Accumulation of reactive oxygen species in arbuscular mycorrhizal roots

We investigated the accumulation of reactive oxygen species (ROS) in arbuscular mycorrhizal (AM) roots from Medicago truncatula, Zea mays and Nicotiana tabacum using three independent staining techniques. Colonized root cortical cells and the symbiotic fungal partner were observed to be involved in the production of ROS. Extraradical hyphae and spores from Glomus intraradices accumulated small levels of ROS within their cell wall and produced ROS within the cytoplasm in response to stress. Within AM roots, we observed a certain correlation of arbuscular senescence and H₂O₂ accumulation after staining by diaminobenzidine (DAB) and a more general accumulation of ROS close to fungal structures when using dihydrorhodamine 123 (DHR 123) for staining. According to electron microscopical analysis of AM roots from Z. mays after staining by CeCl₃, intracellular accumulation of H₂O₂ was observed in the plant cytoplasm close to intact and collapsing fungal structures, whereas intercellular H₂O₂ was located on the surface of fungal hyphae. These characteristics of ROS accumulation in AM roots suggest similarities to ROS accumulation during the senescence of legume root nodules.     

Hyphal N transport by a vesicular-arbuscular mycorrhizal fungus associated with cucumber grown at three nitrogen levels

Cucumis sativus L. cv. Aminex (F1 hybrid) was grown alone or in symbiosis with Glomus intraradices Schenck and Smith in containers with two hyphal compartments (HC_A and HC_B) on either side of a root compartment (RC) separated by fine nylon mesh. Plants received a total of either 100, 200 or 400 mg N which were applied gradually to the RC during the experiment. ¹⁵N was supplied to HC_A 42 d after plating, at 50 mg ¹⁵NH₄ ⁺-N kg^–1 soil. Lateral movement of the applied ¹⁵N towards the roots was minimized by using a nitrification inhibitor and a hyphal buffer compartment.Non-mycorrhizal controls contained only traces of ¹⁵N after a 27 d labelling period irrespective of the amount of N supplied to the RC. In contrast, 49, 48 and 27% of the applied ¹⁵N was recovered in mycorrhizal plants supplied with 100, 200 and 400 mg N, respectively. The plant dry weight was increased by mycorrhizal colonization at all three levels of N supply, but this effect was strongest in plants of low N status. The results indicated that this increase was due partly to the improved inflow of N via the external hyphae. Root colonization by G. intraradices was unaffected by the amount of N supplied to the RC, while hyphal length increased in HC_A compared to HC_B. Although a considerable ¹⁵N content was detected in mycorrhizal roots adjacent to HC_B, only insignificant amounts of ¹⁵N were found in the external hyphae in HC_B. The external hyphae depleted the soil of inorganic N in both HC_A and HC_B, while the concentration of soil mineral N was still high in non-mycorrhizal containers at harvest. An exception was plants supplied with 400 mg N, where some inorganic N was present at 5 cm distance from the RC in HC_A. The possibility of a regulation mechanism for hyphal transport of N is discussed.     

Overlapping expression patterns and differential transcript levels of phosphate transporter genes in arbuscular mycorrhizal,P<Subscript>i</Subscript>-fertilised and phytohormone-treated <Emphasis Type="Italic">Medicago truncatula</Emphasis> roots

A microarray carrying 5,648 probes of Medicago truncatula root-expressed genes was screened in order to identify those that are specifically regulated by the arbuscular mycorrhizal (AM) fungus Gigaspora rosea, by P_i fertilisation or by the phytohormones abscisic acid and jasmonic acid. Amongst the identified genes, 21% showed a common induction and 31% a common repression between roots fertilised with P_i or inoculated with the AM fungus G. rosea, while there was no obvious overlap in the expression patterns between mycorrhizal and phytohormone-treated roots. Expression patterns were further studied by comparing the results with published data obtained from roots colonised by the AM fungi Glomus mosseae and Glomus intraradices, but only very few genes were identified as being commonly regulated by all three AM fungi. Analysis of P_i concentrations in plants colonised by either of the three AM fungi revealed that this could be due to the higher P_i levels in plants inoculated by G. rosea compared with the other two fungi, explaining that numerous genes are commonly regulated by the interaction with G. rosea and by phosphate. Differential gene expression in roots inoculated with the three AM fungi was further studied by expression analyses of six genes from the phosphate transporter gene family in M. truncatula. While MtPT4 was induced by all three fungi, the other five genes showed different degrees of repression mirroring the functional differences in phosphate nutrition by G. rosea, G. mosseae and G. intraradices. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Role of the modification in root exudation induced by arbuscular mycorrhizal colonization on the intraradical growth of Phytophthora nicotianae in tomato

