed in mycorrhizal plants.

Identification of heavy metal-induced genes encoding glutathione S-transferases in the arbuscular mycorrhizal fungus Glomus intraradices

Arbuscular mycorrhizal fungi are able to alleviate the stress for plants caused by heavy metal contamination of soil. To analyze the molecular response of arbuscular mycorrhizal fungi to these pollutants, a subtractive cDNA library was constructed using RNA from Glomus intraradices extraradical hyphae of a root organ culture treated with a mixture of Cd, Zn, and Cu. Screening by reverse Northern blot analysis indicated that, among 308 clones, 17% correspond to genes up-regulated by heavy metals. Sequence analysis of part of the clones resulted, amongst others, in the identification of six genes putatively coding for glutathione S-transferases belonging to two different classes of these enzymes. Expression analyses indicated that the genes are differentially expressed during fungal development and that their RNA accumulation dramatically increases in extraradical hyphae grown in a heavy metal-containing solution.

A limiting source of organic nitrogen induces specific transcriptional responses in the extraradical structures of the endomycorrhizal fungus Glomus intraradices

The molecular bases of organic nitrogen (N) metabolism in arbuscular mycorrhizal (AM) fungi remain so far largely unexplored. To isolate genes responsive to low versus high organic N concentrations, the techniques of suppressive subtractive hybridization (SSH) and reverse Northern dot blot were performed on extraradical structures of the AM fungus Glomus intraradices grown on carrot hairy roots. This approach allowed the identification of 32 up-regulated and 2 down-regulated genes following a 48-h treatment with 2 microM of an amino acid pool (leucine, alanine, asparagine, lysine, tyrosine). The expression profile of eight genes was further confirmed by semi-quantitative and real-time RT-PCR. The majority of the sequences showed no significant similarity to proteins in databases. The other responsive genes code for putative glyoxal oxidases, transcription factors, a subunit of the 20S proteasome, a protein kinase and a Ras protein. This novel set of data indicates that G. intraradices extraradical structures perceive organic N limitation in the surrounding environment leading to a response at transcriptional level and supports the role of N as signalling molecule in AM fungi.

Contribution of the arbuscular mycorrhizal symbiosis to heavy metal phytoremediation

High concentrations of heavy metals (HM) in the soil have detrimental effects on ecosystems and are a risk to human health as they can enter the food chain via agricultural products or contaminated drinking water. Phytoremediation, a sustainable and inexpensive technology based on the removal of pollutants from the environment by plants, is becoming an increasingly important objective in plant research. However, as phytoremediation is a slow process, improvement of efficiency and thus increased stabilization or removal of HMs from soils is an important goal. Arbuscular mycorrhizal (AM) fungi provide an attractive system to advance plant-based environmental clean-up. During symbiotic interaction the hyphal network functionally extends the root system of their hosts. Thus, plants in symbiosis with AM fungi have the potential to take up HM from an enlarged soil volume. In this review, we summarize current knowledge about the contribution of the AM symbiosis to phytoremediation of heavy metals.

The bacterium Paenibacillus validus stimulates growth of the arbuscular mycorrhizal fungus Glomus intraradices up to the formation of fertile spores

Two isolates of Paenibacillus validus (DSM ID617 and ID618) stimulated growth of the arbuscular mycorrhizal fungus Glomus intraradices Sy167 up to the formation of fertile spores, which recolonize carrot roots. Thus, the fungus was capable of completing its life cycle in the absence of plant roots, but relied instead on the simultaneous growth of bacteria. The supernatant of a mixed batch culture of the two P. validus isolates contained raffinose and another, unidentified trisaccharide. Among the oligosaccharides tested, raffinose was most effective in stimulating hyphal mass formation on plates but could not promote growth to produce fertile spores. A suppressive subtractive hybridization library followed by reverse Northern analyses indicated that several genes with products involved in signal transduction are differentially expressed in G. intraradices SY 167 when grown in coculture with P. validus (DSM 3037). The present investigation, while likely representing a significant step forward in understanding the arbuscular mycorrhizal fungus symbioses, also confirms that its optimal establishing and functioning might rely on many, as yet unidentified factors.

Approaches to assess the biodiversity of bacteria in natural habitats

Any attempt to characterize a bacterial community and their functional genes coding for enzymes of the nitrogen cycle is faced with its extreme biodiversity. Novel techniques, based on PCR amplification of target genes in DNA from environmental samples, have been developed for characterizing both cultured and as yet uncultured bacteria in the last few years. Computer-based assignment tools have now been developed utilizing terminal restriction fragments obtained from digestions with multiple restriction enzymes. Such programs allow the gross characterization of bacterial life in any complex bacterial community with confidence.