Establishment of shrub species in a degraded semiarid site after inoculation with native or allochthonous arbuscular mycorrhizal fungi

The re-establishment of native shrub species in the Mediterranean basin serves to restore the characteristic biodiversity and to prevent the processes of erosion and desertification in semiarid areas. A field experiment was carried out in an abandoned semiarid agricultural Mediterranean area to assess the effectiveness of mycorrhizal inoculation, with a mixture of native arbuscular mycorrhizal (AM) fungi or an allochthonous AM fungus (Glomus claroideum), on the establishment of Olea europaea subsp. sylvestris L., Pistacia lentiscus L., Retama sphaerocarpa (L.) Boissier and Rhamnus lycioides L. seedlings in this area. One year after planting, shoot biomass of inoculated O. europaea and P. lentiscus seedlings was greater, by about 630% and 300%, respectively, than that of non-inoculated plants. Shoot biomass of G. claroideum-colonised R. sphaerocarpa plants was significantly greater than that of seedlings inoculated with the mixed native AM fungi after 12 months. The increase of R. lycioides growth due to inoculation with native AM fungi was significantly greater than that of G. claroideum-colonised seedlings during the same growth period. Inoculation with a mix of native AM fungi was the most effective treatment for increasing shoot biomass and N, P and K contents in shoot tissues of R. lycioides seedlings. The mixture of native AM fungi was the most effective with respect to colonisation of the roots of O. europaea and R. lycioides, but the native AM fungi and G. claroideum achieved similar levels of colonisation in P. lentiscus and R. sphaerocarpa. The use of native mycorrhizal potential as a source of AM inoculum may be considered a preferential inoculation strategy to guarantee the successful re-establishment of native shrub species in a semiarid degraded soil.     

Changes in rhizosphere microbial activity mediated by native or allochthonous AM fungi in the reafforestation of a Mediterranean degraded environment

This study was carried out in a semiarid degraded area to assess the effectiveness of mycorrhizal inoculation with a mixture of native arbuscular mycorrhizal (AM) fungi or an allochthonous AM fungus (Glomus claroideum), on the establishment of Olea europaea subsp. sylvestris L. and Retama sphaerocarpa (L.) Boissier in this area. Associated changes in the soil microbiological properties and aggregate stability related to these AM inocula were also recorded. Eighteen months after planting, G. claroideum had increased available P in the rhizosphere of both shrub species. In general, both inoculation treatments increased water-soluble C and water-soluble and total carbohydrates, G. claroideum being the most effective inoculum, particularly in R. sphaerocarpa. The mixture of native AM fungi was the most effective treatment for increasing the aggregate stability of R. sphaerocarpa soil, while that of O. europaea was increased only by G. claroideum. Increased (dehydrogenase, urease, protease-BAA, acid phosphatase and -glucosidase) enzyme activities, in particular of dehydrogenase and acid phosphatase, were recorded in the rhizosphere of both mycorrhizal shrub species. The mixture of native AM fungi was the most effective treatment for stimulating the growth of O. europaea and R. sphaerocarpa (11.6-fold and 3.3-fold, respectively, greater than control plants). The establishment of mycorrhizal shrub species favoured the reactivation of soil microbial activity, which was linked to an increase in aggregate stability.     

Analysis of the mycorrhizal potential in the rhizosphere of representative plant species from desertification-threatened Mediterranean shrublands