We studied the role of modification in root exudation induced by colonization with Glomus intraradices and Glomus mosseae in the growth of Phytophthora nicotianae in tomato roots. Plants were grown in a compartmentalized plant growth system and were either inoculated with the AM fungi or received exudates from mycorrhizal plants, with the corresponding controls. Three weeks after planting, the plants were inoculated or not with P. nicotianae growing from an adjacent compartment. At harvest, P. nicotianae biomass was significantly reduced in roots colonized with G. intraradices or G. mosseae in comparison to non-colonized roots. Conversely, pathogen biomass was similar in non-colonized roots supplied with exudates collected from mycorrhizal or non-mycorrhizal roots, or with water. We cannot rule out that a mycorrhiza-mediated modification in root exudation may take place, but our results did not support that a change in pathogen chemotactic responses to host root exudates may be involved in the inhibition of P. nicotianae.     

Application of in vitro methods to study carbon uptake and transport by AM fungi

Just as multi-compartmented root chambers have advantages over standard plastic pots for the study of nutrient uptake by arbuscular mycorrhizal [AM] fungi in soil, so the split-plate in vitro system has advantages over the standard dual culture system for the study of the physiology of AM fungi. We used the split-plate culture system of Ri T-DNA transformed Daucus carota L. roots and Glomus intraradices Schenck & Smith, in which only the fungus has access to the distal compartment, to study the ability of germ tubes and extraradical and intraradical hyphae to take up ¹³C-labeled substrates. Labeled substrates were added to one side of the plate divider and plates were incubated for 8 weeks while the fungus proliferated on the side from which the root was excluded. Tissues then were recovered from the plate and examined via NMR spectroscopy. Results showed that the morphological phases of the fungus differed in their ability to take up these substrates, most notably that intraradical hyphae take up hexose while extraradical hyphae cannot. In addition, NMR studies indicated that intraradical hyphae actively synthesized lipids while extraradical hyphae did not. These data show that eventual axenic culture of AM fungi is more than a matter of finding the proper substrate for growth. Genetic regulation must be overcome to make extraradical hyphae behave like intraradical hyphae in terms of C uptake and metabolism. This revised version was published online in June 2006 with corrections to the Cover Date.     

Community of arbuscular mycorrhizal fungi in drought-resistant plants, <Emphasis Type="Italic">Moringa</Emphasis> spp., in semiarid regions in Madagascar and Uganda

The community structure of arbuscular mycorrhizal (AM) fungi in the roots of drought-resistant trees, Moringa spp., was examined in semiarid regions in Madagascar and Uganda. Root samples were collected from 8 individuals of M. hildebrandtii and 2 individuals of M. drouhardii in Madagascar and from 21 individuals of M. oleifera in Uganda. Total DNA was extracted from the root samples, and partial nSSU rDNA of AM fungi was amplified using a universal eukaryotic primer NS31 and an AM fungalspecific primer AM1. The PCR products were cloned and divided by restriction fragment length polymorphism (RFLP) analysis with HinfI and RsaI. Some representatives in each RFLP types were sequenced, and a neighbor-joining phylogenetic analysis was conducted for the obtained sequences with analogous sequences of AM fungi. The RFLP and phylogenetic analyses showed that AM fungi closely related to Glomus intraradices or G. sinuosum were detected in many samples. The AM fungal groups frequently detected in the Moringa spp. might be widely distributed species in semiarid environments.     

