GENOMIC CHARACTERIZATION OF QUINIC ACID-INDUCED RESPONSES AND THEIR POTENTIAL ROLES IN THE EXPRESSION OF HYPOVIRULENCE IN RHIZOCTONIA SOLANI RHS 1AP

 GENOMIC CHARACTERIZATION OF QUINIC ACID-INDUCED RESPONSES AND THEIR POTENTIAL ROLES IN THE EXPRESSION OF HYPOVIRULENCE IN RHIZOCTONIA SOLANI RHS 1AP

By Harish Manmathan

Thesis Advisor: Dr. Stellos Tavantzis

A lay Abstract describing the work Presented

in Partial Fulfillment of the Requirements  for the

Degree of Doctor of Philosophy

(in Biological Science)

May, 2010

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 the potato pathogen R. solani. Hypovirulence is a condition in which a pathogen has reduced capacity to cause a disease. A DNA called M2 has been shown to be associated with hypovirulence in R. solani. It was previously shown that the QA pathway was active all the time in M2-containing, hypovirulent R. solani isolates. External application QA not only induces the QA pathway but also leads to dramatic reduction of virulence and expression of M2 DNA. It appears that QA has a bigger role in pathogenesis as it neutralized the effects of externally added chemicals known to increase the virulence. A cDNA subtraction strategy was used to identify and characterize QA-induced genes in the once virulent fungal isolates transformed into hypovirulent 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 potato 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 system and potato system under QA-induction showed either no expression or reduced expression levels. Potential genes (Transporter gene and activator gene) of the QA pathway were identified in QA-induced genetic library in R. solani. Two other potential QA pathway genes, qutb and qutd1 were also identified though bioinformatics. Genome walking in the DNA surrounding putative transporter gene and activator gene showed that in contrast to ascomycetes, a group of well studied fungi, the QA utilization pathway genes in R. solani AG-3 are not clustered. This study provided biochemical insights into the QA induced reduction in virulence of potato fungal pathogen R. solani.