Interaction between a fungal plant disease,fungivorous nematodes and compost suppressiveness

Abstract

We tested the hypothesis that the fungivorous nematodes Aphelenchoides spp. and Aphelenchus avenae can suppress damping-off caused by Rhizoctonia solani in cauliflower seedlings, and enhance the disease-suppressive effect of compost. In greenhouse experiments, we used two different composts mixed with peat (20% + 80%) and pure peat as growth substrates in growing pots. In each substrate, treatments were: (A) with R. solani and nematodes, (B) with R. solani, (C) with nematodes, (D) control without R. solani or nematodes. Treatment effects were measured as percentage of healthy seedlings 7, 10 and 14 days after start of the experiment. We conducted two different experiments with the treatments A–D; one with Aphelenchoides spp. and one with Aphelenchus avenae. Aphelenchoides spp.+R. solani (treatment A) had 85% healthy plants (= control without addition of fungi (D)) compared with 45% in R. solani without nematodes (B). Aphelenchus avenae suppressed damping-off significantly in all substrates, from almost 100% dead plants in peat with R. solani to 65% healthy plants in R. solani+A. avenae. One compost mixture had an intrinsic suppressive effect on damping-off, while plant health in the other compost mixture was not better than in 100% peat as growing substrate. There were no additive suppressive effects (enhancement) between nematode effects and the suppressive compost. The results demonstrate the ability of fungivorous nematodes to suppress plant diseases. The effects of fungivorous nematodes in combination with compost and other control measures on disease suppression need further attention. The usefulness of fungivorous nematodes in agriculture and horticulture is discussed.     

Wheat plants invest more in mycorrhizae and receive more benefits from them under adverse than favorable soil conditions

Soil chemistry and biota heavily influence crop plant growth and mineral nutrition. The stress-severity and optimal resource allocation hypotheses predict mutualistic symbiotic benefits to increase with the degree of metabolic imbalance and environmental stress. Using two cross-factorial pot experiments with the same biologically active calcareous soil, one time highly saline and nutrient-deficient, and the other time partially desalinated and amended with mineral soil fertilizer, we explored whether these general predictions hold true for zinc (Zn) nutrition of bread wheat in mycorrhizal symbiosis. Increased arbuscular mycorrhizal (AM) fungal root colonization positively correlated with plant Zn nutrition, but only when plants were impaired in growth due to salinity and nutrient-deficiency; this was particularly so in a cultivar-responsive to application of mineral Zn fertilizer. Evidence for direct involvement of AM fungi were positive correlations between Zn uptake from soil and frequency of fungal symbiotic nutrient exchange organelles, as well as the quantitative abundance of AM fungi of the genera Funneliformis and Rhizophagus, but not Claroideoglomus. Combined partial soil desalination and fertilization swapped the dominance ranking from Claroideoglomus spp. to Funneliformis spp. Positive growth, nitrogen, and Zn uptake responses to mycorrhization were contingent on moderate soil fertilization with ZnSO₄. In agreement with the predictions of the stress-severity and optimal resource allocation hypotheses, plants limited in growth due to chemically adverse soil conditions invested relatively more into AM fungi, as evident from heavier root colonization, and took up relatively more Zn and nitrogen in response to mycorrhization, than better growing and less mycorrhized plants. It thus appears that crop plant cultivar-dependent mycorrhization and Zn fertilizer-responsiveness may reinforce each other, provided that there is bioavailable Zn in soil and plant growth is impaired by suboptimal chemical soil conditions.     

The significant contribution of mycorrhizal fungi and earthworms to maize protection and phytoremediation in Cd-polluted soils

