Date Created

8-2026

Embargo Date

8-7-2028

Document Type

Thesis

Degree Name

Master of Science

Department

College of Natural and Health Sciences, Biological Sciences, Biological Sciences Student Work

First Advisor

S. Karen Gomez

First Committee Member

Emily Holt

Second Committee Member

Elisa Garzo

Abstract

Climate change is reshaping ecological interactions by altering the environmental conditions under which plants, insect herbivores, and beneficial microorganisms coexist. In natural ecosystems, plants simultaneously engage in both antagonistic and mutualistic relationships, and these interactions are strongly influenced by abiotic factors such as atmospheric carbon dioxide (CO₂). The purpose of this study was to determine how elevated carbon dioxide (eCO₂) influences the tripartite interaction among Medicago truncatula Gaertn., pea aphids (Acyrthosiphon pisum Harris), and arbuscular mycorrhizal fungi (Rhizophagus intraradices (N.C. Schenck & G.S. Sm.) C. Walker & A. Schüßler 2010) by evaluating ecological responses such as arbuscular mycorrhizal fungal colonization and aphid performance, in addition to plant defense responses at molecular levels including the expression of genes involved in salicylic acid (SA), jasmonic acid (JA) and medicarpin biosynthesis. To our knowledge, this is the first study to investigate the combined effects of eCO₂ on the interaction among barrel medic (Medicago truncatula), the pea aphid (Acyrthosiphon pisum), and the arbuscular mycorrhizal fungus (Rhizophagus intraradices). Using controlled growth chamber experiments under ambient (425 ppm) and elevated (700 ppm) CO₂ conditions, combined with treatments involving presence or absence of AM fungal colonization and aphid iv herbivory, we evaluated aphid performance, AM fungal colonization, and the expression of defense-related genes associated with salicylic acid (SA), and jasmonic acid (JA) biosynthesis. Our results demonstrated that eCO₂ reduced AM fungal colonization despite the increased carbon availability typically associated with enhanced AM fungal symbiosis. At the molecular level, eCO₂ promoted the upregulation of SA biosynthesis-associated genes, including isochorismate synthase 1 (MtICS1), and phenylalanine ammonia-lyase (MtPAL), suggesting increased biosynthesis of SA defense mechanisms. In contrast, the JA pathway exhibited differential regulation, with increased expression of allene oxide cyclase (MtAOC) but no significant changes in allene oxide synthase 1 (MtAOS1), indicating selective or primed activation of JA-associated defenses rather than complete suppression of JA signaling. Aphid herbivory further contributed to the activation of both SA- and JA-related responses, supporting the idea that aphid-induced defenses involve complex hormonal crosstalk rather than strict antagonism between these two pathways. Additionally, AM fungal colonization negatively affected aphid fitness under both ambient and eCO₂ conditions, likely through mycorrhiza-induced resistance and enhanced chemical defenses. Overall, this study demonstrates that eCO₂ reshapes the balance among SA and JA, plant interactions with symbionts, and aphid fitness through complex hormonal and metabolic regulation. These findings provide new insight into how climate change may alter three-way interactions in agroecosystems and highlight the importance of integrating ecological and molecular approaches to better understand plant responses under future atmospheric conditions.

Abstract Format

html

Keywords

Climate change; elevated carbon dioxide; Medicago truncatula; pea aphid; arbuscular mycorrhizal fungi; plant defense; gene expression

Subject Categories

Agricultural Science | Agriculture | Cellular and Molecular Physiology | Entomology | Integrative Biology | Molecular Genetics | Plant Biology | Plant Sciences

Language

English

Places

Greeley, Colorado

Extent

115 pages

Rights Statement

Copyright is held by the author.

Digital Origin

Born digital

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Available for download on Monday, August 07, 2028

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