Date of Degree

9-2021

Document Type

Dissertation

Degree Name

Ph.D.

Program

Biology

Advisor

Edward J. Kennelly

Committee Members

Paul D. Matthews

Renuka P. Sankaran

Damon Little

Michael Balick

Subject Categories

Agricultural Science | Agriculture | Biochemistry | Biology | Botany | Food Chemistry | Other Plant Sciences | Plant Biology | Plant Breeding and Genetics | Systems Biology

Keywords

genotype, environment, green leaf volatiles, climate change, beer, multivariate statistics

Abstract

The hop plant (Humulus L., Cannabaceae) is a dioecious, perennial, twining vine with a long history of human use. Nowadays, hop plants are generally grown for their inflorescences (“cones”), which are used in brewing for their phytochemical metabolites. Many of these metabolites are involved in plant stress response and communication. Genetics and environment are two major factors that affect plant metabolism. In three separate metabolomics studies, this project examined the effects of both genetic and environmental factors on hop phytochemistry.

In the first study, 23 hop genotypes were grown in two different locations in the Pacific Northwest region of the United States. Each location has a distinct set of environmental conditions or terroir. Ultrahigh-performance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS) was used to measure the relative amounts of four glycosylated aroma compounds in hop cones previously processed to remove resins and oils. Analysis of variance (ANOVA) showed that the large majority of the variability in levels of linalool, raspberry ketone, and 2-phenylethanol glucosides was attributable to genotype differences. In contrast, difference in growing location had the largest effect on levels of the glycosylated green leaf volatile, hexyl glucoside, although the effect of genetics was still substantial. While the impact of hop aroma glycosides on beer aroma is considered small relative to that of hop essential oils, the influences of genotype and environment on hop aroma glycoside content may still have some effect on beer aroma development as it ages.

In the second study, three levels of water stress were replicated across four hop genotypes in a controlled greenhouse experiment. Untargeted UPLC-QTof-MSE was used to profile a wide range of metabolites in the leaves. Extensive filtering reduced the number of chemical features from 2387 to 278. Principal component analysis (PCA) of the filtered features showed that the drought treatments had a noticeable effect on the overall chemical composition of most genotypes. Still, each genotype appeared distinct, suggesting that genotype (rather than drought stress) had the larger effect on hop leaf chemical composition. Also, the study found 14 phytochemical markers that consistently increased or decreased in response to water stress. Ten of these markers were tentatively identified: five monoacylated glycerolipids, two norisoprenoids (abscisic acid [ABA] and roseoside), pheophorbide A, glutaric acid, and dihydromyricetin. All but dihydromyricetin were more abundant in drought stressed leaves. The glycerolipids, norisoprenoids, and pheophorbide A all have known or likely stress signaling roles, and their accumulation likely indicates respective breakdowns of membrane lipids, carotenoids, and chlorophyll. Many of these compounds are likely drought stress markers in higher plants in general, while some may be more narrow or depend on the type of drought treatment.

In the third study, UPLC-MS was used to compare differences in leaf phytochemical composition and glandular trichome density between two groups of greenhouse-grown hop plants representing two distinct genetic lineages: commercial hop cultivars of pure European or mixed European–North American heritage; and wild hops from the American Southwest (H. neomexicanus). In an untargeted principal component analysis, the phytochemical compositions of the two groups appeared distinct. Bitter acid and prenylflavonoid levels were higher in the H. neomexicanus group; so was the density of glandular trichomes, where bitter acids and prenylflavonoids are biosynthesized. The H. neomexicanus group also had higher flavonol glycoside levels. However, all six phenolic acids measured were higher in the cultivar group, although only one of these differences was statistically significant. Due to the higher levels of bitter acids, prenylflavonoids, and flavonol glycosides in their leaves, the H. neomexicanus genotypes may have greater resistance to insect herbivory, fungal infection, or both and should be evaluated for these characteristics.

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