Mycorrhizal fungi and earthworms can individually or interactively influence plant growth and heavy metal uptake. The influence of earthworms and arbuscular mycorrhizal (AM) fungi either alone or in combination on maize (Zea mays L.) growth and cadmium (Cd) uptake was investigated in a calcareous soil artificially spiked with Cd. Soils were contaminated with Cd (10 and 20 mg Cd kg⁻¹), inoculated or un-inoculated with the epigeic earthworm Lumbricus rubellus and two AM fungal species (Rhizophagus irregularis and Funneliformis mosseae) for two months of growth under greenhouse conditions. Generally, earthworms alone increased both shoot P uptake and biomass but decreased shoot Cd concentration and root Cd uptake. AM fungi individually often increased total maize P uptake, declined shoot Cd concentration, and consequently produced higher total biomass. However, R. irregularis enhanced shoot Cd uptake at low Cd level and root Cd uptake at high Cd level. In plants inoculated with F. mosseae species, earthworms increased shoot biomass and Cd uptake, decreased root biomass and Cd uptake at all Cd levels, and increased shoot Cd concentration at low Cd level. In plants colonized by R. irregularis species, however, earthworm addition decreased maize biomass only at high Cd level and root Cd concentration and total maize Cd uptake at both Cd levels. Earthworm activity decreased Cd transfer from the soil to maize roots at low Cd level, but this was counterbalanced in the presence of F. mosseae. Mycorrhizal symbiosis significantly reduced the transfer of Cd from roots to shoots, independence of earthworm effect. Overall, it is concluded that L. rubellus and AM fungi, in particular F. mosseae isolate, improved maize tolerance to Cd toxicity both individually and interactively by increasing plant growth and P nutrition, and restricting Cd transfer to the aboveground biomass. Consequently, the single and interactive effects of the two soil organisms might potentially be important not only in protecting maize plants against Cd toxicity, but also in Cd phytostabilization in soils polluted by this highly toxic metal.     

Interaction among free-living N-fixing bacteria isolated from Drosera villosa var. villosa and AM fungi (Glomus clarum) in rice (Oryza sativa)

Rice is usually grown in N-deficient soils, demanding that the element be supplied to the field by commercially available N fertilizers. Unfortunately, a substantial amount of the urea-N or NO₃-N applied as fertilizers is lost through different mechanisms, causing environmental pollution problems. Utilization of biological N₂ fixation (BNF) technology can decrease the application of N fertilizers, reducing environmental risks. This study evaluated the effects of four free-living N-fixing bacterial species, isolated from oligotrophic soil conditions, as single inoculants or combined with arbuscular mycorrhizal fungi (Glomus clarum), on the development of rice plants grown as flooded or upland rice, in the greenhouse. Upland rice roots were inoculated with Methylobacterium sp., Burkholderia sp. and Sphingomonas sp., whereas the species Burkholderia sp., Pseudomonas sp. and Sphingomonas sp., were inoculated on flooded rice. Inoculants consisted of individual bacterial species or their mixtures, with or without G. clarum. Controls included non-bacteria/non-AM fungi, and AM fungi alone. Experiments were carried out in five replicates. The presence of G. clarum decreased or did not significantly affect plant growth under the different culture conditions. The presence of AM fungi stimulated the N-fixing bacterial population of upland rice. Bacterial species had different effects, under both culture conditions, and some genera of N-fixing bacteria increased root and shoot growth at different plant growth stages. The level of mycorrhiza colonization had no influence on plant growth     

Arbuscular mycorrhizal fungi regulate plant mineral nutrient uptake and partitioning in iron ore tailings undergoing eco-engineered pedogenesis

Excess available K and Fe in Fe ore tailings with organic matter amendment and water-deficiencies may restrain plant colonization and growth, which hinders the formation of eco-engineered soil from these tailings for sustainable and cost-effective mine site rehabilitation. Arbuscular mycorrhizal (AM) fungi are widely demonstrated to assist plant growth under various unfavorable environments. However, it is still unclear whether AM symbiosis in tailings amended with different types of plant biomass and under different water conditions could overcome the surplus K and Fe stress for plants in Fe ore tailings, and if so, by what mechanisms. Here, host plants (Sorghum sp. Hybrid cv. Silk), either colonized or noncolonized by the AM fungi (Glomus spp.), were cultivated in lucerne hay (LH, C:N ratio of 18)- or sugarcane mulch (SM, C:N ratio of 78)-amended Fe ore tailings under well-watered (55% water-holding capacity (WHC) of tailings) or water-deficient (30% WHC of tailings) conditions. Root mycorrhizal colonization, plant growth, and mineral elemental uptake and partitioning were examined. Results indicated that AM fungal colonization improved plant growth in tailings amended with plant biomass under water-deficient conditions. Arbuscular mycorrhizal fungal colonization enhanced plant mineral element uptake, especially P, both in the LH- and SM-amended tailings regardless of water condition. Additionally, AM symbiosis development restrained the translocation of excess elements (i.e., K and Fe) from plant roots to shoots, thereby relieving their phytotoxicity. The AM fungal roles in P uptake and excess elemental partitioning were greater in LH-amended tailings than in SM-amended tailings. Water deficiency weakened AM fungal colonization and functions in terms of mineral element uptake and partitioning. These findings highlighted the vital role AM fungi played in regulating plant growth and nutrition status in Fe ore tailings technosol, providing an important basis for involvement of AM fungi in the eco-engineered pedogenesis of Fe ore tailings.     

