Date Created

8-2026

Embargo Date

8-7-2027

Document Type

Thesis

Degree Name

Master of Science

Department

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

First Advisor

Susana Karen Gomez

First Committee Member

Patrick Burns

Second Committee Member

Elisa Garzo

Abstract

Arbuscular mycorrhizal (AM) fungi form mutualistic relationships with the majority of terrestrial plant species and are known to improve plant nutrient uptake, increase tolerance to abiotic stress, and regulate defense mechanisms through mycorrhiza-induced resistance (MIR). However, the effect of AM symbiosis on phloem-feeders, such as pea aphids (Acyrthosiphon pisum), is quite variable and largely dependent on environmental conditions, including elevated temperature. This study investigated how elevated temperature and AM fungal colonization by Rhizophagus intraradices interact to influence pea aphid life-history traits and plant defense gene expression in Medicago truncatula. The research objectives were to (1) determine how elevated temperature and AM fungal colonization influence pea aphid life-history traits, and (2) evaluate how AM fungal colonization, aphid herbivory, and elevated temperature influence plant defense responses. Two controlled growth chamber experiments were performed to examine the effects of two temperature conditions, 24°C (ambient) and 27°C (elevated), combined with AM fungal colonization status (mock-inoculated versus AM fungus-inoculated plants). In Experiment 1, six pea aphid life-history traits were measured across all combinations of temperature (24°C and 27°C) and AM fungal colonization (absent versus present), including fecundity rate (Md), pre-reproductive period (d), intrinsic rate of natural increase (rm), mean generation time (Td), relative growth rate (RGR), and final body weight. In Experiment 2, expression of defense-related genes associated with the biosynthesis of salicylic acid; Isochorismate Synthase 1 (MtICS1) and jasmonic acid; Allene Oxide Synthase 1 (MtAOS1) were determined using reverse transcription quantitative real-time PCR (RT-qPCR) under factorial combinations of temperature, AM fungal colonization, and aphid herbivory. The data obtained were analyzed using non-parametric multiplicative regression (NPMR) in order to examine the effect of temperature and colonization by AM fungi. The findings revealed that the combination of AM fungal colonization and temperature was the best predictor for three reproductive and growth-related traits including effective fecundity, intrinsic rate of increase, and relative growth rate indicating that mycorrhizal plants under warming conditions posed a compounded challenge to aphid population growth. Temperature alone best predicted developmental timing traits for pre-reproductive period and mean generation time with elevated temperature (27°C) slightly accelerating aphid development, consistent with thermal acceleration near the pea aphid thermal optimum. Final body weight was not significantly predicted neither by temperature nor AM fungal colonization, with initial aphid body weight identified as the best single predictor, though this relationship did not reach statistical significance. AM fungal colonization rates remained stable across temperature treatments in both experiments, suggesting the AM symbiosis was maintained under the tested temperature range. The gene expression analysis revealed high variation and lack of significant influence of temperature, aphid infestation, and AM fungal colonization on the MtICS1 and MtAOS1 gene expression levels. These findings demonstrate that the interaction between elevated temperature and AM fungal colonization rather than temperature alone shapes key aspects of aphid population dynamics, highlighting the importance of considering multiple interacting factors in plant-insect-microbe systems under climate warming. This study provides baseline knowledge for developing sustainable pest management strategies that leverage beneficial soil fungi under warming conditions.

Abstract Format

html

Keywords

Arbuscular mycorrhizal fungi; Rhizophagus intraradices; Medicago truncatula; pea aphid; Acyrthosiphon pisum; plant defense; aphid resistance; heat stress; elevated temperature; mycorrhiza-induced resistance; plant-insect interactions; climate warming

Subject Categories

Biology | Entomology | Life Sciences | Plant Biology | Plant Sciences

Language

English

Places

Greeley, Colorado

Extent

132 pages

Rights Statement

Copyright is held by the author.

Digital Origin

Born digital

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Available for download on Saturday, August 07, 2027

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