Vesicular-arbuscular Research Articles

Patterns of Nutrient Dynamics within and below the Rootzone of Collard Greens Grown under Different Organic Amendment Types and Rates

The knowledge about nutrient dynamics in the soil is pivotal for sustainable agriculture. A comprehensive research trial can retort unanswered questions. Dynamics of nutrients sourced from organic amendment types (chicken manure, dairy manure, and MilorganiteTM) applied at different rates (0, 168, 336, 672 kg total N/ha) were monitored within and below the rootzone of collard greens cultivated on a sandy loam soil in Prairie View, TX, USA. Macro- and micronutrients (e.g., TN: total nitrogen, P: phosphorous, K: potassium, Na: sodium, Ca: calcium, Mg: magnesium, B: boron, Cu: copper, Fe: iron, and Zn: zinc) were analyzed from soil solution samples collected during six sampling periods from within and below the rootzone. As hypothesized, the organic amendment types and rates significantly (p < 0.05 and/or 0.01) affected nutrient dynamics within and below the crop rootzone. Chicken manure released significantly more TN, P, K, Na, Ca, Mg, B, Cu, and Fe than the other two amendments. The application of chicken manure and MilorganiteTM resulted in higher below-the-rootzone leachate concentration of TN, Na, Mg, and Ca than in the leachates of dairy manure. Dairy manure treatments had the lowest concentrations of TN, Ca, and Mg; whereas, MilorganiteTM had the lowest concentrations of P, K, Na, B, and Cu in the collected leachates. The higher level of P (i.e., 4% in MilorganiteTM as compared to 2 and 0.5% in chicken and dairy manures, respectively, might have reduced the formation of Vesicular-Arbuscular (VA) mycorrhizae—a fungus with the ability to dissolve the soil P, resulting in slow release of P from MilorganiteTM treatment than from the other two treatments. Patterns of nutrient dynamics varied with rain and irrigation events under the effects of the soil water and time lapse of the amendment applications’ rates and types. All the macronutrients were present within the rootzone and leached below the rootzone, except Na. The dynamic of nutrients was element-specific and was influenced by the amendments’ type and application rate.

Root colonization and spore population by VA-mycorrhizal fungi in four grapevine rootstocks

The occurrence of vesicular arbuscular (VA) mycorrhizae was investigated in an experimental vineyard at the Thessaloniki prefecture, in a soil which was poor in available P. The soil had not received any phosphorus fertilization for several years, nevertheless the leaves had an adequate P content (0.17 - 0.31 % of d.w.). The vineyard was planted with 4 introduced (Cinsaut, Syrah, Grenache, Carignan) and 4 local cultivars (Athiri, Roditis, Assyrtiko, Malagouzia), grafted to the rootstocks 110 R, 41 B, 140 Ru and 1103 P. Microscopic examination showed that all 4 rootstocks were colonized by VA-mycorrhizal fungi at frequencies ranging from 45 to 75 %. The number of vesicles varied between 16 and 47 and the number of arbuscules between 5 and 26 per cm of infected root. Spore number produced by the mycorrhizal fungi in the rhizosphere ranged from 196 to 280 per 100 g of soil. Spores of the genus Glomus were more commonly encountered, in particular those of G. mosseae and G. macrocarpum. The roots of 1103 P, followed by 41 B, 140 Ru and 110 R were most heavily colonized. The same pattern was observed with regard to the number of spores in the rhizosphere of these rootstocks. Grafted cultivars were found to have some influence on the degree of colonization of these rootstocks and on the population of spores, but had no effect on the formation of vesicles and arbuscules. The degree of colonization of roots by endomycorrhizal fungi was inversely related to the available P content of the soil. At the time of root sampling the P content of leaves was not directly related neither to the degree of root colonization nor to the P content of the soil. However, leaves were found to be adequately supplied with P in all cases. Therefore, the differences observed among the 8 cultivars can be attributed primarily to a variety-specific demand for P.