Effects of anthracene on development of an arbuscular mycorrhizal fungus and contribution of the symbiotic association to pollutant dissipation

The influence of anthracene, a low molecular weight polycyclic aromatic hydrocarbon (PAH), on chicory root colonization by Glomus intraradices and the effect of the root colonization on PAH degradation were investigated in vitro. The fungus presented a reduced development of extraradical mycelium and a decrease in sporulation, root colonization, and spore germination when exposed to anthracene. Mycorrhization improved the growth of the roots in the medium supplemented containing 140 mg l⁻¹ anthracene, suggesting a positive contribution of G. intraradices to the PAH tolerance of roots. Anthracene disappearance from the culture medium was quantified; results suggested that nonmycorrhizal chicory roots growing in vitro were able to contribute to anthracene dissipation, and in addition, that mycorrhization significantly enhanced anthracene dissipation. These monoxenic experiments demonstrated a positive contribution of the symbiotic association to anthracene dissipation in the absence of other microorganisms. In addition to anthracene dissipation, intracellular accumulation of anthracene was detected in lipid bodies of plant cells and fungal hyphae, indicating intracellular storage capacity of the pollutant by the roots and the mycorrhizal fungus.     

Beyond the rhizosphere: growth and function of arbuscular mycorrhizal external hyphae in sands of varying pore sizes

Research on nutrient acquisition by symbiotic arbuscular mycorrhizal (AM) fungi has mainly focused on the root–fungus interface and less attention has been given to the growth and functioning of external hyphae in the bulk soil. The growth and function of external hyphae may be affected by unfavourable soil environments, such as compacted soils in which pores may be narrow. The effects of pore size on the growth of two AM fungi (Glomus intraradices and G. mosseae) and their ability to transport ³³P from the bulk soil to the host were investigated. Trifolium subterraneum L. plants were grown individually in `single arm cross-pots' with and without AM fungi. The side arm was separated from the main compartment by nylon mesh to prevent root penetration. It contained three zones: 5 mm of soil:sand mix (HC1); 25 mm of media treatment (HC2); and 20 mm of ³³P-labelled soil (HC3). There were four media treatments; soil and three types of quartz sand with most common continuous pore diameters of 100, 38 and 26 m. AM plants had similar growth and total P uptake in all treatments. However, plants grown with G. intraradices contained almost three times more ³³P than those grown with G. mosseae, indicating G. intraradices obtained a greater proportion of P at a distance from the host roots. Differences in P acquisition were not correlated with production of external hyphae in the four media zones and changes in sand pore size did not affect the ability of the fungi studied to acquire P at a distance from the host roots. Production of external hyphae in HC2 was influenced by fungal species and media treatment. Both fungi produced maximum amounts of external hyphae in the soil medium. Sand pore size affected growth of G. intraradices (but not G. mosseae) and hyphal diameter distributions of both fungi. The results suggest that not only are G. mosseae and G. intraradices functionally complementary in terms of spatial phosphorus acquisition, they are also capable of altering their morphology in response to the soil environment.     

Mycorrhizal fungi enhance accumulation and tolerance of chromium in sunflower (Helianthus annuus)

