Date of Award

8-2010

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Botany and Plant Pathology

Advisor

Stellos Tavantzis

Second Committee Member

Benildo G. de los Reyes

Third Committee Member

Keith Hutchison

Abstract

Rhizoctonia solani, teleomorph Thanatephoris cucumeris, is a nectrotrophic plant pathogen of the Basidiomycete order that is split into fourteen different anastomosis groups (AGs) based on hyphal interactions. Due to its extensive host range, this ubiquitous pathogen has commonly been described as 'infecting everything green.' Many of the plants that fall prey to Rhizoctonia-induced disease are of anthropogenic importance for agricultural, economic, utilitarian, and aesthetic reasons. Chief amongst these are rice (Oryza sativa) and potato (Solarium tuberosum), which represent the first and fourth most important crops for human consumption grown worldwide, respectively. Sheath blight of rice caused by R. solani AGl-IA is the second leading cause of yield loss in most major rice growing areas worldwide. Similarly, the R. solani AG3-induced diseases stem canker and black scurf of potato can cause significant reductions in yield and quality, as well as interfere with seed certification. Currently, little is known about the methods by which R. solani infects its multitude of hosts, particularly at the molecular level. Such studies have been performed in a number of different host/pathogen systems and have provided valuable insight as to the genes and their products involved in pathogenesis. This has improved understanding of the means by which these pathogens infect their hosts and enabled the development of highly effective methods for disease control and prevention in these pathosytems. To this end, it was the purpose of the current investigation to determine factors potentially involved in R. solani pathogenesis and evaluate their expression during the early stages of infection. A comparative study of the rice and potato pathosystems was undertaken to allow for the identification of genes important for R. solani pathogenesis in general and not specific to a particular host. Factors potentially involved in the interaction between R. solani and its hosts were determined by mining for sequences of pathogen origin in a library of rice tissue infected with the sheath blight pathogen, AG1-IA. Comparison of these sequences against the recently released R. solani AG3 genome allowed for the identification of genes with significant homology between the two AGs. This collection was subsequently narrowed down by review of the currently available literature to include only those elements most likely to be involved in pathogenesis. Ten of these genes common to both AGs and two specific to AG1-IA were selected for expression analysis by quantitative real-time PCR. The results of this research indicate that a number of genes are similarly expressed by AG1 and AG3 during the early stages of pathogenesis in their respective hosts. Grouping of these virulence factors based on relatedness of expression profiles suggests three key events are involved in the R.solani pathogenesis process: early host contact and infiltration, adjustment to the host environment, and disease proliferation through necrosis. Further studies, including knock-out and overexpression analyses, of the pathogenesis-associated genes identified in this project will enable more precise elucidation of the molecular mechanisms that allow for the wide-spread success of R. solani as a phytopathogen and allow for more targeted, effective methods of control.

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