Nematode assemblages in the rhizosphere of spring barley (Hordeum vulgare L.) depended on fertilisation and plant growth phase

Nematodes from rhizosphere soil of barley grown at three fertiliser treatments (control (0), NK and NPK) were studied in a field experiment. Sampling was done twice, during vegetative growth and flowering, respectively, to determine how fertiliser effects on nematode assemblages depended on plant growth phase. At the growth stage the proportion of fungal feeding nematodes (dominated by Aphelenchoides spp. and Aphelenchus sp.) was highest in NK. During flowering, the abundance and proportion of fungal feeders in the 0 and NPK plots had increased and reached a level similar to the NK plot. Overall densities of bacterial feeders (mainly Cephalobidae and Rhabditidae) were similar, but opportunistic bacterial feeders constituted a higher proportion in the fertilised plots compared to the unfertilised. Ectoparasitic plant feeders (Tylenchorhynchus sp.) were more numerous in NK and NPK than in the control at both sampling dates. Endoparasite (Pratylenchus spp.) numbers were lower in the NPK plot at the growth stage. Numbers of Tylenchidae increased between samplings. The classification of Tylenchidae as epidermal cell and root hair feeders as opposed to hyphal feeders is discussed. Results thus indicate that: (i) bacterial and especially fungal feeding nematodes are stimulated by unbalanced fertilisation; (ii) ectoparasitic plant feeders are stimulated by N-fertilisation, while migratory endoparasites are inhibited at high and balanced fertilisation; (iii) nutrient effects diminish after plants reach the flowering stage.     

Relationship between root-endophytic microbial communities and replant disease in specialized apple growing areas in Europe

Apple replant disorders are one of the causes of the downward trend of land planted with apple orchards in Central Europe. A specific transnational survey was thus conducted on apple root associated microorganisms, aimed at increasing the knowledge regarding crop management to counteract this tendency. Soil health was evaluated using a bioassay test with root cuttings of the clonal M9 rootstock, one of the most commonly used rootstocks in Europe. Plant growth response in replant, fallow and gamma ray-sterilized soil was evaluated using soil samples taken from nine orchards selected (three per country) from specialized apple growing areas of Germany, Austria and Italy. Plant growth significantly differed between treatments but not between countries. Root endophytic fungi were confirmed as one of the main components of growth reduction, while endophytic nematodes (Pratylenchus sp.) were not. The large set of quantitative and qualitative data of root endophytic fungi allowed to clarify that Cylindrocarpon-like fungi (Ilyonectria spp. and Thelonectria sp.) had a major pathogenic role in the three countries, while Pythium spp. prevailed as pathogen only in German orchards. Gamma ray-sterilized soil resulted in a more-than-proportional increase of plant growth (42% and 31% respectively vs. replant and fallow) as compared to that observed between fallow and replant soil (18%). This best performance in gamma ray-sterilized soil, in addition to the lowest root colonization by Cylindrocarpon-like fungi in this treatment, appeared to be due to the different composition of fungal communities as compared to two other treatments (fallow and replant soil), which instead were highly similar to each other. This difference was due to a shift of community composition toward Fusarium and binucleate Rhizoctonia, which prevailed in sterilized soil.     

Interactions of arbuscular-mycorrhizal fungi and Bacillus strains and their effects on plant growth,microbial rhizosphere activity (thymidine and leucine incorporation) and fungal biomass (ergosterol and chitin)