Cryo-scanning electron microscopy (CSEM) in the advancement of functional plant biology: energy dispersive X-ray microanalysis (CEDX) applications

The capacity to make measurements of elemental concentrations at the level of single cells by energy dispersive X-ray microanalysis of cryo-fixed, inherently-hydrated plant parts (CEDX) is changing or extending our understanding of many plant functions. We include in this review a wide-ranging catalogue of studies that have used CEDX which provides access to the literature on elements measured, plants and tissues studied, techniques used, level of quantitation and the significant findings. These findings include new perspectives on the following areas: salt tolerance; xylem maturation and solute content, root pressure and embolism refilling; the contents of intercellular spaces; sequestration of toxic elements; biomineralisation with silicon; movement of tracer homologues of native cations; indirect localisation of molecules with a distinctive element component; transfer of nutrients from vesicular-arbuscular (VA) mycorrhizas; the role of mucilages in protection and in generating mechanical force. In an Appendix we discuss the procedures involved in CEDX: cryo-fixation, specimen planing, etching, elemental quantitation and mapping. Limitations on sample numbers, elements measurable, spatial resolution, sensitivity and threshold concentrations quantifiable are outlined. A brief discussion of the potential of emerging technologies for cell-specific analysis of cryo-fixed, hydrated specimens is included. In the Accessory Publication we list our standard protocol for CEDX.

Tolerance to cadmium of vesicular arbuscular mycorrhizae spores isolated from a cadmium-polluted and unpolluted soil

We carried out a trial to investigate if vesicular arbuscular (VA) mycorrhizae from a Cd-polluted soil can increase both their Cd tolerance and their influence on the host plant, compared with the same strain from unpolluted soil. Inocula (S0, S1 and S2, respectively), were prepared from soil containing 0, 10, 100 mg/kg Cd (T0, T1 and T2, respectively). Each inoculum was added to each soil giving nine treatments. The percentage infection of barley roots and the spore numbers decreased proportionally with increasing Cd concentration in soil. However, the S2 inoculum showed the highest capacity to colonise the barley roots. This inoculum also created the best biotic barrier against the movement of Cd from the roots to the shoot in the T2 soil. In the T1 soil, the S1 and S2 inocula were less efficient than the S0 inoculum, probably because the level of soil pollution was not sufficiently high for any difference to be seen. We concluded that VA mycorrhizae can adapt to heavy metals and that their performance can influence metal translocation in the plant, providing a biological barrier. This is very important for the use of VA mycorrhizae in the reclamation of polluted soils.

Mycorrhizal association of the extinct conifer Metasequoia milleri

Vesicular-arbuscular (VA) mycorrhizas are described from anatomically preserved roots of the Middle Eocene taxodiaceous conifer Metasequoia milleri and compared to those of the living species M. glyptostroboides. Virtually identical VA mycorrhizal structures occur in the root cortex of both species, where they conform to the Paris-type. Coiled hyphae are most common within cells of the inner cortical region, and these produce numerous, highly branched arbuscules. Close similarity of fungal position and structure within roots of the living and fossil Metasequoia species demonstrates that modern Paris-type VA mycorrhizal associations characterized taxodiaceous conifers by the early Tertiary.

Interactions between a vesicular-arbuscular mycorrhizal fungus ( Glomus intraradices ) and Streptomyces coelicolor and their effects on sorghum plants grown in soil amended with chitin of brawn scales

The effect of the interaction between a vesicular-arbuscular (VA)-mycorrhiza (Glomus intraradices no. LAP8) and Streptomyces coelicolor strain no. 2389 on the growth response, nutrition and metabolic activities of sorghum (Sorghum bicolor) plants grown in non-sterilized soil amended with chitin waste was studied in a greenhouse over 8 weeks. Chitin amendment resulted in an increase in the microbial population and chitinase activity in soils. Growth of mycorrhizal G. intraradices no. LAP8 and non-mycorrhizal sorghum plants increased as compared with other treatments either in the presence or absence of S. coelicolor strain 2389. VA-mycorrhizal inoculation significantly increased the growth, photosynthetic pigments, total soluble protein and nutrient contents of sorghum compared to non-mycorrhizal sorghum. Such increases were related to increased mycorrhizal colonization. Inoculation with S. coelicolor 2389 significantly increased the intensity of mycorrhizal root colonization and arbuscular formation, but the levels of mycorrhizal infection and their beneficial effects were significantly reduced with the addition of chitin waste to the soil. Analysis of the content of total amino acids and ammonia in leaves on the basis of dry matter production showed that, in most instances, total amino acids of mycorrhizal plants were significantly higher than those of non-inoculated plants. The microflora of the rhizosphere was highly affected by mycorrhizal inoculation. Quantitative changes in acid and alkaline phosphatase activities of the roots in response to the mycorrhizal inoculation are discussed.

