Gabor J. Bethlenfalvay

Bacteria from rhizosphere and hyphosphere soils of different arbuscular-mycorrhizal fungi

Effects of roots and of arbuscular-mycorrhizal (AM) fungi on the composition of soil bacterial colonies and the combined effects of AM fungus-rhizobacterium associations on plant and soil development are little-known. We grew sorghum (Sorghum bicolor L.) either nonsymbiotically or colonized by one of two isolates of the AM fungi Glomus etunicatum, Glomus intraradices, or Glomus mosseae. The isolates were either exotic or native to the test soil. Soils adhering (rhizosphere) or not adhering (hyphosphere) to the roots were sampled 45 days after planting. Total populations of bacteria were estimated by counting colony-forming units on a nonselective medium and grouped by colony and cell morphology. Rhizosphere populations of fluorescent pseudomonads were determined on P1 medium. Visually distinct isolates were selected for identification by Fatty-Acid-Methyl-Esther analysis; of these 25 were found to be separate species. Bacterial numbers were greater in rhizo- than in hyphosphere soil. Isolates of Bacillus and t Arthrobacter were most frequent in hyphosphere and Pseudomonas in rhizosphere soils. More bacterial species were encountered in hyphosphere than in rhizosphere soil, and bacterial communities varied within and among AM treatments. The development of the AM mycelium in soil had little influence on the composition of the microflora in the hyphosphere, while AM root colonization was positively related with bacterial numbers in the hyphosphere and with the presence of Pseudomonas in the rhizosphere. The results suggest that qualitative effects of the AM fungal taxon on the hyphosphere, such as the nature of exudates, are more important to composition and proliferation of rhizobacteria than the quantitative development of AM soil mycelia.

Mycorrhizal fungi influence plant and soil functions and interactions

Potted soybean (Glycine max (L.) Merr.) plants were grown in P-fertilized (+P) or low-P soil (-P), or colonized in -P soil by one of the arbuscular mycorrhizal (AM) fungi Glomus etunicatum (Ge), Glomus mosseae (Gm), or Gigaspora rosea (Gr). Treatment effects on plant development, on the soil microflora, and on the status of water-stable soil aggregates (WSA) were evaluated for all 5 treatments or for the 3 AM treatments alone. Dry weights of the AM plants, as a group, were half-way between the dry weights of the +P and -P plants, but within the AM group, Gm plants had the highest pod dry weights and pod/stem and root/stem ratios and the lowest specific root lengths, while Ge plants had high stem dry weights and were highly nodulated. High reproductive development and coarse roots in the Gm plants were associated with the most extensive growth of AM soil hyphae (km pot-1: Gm, 20; Gr, 12; Ge, 8), while nodulation was inversely related with AM-colonized root length. The soils colonized by AM fungi had significantly higher levels of WSA (size classes 1 to 2 and 2 to 4 mm), and within the larger size class, Gm soils had the highest percentage of WSA. Proliferation (plate counts) of Gram positive (G+) and Gram negative (G-) bacteria, Arthrobacter sp. (G+), and Pseudomonas sp. (G-) was greatest in the -P soils, but the bacterial populations of the +P and the AM soils were generally not significantly different. There were, however, differences among the AM treatments, where Gm soils had the lowest G- bacterial populations, while Ge soils had the highest populations of both G+ and G- bacteria. Correlations between plant and soil traits indicated that interactions within the plant-soil system were mediated by the AM fungi.