The effects of two Bacillus strains (Bacillus pumillus and B. licheniformis) on Medicago sativa plants were determined in single or dual inoculation with three arbuscular-mycorrhizal (AM) fungi and compared to P-fertilization. Shoot and root plant biomass, values of thymidine and leucine incorporation as well as ergosterol and chitin in rhizosphere soil were evaluated to estimate metabolic activity and fungal biomass, respectively, according to inoculation treatments. For most of the plant parameters determined, the effectiveness of AM fungal species was influenced by the bacterial strain associated. Dual inoculation of Bacillus spp. and AM fungi did not always significantly increase shoot biomass compared to single AM-colonized plants. The most efficient treatment in terms of dry matter production was the dual Glomus deserticola plus B. pumillus inoculation, which produced similar shoot biomass and longer roots than P-fertilization and a 715% (shoot) and 190% (root length) increase over uninoculated control. The mycorrhizas were more important for N use-efficiency than for P use-efficiency, which suggests a direct mycorrhizal effect on N nutrition not mediated by P uptake. Both chemical and biological treatments affected thymidine and leucine incorporation in the rhizosphere soil differently. Thymidine was greater in inoculated than in control rhizospheres and B. licheniformis was more effective than B. pumillus in increasing thymidine. Non-inoculated rhizospheres showed the lowest thymidine and leucine values, which shows that indigenous rhizosphere bacteria increased with introduced inocula. The highest thymidine and leucine values found in P-fertilized soils indicate that AM plants are better adapted to compete with saprophytic soil bacteria for nutrients than P-amended plants. Chitin was only increased by coinoculation of B. licheniformis and G. intraradices. B. pumillus increased ergosterol (indicative of active saprophyte fungal populations) in the rhizosphere of AM plants and particularly when colonized by G. mosseae. The different AM fungi have different effects on bacterial and/or fungal saprophytic populations and for each AM fungus, this effect was specifically stimulated or reduced by the same bacterium. This is an indication of ecological compatibilities between microorganisms. Particular Glomus–bacterium interactions (in terms of effect on plant growth responses or rhizosphere population) do not seem to be related to the percentage of AM colonization. The effect on plant growth and stimulation of rhizosphere populations, as a consequence of selected microbial groups, may be decisive for the plant establishment under limiting soil conditions.     

Effects of soil compaction and arbuscular mycorrhiza on corn (Zea mays L.) nutrient uptake

Soil compaction is of great importance, due to its adverse effects on plant growth and the environment. Mechanical methods to control soil compaction may not be economically and environmentally friendly. Hence, we designed experiments to test the hypothesis that use of plant symbiotic fungi, arbuscular mycorrhiza (AM) may alleviate the stressful effects of soil compaction on corn (Zea mays L.) growth through enhancing nutrient uptake. AM continuously interact with other soil microorganisms and its original diversity may also be important in determining the ability of the fungi to cope with the stresses. Hence, the objectives were: (1) to determine the effects of soil compaction on corn nutrient uptake in unsterilized (S1) and sterilized (S2) soils, and (2) to determine if inoculation of corn with different species of AM with different origins can enhance corn nutrient uptake in a compacted soil. Using 2 kg weights, soils (from the field topsoil) of 10 kg pots were compacted at three and four levels (C1, C2, C3 and C4) (C1 = non-compacted control) in the first and second experiment, respectively. Corn (cv. 704) seeds were planted in each pot and were inoculated with different AM treatments including control (M1), Iranian Glomus mosseae (M2), Iranian G. etunicatum (M3), and Canadian G. mosseae, received from GINCO (Glomales In Vitro Collection), Canada (M4). Corn leaf nutrient uptake of N, P, K, Fe, Mn, Zn and Cu were determined. Higher levels of compaction reduced corn nutrient uptake, however different species of AM and soil sterilization significantly increased it. The highest increase in nutrient uptake was related to P (60%) and Fe (58%) due to treatment M4S2C3. Although it seems that M3 and M4 may be the most effective species on corn nutrient uptake in a compacted soil, M2 increased nutrient uptake under conditions (C3 and C4 in unsterilized soil) where the other species did not. Through increasing nutrient uptake AM can alleviate the stressful effects of soil compaction on corn growth.     

Effects of soil application of olive mill wastewaters on the structure and function of the community of arbuscular mycorrhizal fungi