Chromium (Cr) is a heavy metal risk to human health, and a contaminant found in agricultural soils and industrial sites. Phytoremediation, which relies on phytoextraction of Cr with biological organisms, is an important alternative to costly physical and chemical methods of treating contaminated sites. The ability of the arbuscular mycorrhizal fungus (AM),Glomus intraradices, to enhance Cr uptake and plant tolerance was tested on the growth and gas exchange of sunflower (Helianthus annuus L.). Mycorrhizal-colonized (AM) and non-inoculated (Non-AM) sunflower plants were subjected to two Cr species [trivalent cation (Cr³⁺) Cr(III) , and divalent dichromate anion (Cr₂O₇⁻) Cr(VI) ]. Both Cr species depressed plant growth, decreased net photosynthesis (A) and increased the vapor pressure difference; however, Cr(VI) was more toxic. Chromium accumulation was greatest in roots, intermediate in stems and leaves, and lowest in flowers. Greater Cr accumulation occurred with Cr(VI) than Cr(III). AM enhanced the ability of sunflower plants to tolerate and hyperaccumulate Cr. At higher Cr levels greater mycorrhizal dependency occurred, as indicated by proportionally greater growth, higherA and reduced visual symptoms of stress, compared to Non-AM plants. AM plants had greater Cr-accumulating ability than Non-AM plants at the highest concentrations of Cr(III) and Cr(VI), as indicated by the greater Cr phytoextraction coefficient. Mycorrhizal colonization (arbuscule, vesicle, and hyphae formation) was more adversely affected by Cr(VI) than Cr(III), however high levels of colonization still occurred at even the most toxic levels. Arbuscules, which play an important role in mineral ion exchange in root cortical cells, had the greatest sensitivity to Cr toxicity. Higher levels of both Cr species reduced leaf tissue phosphorus (P). While tissue P was higher in AM plants at the highest Cr(III) level, tissue P did not account for mycorrhizal benefits observed with Cr(VI) plants.     

Presence of different arbuscular mycorrhizal infection patterns in roots of<Emphasis Type="Italic"> Lotus glaber</Emphasis> plants growing in the Salado River basin

Morphological types of arbuscular mycorrhizal (AM) fungi associated with Lotus glaber in sodic soils of the Salado River basin were studied. At least eight colonization patterns (IP) of AM fungi in roots of L. glaber were observed after 30 plants were analyzed. Arum- and Paris-type infection were found in the same plant species. This result supports the idea that AM morphology is not solely under plant control, but is also influenced by fungal identity. One infection pattern, presumably corresponding to Glomus intraradices, and a second, possibly assignable to Glomus tenue, were the most commonly found. Our results reinforce previous suggestions that G. intraradices is well adapted to sodic-saline conditions and may play a role in the resistance of L. glaber to these soils.     

Seasonal variation of arbuscular mycorrhizal fungi in temperate grasslands along a wide hydrologic gradient

We studied seasonal variation in population attributes of arbuscular mycorrhizal (AM) fungi over 2 years in four sites of temperate grasslands of the Argentinean Flooding Pampas. The sites represent a wide range of soil conditions, hydrologic gradients, and floristic composition. Lotus glaber, a perennial herbaceous legume naturalised in the Flooding Pampas, was dominant at the four plant community sites. Its roots were highly colonised by AM fungi. Temporal variations in spore density, spore type, AM root colonisation, floristic composition and soil chemical characteristics occurred in each site and were different among sites. The duration of flooding had no effect on spore density but depressed AM root colonisation. Eleven different types of spores were recognized and four were identified. Two species dominated at the four sites: Glomus fasciculatum and Glomus intraradices. Spore density was highest in summer (dry season) and lowest in winter (wet season) with intermediate values in autumn and spring. Colonisation of L. glaber roots was highest in summer or spring and lowest in winter or autumn. The relative density of G. fasciculatum and G. intraradices versus Glomus sp. and Acaulospora sp. had distinctive seasonal peaks. These seasonal peaks occurred at all four sites, suggesting differences among AM fungus species with respect to the seasonality of sporulation. Spore density and AM root colonisation when measured at any one time were poorly related to each other. However, spore density was significantly correlated with root colonisation 3 months before, suggesting that high colonisation in one season precedes high sporulation in the next season.     

