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Dissertation/Thesis Abstract

The effect of conditionally dispensable chromosomes of Nectria haematococca MPVI on rhizosphere colonization and the identification of a gene cluster for homoserine utilization
by White, Gerard Joseph, Ph.D., The University of Arizona, 2008, 181; 3325315
Abstract (Summary)

The habitat diversity of the fungus Nectria haematococca MPVI has been shown to be due in part to conditionally dispensable (CD) chromosomes that carry habitat-defining genes. From a biological perspective, the CD chromosomes are analogous to plasmids that possess genes that determine the habitats of plant-associated bacteria. This study establishes that the N. haematococca CD chromosome that contains the genes for Pea Pathogenicity ( PEP cluster) also carries genes for the utilization of homoserine, an amino acid found in pea root exudates. Competition studies presented here demonstrate that an isolate that lacks the PEP cluster, but carries a portion of the CD chromosome containing the homoserine utilization (HUT) genes, is more competitive in the pea rhizosphere than an isolate without the CD chromosome.

Further competition studies show that both the PDA1 and PDA6 CD chromosomes confer a competitive advantage in the rhizosphere of soybean, whereas only the PDA6CD chromosome confers a competitive advantage in the rhizospheres of tomato and alfalfa, and only the PDA1 CD chromosome confers a competitive advantage in the rhizosphere of pea. These studies suggest the presence of genes on the PDA6 and PDA1 CD chromosomes that enhance the ability of N. haematococca to expand its habitat and support the idea that fungal CD chromosomes are analogous to host-specifying plasmids in plant-associated bacteria.

Transformation, insertional mutagenesis, and bioinformatics were used to identify a cluster of five genes on the PDA1 CD chromosome that was responsible for the HUT phenotype in N. haematococca. One of the genes was found only in N. haematococca, another was a fungal transcription factor, and the other three had homologs involved in the synthesis of the amino acids methionine, threonine, and isoleucine, in which homoserine is an intermediate. Competition experiments that compared isolates with or without the HUT cluster showed that the HUT cluster is responsible for increased competitive ability of HUT + N. haematococca isolates in the rhizosphere of pea. This study establishes that homoserine utilization can be a rhizosphere competency trait for N. haematococca and, to our knowledge, is the first example of a rhizosphere competency trait identified in a fungus.

Indexing (document details)
Advisor: VanEtten, Hans D.
Commitee: Hawes, Martha C., Olsen, Mary W., Orbach, Marc J., Rensing, Christopher
School: The University of Arizona
Department: Plant Pathology
School Location: United States -- Arizona
Source: DAI-B 69/08, Dissertation Abstracts International
Subjects: Ecology, Microbiology, Plant pathology
Keywords: Chromosomes, Competition, Fungi, Gene clusters, Homoserine, PCR, Plant-pathogen interactions, Rhizosphere
Publication Number: 3325315
ISBN: 978-0-549-79351-9
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