Date of Award:

5-2008

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Committee Chair(s)

Bradley R. Kropp

Committee

Bradley R. Kropp

Committee

Daryll B. DeWald

Committee

Anne J. Anderson

Committee

Yajun Wu

Committee

Kent Evans

Abstract

The kinetics and amplitude of the salicylic acid-responsive pathogenesis-related (PR) genes and the cytochrome P450 gene ItCYP79B2 in the compatible interaction between Puccinia thlaspeos and dyer’s woad (Isatis tinctoria) during the first 72 hours of inoculation were examined. Immediately following penetration of the host by the rust pathogen, there was a modest up-regulation of PR genes but a significant down-regulation of ItCYP79B2 expression. During haustoria formation, a significant pathogen-mediated suppression of PR genes was observed with a corresponding up-regulation of ItCYP79B2. This potentially facilitates haustoria formation by P. thlaspeos. After haustoria formation, a more significant up-regulation of PR genes was observed that was followed by a second pathogen-induced suppression of both PR and ItCYP79B2 genes. This suppression of defense responses by the pathogen was sustained and was potentially responsible for successful infections in dyer’s woad.

Exogenous application of salicylic acid was done to experimentally trigger the expression of defense-related genes in dyer’s woad. In treatments not involving the pathogen, exogenous application of salicylic acid led to rapid activation of defense responses. In treatments involving both salicylic acid and the rust pathogen, a differential response was observed based on the timing of salicylic acid application. During the pre-haustorial and post-haustorial phases, the up-regulation of defense genes by salicylic acid application increased protection against rust. However, salicylic acid application during the haustorial formation could not override the pathogen-mediated suppression of defense responses. Suppression of pathogen-induced defense responses during and after haustoria formation is postulated to be vital in the establishment of biotrophy in this system.

In addition, the effects of varying levels of a number of different abiotic stresses (oxidative, salt, osmotic, dehydration, and cold stresses) on the host during the infection process were examined. While moderate-to-severe levels of salinity, osmotic, dehydration, and cold stress did not decrease infection, mild abiotic stress appears to help dyer’s woad develop cross-tolerance to the rust pathogen, thereby affecting its efficacy as a biocontrol agent.

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