Stephen E. Williams

Population variation in the mycorrhizal fungus Glomus mosseae: uniform garden experiments.

A series of experiments was conducted in the field, greenhouse, and laboratory to investigate population variation among three geographic isolates of the widely distributed vesicular-arbuscular mycorrhizal fungus Glomus mosseae. Uniform garden experiments were used to determine whether these populations from dissimilar environments are physiologically different ecotypes or phenotypic variants. Using Melilotus officinalis as host, comparisons were made of the mycorrhizas formed, spore production, spore germination, propagule density, and the influence of the endophytes on host growth and physiology. Large disparities were observed in the responses of the three fungal populations and in their effects under uniform environmental conditions. Analysis of variance indicated highly significant differences among the G. mosseae populations in terms of amounts of mycorrhizas formed, spore production and their effect on biomass production, shoot phosphorus concentration and water relations of M. officinalis. These results support the hypothesis that populations of G. mosseae from dissimilar environments are genetically different races or ecotypes and suggest that there may be significant genetic and physiological diversity within this morphologically defined taxon.

Oil shale process water affects activity of vesicular-arbuscular fungi and rhizobium 4 years after application to soil

The effects of different concentrations of water from oil shale processing on the vesicular-arbuscular (VA) fungal and Rhizobium activity of an arid land soil were investigated 4 yr after contamination. Effects were assessed in field plots and by a greenhouse bioassay. Addition of process water had marked effects on the chemistry of the soil and increased concentrations of Ca, Mg, Na, NO3 and NH4 as well as raising the electrical conductivity. VA infection analysis of roots from field plots and the bioassay combined with counts of VA fungal spores indicate reduced mycorrhizal activity in treated soils. The individual species of VA fungi were found to be affected differently by the process water. Roots of the legume yellow sweetclover developed fewer nodules in soils treated with retort water. An acetylene reduction assay indicated that nitrogenase activity was reduced in nodules from soils treated with undiluted process water.

Long-Term Reestablishment of Arbuscular Mycorrhizal Fungi in a Drastically Disturbed Semiarid Surface Mine Soil

Reestablishment of arbuscular mycorrhizal (AM) fungi in severely disturbed soil on a surface mine reclamation site inoculated with native endophytes in 1983 was examined in 1997. At that earlier time (1983) research had been initiated on a newly reclaimed site to test the hypothesis that native AM fungi would form effec tive mycorrhizae and would improve growth of W yoming big sagebrush (Artemisia tridentata ssp. wyomingensis) in severely disturbed reclamation soil. Data collected during 1984 and 1985 did not support that hypothesis and it was concluded that native AM fungi inoculated onto the site were not tolerant of the altered soil environmental conditions in the reclamation soil. Reexamination of the site in 1997 indicated that soil environmental conditions were becoming more similar to those before disturbance and AM fungal populations were recovering. Soil analyses in 1997 show an approximate 1 % increase in soil organic matter content, improvement in soil structure compared to 1983, and decreases ...

Renodulation and characterization of Rhizobium isolates from cicer milkvetch (Astragalus cicer L.)

Abstract In 1993 and 1994, 12 bacterial isolates were isolated from root nodules of cicer milkvetch (Astragalus cicer). In the tests for nodulation of A. cicer by these bacterial isolates, five were found to form hypertrophic structures, while only two formed true nodules. These true nodules were formed in a sterilized soil system. This system might be able to act as a DNA donor to provide residual DNA to other microbes in the soil. The rhizobial isolates were thought to have lost genetic material crucial to nodulation during the isolation process. This hypothesis was supported by an experiment in which isolate B2 was able to nodulate A. cicer in vermiculite culture after being mixed with heat-killed rhizobia, Rhizobium leguminosarum bv. trifolii and R. loti. The nodulation would not occur in vermiculite culture system without the heat-killed rhizobia. Based on the biochemical data, the B2 and 9462L, which formed true nodules with A. cicer, were closely related. The rhizobia type cultures that nodulate A. cicer include Bradyrhizobiumjaponicum, Rhizobium leguminosarum bv. trifolii, R. leguminosarum bv. viceae, and R. loti. All of these rhizobia were from different cross-inoculation groups. The B2 and 9462L isolates could only nodulate Medicago sativa, Phaseolus vulgaris, and Melilotus officinalis, but not these species within the genus from which they were isolated: Astragalus. The traditional cross-inoculation group concept obviously does not fit well in the classification of rhizobia associated with Astragalus. The rhizobia isolated from A. cicer can be quite different, and the rhizobia able to renodulate A. cicer also quite diverse.

The arbuscular mycorrhizal fungi colonising roots and root nodules of New Zealand kauri Agathis australis

As the only endemic member in New Zealand of the ancient conifer family, Araucariaceae, Agathis australis is an ideal species to study putatively long-evolved mycorrhizal symbioses. However, little is known about A. australis root and nodular arbuscular mycorrhizal fungi (AMF), and how mycorrhizal colonisation occurs. We used light, scanning and transmission electron microscopy to characterise colonisation, and 454-sequencing to identify the AMF associated with A. australis roots and nodules. We interpreted the results in terms of the edaphic characteristics of the A. australis-influenced ecosystem. Representatives of five families of Glomeromycota were identified via high-throughput pyrosequencing. Imaging studies showed that there is abundant, but not ubiquitous, colonisation of nodules, which suggests that nodules are mostly colonised by horizontal transmission. Roots were also found to harbour AMF. This study is the first to demonstrate the multiple Glomeromycota lineages associated with A. australis including some that may not have been previously detected.