Cereal phosphate transporters associated with the mycorrhizal pathway of phosphate uptake into roots

A very large number of plant species are capable of forming symbiotic associations with arbuscular mycorrhizal (AM) fungi. The roots of these plants are potentially capable of absorbing P from the soil solution both directly through root epidermis and root hairs, and via the AM fungal pathway that delivers P to the root cortex. A large number of phosphate (P) transporters have been identified in plants; tissue expression patterns and kinetic information supports the roles of some of these in the direct root uptake pathways. Recent work has identified additional P transporters in several unrelated species that are strongly induced, sometimes specifically, in AM roots. The primary aim of the work described in this paper was to determine how mycorrhizal colonisation by different species of AM fungi influenced the expression of members of the Pht1 gene families in the cereals Hordeum vulgare (barley), Triticum aestivum (wheat) and Zea mays (maize). RT-PCR and in-situ hybridisation, showed that the transporters HORvu;Pht1;8 (AY187023), TRIae;Pht1;myc (AJ830009) and ZEAma;Pht1;6 (AJ830010), had increased expression in roots colonised by the AM fungi Glomus intraradices,Glomus sp. WFVAM23 and Scutellospora calospora. These findings add to the increasing body of evidence indicating that plants that form AM associations with members of the Glomeromycota have evolved phosphate transporters that are either specifically or preferentially involved in scavenging phosphate from the apoplast between intracellular AM structures and root cortical cells. Operation of mycorrhiza-inducible P transporters in the AM P uptake pathway appears, at least partially, to replace uptake via different P transporters located in root epidermis and root hairs. Electronic Supplementary Material Supplementary material is available for this article at     

Effects of preplant inoculation of apple (Malus domestica Borkh.) with arbuscular mycorrhizal fungi on population growth of the root-lesion nematode,Pratylenchus penetrans

Six species of arbuscular mycorrhizal (AM) fungi (Glomus aggregatum, G. clarum, G. etunicatum, G. intraradices, G. mosseae and G. versiforme) were evaluated, in three greenhouse experiments, for their effects on reproduction of the root-lesion nematode, Pratylenchus penetrans, and growth of Ottawa 3 apple rootstock. Glomus mosseae increased total dry weights of nematode-inoculated and non-inoculated rootstock in all three greenhouse experiments, and G. intraradices increased dry weights in two of three greenhouse experiments. Plants inoculated with G. mosseae generally supported fewer P. penetrans per gram of root than plants inoculated with other AM fungi, but did not differ significantly from the controls in any greenhouse experiment. Colonization of roots by AM fungi was reduced by P. penetrans at initial inoculum densities greater than 250 nematodes/L soil. In field trials, preplant inoculation with either G. intraradices or G. mosseae increased rootstock growth and leaf concentrations of P, Mg, Zn and Cu in fumigated plots but not in non-fumigated plots, indicating that colonization by native AM fungi in non-fumigated plots may have been sufficient for adequate nutrient acquisition. The abundance of vesicles and arbuscules was greater in roots of plants inoculated with AM fungi before planting than in roots of non-inoculated plants, in both fumigated and non-fumigated plots. P. penetrans per gram of root and per 50 ml soil were significantly lower for G. mosseae- inoculated plants than for non-inoculated plants in fumigated soil but not in non-fumigated soil.     

Cold-storage of mixed inoculum of Glomus intraradices enhances root colonization,phosphorus status and growth of hot pepper

Growth response of hot pepper (Capsicum annuum L.) inoculated with the arbuscular mycorrhizal (AM) fungus, Glomus intraradices Schenck and Smith was evaluated in a greenhouse study. Three treatments in a soil-based medium amended with rock phosphate were: (1) control (CON), (2) inoculation of G. intraradices as a freshly prepared soil mixture of spores, hyphae and colonized roots of Sorghum vulgare (FM), and (3) inoculation of the fungus as cold-stored mixed inoculum (CM). Colonization at 14 weeks after inoculation with CM was 42.5%, but was significantly lower with FM (14.5%). Inoculation with G. intraradices as FM and CM increased growth of pepper, and total phosphorus and nitrogen uptake in shoots and roots compared with the CON treatment. Inoculation with CM resulted in significant increases in plant dry weight and chlorophyll concentration compared to the FM and CON treatments. Acid phosphatase activity in the rhizosphere was generally increased by AM fungal treatments. Highest acid phosphatase activity occurred at 14 weeks after inoculation with CM. Alkaline phosphatase activity in the CM treatment was significantly higher compared to that in CON and FM treatments throughout the growth period. Thus, cold storage of mixed inoculum enhanced colonization and growth-promoting activity of G. intraradices compared to freshly prepared inoculum.     