Symbiotic fungal associations in ‘lower’ land plants

An analysis of the current state of knowledge of symbiotic fungal associations in 'lower' plants is provided. Three fungal phyla, the Zygomycota, Ascomycota and Basidiomycota, are involved in forming these associations, each producing a distinctive suite of structural features in well-defined groups of 'lower' plants. Among the 'lower' plants only mosses and Equisetum appear to lack one or other of these types of association. The salient features of the symbioses produced by each fungal group are described and the relationships between these associations and those formed by the same or related fungi in 'higher' plants are discussed. Particular consideration is given to the question of the extent to which root fungus associations in 'lower' plants are analogous to 'mycorrhizas' of 'higher' plants and the need for analysis of the functional attributes of these symbioses is stressed. Zygomycetous fungi colonize a wide range of extant lower land plants (hornworts, many hepatics, lycopods, Ophioglossales, Psilotales and Gleicheniaceae), where they often produce structures analogous to those seen in the vesicular-arbuscular (VA) mycorrhizas of higher plants, which are formed by members of the order Glomales. A preponderance of associations of this kind is in accordance with palaeohbotanical and molecular evidence indicating that glomalean fungi produced the archetypal symbioses with the first plants to emerge on to land. It is shown, probably for the first time, that glomalean fungi forming typical VA mycorrhiza with a higher plant (Plantago lanceolata) can colonize a thalloid liverwort (Pellia epiphylla), producing arbuscules and vesicles in the hepatic. The extent to which these associations, which are structurally analogous to mycorrhizas, have similar functions remains to be evaluated. Ascomycetous associations are found in a relatively small number of families of leafy liverworts. The structural features of the fungal colonization of rhizoids and underground axes of these plants are similar to those seen in mycorrhizal associations of ericaceous plants like Vaccinium. Cross inoculation experiments have confirmed that a typical mycorrhizal endophyte of ericaceous plants, Hymenoscyphus ericae, will form associations in liverworts which are structurally identical to those seen in nature. Again, the functional significance of these associations remains to be examined. Some members of the Jungermanniales and Metzgeriales form associations with basidiomycetous fungi. These produce intracellular coils of hyphae, which are similar to the pelotons seen in orchid mycorrhizas, which also involve basidiomycetes. The fungal associates of the autotrophic Aneura and of its heterotrophic relative Cryptothallus mirabilis have been isolated. In the latter case it has been shown that the fungal symbiont is an ectomycorrhizal associate of Betula, suggesting that the apparently obligate nature of the association between the hepatic and Betula in nature is based upon requirement for this particular heterotroph.

Inhibition of hyphal growth of a vesicular-arbuscular mycorrhizal fungus in soil containing sodium chloride limits the spread of infection from spores

The hyphal growth of some vesicular-arbuscular (VA) mycorrhizal fungi is inhibited by increasing concentrations of NaCl in soil, and this may influence the formation of mycorrhizas. Our objective was to examine the effect of NaCl on the distance that hyphae of a VA mycorrhizal fungus, Gigaspora decipiens Hall and Abbott can grow through soil and infect roots of a known host plant, Trifolium resupinatum. L. cv. Kyambro. An experimental procedure was developed to enable the effect of NaCl on the pre-symbiotic growth of hyphae of a VA mycorrhizal fungus to be studied separately from other phases of the life-cycle. Roots 1 cm from the spores were infected after 15 d in soils moistened with 0 or 75 m m NaCl, indicating that the initial hyphal growth was not inhibited in soil moistened with 75 m m NaCl. With time, hyphal growth became increasingly inhibited by NaCl, as roots 2.5 cm from the spores were not infected after 26 d in soil moistened with 75 m m NaCl, but were infected in soil without NaCl added. In soil moistened with 150 m m NaCl, roots 1 cm from the spores were infected after 26 d, but roots 2 cm from the spores were not infected after 36 d. Increasing concentrations of NaCl inhibited either the hyphal growth or the infectivity of G. decipiens hyphae after they had grown through soil. The latter may be unlikely, because the infectivity of G. decipiens hyphae placed directly onto the roots was not affected by NaCl. Increasing concentrations of NaCl also inhibited the spread of infection after the initial infection had occurred, and this is probably due to NaCl in the soil inhibiting hyphal growth as well as possibly affecting the supply of carbohydrates from the plant to the fungus.