An evaluation of the mycorrhizal status of desertification-threatened ecosystems has been recommended as a first step in rehabilitation/restoration approaches based on revegetation strategies using arbuscular mycorrhizal (AM) technology. Representative desertified semiarid areas were selected from southeast Spain where the vegetation is dominated by grasses, with Stipa tenacissima usually present, and with some patches of the shrubs Pistacia lentiscus, Rhamnus lycioides, Olea europaea subsp. sylvestris and Retama sphaerocarpa. The objective of this study was to evaluate the mycorrhizal potential in these soils, the contribution of the different species established to the mycorrhizal potential of the soils and to assess the main mycorrhizal propagules involved. There were more AM fungal propagules in the rhizospheres of all the shrub species studied compared with adjacent fallow soils, suggesting that AM propagules can be considered as a functional component of the resource islands developing around plant roots. R. sphaerocarpa and O. europaea had a higher capacity to enhance the development of mycorrhizal propagules in their rhizospheres than R. lycioides and P. lentiscus. Correlation analyses showed that the number of spores of the most representative AM fungal species, i.e. Glomus constrictum, and the total length of extraradical AM mycelium are the propagule sources which were best correlated with the mycorrhizal potential in terms of the number of “infective” AM propagules in the rhizosphere of the target plant species. The contribution of AM symbiosis to the potentiality of S. tenacissima as nurse plant was site dependent. Diversity of AM fungi present in the test area is rather low, indicating the high degree of degradation of the ecosystem. At most, only four AM fungal spore morphoecotypes were consistently detected in the rhizosphere of the target plant species.     

Plant type differently promote the arbuscular mycorrhizal fungi biodiversity in the rhizosphere after revegetation of a degraded, semiarid land

It is suggested that the diversity of arbuscular mycorrhizal fungi (AMF) and their association with distinct plants species are crucial in the early stages of revegetation procedures since the AMF roots colonisation plays an important role improving plant establishment and growth. We carried out a study where we analyse the AMF community composition in the roots of Ephedra fragilis, Rhamnus lycioides, Pistacia lentiscus and Retama sphaerocarpa fourteen months after revegetation in a Mediterranean semiarid degraded area of southeast Spain in order to verify whether different plant species can variably promote the diversity of AM fungi in their rhizospheres after planted. We analysed a portion of approximately 795 bases pairs of the small-subunit ribosomal DNA by means of nested PCR, cloning, sequencing and phylogenetic analyses. Eight fungal sequence types belonging to Glomus group A and B and to the genus Paraglomus were identified. The different plant species had different AM fungal community composition. Thus, R. lycioides harboured the highest number of four fungal sequence types while from E. fragilis only two types could be characterized that were specific for this plant species. P. lentiscus and R. sphaerocarpa harboured each one three sequence types and two of them were shared. All AMF sequence types were found in the natural soil. These results show that one effective way of restoring degraded lands is to increase the number of plant species used, which would increase the AMF diversity in the soil and thus the below-ground, positive interactions.     

Establishment of Retama sphaerocarpa L. seedlings on a degraded semiarid soil as influenced by mycorrhizal inoculation and sewage‐sludge amendment

A field experiment was carried out to evaluate the effectiveness of mycorrhizal inoculation with three arbuscular mycorrhizal (AM) fungi (Glomus intraradices Schenck & Smith, Glomus deserticola (Trappe, Bloss. & Menge), and Glomus mosseae (Nicol & Gerd.) Gerd. & Trappe) and the addition of composted sewage sludge (SS) with respect to the establishment of Retama sphaerocarpa L. seedlings, in a semiarid Mediterranean area. Associated changes in soil chemical (nutrient content and labile carbon fractions), biochemical (enzyme activities), and physical (aggregate stability) parameters were observed. Six months after planting, both the addition of composted SS and the mycorrhizal‐inoculation treatments had increased total N content, available‐P content, and aggregate stability of the soil. Values of water‐soluble C and water‐soluble carbohydrates were increased only in the mycorrhizal‐inoculation treatments. Rhizosphere soil from the mycorrhizal‐inoculation treatments had significantly higher enzyme activities (dehydrogenase, protease‐BAA, acid phosphatase, and β‐glucosidase) than the control soil. In the short‐term, mycorrhizal inoculation with AM fungi was the most effective treatment for enhancement of shoot biomass, particularly with G. mosseae (about 146% higher with respect to control plants). The addition of the composted SS alone was sufficient to restore soil structural stability but was not effective with respect to improving the performance of R. sphaerocarpa plants.     