Comparison of <Superscript>233</Superscript>U and <Superscript>33</Superscript>P uptake and translocation by the arbuscular mycorrhizal fungus<Emphasis Type="Italic"> Glomus intraradices</Emphasis> in root organ culture conditions

This study aimed to quantify and compare ²³³U and ³³P uptake and translocation by hyphae of the arbuscular mycorrhizal (AM) fungus Glomus intraradices in root organ culture conditions with transformed carrot (Daucus carota L.) roots as host. Mycorrhizal roots were grown in two-compartment Petri dishes to spatially separate a root compartment (RC) and a hyphal compartment (HC). The HC was labelled with 8.33 Bq ²³³U ml^–1 and 13.33 Bq ³³P ml^–1. After 2 weeks contact between hyphae and the labelled solution, ²³³U and ³³P activities were measured in the RC and in the HC. ²³³U and ³³P were taken up by the extraradical AM mycelium grown in the HC and this uptake represented 4.4% and 16% of the initial isotope supply, respectively. The translocation into roots developing in the RC via hyphae accounted for 5.9% and 72% of the initial isotope supply, respectively. Thus, both uptake and translocation were much higher for ³³P than for ²³³U. This suggests (1) the existence in hyphal tissues of efficient mechanisms limiting the uptake and translocation of non-essential elements such as U, and (2) that the hyphae have a higher sequestration than translocation function for U, and the converse for P.     

Interactions between the arbuscular mycorrhizal (AM) fungus Glomus intraradices and nontransformed tomato roots of either wild-type or AM-defective phenotypes in monoxenic cultures

Monoxenic symbioses between the arbuscular mycorrhizal (AM) fungus Glomus intraradices and two nontransformed tomato root organ cultures (ROCs) were established. Wild-type tomato ROC from cultivar “RioGrande 76R” was employed as a control for mycorrhizal colonization and compared with its mutant line (rmc), which exhibits a highly reduced mycorrhizal colonization (rmc) phenotype. Structural features of the two root lines were similar when grown either in soil or under in vitro conditions, indicating that neither monoxenic culturing nor the rmc mutation affected root development or behavior. Colonization by G. intraradices in monoxenic culture of the wild-type line was low (<10%) but supported extensive development of extraradical mycelium, branched absorbing structures, and spores. The reduced colonization of rmc under monoxenic conditions (0.6%) was similar to that observed previously in soil. Extraradical development of runner hyphae was low and proportional to internal colonization. Few spores were produced. These results might suggest that carbon transfer may be modified in the rmc mutant. Our results support the usefulness of monoxenically obtained mycorrhizas for investigation of AM colonization and intraradical symbiotic functioning.     

葡萄糖、根浸出液对丛枝菌根真菌吸收不同外源氮产生精氨酸的影响

以大葱(Allium fistulosum)为宿主植物, 接种丛枝菌根(Arbuscular mycorrhizal, AM)真菌Glomus intraradices, 采用三室隔离盆栽培养系统, 在菌丝室施加浓度为4 mmol/L的不同形态外源氮、1%葡萄糖及根浸出液, 通过测定根外菌丝(Extraradical mycelium, ERM)和菌根中精氨酸的含量, 探究葡萄糖、根浸出液对AM真菌吸收不同形式外源氮产生精氨酸的影响。结果表明, 不同外源氮对ERM中精氨酸含量的影响为尿素>Gln>NH₄NO₃>Arg/Gly>NH₄Cl>KNO₃, 对菌根中精氨酸含量的影响为Arg>Gln>尿素>NH₄NO₃>Gly>NH₄Cl>KNO₃; 施加葡萄糖和根浸出液在不同程度上提高ERM干重和菌丝室孢子数量, 但使ERM和菌根中的精氨酸含量降低。说明AM真菌吸收同化不同外源氮产生精氨酸的能力不同, 葡萄糖和根浸出液降低AM真菌吸收同化外源氮产精氨酸的能力。     