A mutant in Lycopersicon esculentum Mill. with highly reduced VA mycorrhizal colonization: isolation and preliminary characterisation.

This paper reports the successful isolation and preliminary characterisation of a mutant of Lycopersicon esculentum Mill. with highly reduced vesicular-arbuscular (VA) mycorrhizal colonization. The mutation is recessive and has been designated rmc . Colonization by G. mosseae is characterised by poor development of external mycelium and a few abnormal appressoria. Vesicles were never formed by this fungus in association with the mutant. Gi. margarita formed large amounts of external mycelium, complex branched structures and occasional auxiliary cells. Small amounts of internal colonization also occurred. Laser scanning confocal microscopy (LSCM) gave a clear picture of the differences in development of G. intraradices and Gi. margarita in mutant and wild-type roots and confirmed that the fungus is restricted to the root surface of the mutants. The amenability of tomato for molecular genetic characterisation should enable us to map and clone the mutated gene, and thus identify one of the biochemical bases for inability to establish a normal mycorrhizal symbiosis. The mutant represents a key advance in molecular research on VA mycorrhizal symbiosis.

Mycorrhizal sink strength influences whole plant carbon balance of Trifolium repens L.

A comparative analysis of daily carbon (C) budgets and aspects of the C physiology of clover (Trifolium repens L.) colonized by vesicular‐arbuscular (VA) mycorrhizal fungi was carried out over a 70 d growth period under conditions designed to ensure that shoots of mycorrhizal (M) and non‐mycorrhizal (NM) plants were of similar nutrient status. C budgets did not differ on day 24 but by day 42 M plants had a significantly higher rate of photosynthesis than their NM counterparts when expressed on a whole shoot basis or unit dry weight basis. As both sets of plants were of the same size it was concluded that this greater C gain was the result of increased sink strength provided by the mycorrhizal fungus. By day 53 M plants had become larger than their uncolonized counterparts and a sink‐induced stimulation in the rate of photosynthesis was no longer apparent. M plants had higher root sucrose, glucose and fructose pools from day 24. Analyses suggested that these sugars were utilized for trehalose and lipid synthesis, for the production of the large extramatrical mycelium and for the support of the respiratory demands of the M root system. Increased C allocation to roots of M plants was associated with a stimulation of the activities of cell wall and cytoplasmic invertases and of sucrose synthase in roots colonized by VA fungi. Such increases in enzyme activity may provide the mechanism enabling increased partitioning of carbohydrate both to the M root system and the fungal symbiont.

Changes in the wall potential ofScutellospora calosporaassociated with colonization ofAllium porrumroots are not accompanied by equivalent changes in the host

Cell wall electrical potentials were recorded from external hyphae, appressoria, and associated hyphae of the vesicular-arbuscular (VA) mycorrhizal fungus Scutellospora calospora (Nicolson & Gerdemann) Walker & Sanders colonizing leek (Allium porrum L.) roots. As colonization progressed the hyphal walls became more polarized with similar values to the walls of the leek root. These changes, which may reflect changes in fungal wall composition, have important implications for nutrient uptake by the fungus. There were no differences in wall or intracellular potentials between leek roots colonized by either S. calospora orGlomus sp. ("City Beach") and non-mycorrhizal roots. These results are further evidence of the close compatibility between roots and VA mycorrhizal fungi. Key words: vesicular-arbuscular mycorrhiza, cell wall, electrical potential difference, leek, Allium porrum, Scutellospora calospora.

