 Morgan Thompson written by: Amanda Rae Brucchieri Back in the UMD Entomology Department where she previously earned her Master’s degree, Dr. Morgan Thompson is now a Postdoctoral Research Associate in the Burghardt Lab. Upon returning to the department, she shared some interesting findings from her doctoral research. At this week’s Colloquium, Dr. Thompson told three stories: undercover operation, eavesdropping plants, and phytochemical diversity. Altogether, these research tales earned her the prestigious title of PhD from Texas A&M University. These stories focused on the relationships between plants and herbivores. More specifically, how do plants handle herbivory? A plant's success is often connected to its ability to resist or tolerate herbivores. Resistance includes direct forms, such as physical barriers (e.g., spines) and chemical compounds (e.g., toxins) or indirect forms including recruitment of assistance (e.g., signaling parasitoids), while tolerance corresponds with plant regrowth or resource reallocation after herbivory. These defenses can be scaled across the plant and are important when considering plant defenses in an agricultural system. Understanding these scales within plants—broadly described as local (i.e., immediate area of reaction to herbivory) or systemic (i.e., throughout the whole-plant ranging from above- to belowground components)—can provide important insights into plant/pest relationships in a variety of settings, including agricultural systems. Dr. Thompson, curious about these relationships, dug deeper into the mechanisms and multitrophic consequences of plant defense against insect herbivory above- and below-ground. The main characters of her three stories included various plant species within the family Cucurbitaceae (Figure 1), striped cucumber beetles (Figure 2), and squash bugs (Figure 3). These insects not only cause damage by consuming plants but can also transmit plant diseases.
Undercover Operation. Dr. Thompson investigated how belowground herbivory by striped cucumber beetle larvae indirectly impacts aboveground herbivores, including adult striped cucumber beetles and squash bugs, through cucurbit plant defenses. With herbivore plant preference experiments, Dr. Thompson noted that adult beetles and bugs avoided larvae-damaged plants, but why? To determine this, Dr. Thompson and her team collected volatile chemical cues from the plants, considering plant chemical cues are often used by foraging herbivores to select their hosts. Dr. Thompson and her team noticed leaves of plants with belowground damage from larvae increased emission of a chemical compound, (E)-β-Ocimene. Further investigation showed that adult striped cucumber beetles and squash bugs did not react to this volatile chemical compound, so Dr. Thompson looked at carbohydrate and protein levels in plant leaves. With root damage, carbohydrate levels decreased and proteins increased in leaves. Overall, these findings indicate leaf herbivores avoided root-damaged plants and likely relied on leaf nutrients to make their foraging decisions rather than plant chemical cues. Dr. Thompson proposed that further inquiries into how leaf herbivores respond to shifting nutrients could be an interesting topic for future research (Thompson et al. 2022).
Eavesdropping Plants. Lacking common communication methods such as producing sound or motion, plants use other techniques to communicate with themselves and each other, including the emission and detection of volatile chemical cues. If root damage caused by herbivory produces an increase in volatile compounds, are neighboring plants able to “listen” and react to the threat by enhancing their defenses? In summary, the answer is yes and maybe. Dr. Thompson showed that exposure to volatile chemical cues increased neighboring plant defense against squash bugs, but not adult striped cucumber beetles. Further investigations by Dr. Thompson and her team revealed β-Ocimene plays a key role in this plant communication about insect herbivory. Ultimately, Dr. Thompson determined below-ground root-feeding herbivores can alter chemical signaling between aboveground leaves, revealing a novel form of plant communication (Thompson et al. 2023). Phytochemical Diversity. Variation in plant emission of volatile chemical cues often depends on the nature of herbivore damage. For example, when a leaf is damaged by herbivores, plants emit volatile chemical cues that attract parasitoids and if roots are damaged by herbivores, plants can produce different volatile chemical cues that attract predators in the soil such as entomopathogenic nematodes. Dr. Thompson looked at the quantitative and qualitative changes in volatile chemical cues of six different gourd plant species following squash bug leaf herbivory and striped cucumber beetle larval root herbivory. Herbivore damage increased volatile chemical emissions for both leaves and roots, but the foliar emissions were significantly higher than root emissions. Qualitatively, the number of volatiles emitted after herbivory increased to a greater degree from leaves compared to roots. This suggests cucurbit plants react differently to leaf and root herbivory. In conjunction with experiments on plant regrowth after herbivory, these findings show cucurbit plants resist aboveground herbivore damage but tolerate belowground herbivory (Thompson et al. 2024; Thompson et al. In Revision). By better understanding the interactions between plants and their herbivores, there are potential developments that can be made to help plants resist or tolerate herbivory. In an agricultural system, these developments could result in decreased crop damage and reduced pesticide applications. While still fond of the cucurbit study system, Dr. Thompson expressed excitement at joining the ranks of the Burghardt Lab and beginning her work investigating the complexities of insect communities on trees. It was clear that, while Dr. Thompson is happy to be returning to her graduate student roots at UMD, she is eager to branch out and expand her knowledge and expertise. Citations: Thompson, M.N., Z.P. Cohen, D. Merrell, and A.M. Helms (2024) Eco-evolutionary factors contribute to chemodiversity in aboveground and belowground cucurbit herbivore-induced plant volatiles. Plant Biology. http://doi.org/10.1111/plb.13709 Thompson, M.N., J. Arriaga, B.J. Bradford, R. Kurian, G. Strozier, and A.M. Helms (2023) Belowground insect herbivory induces systemic volatile emissions that strengthen neighboring plant resistance aboveground. Plant, Cell & Environment. https://doi.org/10.1111/pce.14762 Thompson, M.N., J.M. Grunseich, L.O. Marmolejo, N.M. Aguirre, P.A. Bradicich, S.T. Behmer, C.P.-C. Suh, and A.M. Helms (2022) Undercover operation: Belowground insect herbivory modifies systemic plant defense and repels aboveground foraging insect herbivores. Frontiers in Ecology and Evolution. https://doi.org/10.3389/fevo.2022.1033730 Comments are closed.
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