Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biology

Supervisor

Dr. Mark A. Bernards

Abstract

Ginsenosides, the triterpenoid saponins produced by American ginseng (Panax quinquefolius L.), have been extensively studied for their medicinal value, however, their function in the rhizosphere remains largely unknown. Like other saponins, ginsenosides possess mild fungitoxic activity toward some common ginseng pathogens. However, numerous oomycete root pathogens of ginseng, most notably Pythium irregulare Buisman, are able to partially deglycosylate the 20 (S)-protopanaxadiol ginsenosides Rb1, Rd and gypenoside XVII via extracellular glycosidases (ginsenosidases), leading to the formation of a common product, ginsenoside F2. In this thesis the potential role(s) of these extracellular ginsenosidases and the ginsenoside products they produce (ie. ginsenoside F2) in the ginseng-P. irregulare pathosystem are examined. Treatment of the roots of ginseng plants with a relatively high dose of ginsenosides results in a delay of infection by P. irregulare, as monitored in vivo by a non-invasive chlorophyll fluorescence imaging method adapted to track the progression of a root pathogen in perennial plants. Furthermore, there is a correlation between the virulence of P. irregulare on ginseng and ginsenosidase activity and the presence of ginsenosides may act as a trigger for the production of these specific ginsenosidases, although the mechanism regulating this is unknown. Previously it was hypothesized that ginsenoside F2 acts as the main chemoattractant/growth stimulator in P. irregulare, however, data obtained showed in vitro inhibition of P. irregulare growth caused by ginsenoside F2, which was overcome with time. This has cast doubt on the initial hypothesis however, it is possible that the inhibition in growth may be overcome through a physiological/structural change in P. irregulare signalled by exposure to ginsenoside F2 which may be related to signalling and increased pathogenicity. This suggests that ginsenosides may serve to alter the growth and metabolism of P. irregulare which may serve to facilitate infection. In conjunction with the above work, amino-acid sequence data of the purified ginsenosidases was used to clone and sequence a putative ß-glucosidase designated as PiGH1-x from P. irregulare which in silico analysis suggests may encode previously isolated ß-1,2 glycosidases from P. irregulare. This will allow for future studies of the molecular triggers that that could be involved in facilitating the infection of ginseng by P. irregulare.


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