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Document Type

Dissertation - University Access Only

Award Date


Degree Name

Doctor of Philosophy (PhD)


Biology and Microbiology

First Advisor

Heike Bucking


The 400 million year old arbuscular mycorrhizal (AM) mutualism is a symbiosis that is formed between the roots of 65% of all land plant species and an exclusively subterranean fungus. In this mutualistic interaction the fungus transfers nutrients, such as phosphate (P) and nitrogen (N), sulfur and zinc, and in addition provides the host with a higher stress tolerance in exchange for photoassimilates. Previous studies indicated a direct link between the carbon (C) and P exchange in the symbiosis, but it is unknown whether C also acts as a trigger for fungal N transport. It has been suggested that biological market dynamics could contribute to the evolutionary stability in the AM symbiosis. However, in order for these mechanisms to work, the host plant and the fungus must be able to discriminate between partners that differ in the mycorrhizal benefit that they provide. So far, it is unknown whether host plants can, for example, discriminate between cocolonizing fungi on a fine enough scale to reciprocate accordingly. We developed two hypotheses to address these gaps in our knowledge: 1. Host C has an effect on AM fungal N uptake and transport to the host. 2. Host plants and AM fungi can discriminate between beneficial and less beneficial partners, and reciprocate accordingly. We tested our hypotheses in root organ cultures and whole plant systems at the community, physiological, and molecular level. We demonstrate that host C stimulates fungal N uptake and transfer to the host. We demonstrate that plants and fungi can preferentially allocate resources to partners that provide more benefit. Our data reinforce our hypothesis that biological market theory provides a suitable context for understanding nutrient exchange between partners and the evolutionary stability of the AM symbiosis.

Library of Congress Subject Headings

Vesicular-arbuscular mycorrhizas
Mutualism (Biology)
Plant-fungus relationships


Includes bibliographical references (pages 179-180)



Number of Pages



South Dakota State University


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