Nitrifiers and mycorrhizae in pristine and grazed pinyon‐juniper ecosystems

We compared soils from grazed and pristine pinyon‐juniper ecosystems for the numbers of chemoautotrophic nitrifying bacteria and determined the distribution of vesicular‐arbuscular (VA) mycorrhizae and ectomycorrhizae (EM). Additionally, we report on the presence of allelopathic substances in organic horizons. Ammonium (NH4+)‐oxidizing bacteria were in greater numbers in the interspaces between the trees, ranging from 9.60 × 104 to 2.13 × 105 bacteria‐1 soil, than under canopies, 3.5 × 104 to 4.8 × 104 bacteria‐1 soil. Grazed interspace soils had over twice the number of NH4+oxidizing bacteria than any other location. There were no differences in the numbers of nitrite‐oxidizing bacteria between either vegetative covers or sites. The rate of nitrate production did not correlate with the number of nitrifiers. However, there was a significant correlation (r2 = 0.85) between mineralization coefficients of total nitrogen and the total number of nitrifiers. Fourteen known and ten unknown monoterpenes were found in soils from under pinyon pine (Pinus edulis Engelm.) and juniper [Juniperus osteosperma (Torr.) Little] canopies. VA mycorrhizal spore numbers were significantly greater under canopies than in interspace soils at both sites, with the lowest number recorded in grazed interspace soils. No differences were found in the degree of colonization of either EM or VA mycorrhizae on pinyon pine or juniper, respectively, between each site. The differences in the spatial distribution of nitrifiers and mycorrhizae emphasize the complexities of the belowground ecosystem in these woodlands despite their apparent aboveground structural simplicity.

Earthworm Aporrectodea trapezoides had no effect on the dispersal of a vesicular-arbuscular mycorrhizal fungi, Glomus intraradices

We investigated the interactions between earthworms and vesicular-arbuscular (VA) mycorrhizal fungi. The effects of earthworms ( Aporrectodea trapezoides) at three densities, on the initiation of mycorrhizal colonization of roots of Trifolium subterraneum L. were examined in Experiment 1. An increasing density of earthworms was associated with a decrease in proportional colonization of the roots by mycorrhizal fungi, with a significant difference between the treatment with no earthworms and that with three earthworms per pot (equivalent to 500 earthworms m −2). Experiment 2 was established to examine the effect of earthworms on the lateral spread of VA mycorrhizal fungi from localized inoculum through a sward of T. subterraneum. A reduction in colonization was again observed at 14 and 21 days (Experiment 2A) or 14 and 28 days (Experiment 2B) but was not evident at a later harvest. The effects of earthworms on the infectivity of an established fungal hyphal network was examined in Experiment 3. The mean proportional colonization was consistently lower in the earthworm treatment compared to the no-earthworm treatment, except in the 1–3-cm section of soil, but none of the differences was statistically significant. We obtained no evidence that earthworms increase the rate of spread of mycorrhizal fungi between plants. Our results indicate that earthworms reduce the infectivity of the soil, with respect to VA mycorrhizal colonization of roots. However, the reduction may only be transient. Possible mechanisms for this effect include grazing on or disturbance of the hyphal network in soil.

Effects of soil compaction on plant growth phosphorus uptake and morphological characteristics of vesicular—arbuscular mycorrhizal colonization of Trifolium subterraneum

SUMMARYWe investigated the effect of soil compaction and phosphorus (P) application on morphological characteristics of mycorrhizal colonization and growth responses, to determine the reasons for reduced responses observed in our previous work with compacted soil. Growth, phosphorus (P) uptake and intensity of vesicular–arbuscular (VA) mycorrhizal colonization were studied in clover plants (Trifolium subterraneum L.) with and without VA mycorrhizal colonization at two P applications and three levels of soil compaction. Phosphorus was supplied either at constant mass concentration (mg P kg−1 soil) or at constant volume concentration (mg P dm−3 soil). Increasing bulk density of the soil from 1·1 to 1.6 Mg m−3 significantly decreased root length and shoot d. wt, but increased the diameter of both main axes and first order lateral roots regardless of P application. Total P uptake and shoot d. wt of clover plants colonized by Glomus intraradices (Schenck & Smith) were significantly greater than those of non‐mycorrhizal plants at all levels of soil compaction and both P applications. However, soil compaction to a bulk density of 1.6 Mg m−3 (penetrometer resistance = 3.5 MPa at a matric potential of − 33 kPa) significantly decreased mycorrhizal growth response. There was no evidence that the increased volume concentration of P at high bulk densities was responsible for the reduced responses. Soil compaction had no significant effect on the fraction of root length containing arbuscules and vesicles, but total root length colonized by arbuscules, vesicles or by any combination of arbuscules, vesicles and intra‐radical hyphae significantly decreased as soil compaction was increased. The air‐filled porosity of highly compacted soil, which varied from 0.07 to 0.11 over the range of matric potentials encountered (− 33 and − 100 kPa), had no significant effect on the intensity of internal colonization.