The differential behavior of arbuscular mycorrhizal fungi in interaction with Astragalus sinicus L. under salt stress

Three arbuscular mycorrhizal (AM) fungi (Glomus mosseae, Glomus claroideum, and Glomus intraradices) were compared for their root colonizing ability and activity in the root of Astragalus sinicus L. under salt-stressed soil conditions. Mycorrhizal formation, activity of fungal succinate dehydrogenase, and alkaline phosphatase, as well as plant biomass, were evaluated after 7 weeks of plant growth. Increasing the concentration of NaCl in soil generally decreased the dry weight of shoots and roots. Inoculation with AM fungi significantly alleviated inhibitory effect of salt stress. G. intraradices was the most efficient AM fungus compared with the other two fungi in terms of root colonization and enzyme activity. Nested PCR revealed that in root system of plants inoculated with a mix of the three AM fungi and grown under salt stress, the majority of mycorrhizal root fragments were colonized by one or two AM fungi, and some roots were colonized by all the three. Compared to inoculation alone, the frequency of G. mosseae in roots increased in the presence of the other two fungal species and highest level of NaCl, suggesting a synergistic interaction between these fungi under salt stress.     

Effects of AM colonization on “wild tobacco” plants grown in zinc-contaminated soil

This greenhouse study aimed to determine the effect of colonization by the arbuscular mycorrhizal (AM) fungus (Glomus intraradices Schenck & Smith) on the “wild” tobacco (Nicotiana rustica L. var. Azteca), under soil–zinc (Zn) conditions. Plants of N. rustica were grown in AM or non-AM inoculated substrate and subjected to four soil–[Zn] concentrations (0, 50, 100, and 250 mg Zn kg⁻¹ dry soil). The AM root colonization increased markedly from 14 to 81% with the increasing soil–[Zn] and the mycorrhizal structures were significantly more abundant at the highest soil–[Zn], suggesting that Zn may be involved directly or indirectly in AM root colonization. In addition, total Zn content or Zn concentrations in shoots and roots were shown to increase as soil–[Zn] increased in both AM and non-AM plants. As for the growth parameters studied, there were no significant differences between treatments despite the increase in Zn content or concentration. The AM roots subjected to the highest soil–[Zn] had a significant reduction by about 50% of total Zn content and Zn concentration compared to non-AM roots. Still, the relative extracted Zn percentage decreased dramatically as soil–[Zn] increased. Soil pH was significantly lower in non-AM than AM treatments at the highest soil–[Zn]. In summary, AM plants (particularly roots) showed lower Zn content and concentration than non-AM plants. In this regard, the AM fungi have a protective role for the host plant, thus playing an important role in soil-contaminant immobilization processes; and, therefore, are of value in phytoremediation, especially when heavy metals approach toxic levels in the soil.     

Alleviation of salt stress in Lotus glaber by Glomus intraradices

Lotus glaber is a glycophytic, perennial legume from Europe that occurs widely in saline habitats. We evaluated the effect of mycorrhizal fungus colonization on the response to salt stress of two genotypes of L. glaber differing in their tolerance to salinity. The experiment consisted of a randomized block design with two factors: (1) mycorrhizal fungus treatments (with or without AM fungus) and (2) two salinity levels of 0 and 200 mM NaCl. Our results indicated that Glomus intraradices established a more efficient symbiosis with the tolerant than with the sensitive genotype. G. intraradices improved growth of L. glaber plants under saline conditions. They showed higher values of net growth, shoot/root and K⁺/Na⁺ ratios, and protein concentrations than controls. Tolerant AM plants also showed higher chlorophyll levels than non-AM ones. Prevention of Na⁺ accumulation in the plant and enhancement of K⁺ concentrations in roots observed in this work could be part of the general mechanism of salt stress alleviation of L. glaber by G. intraradices.