Soil aggregation status and rhizobacteria in the mycorrhizosphere

Soil aggregation is a dynamic process in which plants and the soil microbiota play a major role. This experiment was conducted to determine whether the effects of mycorrhizae on the stability of water-stable soil aggregates (WSA) and on selected groups of soil microorganisms are interrelated. Soil containers consisting of four compartments were utilized. Two compartments on each side of a solid barrier were separated by a 43 μm screen that permitted the passage of hyphae, but not of roots. The roots of Sorghum bicolor plants were split over the center barrier, and the roots on one side were inoculated with an arbuscular-mycorrhizal (AM) fungus. This design produced mycorrhizosphere soils (M) by AM roots or hyphosphere (H) soils by AM hyphae in the two compartments on the one side of the barrier, and rhizosphere soils (R) by nonAM roots or root- and hypha-free bulk soil (S) in the two compartments on the other side. At harvest (10 wk), there were significant differences in WSA between soils in the order: M>R>H>S, and WSA stability was significantly correlated with root or hyphal length. Numbers of colony-forming units of the microflora (total bacteria, actinomycetes, anaerobes, P solubilizers, and nonAM fungi) were in general not correlated with root or hyphal length, but in some cases were significantly correlated with WSA. Bacteria isolated from the water-stable soil-aggregate fraction tended to be more numerous than from the unstable fraction. The difference was significant in the M soil for total bacteria and P solubilizing bacteria. NonAM fungi were more numerous in the unstable fraction of the M soil. The data show that the root and fungal components of mycorrhizae enhance WSA stability individually and additively in concert, and suggest that they affect microorganism numbers indirectly by providing a favorable and protective habitat through the creation of habitable pore space in the WSA.

Bacterial associations with the mycorrhizosphere and hyphosphere of the arbuscular mycorrhizal fungus Glomus mosseae

Roots and mycorrhizal fungi may not associate with soil bacteria randomly, but rather in a hierarchical structure of mutual preferences. Elucidation of such structures would facilitate the management of the soil biota to enhance the stability of the plant-soil system. We conducted an experiment utilizing two isolates of soil bacteria to determine their persistence in distinct mycorrhizal regions of the root zone, and their effects on general rhizosphere populations of fluorescent pseudomonads (FP). Split-root sorghum (Sorghum bicolor L.) plants were grown in four-compartment containers, constructed so that the soils in individual compartments held either (1) roots colonized by the arbuscular-mycorrhizal (AM) fungus Glomus mosseae (M), (2) nonAM roots only (R), (3) hyphae of G. mosseae (H), or (4) no mycorrhizal structures (S). The soils were inoculated (107 cells g-1 dry soil) with antibiotic-resistant (rifampicin, rif; streptomycin, sm) strains of the soil bacteria, Alcaligenes eutrophus (rifr50) or Arthrobacter globiformis (smr250), or were left uninoculated as control. A. eutrophus had been isolated from a specific source (hyphosphere soil of G. mosseae), and A. globiformis from mycorrhizosphere soils of two AM fungi. After 10 wk of growth, the presence of A. eutrophus was barely detectable (<10 cfu g-1 dry soil) in nonAM (R and S) soils, but persisted well (104 cfu g-1 dry soil) in AM (H and M) soils. Numbers of A. globiformis were more evenly distributed between all soils, but were highest in the presence of AM roots (M soil). There were varied bacterial effects on root and AM-hyphal development: A. eutrophus decreased hyphal length in H soil, while A. globiformis stimulated root length in M soil. The two bacterial inoculants did not affect numbers of FP in H, R, and M soils, but the AM status of the soils did: the numbers of FP increased in the order M>R>H>S. There was a positive correlation of FP numbers with both bacterial inoculants in M and H soils. Numbers of FP changed with root or hyphal lengths, an effect that was related to changes in the numbers of the inoculated bacteria. The results indicate that the hyphosphere-specific A. eutrophus depended on the presence of G. mosseae, but that the nonspecific A. globiformis did not. The mycorrhizal status of soils may selectively influence persistence of bacterial inoculants as well as affecting the numbers of other native bacteria.

Mycorrhizal Interactions in Sustainable Agriculture

AbstractVesicular-arbuscular mycorrhizal (VAM) fungi are an intimate link between the roots of most crop plants and soils, thereby affecting the development of host plants and host soils. The role of VAM fungi in improving plant nutrition and their interactions with other soil biota have been investigated with reference to host plant growth, but little is known about how these interactions affect soil structure. The impact of cultural practices and the particular role that VAM fungi play in improving soil structure are discussed in the context of sustainable farming.

Measurement of the extraradical mycelium of a vesicular-arbuscular mycorrhizal fungus in soil by chitin determination

SummaryDevelopment of a vesicular-arbuscular mycorrhizal (VAM) fungus in association with soybean was determined in a greenhouse soil mix by chitin assay. Samples were sieved to eliminate hexosamine-containing contaminants. This preparation reduced the interference caused by extraneous soil substances and permitted quantitative measurement of extraradical VAM fungal mycelium in the soil mix by colorimetric assay. Recovery of added chitin, used as an internal standard, was greater in the soil mix than in an inert medium indicating that some hexosamine was stabalized from chemical degradation by other soil components.