Recycling of olive mill wastewaters (OMW) into agricultural soils is a controversial issue since benefits to soil fertility should counterbalance potential short-term toxicity effects. We investigated the short-term effects of OMW on the soil-plant system, regarding the diversity, structure and root colonization capacity of arbuscular mycorrhizal (AM) fungi and the respective growth response of Vicia faba L, commonly used as green manure in olive-tree plantations. A compartmentalized pot system was used that allowed the establishment of an AM fungal community in one compartment (feeder) and the application of three OMW dose levels in an adjacent second compartment (receiver). At 0, 10, and 30 days after OMW treatment (DAT), V. faba pre-germinated seeds were seeded in the receiver compartment. At harvest, shoot and root dry weights, AM fungal root colonization, soil hyphal length and P availability were recorded in the receiver compartment. In addition, OMW effects on AM fungal diversity in plant roots were studied by DGGE. A transient effect of OMW application was observed; plant growth and AM fungal colonization were initially inhibited, whereas soil hyphal length was stimulated, but in most cases differences were absent when seeding was performed 30 DAT. Similarly, changes induced in the structure of the root AM fungal community were of transient nature. Cloning and sequencing of all the major DGGE bands showed that roots were colonized by Glomus spp. The transient effects of OMW on the structure and function of AM fungi could be attributed to OMW-derived phytoxicity to V. faba plants or to an indirect effect via alteration of soil nutritional status. The high OMW dose significantly increased soil P availability in the presence of AM fungi, suggesting efficient involvement of AM fungi in organic-P minerilization. Overall our results indicate that soil application of OMW would cause transient changes in the AM fungal colonization of V. faba plants, which, would not impair their long-term plant growth promoting ability.     

Effects of fungivorous nematodes on corky root disease of tomato grown in compost-amended soil

Abstract

The effect of fungivorous nematodes, Aphelenchus avenae and Aphelenchoides spp., against corky root disease of tomato caused by Pyrenochaeta lycopersici was investigated. Three different greenhouse trials were conducted using soil naturally infested with P. lycopersici, alone or mixed with four different types of compost consisting of green manure, garden waste and horse manure (20% compost by volume). The fungivorous nematodes were propagated in cultures of the fungus Pochonia bulbillosa and inoculated (3 or 23 nematodes ml^?1 substrate) into the soil and soil-compost mixtures one day after transplanting of tomato seedlings. Greenhouse experiments were terminated after ten weeks and disease was measured from infected roots after harvesting. Aphelenchus avenae significantly reduced the disease severity when added to infested soil without compost in all experiments. Aphelenchoides spp. did not suppress the disease either in the presence or absence of compost. Among the composts tested, only a garden waste compost was found to be suppressive to the disease. Neither A. avenae nor Aphelenchoides spp. improved the suppressive effect of the compost.     

Fraxinus-Glomus-Pisolithus symbiosis: Plant growth and soil aggregation effects

It has been established that arbuscular mycorrhizal (AM) fungi are involved in the conservation of soil structure. However, the effect of ectomycorrhizal (EM) fungi alone or in interaction with AM fungi in soil structure has been much less studied. This experiment evaluated EM and AM fungi effects on soil aggregation and plant growth. Ash plants (Fraxinus uhdei) were grown in pots, and were inoculated with Glomus intraradices and Pisolithus tinctorius separately but also in combination. Our results showed that F. uhdei established a symbiotic association with EM and AM fungi, and that these organisms, when interacting, showed synergistic and additive effects on plant growth compared to singly inoculated treatments. EM and AM fungi prompted changes in root morphology and increased water-stable aggregates. AM fungi affect mainly small-sized macroaggregates, while EM and EM-AM fungi interaction mainly affected aggregates bigger than 0.5 mm diameter. These results suggest that ectomyccorrhizal as well as arbuscular mycorrhizal fungi should be considered in restoration programs with Fraxinus plants.     

Differential effects of soil disturbance and plant residue retention on function of arbuscular mycorrhizal (AM) symbiosis are not reflected in colonization of roots or hyphal development in soil

The effects of soil disturbance and residue retention on the functionality of the symbiosis between medic (Medicago truncatula L.) and arbuscular mycorrhizal fungi (AMF) were assessed in a two-stage experiment simulating a crop rotation of wheat (Triticum aestivum L.) followed by medic. Plants were inoculated or not with the AMF, Glomus intraradices and Gigaspora margarita, separately or together. The contribution of the arbuscular mycorrhizal (AM) pathway for P uptake was determined using ³²P-labeled soil in a small hyphal compartment accessible only to hyphae of AMF. In general AM colonization was not affected by soil disturbance or residue application and disturbance did not affect hyphal length densities (HLDs) in soil. At 4 weeks disturbance had a negative effect on growth and phosphorus (P) uptake of plants inoculated with G. margarita, but not G. intraradices. By 7 weeks disturbance reduced growth of plants inoculated with G. margarita or AMF mix and total P uptake in all inoculated plants. With the exception of plants inoculated with G. margarita in disturbed soil at 4 weeks, the AM pathway made a significant contribution to P uptake in all AM plants at both harvests. Inoculation with both AMF together eliminated the negative effects of disturbance on AM P uptake and growth, showing that a fungus insensitive to disturbance can compensate for loss of contribution of a sensitive one. Application of residue increased growth and total P uptake of plants but decreased ³²P in plants inoculated with the AMF mix in disturbed soil, compared with plants receiving no residue. The AMF responded differently to disturbance and G. intraradices, which was insensitive to disturbance, compensated for lack of contribution by the sensitive G. margarita when they were inoculated together. Colonization of roots and HLDs in soil were not good predictors of the outcomes of AM symbioses on plant growth, P uptake or P delivery via the AM pathway.     

