First Advisor

Gomez, Susana Karen

Date Created

5-2017

Abstract

Arbuscular mycorrhizal fungi (AMF) are integral components of ecosystems and form root associations with the majority of land plants (>80%). In these relationships, AMF provide essential nutrients to their hosts, primarily phosphorus, in exchange for photosynthates, which enhance plant growth and help plants overcome environmental stress. The below-ground interaction between plants and AMF can indirectly alter aboveground plant interactions with insect herbivores. Potatoes (Solanum tuberosum) which are considered one of the most important vegetable crops worldwide, naturally form symbioses with AMF. However, it is not well understood how the association between potatoes and AMF can potentially affect leaf-chewing insect herbivory. This study examined the interactions between potatoes, the generalist Lepidopteran larvae of the cabbage looper (Trichoplusia ni), and a generalist AM fungus (Glomus intraradices). The research objectives were to: a) determine the impact of the tripartite interaction involving an AM fungus, potatoes, and cabbage loopers on each organism involved, b) examine gene expression of a group of defense-related plant genes during a tripartite interaction, and c) assess changes in potato physiology during the tripartite cabbage looper-potatoAM fungus interaction. The results indicate that larval growth was negatively impacted after feeding on mycorrhizal potato plants at the low level of G. intraradices root colonization (20-40% colonized at time of insect exposure) in the first experiment. Larvae gained significantly less weight after seven days of feeding on mycorrhizal plants at the low level of G. intraradices colonization compared to those that fed on highly colonized plants. Mycorrhizal plants at high levels of G. intraradices root colonization accumulated more shoot biomass, however, root biomass was not altered by the AM symbiosis. While defense-related genes were upregulated in shoots of mycorrhizal plants, their expression levels were not significantly different compared to non-mycorrhizal plants. The second and third experiments were designed using the low level of G. intraradices root colonization. Similarly, cabbage looper larvae gained less mass after eight days of feeding on mycorrhizal plants compared to those that fed on nonmycorrhizal plants. In this case, increased levels of transcripts of defense-related genes were detected in above-ground tissues. Interestingly, cabbage looper herbivory caused an ‘apparent’ stimulation of the AM fungus root colonization. Results from the third experiment revealed that while insects were negatively affected by the AM symbiosis, there were not substantial changes in potato plant physiology. Overall, this research showed that potato root colonization by G. intraradices indirectly altered cabbage looper growth (measured as weight), but the effect is dependent on the mycorrhizal stage. At the low level of G. intraradices colonization, the physiology of potato plants was not altered, but again, insects gained less mass after feeding on mycorrhizal plants. At the high levels of G. intraradices colonization, potato shoots accumulated more mass, but also insects gained more mass after feeding on mycorrhizal plants. Taken together, these data suggest that potatoes may transition from insect resistance to tolerance when progressing from low to high levels of G. intraradices root colonization.

Keywords

Plants, Fungi, Insects

Extent

105 pages

Local Identifiers

SchoenherrThesis_2017

Rights Statement

Copyright is held by the author.

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