Graduate Defense: Bryanna Bright

Thesis Information

Title: Understanding Temporal Dynamics of Plant Specialized Metabolites in Response to Simulated Browsing in Winter

Program: Master of Science in Biology

Advisor: Dr. Jennifer Forbey, Biological Sciences

Committee Members: Dr. Amy Ulappa, Biological Sciences; and Dr. Sven Buerki, Biological Sciences

Abstract

Plants are continually defending themselves from the herbivores that consume them, often using an array of plant specialized metabolites (PSMs). Volatile organic compounds, including monoterpenes, are one such array of PSMs that can be emitted and induced by plants in response to mechanical damage and herbivory. These volatiles serve as direct defenses against herbivores and can alert neighboring plants about potential threats, resulting in protection against future attacks. However, how these chemicals change over time in response to browsing by vertebrates in the winter has received limited attention and is crucial to interpreting monoterpene patterns as a defense against vertebrate herbivores. To assess induced defenses of plants in the winter, we investigated temporal changes of monoterpenes in Wyoming big sagebrush (Artemisia tridentata spp. wyomingensis) following either a single event of mechanical damage (Acute damage) or repeated damage (Chronic damage) to leaves that simulated “bites” by the avian herbivore, Greater sage-grouse (Centrocercus urophasianus). We hypothesized that plants would exhibit an initial increase in monoterpene emission and therefore reduced concentration in leaf material followed by increases in specific monoterpenes as the time-course progressed. This hypothesis is based on predicted immediate emission of PSMs released from plant tissue upon the onset of herbivory associated with conspecific communication and longer-term enzymatic biosynthesis for induction of specific compounds that deters herbivores over time. We also hypothesized that Treatment plants that received simulated browsing would have lower variability in the different types of monoterpenes present compared to Control plants. This hypothesis is based on predictions that plants up-regulate a specific suite of monoterpenes that more precisely target herbivores rather than a breadth of baseline monoterpenes associated with a variety of biotic and abiotic factors. We also hypothesized that repeated damage to plants in our Chronic Treatment would result in greater changes in monoterpenes than Acute Treatment plants. This hypothesis is based on predictions that repeated browsing would have a longer lasting effect than a single browsing event which would allow the plant to recover and return to baseline monoterpene production. Similarly, we hypothesized that plants with the highest levels of damage will have greater changes in monoterpenes than plants with lower levels of Collection Intensity events. This hypothesis reflects conditions in nature where an herbivore may continually return to the same plant which may cause greater induction of defenses compared to a single browsing event. Multiple Principal Component Analyses (PCA) were used to assess which monoterpenes had the largest vector loadings in a particular PCA indicating a compound of interest and to compare ellipses over Time Points (0, 1, 24, 48, and 144 hr after simulated browsing) within Treatments, between Treatment groups (Control, Acute, and Chronic), and between levels of Collection Intensity (Low, Medium, and High) at 144 hr after an initial simulated browsing event. We identified fourteen compounds of interest with the largest vector loadings from the PCAs. These were reduced to six compounds of interest (C26, C28, C34, C38, C44, and C61) based on strong correlation with other compounds. There was no evidence that any compound of interest changed over time after simulated browsing within individual Treatment groups. We found no evidence that monoterpene profiles differed between plants receiving Acute or Chronic simulated browsing and Control plants. We also found no difference in monoterpene profiles among levels of Collection Intensity. Results suggest that damage to sagebrush in the winter results in only minor changes in monoterpenes over a six-day time-course. However, review of correlation among clusters of compounds following browsing identified several compounds worthy of further investigation as potential biomarkers of damage. Lack of induced defenses associated with winter browsing is inconsistent with observed induced defenses in sagebrush damaged in the summer and suggests stable winter chemistry following leaf damage that may explain the relatively high winter foraging fidelity observed in sage-grouse. We recognize this experiment was conducted on a single subspecies of sagebrush and single type of damage. It is possible that other species of sagebrush are more chemically dynamic after damage or that other types of damage that includes damage to stems or flowering stalks or damage that includes the saliva of the herbivore might have elicited an induced defense responses. However, this research demonstrates the value of a multivariate approach to detect chemicals that might normally be ignored due to their relatively low concentration and rarity in a plant-herbivore system. Finally, results may inform management decisions of what plants to select for restoration of disturbed areas by understanding how seasonality, damage by herbivores that differ in browsing behavior (e.g., leaves or stems, time spent at each plant, potential for repeat visits), and plant responses impact the chemical traits of plants that can cascade up to influence species of conservation concern and entire communities.