Belowground responses by AM fungi and animal trophic groups to repeated defoliation in an experimental grassland community

We tested a hypothesis that the effects of defoliation on plants and soil organisms vary with the number of successive defoliations. We established a 23-week greenhouse experiment using replicated grassland microcosms that were composed of three plant species, Trifolium repens, Plantago lanceolata and Phleum pratense, growing together in grassland soil with a diverse soil community. The experiment consisted of two treatment factors-defoliation and harvest time-in a fully factorial design. The defoliation treatment had two levels, i.e. no trimming and trimming of plants every 2 weeks, and the harvest time five levels, i.e. harvests after 1-3, 5 and 7 trimmings. Shoot production (trimmed plus harvested shoot mass), harvested shoot and root mass and root N and C concentrations increased with time but were reduced by defoliation. Colonization rates of arbuscular mycorrhizal (AM) fungi decreased with time in T. repens roots but were enhanced by defoliation, whereas AM colonization rates in P. pratense roots were not affected by harvest time or defoliation. The abundance of bacterivorous and fungivorous nematodes decreased and that of herbivorous and predatory nematodes increased with time, while the abundance of omnivorous nematodes and detritivorous enchytraeids varied in time without a linear trend. Defoliation had no effect on fungivores and predators but increased the abundance of bacterivores. Defoliation also increased the abundance of herbivores, omnivores and detritivores after 2 trimmings and that of omnivores and detritivores after 5 trimmings, but had a negative effect on omnivores after 3 trimmings and on herbivores after 7 trimmings. Among nematode genera, some deviation from the trophic group responses existed: for instance, defoliation reduced the abundance of bacterivorous Acrobeloides spp. and did not affect the abundance of herbivorous Filenchus spp. and Paratylenchus spp. Our results show that the effects of defoliation on plants, AM fungi and some soil animal trophic groups may remain constant all the way through several defoliations, whereas other animal trophic groups may have different and even opposite responses to defoliation depending on the length of the defoliation period before monitoring. This shows how separate studies with defoliation periods of different length can produce contradictory results of the effects of defoliation on the abundance of soil animals.     

Direct and indirect influence of arbuscular mycorrhizal fungi on abundance and community structure of ammonia oxidizing bacteria and archaea in soil microcosms

Both arbuscular mycorrhizal (AM) fungi and ammonia oxidizers are important soil microbial groups in regulating soil N cycling. However, knowledge of their interactions, especially the direct influences of AM fungi on ammonia oxidizers is very limited to date. In the present study, a controlled microcosm experiment was established to examine the effects of AM fungi and N supply level on the abundance and community structure of ammonia oxidizing bacteria (AOB) and archaea (AOA) in the rhizosphere of alfalfa plants (Medicago sativa L.) inoculated with AM fungus Glomus intraradices. Effects were studied using combined approaches of quantitative polymerase chain reaction (qPCR) and terminal-restriction fragment length polymorphism (T-RFLP). The results showed that inoculation with AM fungi significantly increased the plant dry weights, total N and P uptake. Concomitantly, AM fungi significantly decreased the amoA gene copy numbers of AOA and AOB in the root compartment (RC) but not in the hyphal compartment (HC). Moreover, AM fungi induced some changes in AOA community structure in HC and RC, while only marginal variations in AOA composition were observed to respond to N supply level in HC. Neither RC nor HC showed significant differences in AOB composition irrespective of experimental treatments. The experimental results suggested that AM fungi could directly shape AOA composition, but more likely exerted indirect influences on AOA and AOB abundance via the plant pathway. In general, AM fungi may play an important role in mediating ammonia oxidizers, but the AOA community appeared to be more sensitive than the AOB community to AM fungi.     

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.     

