Bibliography: leaves 146-170. / xix, 170 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--Dept. of Soil and Water, Waite Agricultural Research Institute, University of Adelaide, 1994

Includes bibliographical references.

Copies of author's previously published articles inserted. Bibliography: leaves 184-197.

Bibliography: leaves 71-79. Almost 80 percent of plant taxa develop vesicular-arbuscular mycorrhizae (VAM) which are symbiotic associations between plant roots and soil fungi. The fungi are biotropic-obligate symbionts. Identification of VAM fungi is currently based on spore characteristics. Molecular techniques provide tools for better and more accurate identification of species, as well as for the examination of genetic variability occuring between individual spores of a single species.

Bibliography: leaves 153-185. Vesicular-arbuscular (VA) mycorrhiza formation is a complex process which is under the genetic control of both plant and fungus. This project aims to develop a model infection system in Hordeum vulgare L. (barley) suitable for molecular analysis; to identify host plant genes differentially expressed during the early stages of the infection process; and to screen a mutant barley population for phenotypes which form abnormal mycorrhizas.

Graduation date: 1991

Graduation date: 1991

Arbuscular mycorrhizal (AM) fungi have been living in association with land plants for at least 400 million years. Because they are obligate symbionts, the study of AM symbiosis has focused primarily on its plant host and progress in the molecular biology of AM fungi has been very slow. Using two different approaches, library screening and direct PCR-based assays, genetic information of AM fungi was compared across isolates and species. This allowed the study of novel DNA regions previously unexplored in AM fungi. The following species were investigated: Glomus intraradices, Glomus mosseae, Gigaspora margarita, Scutellospora calospora, Acaulospora scrobiculata and Entrophosphora colombiana. In the first approach, using in vitro grown G. intraradices, a DNA extraction protocol was developed for the construction of a partial genomic library. This library was screened for the presence of microsatellite-containing loci. PCR primers were designed based on five identified loci. Two of these loci were monomorphic for all isolates and species. The second approach used a combination of degenerate and specific primers for fungal chitin synthase genes to explore the variability of this gene family in AM fungi. A total of 21 AM sequences were isolated and sequenced, covering class I and II chitin synthases. RT-PCR with G. intraradices revealed differential expression of chitin synthases in spores and mycelium, as compared to mycorrhized roots. In addition, using primers designed from a highly conserved sequence for plant resistance genes, classical PCR and RT-PCR allowed the detection of a genomic sequence and its cDNA counterpart encoding a putative serine/lysine rich protein in G. intraradices. We have thus investigated genetic variability in AM fungi in functional genes as well as in repetitive DNA regions. Study of gene expression was also possible using in-vitro grown G. intraradices.

N-transfer from legume to grass when the two were intercropped in the field and the mechanisms of this transfer were studied. Studies involving either alfalfa-grasses or soybean-corn intercrops, were undertaken. Mycorrhizal and P-supplemented (to compensate for the lack of mycorrhizae) intercrops were compared. In these studies, the legume component of intercrops was labelled with $ sp{15}$N and any excess of the label was looked for in the associated grass plants. / There was no reversal of N transport at the legume-fungus interface. N-transfer from legume to grass must therefore, proceed via excretion of N by legume roots and subsequent uptake by grass roots. Soil microorganisms and proximity of plant root systems are important factors affecting N-transfer. Mycorrhizae enhance the transfer by increasing the absorptive efficiency of the receiver plants. / Mycorrhizal fungi enhance the competitiveness of the most mycotrophic component of the mixtures by either improving P uptake or the general nutrient balance of the plant. Mycorrhizal inoculation can decrease the level of P competition between corn and soybean by increasing the availability of P. / The observation that mycorrhizal plants differ in many regards from P-supplemented plants, emphasize the generally poor comprehension of the mycorrhizal symbiosis.

There has been little research into the compatibility of commonly utilized apple rootstocks and VA-fungal types, and even less research regarding the effects of herbicides used in orchards, on the VAM symbiosis of apple trees. Studies demonstrated that early inoculation of young apple plants, prior to transplanting, results in healthy and vigorous plants with better growth and nutrition than uninoculated plants. We studied the efficiency of VA-fungal species and isolates on young apple rootstocks, produced by in vitro propagation. Mycorrhizal inoculation promoted plant growth, dry mass production and leaf P concentration. Mycorrhizal efficiency was associated with larger external hyphal network but showed no relation with the internal colonization. Despite the high P-fertility of the soil used, growth enhancement due to mycorrhizal inoculation was attributed to an improved P nutrition. / In a second experiment, the effect of herbicides currently used in orchards was tested on the mycorrhizal symbiosis. Paraquat, simazine and dichlobenil were applied to soil with mycorrhizal and non-mycorrhizal apple plants. Mycorrhizae increased herbicide toxicity in apple, as demonstrated by the greatly increased plant mortality. While both paraquat and simazine decreased spore germination in vitro, none of the herbicide treatments affected root colonization in soil. Effects on the photosynthetic rate, measured after herbicide application, indicated a physiological interaction between mycorrhizal colonization and dichlobenil, involved in the toxic response of apple plants.