Mycotrophic growth and mutualistic development of host plant and fungal endophyte in an endomycorrhizal symbiosis

SummarySoybean plants colonized by the vesicular-arbuscular mycorrhizal (VAM) fungusGlomus fasciculatum were grown in pot cultures utilizing a composite greenhouse rooting medium. Development of fungal mycelia inside and outside the host root and total fungal biomass were determined from assays of fungal chitin. Growth and phosphorus uptake by VAM plants and uncolonized controls were compared. Mycotrophic growth in VAM plants occurred during the final six weeks of the 19-week growth period, when the concentration of available soil P fell below 10 μg P/g soil. Growth enhancement was most pronounced in the reproductive organs. The data suggest a relationship between the initiation of the reproductive phase in the host and the cessation of growth in the endophyte. Source-sink relationships and P availability appear to be factors influencing interactions between the symbionts.

Nutrition of sorghum plants fertilized with nitrogen or inoculated withAzospirillum brasilense

SummarySorghum plants were inoculated withAzospirillum brasilense or received an N-amended nutrient solution. Azospirillum inoculation increased plant dry weight and nitrogen assimilation by 25%. Most plant growth responses to Azospirillum were comparable to application of 2.0 mM N. Increased scavenging of nutrients, altered root permeability or nitrogen fixation are possible explanations for these effects.

Plant and soil response to single and mixed species of arbuscular mycorrhizal fungi under fungicide stress

Abstract Defining sustainable agricultural practices requires an understanding of both above- and below-ground consequences to management strategies. While alternatives to biocides are sought for the control of weeds, insects and pathogens, biocide use will continue with the goal of reducing quantities used in an integrated approach. The impact of three fungicides on plant growth, seed yield, seed nutrient composition, and on soil aggregation as mediated by arbuscular mycorrhizal (AM) fungi was studied in a silty-clay loam soil with a high extractable P concentration. Shoot dry mass, seed yield and seed nutrient (N, P, K) contents of pea (Pisum sativum L.) plants were enhanced by three AM fungi and a mixture of three fungi compared with nonAM plants. Each AM fungus produced a distinct pattern of shoot responses of plants, but the mixed inoculum treatment was as good or better than each single species for all of the above-ground measures of plant performance. Soil aggregation was improved by two of the three AM fungi as individual inocula, and was further increased in the mixed inoculum treatment. Two of the three fungicides reduced shoot dry mass and seed yield, but none of the fungicides affected soil aggregation. Fungicides inhibited mycorrhiza formation least in the mixed inoculum treatment which gave the best overall plant and soil responses. Since the three fungi together were more tolerant of fungicides than each fungus alone, it appeared that as a community, AM fungi modified and alleviated fungicide stress, resulting in high levels of plant performance and soil aggregation.

Vesicular-arbuscular mycorrhizal fungi in nitrogen-fixing legumes : problems and prospects

Publisher Summary This chapter discusses the problems and prospects of vesicular-arbuscular mycorrhizal fungi in nitrogen-fixing legumes. A number of methods are available to determine both the internal and external portion of vesicular-arbuscular mycorrhizal fungal mycelium. All of these methods involve estimates, to some extent. Recently, techniques are developed to estimate also the viability of the external (extraradical) hyphae of vesicular-arbuscular mycorrhizal fungi, adding little to the cumbersomeness, but a great deal to the precision and validity of mycorrhizal work. The difficulty of quantifying vesicular-arbuscular mycorrhizal soil hyphae generally prevents workers from including statements in their reports on this important organ of the fungus-root, even though the tenuous nature of the relationship between root colonization and the growth effect is well-known. The chapter also describes parasitism. Growth depression in vesicular-arbuscular mycorrhizal plants is different from pathogenic parasitism, in that neither irreversible tissue damage nor necrosis is observed as a result of vesicular-arbuscular mycorrhizal colonization. In the vesicular-arbuscular mycorrhizal association, where the host plant and its fungal endophyte live together in an intimate, balanced relationship, symptoms of symbiotic (non-pathogenic) parasitism occur when the balance is disturbed.