Gene expression of agronomically important secondary metabolites in cv. ‘USDA Cascade’ hop (Humulus lupulus L.) cones during critical developmental stages

Authors: Eriksen, Renee; PADGITT-COBB, LILLIAN - Oregon State University; RANDAZZO, ANGELA - Oregon State University; HENDRIX, DAVID - Oregon State University; Henning, John

Submitted to: Journal of the American Society of Brewing Chemists
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/2021
Publication Date: 10/26/2021
Citation: Eriksen, R.L., Padgitt-Cobb, L.K., Randazzo, A., Hendrix, D., Henning, J.A. 2021. Gene expression of agronomically important secondary metabolites in cv. ‘USDA Cascade’ hop (Humulus lupulus L.) cones during critical developmental stages. Journal of the American Society of Brewing Chemists. https://doi.org/10.1080/03610470.2021.1973328.
DOI: https://doi.org/10.1080/03610470.2021.1973328

Interpretive Summary: Hop cones are valued for the flavor they impart to beer. These flavor compounds are derived from secondary metabolites produced by hop cones. Secondary metabolites are produced following a series of biochemical steps called a biosynthesis pathway in which a precursor molecule is converted to the final product by the activity of a series of proteins within the plant cell. These proteins are coded for by genes which can be up-regulated/"turned on" or down-regulated/"turned down" based on their importance in the cell at any given time. We sampled hop cones from the cultivar 'Cascade' at three different stages in development (early, mid, and late) in order to understand how genes within the biosynthesis pathways of important secondary metabolites are regulated during these developmental periods. We find that many genes involved in these pathways are up-regulated during the mid-stage of cone development, which we determine is therefore critical for development of many important flavor compounds, including the bitter acids and the "hop oils." Other flavors that are described as "tropical" or "black current-like" are derived from thiol precursors in the hop cones, and the genes that are thought to be involved in the biosynthesis of these compounds are up-regulated during the later stage of cone development. Our results are supported by previous research that found strong increases in the concentrations of many important flavor compounds during the mid-stage of cone development. Because many of these genes are known from previous research to be sensitive to low-water stress and high temperature stress, we hypothesize that these kinds of stress could be most detrimental to hop flavor compounds if they are experienced during this critical mid-stage period of cone development.

Technical Abstract: We sequenced the transcriptome from lupulin glands and associated bracts from cone tissue of hop (Humulus lupulus) c.v. ‘Cascade’ during three stages of development: early, mid, and late or near-harvest. We found significant increases in expression patterns of genes involved in biosynthesis of bitter acids, xanthohumol, and volatile secondary metabolites or “hop oils” during the middle stage of cone development. The biosynthesis of thiol precursors responsible for popular “tropical fruit” flavors in beer is not well known, but homologs of genes hypothesized to be involved in this process tend to be up-regulated during the late stage in hop cones. We propose that the mid-stage of cone development is critical for development of important flavor-producing secondary metabolites in hops, and this is supported by previous research describing concentrations of secondary metabolites. We hypothesize that abiotic stress during the mid-stage of cone development may be quite detrimental to bitter acid concentrations ultimately found in hops.