Date of Award

5-2010

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Sciences

Advisor

Stellos Tavantzis

Second Committee Member

Benildo G. De los Reyes

Third Committee Member

Seanna Annis

Abstract

Rhizoctonia solani (AG3) is a soil-borne fungal pathogen that causes diseases in numerous plants. This study is focused on the analysis of the role of quinic acid (QA) in regulating the expression of hypovirulence in Rhizoctonia solani AG 3. A 3.6 kb dsRNA (M2) has been shown to be associated with constitutive hypovirulence in R. solani. It was previously shown that the QA pathway is constitutively expressed in M2-containing, hypovirulent R. solani isolates such as the Rhs 1A1. In the virulent M2-lacking isolates such as Rhs 1AP, this pathway is inducible by the addition of QA to the culture media resulting in a dramatic reduction of virulence and induction of synthesis of the M2-encoded polypeptide pA. It appears that the effect of QA on Rhs 1AP is regulatory in nature because QA suppresses the dramatic virulence enhancement mediated by chorismate, the end-product of the shikimic acid pathway. A cDNA subtraction strategy was used to identify and characterize QA-induced genes in the once virulent Rhs 1AP transformed into hypovirulent Rhs 1AP by QA-induction in sprouted potato tubers. The data show that upon QA-induction, a major shift in gene expression occurred reflecting a switch in carbon source despite the presence of the host plant. QA-induction appears to interfere with the general metabolic process bringing about changes in nitrogen and carbon metabolism, protein recycling and energy metabolism. In addition, expression of genes involved in the glyoxylate pathway and those involved in oxalate degradation was enhanced by QA-induction in R. solani. Comparison of the expression of R. solani pathogenesis-related genes in the rice-AG 1 system and Rhs 1AP under QA-induction showed either no expression or reduced expression levels. Putative quta and qutd genes of the QA utilization (QUT) pathway identified in QA-induced EST library in R. solani were characterized. Two other QA pathway genes, qutb and qutdl were also identified in silico. Genome walking in the genomic region surrounding putative quta and qutd showed that in contrast to ascomycetes, the QA utilization pathway genes in R. solani AG-3 are not clustered.

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