Early vesicular-arbuscular mycorrhizal colonisation in soil collected from an annual clover-based pasture in a Mediterranean environment: soil temperature and the timing of autumn rains

The results of 2 experiments investigating the early stages of the formation of vesicular- arbuscular (VA) mycorrhizas in response to both soil temperature and the timing of autumn rains are reported for a Mediterranean environment in the south-west of Western Australia. In Expt 1, treatments including an early break, a late break, and a false break followed by a late break were applied to a mixed and sieved field soil collected dry in the summer and placed in pots in a glasshouse. In each break, pots were watered to field capacity and planted with subterranean clover (Trifolium subterraneum) or capeweed (Arctotheca calendula). In early and false breaks, both initiated on the same day in early autumn, the soil temperature was maintained at 30°C, and in the late break, initiated 50 days later in autumn, the soil temperature was maintained at 18°C. In both early and late breaks, pots were watered to field capacity for either 21 or 42 days when plant and mycorrhizal variables were assessed. In a false break, pots were watered to field capacity for 7 days after which the soil was allowed to dry and newly emerged plants died. These pots were then rewatered and replanted at the same time as pots receiving a late break, and subjected to the same soil temperature (18°C). In Expt 2 performed the following year, soil temperature was maintained at 31 or 18°C in both early and late breaks. Pots were planted with clover and watered to field capacity for 21 or 42 days, when plant and mycorrhizal variables were assessed. In Expt 1, VA mycorrhizal colonisation of both clover and capeweed was initially low in an early break compared with levels observed in a late break. Only mycorrhizas formed by Glomus spp. were observed in the early break, whereas mycorrhizas of Glomus, Acaulospora, and Scutellospora spp. and fine endophytes were observed in the late break. Colonisation was decreased by a false break, predominantly because of a decrease in formation of mycorrhizas of Glomus spp. In Expt 2, mycorrhizas of Glomus spp. predominated in warm soil in both early and late breaks and mycorrhizas of Acaulospora spp., Scutellospora spp., and fine endophytes were observed in greater abundance in cool soil in early and late breaks. These experiments indicate that soil temperature at the time of the break will have a large impact on both the overall levels of VA mycorrhizal colonisation of pasture plants and colonisation by different fungi. In addition, fungi that remain quiescent in warm soil may avoid damage in a false break.

Nitrogen fixation and CO2 exchange in soybeans (Glycine max L.) inoculated with mixed cultures of different microorganisms

N2 fixation, photosynthesis of whole plants and yield increases in soybeans inoculated with mixed cultures of Bradyrhizobium japonicum 110 and Pseudomonas fluorescens 20 or P. fluorescens 21 as well as Glomus mosseae were found in pot experiments in gray forest soil carried out in a growth chamber. The effects of pseudomonads and vesicular-arbuscular (VA) mycorrhizal fungus on these parameters were found to be the same. Dual inoculation of soybeans with mixed cultures of microorganisms stimulated nodulation, nitrogenase activity of nodules and enhanced the amount of “biological” nitrogen in plants as determined by the 15N dilution method in comparison to soybeans inoculated with nodule bacteria alone. An increased leaf area in dually infected soybeans was estimated to be the major factor increasing photosynthesis. P. fluorescens and G. mosseae stimulated plant growth, photosynthesis and nodulation probably due to the production of plant growth-promoting substances. Increasing phosphorus fertilizer rates within the range of 5–40 mg P 100 g-1 1:1 (v/v) soil: sand in a greenhouse experiment led to a subsequent improvement in nodulation, and an enhancement of N2 fixation and yield in soybeans dually inoculated with B. japonicum 110 and P. fluorescens 21. These indexes were considerably higher in P-treated plants inoculated with mixed bacterial culture than in plants inoculated with nodule bacteria alone.