Effects of mycorrhizal inoculation of shrubs from Mediterranean ecosystems and composted residue application on transplant performance and mycorrhizal developments in a desertified soil

Arbuscular mycorrhizal inoculation and composted residue application are being assayed to aid restoration of desertified areas under Mediterranean climate. The particular objective of the present study was to assess the short-term (8 months) effects on the initial stages of plant performance and on mycorrhizal propagule release, key factors to decide further developments in the restoration process. Mycorrhizal inoculation, with Glomus intraradices, was practised during nursery production of representative shrub species from Mediterranean ecosystems and composted residues were added to soil before transplanting to a desertified area in southern Spain. Pistacia lentiscus, Rhamnus lycioides, Olea europaea subsp. sylvestris and Retama sphaerocarpa, key species from the natural succession in the target area, were the test plants. Mycorrhizal inoculation, and in some cases compost addition, improved the ability for nutrient acquisition by plants upon transplanting in the field. The number of "infective" mycorrhizal propagules was higher in soil around mycorrhiza-inoculated shrubs than that around the corresponding non-inoculated controls. The organic amendment significantly increased propagule production in the rhizosphere of mycorrhiza-inoculated plants. The number of mycorrhizal spores was relatively low in soil around transplants, being hardly affected by treatments. Only three distinguishable glomalean spore morphotypes were found, belonging to the species Glomus geosporum, G. contrictum and Scutellospora calospora, with very few unidentified spores, corroborating the low diversity in degraded ecosystems. An increased development of the extramatrical AM mycelium was found in soil around the roots of the four mycorrhiza-inoculated test plants, probably the main source of AM fungal propagules in the ecosystem at this stage of plant development. In conclusion, the tailored AM inoculation assayed was functioning under field conditions to enhance nutrient acquisition by the target indigenous shrubs and, in interaction with organic amendments, promoted mycorrhizal propagule production in soil, critical factors to benefit further stages of the revegetation process.     

Growth and nitrate reductase activity in Juniperus oxycedrus subjected to organic amendments and inoculation with arbuscular mycorrhizae

The effectiveness of reforestation programs on degraded soils in the Mediterranean region is frequently limited by a low soil availability and a poor plant uptake and assimilation of nutrients. While organic amendments can improve the nutrient supply, inoculation with mycorrhizal fungi can enhance plant nutrient uptake. A pot experiment was conducted in 2004 to study the influence of inoculation with an arbuscular mycorrhizal (AM) fungus (Glomus intraradices Schenck & Smith) or with a mixture of three AM fungi (G. intraradices, G. deserticola Trappe, Bloss. & Menge, and G. mosseae (Nicol & Gerd.) Gerd. & Trappe) and of an addition of composted sewage sludge or Aspergillus niger–treated dry‐olive‐cake residue on plant growth, nutrient uptake, mycorrhizal colonization, and nitrate reductase (NR) activity in shoot and roots of Juniperus oxycedrus L. Six months after planting, the inoculation of the seedlings with G. intraradices or a mixture of three AM fungi was the most effective treatment for stimulating growth of J. oxycedrus. There were no differences between the two mycorrhizal treatments. All treatments increased plant growth and foliar N and P contents compared to the control plants. Mycorrhizal inoculation and organic amendments, particularly fermented dry olive cake, increased significantly the NR activity in roots.     

