Date of Award

Summer 8-30-2017

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ecology and Environmental Sciences

Advisor

Jianjun Hao

Second Committee Member

Gregory Porter

Third Committee Member

Seanna Annis

Additional Committee Members

Claudia Goyer

Tsutomu Ohno

Abstract

Phytophthora erythroseptica is the causal agent of potato pink rot, which can enter roots, stolons and stem bases of potato plants to initiate disease. Phytophthora erythroseptica produces sporangia in asexual reproduction. Although sporangia can germinate directly and infect host plants, the majority of infection is initiated by zoospores produced from sporangia. In this study, zoosporic germination and host infection of P. erythroseptica were found to respond to the extracellular products in zoospore exudates (ZE) derived from highly concentrated zoospores (> 103 ml-1). Zoospores did not germinate under low concentration until they reached a threshold (103 ml-1). This quorum sensing phenomenon indicated that the density dependency was associated with chemical concentration produced by zoospores. Concentrated ZE can induce zoosporic germination and host infection even when the density of the population was low. Thus, ZE regulated communal behaviors of zoospores and host infection. The induction for zoosporic germination by ZE was not affected when ZE was treated at temperatures between -20 and 121ºC. High performance liquid chromatography (HPLC) and liquid chromatography-mass spectrometry (LC-MS) were conducted in analyzing the signal chemical in ZE. Several chromatographic peaks were detected in HPLC with relatively high absorption at 230 um UV light. The residua at 12 to 14 min had the highest activity in inducing zoosporic germination. LC-MS detected several amino acids in the 12 to 14 min residue, including phenylalanine, and isoleucine or leucine. The germination assay determined that isoleucine and leucine induced zoosporic germination. The presence of the signal molecules or homologs has been detected not only in P. erythroseptica, but also in other organisms, such as potato (host) and soil microbes. Root exudates of potato induced zoosporic germination, possibly by providing signal molecules that regulated the zoosporic behavior. Potato varieties susceptible to pink rot tended to produce signal molecules, and tolerant varieties barely did so. Many microorganisms, including fungi, oomycetes, and bacteria produced extracellular compounds that had similar activity as ZE, which provided knowledge on the ecological perspectives of P. erythroseptica. Microbial communities in the root areas of potato varies by varieties. Potato plants may affect ecology of P. erythroseptica by affecting soil microbial community that contributed differently to regulation of pathogen behavior in the course of plant infection and ecology.

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