Gene Expression of Agronomically Important Secondary Metabolites in cv. ‘USDA Cascade’ Hop (Humulus lupulus L.) Cones during Critical Developmental Stages

Abstract 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, was sequenced. Significant increases were found in expression patterns of many genes involved in the 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. More research needs to be performed to describe the pathway of thiol precursor biosynthesis in hops. Hierarchical clustering revealed overlap of samples taken from each developmental stage, likely due to non-uniform ripening of cones on the plant. It is proposed that the mid-stage of cone development is critical for the development of important flavor-producing secondary metabolites in hops, and this is supported by previous research describing concentrations of secondary metabolites. It is hypothesized that abiotic stress during the mid-stage of cone development may be quite detrimental to the bitter acid concentrations ultimately found in hops.