Stability of desiccated rhizosphere soil aggregates of mycorrhizal Juniperus oxycedrus grown in a desertified soil amended with a composted organic residue

Adequate soil structural stability favours the establishment and viability of a stable plant cover, protecting the soil against water erosion in desertified Mediterranean environments. We studied the effect of soil drying-rewetting, inoculation with a mixture of three exotic arbuscular mycorrhizal (AM) fungi (Glomus intraradices Schenck & Smith, Glomus deserticola (Trappe, Bloss. & Menge) and Glomus mosseae (Nicol & Gerd.) Gerd. & Trappe) and addition of a composted organic residue on aggregate stabilisation of the rhizosphere soil of Juniperus oxycedrus. The AM fungi and composted residue produced similar increases in plant growth, independently of the water conditions. Under well-watered conditions, the highest percentages of stable aggregates were recorded in the amended soil, followed by the soil inoculated with AM fungi. Excepting microbial biomass C, the soil drying increased labile C fractions (water soluble C, water soluble and total carbohydrates), whereas the rewetting decreased significantly such C fractions. Desiccation caused a significant increase in aggregate stability of the rhizosphere soil of all plants, particularly in the amended and inoculated plants. In all treatments, the aggregates formed after soil drying were unstable, since, in the rewetting, they disappear, reaching the initial levels before soil drying. Our results suggest that the aggregation mechanisms developed by rhizosphere microbial community of the amended and inoculated plants under water stress can be particularly relevant in desertified soils exposed to long desiccation periods.     

Changes in biological activity of a degraded Mediterranean soil after using microbially-treated dry olive cake as a biosolid amendment and arbuscular mycorrhizal fungi

A field experiment was carried out to assess the effect of a combined treatment involving addition of Aspergillus niger-treated dry olive cake (DryOC) in the presence of rock phosphate, plus pre-transplant inoculation of seedlings with the arbuscular mycorrhizal (AM) fungi Glomus intraradices, Glomus deserticola or Glomus mosseae, on the establishment of Dorycnium pentaphyllum L., in a degraded semiarid Mediterranean area. Associated changes in soil labile C fractions, enzyme activities and aggregate stability were also observed. One year after planting, the combined treatment of fermented DryOC addition and inoculation with AM fungi, particularly with G. mosseae (on average 328% greater than control plants), had the strongest effect on the shoot biomass of D. pentaphyllum. Only the fermented DryOC addition increased assimilable P, total N and aggregate stability, the greatest increase being in the soil available P content (about four-fold higher than in the non-amended soil). Both the addition of fermented DryOC and the mycorrhizal inoculation treatments significantly increased enzyme activities of rhizosphere soil (dehydrogenase, protease-BAA, acid phosphatase and β-glucosidase). The microbially-treated DryOC proved to be an effective amendment for improving the soil quality which, in turn, enhanced the success of revegetation with mycorrhizal D. pentaphyllum seedlings.     

Assessing the tolerance to heavy metals of arbuscular mycorrhizal fungi isolated from sewage sludge-contaminated soils

Different fungal ecotypes were isolated from soils which had received long-term applications of metal-contaminated sewage sludge with the aim of studying the degree of tolerance and adaptation to heavy metals of arbuscular mycorrhizal (AM) fungi. The development and structural aspects of AM colonization produced by the different fungal isolates were studied using two host plants, Allium porrum and Sorghum bicolor, which were grown in either contaminated or non-contaminated soils. Four different AM fungi were successfully isolated from the experimental field plots: (i) Glomus claroideum, isolated from plots receiving only inorganic fertilizer; (ii) another apparently similar ecotype of Glomus claroideum, but isolated from plots with 300 m³ ha⁻¹ year⁻¹ of contaminated sludge added, (iii) an unidentified Glomus sp., present only in the less contaminated plots (100 m³ ha⁻¹ year⁻¹ of unamended sludge) and (iv) Glomus mosseae, isolated from plots receiving 100 or 300 m³ ha⁻¹ year⁻¹ of amended or unamended sludge (intermediate rates of contamination). There were consistent differences in behaviour among the four AM fungi tested with regard to the colonization levels they produced in non-contaminated and contaminated soils. Both total and arbuscular colonization were affected by heavy metal contamination. The main conclusions of this study are that Glomus sp. and G. mosseae isolates are strongly inhibited by heavy metals, which acted mainly by interfering with the growth of the external mycelium, and also by limiting the production of arbuscules. Our results suggest that G. claroideum isolates, particularly the ecotype which was isolated from the plots receiving the highest dose of metal-contaminated sludge, shows a potential adaptation to increased metal concentration in soil.     