Increased abundance of arbuscular mycorrhizal fungi in soil coincides with the reproductive stages of maize

Arbuscular mycorrhizal (AM) fungi are recognized for their positive effects on plant growth, playing an important role in plant P nutrition. We used C16:1cis11 and C18:1cis11 fatty acid methyl ester (FAME) biomarkers to monitor the dynamics of AM fungi during the reproductive stages of maize (Zea mays L.) grown at high yield in Nebraska, USA. Two fields with four different levels of P availability were sampled throughout the reproductive stages. Chambers, made of PVC enclosed mesh fabric to allow passage of roots and hyphae (+R) or hyphae alone (-R) and amended with either KH₂PO₄(+P) or distilled water (-P), were installed in the field at tasselling and removed after three, six and nine weeks. Our objectives were (i) to provide evidence for C allocation to AM fungi during the reproductive stages of high productivity maize and (ii) to link AM fungal growth dynamics with changes in soil P availability. We observed that initial AM FAME concentration was lower at sites with a high availability of P. During the reproductive growth of maize, AM biomarkers increased inside the chambers and were consistent with the biomarker increase observed in adjacent field soil. This confirms that there is C allocation from the plant to the symbiont during the reproductive stages of maize. We also observed a reduction in available P in +R and -R chambers. This observation implies that hyphae were as efficient as roots and hyphae in reducing the P concentration in chambers. These results demonstrate that AM fungi are active during the reproductive growth stages of maize and may benefit high productivity maize crops by facilitating P uptake.     

Crop residue and Collembola interact to determine the growth of mycorrhizal pea plants

The effects of collembolan grazing on arbuscular mycorrhizal (AM) fungi and plant growth were studied in a controlled experiment utilizing a mix of AM fungi and the dominant collembolan species (Isotoma sp.) indigenous to the experimental soil. Collembolan (+/– Col) effects were examined in the presence and absence of crop residue (+/– Litter) incorporated into the experimental soil. Significant interactions between collembolans and crop residue occurred for mycorrhizal colonization of roots and plant growth. In the absence of crop residue, collembolans reduced root length colonized by AM fungi, total plant dry mass and seed pod yield. However, in the presence of crop residue, collembolans had no effect on root colonization by AM fungi, and increased total plant mass and pod yield. Crop residue increased root colonization by AM fungi, numbers of bacteria and saprophytic fungi (colony forming units), small- (<5 m) and large- (>5 m) diameter hyphal lengths in soil, and the final population of collembolans in soil. Collembolans reduced both small- and large-diameter hyphae in soil and the number of saprophytic fungi (colony forming units, p =0.052). Feeding preference experiments conducted in vitro showed that Isotoma sp. preferred to graze on mycorrhizal roots over nonmycorrhizal roots when given no other food choice. However, when crop residue was added as a food choice, Isotoma sp. showed a clear feeding preference for crop residue. We conclude that collembolan grazing on mycorrhizae can be detrimental to plant growth when other fungal food sources are limited, but grazing on mycorrhizal fungi does not occur when ample organic matter and associated saprophytic fungi are present in soils.     

Mycorrhizal phosphorus enhancement of plants in undisturbed soil differs from phosphorus uptake stimulation by arbuscular mycorrhizae over non-mycorrhizal controls

Previous greenhouse and field studies have shown arbuscular mycorrhizal (AM) plants usually have greater P uptake and growth when raised in undisturbed soil compared to soil disturbed between plantings, such as by tillage. We report here for the first time that AM fungi able to stimulate shoot P uptake in experimental comparisons to non-mycorrhizal plants differ in their ability to bring about similar responses in undisturbed soil compared to disturbed soil. This outcome indicates a difference in functional character between the two stimulation processes. Three isolates of AM fungi were tested for growth promotion of maize (Zea mays L.) in pots in a soil disturbance experiment that included non-mycorrhizal controls. All three fungi colonized roots well and promoted shoot P uptake compared to non-inoculated controls, but only Glomus mosseae was able to stimulate growth in undisturbed soil compared to disturbed soil. This effect was seen when Glomus mosseae was alone or in combination with Gigaspora margarita. However, the presence of Glomus aggregatum in combination with Glomus mosseae prevented any stimulation, presumably due to domination by Glomus aggregatum. The ability of AM fungi to be beneficial to plants in comparison to non-mycorrhizal situations likely relates to the spread of mycelium in the soil and the capacity for nutrient transfer to the root. The ability of an AM fungus to promote growth in undisturbed soil appears to be related to these features and, in addition, a capacity for persistence and retention of functional capacity of the extraradical mycelium from one plant generation to the next.     

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.