Growth Responses to Vesicular-Arbuscular Mycorrhizae and Elevated CO 2 in Seedlings of a Tropical Tree, Beilschmiedia pendula

1. Vesicular-arbuscular (VA) mycorrhizae increased relative growth rates (RGR) of the shade-tolerant tropical tree species Beilschmiedia pendula at both ambient and doubled CO2 concentrations.

Vesicular-arbuscular mycorrhizal fungi in earthworm casts and surrounding soil in relation to soil management of a semi-arid tropical Alfisol

Spores and infective propagules of vesicular-arbuscular (VA) mycorrhiza were examined in earthworm casts and field soils collected from three different soil management treatments (zero tillage with straw amendment, deep tillage without amendment (bare), and zero tillage with previous perennial cropping of Stylosanthes hamata) on an Alfisol in the semi-arid tropics (SAT) of India. The average mean of spore counts and most probable number (MPN) of infective propagules of VA mycorrhiza (VAM) were significantly ( P < 0.05) higher in earthworm casts than in field soil across the three soil management treatments. There was no significant difference in the number of VAM spores or propagules among field soils from the three different soil management treatments, but the number of VAM spores and propagules in the earthworm casts from the deep tillage bare treatment was significantly higher ( P ≤ 0.05) than in the earthworm casts from the other two treatments. In the deep tillage bare treatment, the number of spores and MPN of infective propagules were significantly ( P < 0.05) higher in earthworm casts than in field soil. Therefore, it may be concluded that earthworms can concentrate VA mycorrhizal spores and propagules in their casts.

Mutualism and antagonism in the mycorrhizal symbiosis, with special reference to impacts on plant community structure

Examination of the roots of land plants has revealed the occurrence of mycorrhiza in the majority of species, over 70% of which are hosts to zygomycetous fungi that form vesicular–arbuscular (VA) associations. On the basis of experiments with a small number of host species showing enhancement of growth following colonization, it is widely assumed that wherever mycorrhizas are observed, the symbiosis is of the mutualistic type. The value of definitions based on structural rather than functional attributes is here brought into question by experiments simulating the ecologically realistic circumstance in which seeds germinate in soil in the presence or absence of established VA mycelium. These reveal a spectrum of fungal impacts in which some species respond mutualistically, while others, putative hosts or nonhosts, are antagonised, showing reduction of yield and survivorship and, hence, a loss of fitness relative to plants grown without VA fungi. Antagonised species normally grow in disturbed, open habitats and fail to establish in closed communities. It is hypothesised that their turf incompatibility arises from a sensitivity to interference by VA fungi, which consigns them to ruderal habitats. Mycorrhizal fungi, thus, play a role in defining the ecological niches occupied by plants and in determining of plant community composition. Key words: mycorrhiza, vesicular–arbuscular, mutualism, symbiosis, antagonism, plant community.

A phosphate transporter from the mycorrhizal fungus Glomus versiforme.

Vesicular-arbuscular (VA) mycorrhizal fungi form symbiotic associations with the roots of most terrestrial plants, including many agriculturally important crop species. The fungi colonize the cortex of the root to obtain carbon from their plant host, while assisting the plant with the uptake of phosphate and other mineral nutrients from the soil. This association is beneficial to the plant, because phosphate is essential for plant growth and development, especially during growth under nutrient-limiting conditions. Molecular genetic studies of these fungi and their interaction with plants have been limited owing to the obligate symbiotic nature of the VA fungi, so the molecular mechanisms underlying fungal-mediated uptake and translocation of phosphate from the soil to the plant remain unknown. Here we begin to investigate this process by identifying a complementary DNA that encodes a transmembrane phosphate transporter (GvPT) from Glomus versiforme, a VA mycorrhizal fungus. The function of the protein encoded by GvPT was confirmed by complementation of a yeast phosphate transport mutant. Expression of GvPT was localized to the external hyphae of G. versiforme during mycorrhizal associations, these being the initial site of phosphate uptake from the soil.