Organic acid exudation by mycorrhizal Andropogon virginicus L. (broomsedge) roots in response to aluminum

Elevated aluminum (Al) availability limits plant growth on acidic soils. Although this element is found naturally in soils, acidic conditions create an environment where Al solubility increases and toxic forms of Al impact plant function. Plant resistance to Al is often attributed to organic acid exudation from plant roots and the chelation of cationic Al in the rhizosphere. The association of arbuscular mycorrhizal (AM) fungi with the roots of plants may alleviate Al toxicity by altering soil Al availability or plant exposure through the binding of Al to fungal structures or through the influence of fungi on exudation from roots. Diverse communities of AM fungi are found in soil ecosystems and research suggests that AM fungi exhibit functional diversity that may influence plant performance under varying edaphic environments. In the present study, we evaluated acidic isolates of six AM species in their responses to Al. Andropogon virginicus (broomsedge), a warm-season grass that commonly grows in a range of stressful environments including acidic soils, was used as a plant host for Acaulospora morrowiae, Glomus claroideum, Glomus clarum, Glomus etunicatum, Paraglomus brasilianum, and Scutellospora heterogama. Fungal spores were germinated and exposed to 0 or 100 μM Al on filter paper in sand culture or were grown and exposed to Al in sand culture in association with A. virginicus. Short- and long-term responses to Al were evaluated using direct measurements of fungal spore germination, hyphal elongation, and measurements of A. virginicus colonization and plant growth as a phytometer of AM function in symbio. Spore germination and hyphal elongation varied among AM species in response to Al, but patterns were not consistent with the influences of these AM species on A. virginicus under Al exposure. Exposure to Al did not influence colonization of roots, although large differences existed in colonization among fungal species. Plants colonized by G. clarum and S. heterogama exhibited the least reduction in growth when exposed to Al, produced the highest concentrations of Al-chelating organic acids, and had the lowest concentrations of free Al in their root zones. This pattern provides evidence that variation among AM fungi in Al resistance conferred to their plant hosts is associated with the exudation of Al-binding organic acids from roots and highlights the role that AM fungal diversity may play in plant performance in acidic soil environments.     

Arbuscular mycorrhizal fungi associated with wild forage plants in typical steppe of eastern Inner Mongolia

A preliminary investigation was conducted on the arbuscular mycorrhizal (AM) status of the dominant and common wild forage plants in typical steppe of eastern Inner Mongolia, a major semi-arid grassland region in China. Fifty-four wild forage plant species were collected and examined, and 27 of these were colonized by AM fungi. Some plants belonging to families that are presumed to lack mycorrhizas (Cyperaceae, Caryophyllaceae and Chenopodiaceae) were also found to be mycorrhizal. Higher proportions of arbuscular mycorrhizal plants were found in perennial (56.1%) and monocotyledonous (64.7%) forage species. However, neither percentage of root length colonized nor spore density varied significantly between the two life forms or cotyledon types. Twenty-seven species belonging to 7 genera of AM fungi were identified in total according to the morphological characteristics of the spores from field soil and trap cultures, and the results indicate that Glomus was the dominant AM genus and Glomus geosporum (Nicolson & Gerdemann) Walker and Glomus mosseae (Nicolson & Gerdemann) Gerdemann & Trappe were the dominant species in field soil and trap cultures, respectively. Glomus intraradices Schenck & Smith, Glomus etunicatum Becher & Gerdemann, Glomus claroideum Schenk & Smith emend Walker & Vestberg, Glomus clarum Nicolson & Schenck and Scutellospora callospora (Nicolson & Gerdemann) Walker & Sanders also occurred with high frequencies.     

Interaction between AM fungi and a liquid organic amendment with respect to enhancement of the performance of the leguminous shrub Retama sphaerocarpa

This study examined the interactions between the inoculation with three arbuscular mycorrhizal fungi, namely, Glomus intraradices, Glomus deserticola and Glomus mosseae, and the addition of a liquid organic amendment at different rates (0, 50, 100 or 300 mg C of liquid amendment per kilogram soil) obtained by alkaline extraction of composted dry olive residue with respect to their effects on growth of Retama sphaerocarpa seedlings and on some microbiological and physical properties of soil. One year after planting, both mycorrhizal inoculation treatments and the addition of amendment had increased plant growth and dehydrogenase, urease and benzoyl argininamide hydrolysing activities. The inoculation with G. mosseae increased plant growth to a greater extent than the addition of the amendment (about 35% greater than plants grown in the amended soil and about 79% greater than control plants) and both treatments produced similar increases in soil aggregate stability (about 31% higher than control soil). The organic amendment produced a very significant decrease in the levels of microbial biomass C and a strong increase in soil dehydrogenase and urease activities, which were proportional to the amendment rate. Only the combined treatment involving the addition of a medium dose of amendment (100 mg C kg⁻¹ soil) and the mycorrhizal inoculation with G. intraradices or G. deserticola produced an additive effect on the plant growth with respect to the treatments applied individually (about 77% greater than plants grown in the amended soil and about 63% greater than inoculated plants).     

Changes in microbial communities and carbohydrate profiles induced by the mycorrhizal fungus (Glomus intraradices) in rhizosphere of olive trees (Olea europaea L.)

The influence of inoculation of olive trees with arbuscular mycorrhizal (AM) fungi, Glomus (G) intraradices, on microbial communities and sugar concentrations, were examined in rhizosphere of olive trees (Olea europaea L.). Analyses of phospholipid and neutral lipid fatty acids (PLFA and NLFA, respectively) were then used to detect changes in microbial community structure in response to inoculation of plantlets with G. intraradices.Microscopic observations studies revealed that the extraradical mycelium of the fungus showed formation of branched absorbing structures (BAS) in rhizosphere of olive tree. Root colonization with the AM fungi G. intraradices induced significant changes in the bacterial community structure of olive tree rhizosphere compared to non-mycorrhizal plants. The largest proportional increase was found for the fatty acid 10Me18:0, which indicated an increase in the number of actinomycetes in mycorrhizal rhizosphere soil, whereas the PLFAs i15:0, a15:0, i16:0, 16:1ω7 and cy17:0 which were used as indicators of bacteria decreased in mycorrhizal treatment compared to non-mycorrhizal control treatment. A highest concentration of glucose and trehalose and a lowest concentration of fructose, galactose, sucrose, raffinose and mannitol were detected in mycorrhizal rhizosphere soil. This mycorrhizal effect on rhizosphere communities may be a consequence of changes in characteristics in the environment close to mycorrhizal roots.     

Impact of arbuscular mycorrhizal fungi on the growth and expression of gene encoding stress protein – metallothionein BnMT2 in the non‐host crop Brassica napus L.

Arbuscular mycorrhizal (AM) fungi are an important component of the soil biota in most agroecosystems, and their association can directly or indirectly affect the diversity of soil microorganisms, nutrient cycling, and growth of host plants. Since not all crops are symbiotic, we hypothesized that the presence of AM fungi can: (1) inhibit the growth of non‐host plants by resulting in biotic stress, or (2) promote their growth indirectly by increased nutrient mobilization. These hypotheses were tested in the present study on the non‐mycorrhizal crop canola (Brassica napus L.) in the presence and absence of other autochthonous soil microorganisms. The soil was inoculated with a mixture of AM fungi (Acaulospora longula, Glomus geosporum, G. mosseae, Scutellospora calospora) and as a control, a non‐inoculated soil was used. The impact of inoculation on plant growth (biomass production, nutrient concentrations) and expression of the stress protein metallothionein gene BnMT2 was investigated in the shoots. B. napus L. did not form mycorrhizal associations on its roots, but its growth was promoted after inoculation with AM fungi. In the soil with autochthonic microorganisms, growth inhibition after inoculation was observed compared to the control. The concentrations of N, P, K, and S in the shoot were always significantly increased after inoculation with AM fungi. However, this was partly combined with reduced growth and thereby decreased total uptake of nutrients. Expression of BnMT2 in the leaves was increased after inoculation with AM spores at the soil devoid of indigenous microorganisms, but decreased in their presence. The expression of stress proteins (BnMT2) significantly increased with increasing length and biomass of shoots. In conclusion, the inhibition of the non‐host plant B. napus L. following inoculation with AM fungi was confirmed, however, only in combination with autochthonous microorganisms. Growth promotion of B. napus L. in the presence of AM fungi in the absence of autochthonous soil microorganisms suggest that plant growth depression in the presence of AM fungi was based on interactive effects of AM fungi with the autochthonous microorganisms in the soil rather than on a direct impact of the AM fungi.     

Interactions between arbuscular mycorrhizal fungi and fungivorous nematodes on the growth and arsenic uptake of tobacco in arsenic-contaminated soils

The effects of inoculation with two AM fungi (M1, Glomus caledonium; M2, Glomus spp. and Acaulospora spp.) and a fungivorous nematode Aphelenchoides sp. on growth and arsenic (As) uptake of Nicotiana tabacum L. were investigated in soils contaminated with a range of As. The reproduction of Aphelenchoides sp. was triggered by the co-inoculation of AM fungi regardless of AM fungal isolates and As levels. Stimulative effects of Aphelenchoides sp. on the development of mycorrhiza, slightly different between two AM fungi, were found particularly at the lowest As level. Irrespective of mycorrhizal inoculi, increasing soil As level decreased plant growth, but increased plant As uptake. Co-inoculation of AM fungi and Aphelenchoides sp. led plants to achieving further growth and greater As accumulation at the lowest As level. Results showed that the interactions between AM fungi and fungivorous nematodes were important in plant As tolerance and phytoextraction at low level As-polluted soil.     

The bacterial community of tomato rhizosphere is modified by inoculation with arbuscular mycorrhizal fungi but unaffected by soil enrichment with mycorrhizal root exudates or inoculation with Phytophthora nicotianae

Arbuscular mycorrhizal (AM) fungi have been shown to induce the biocontrol of soilborne diseases, to change the composition of root exudates and to modify the bacterial community structure of the rhizosphere, leading to the formation of the mycorrhizosphere. Tomato plants were grown in a compartmentalized soil system and were either submitted to direct mycorrhizal colonization or to enrichment of the soil with exudates collected from mycorrhizal tomato plants, with the corresponding negative controls. Three weeks after planting, the plants were inoculated or not with the soilborne pathogen Phytophthora nicotianae growing through a membrane from an adjacent infected compartment. At harvest, a PCR-Denaturing gradient gel electrophoresis analysis of 16S rRNA gene fragments amplified from the total DNA extracted from each plant rhizosphere was performed. Root colonization with the AM fungi Glomus intraradices or Glomus mosseae induced significant changes in the bacterial community structure of tomato rhizosphere, compared to non-mycorrhizal plants, while enrichment with root exudates collected from mycorrhizal or non-mycorrhizal plants had no effect. Our results support that the effect of AM fungi on rhizosphere bacteria would not be mediated by compounds present in root exudates of mycorrhizal plants but rather by physical or chemical factors associated with the mycelium, volatiles and/or root surface bound substrates. Moreover, infection of mycorrhizal or non-mycorrhizal plants with P. nicotianae did not significantly affect the bacterial community structure suggesting that rhizosphere bacteria would be less sensitive to the pathogen invasion than to mycorrhizal colonization. Of 96 unique sequences detected in the tomato rhizosphere, eight were specific to mycorrhizal fungi, including two Pseudomonas, a Bacillus simplex, an Herbaspirilium and an Acidobacterium. One Verrucomicrobium was common to rhizospheres of mycorrhizal plants and of plants watered with mycorrhizal root exudates.     

Screening of Arbuscular Mycorrhizal Fungi for Symbiotic Efficiency with Sweet Potato

A greenhouse study was conducted to study the efficiency of 14 isolates of arbuscular mycorrhizal (AM) fungi isolated from a local agricultural soil on the productivity of sweet potato (Ipomoea batatas). The different AM fungi enhanced the biomass and nutritional status of sweet potato seedlings to different extents. The genus Glomus was more effective than Acaulospora or Scutellospora. Efficiency also varied among isolates of Glomus irrespective of individual host plant or location of origin. Intraspecific differences were sometimes greater than interspecific differences. Benefits deriving from fungal isolates were positively correlated with the root-colonization rate and the abundance of extraradical propagules of the AM fungi. Taking plant yield parameters, nutritional status of the plants, and fungal attributes into consideration, GEGM (Glomus etunicatum together with Glomus mosseae) and GE6 (Glomus etunicatum) were the most effective AM symbionts for sweet potato under the experimental conditions.     

Mycorrhiza and phosphate protection of tropical grass species against heavy metal toxicity in multi-contaminated soil

In this paper, the effects of arbuscular mycorrhizal (AM) fungi and phosphate amendments on protection of the tropical grass Brachiaria decumbens Stapf. against metal toxicity caused by Zn, Cd, Cu, and Pb were studied in a sterilized soil. Plants inoculated with a mixture of AM fungi (Acaulospora morrowiae, Gigaspora albida, and Glomus clarum) isolated from a heavy-metal-contaminated site or amended with P (added as triple superphosphate) exhibited marked positive growth responses, indicating the ameliorating effects of these two factors. Soil metal concentrations needed to inhibit plant growth by 50% were around twofold higher for AM plants as compared to those for non-inoculated ones. Similarly, phosphate showed ameliorating effects for B. decumbens, but its effects were not related to mycorrhizal conditions. Although mycorrhiza and phosphate act independently, their protecting effects were additive. Metal bioaccumulation factor of B. decumbens is high, especially for Cd; but AM inoculation prevents metal transference from roots to shoots, retaining these metals in the roots. AM fungus and phosphate represent a promising tool for enhancing ground vegetation in heavy-metal-contaminated sites.     

The development of arbuscular mycorrhiza in two simulated stages of spoil-bank succession

A greenhouse experiment based on a dual mode of mycorrhizal inoculation simulated the formation of mycorrhizal symbiosis at two different stages of plant succession on coalmine spoil banks. The model plants were inoculated either with propagules of the arbuscular mycorrhizal fungi (AMF) Glomus mosseae BEG95, which represented the initial stages of succession, or were provided with the pre-established extraradical mycelium (ERM) network of the same AMF isolate, which simulated later succession stages. The plant species used – non-mycotrophic Atriplex sagittata and Sisymbrium loeselii, and mycotrophic Tripleurospermum inodorum, Calamagrostis epigejos and Elytrigia repens – represented succession dominants at those sites. Even though the grasses were colonised in both mycorrhizal treatments, the presence of an established ERM network increased the intensity of their colonisation and arbuscular abundance. No trace of colonisation of non-mycotrophic plants was found in the treatment inoculated with propagules. Surprisingly, marked colonisation, including abundant arbuscules, was observed when non-mycotrophic plants were grown in the presence of a pre-established ERM network. In A. sagittata, arbuscules were found at maturity and senescence of the plants after 16 weeks of growth. In S. loeselii, however, the arbuscules were found at the vegetative stage of the leaf rosette after 8 weeks and then completely disappeared during the following weeks. When the ability of propagules and ERM to induce mycorrhizal colonisation is compared, it seems that the established mycelium probably has an enhanced potential to colonise roots of plants, even if the plants belong to species usually not hosting mycorrhizal fungi.