Gene expression for secondary metabolite biosynthesis in hop (Humulus lupulus L.) leaf lupulin glands exposed to heat and low-water stress

Authors: Eriksen, Renee; PADGITT-COBB, LILLIAN - Oregon State University; TOWNSEND, M. SHAUN - Oregon State University; Henning, John

Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2021
Publication Date: 3/4/2021
Citation: Eriksen, R.L., Padgitt-Cobb, L., Townsend, M., Henning, J.A. 2021. Gene expression for secondary metabolite biosynthesis in hop (Humulus lupulus L.) leaf lupulin glands exposed to heat and low-water stress. Scientific Reports. 11. Article 5138. https://doi.org/10.1038/s41598-021-84691-y.
DOI: https://doi.org/10.1038/s41598-021-84691-y

Interpretive Summary: Hops are valued for their bitter acids, essential oils, and other complex compounds that impart flavor in beer. Previous studies have shown that hop yield and bitter acid content declines with increased temperatures and low-water stress, and these declines threaten the supply chain of the brewing industry. We looked at gene expression in leaf and root tissue from hop (Humulus lupulus) cultivar ‘USDA Cascade’ in plants exposed to high temperature stress, low-water stress, and a combined treatment of both high temperature and low-water stress for six weeks. We found increased growth and photosynthesis rates at high temperatures, but indications of damage to the photosynthetic system. Low-water stress and the combined stress treatment significantly reduced growth and photosynthesis, and there was stronger evidence of damage to the photosynthetic system. These treatments caused many changes in the genes that were expressed (turned on or off), including a number of genes related to agronomically important traits such as the flavor compounds. Of the genes involved in bitter acid production, the gene for critical protein valerophenone synthase experienced substantial and significant reductions in expression levels across stress treatments, suggesting stress-induced gene expression of this protein may be at least partially responsible for declines in bitter acid content measured by previous researchers. We also identified a number of putative genes that are similar to genes from other plants that are known to improve abiotic stress tolerance in other plants. These genes may be useful as markers for breeding improved abiotic stress tolerance in hop. We also provide the first published collection of expressed genes from hop root tissue.

Technical Abstract: Hops are valued for their bitter acids, oils, and polyphenol content that impart flavor in beer. Previous studies have shown that hop yield and bitter acid content declines with increased temperatures and low-water stress, and these declines threaten the supply chain of the brewing industry. We looked at differential gene expression in leaf and root tissue from hop (Humulus lupulus) cv. Cascade in plants exposed to high temperature stress, low-water stress, and a compound treatment of both high temperature and low-water stress for six weeks. We found increases in carbon assimilation and no reductions in biomass under high temperature treatments, but these were offset by declines in FV/FM, indicating damage to the photosystems due to high temperature. Low-water stress and the compound stress significantly reduced biomass, carbon assimilation, and FV/FM. There was a substantial reaction from the transcriptome to the stress conditions imposed in these experiments, with significant reductions in expression of a number of genes related to secondary metabolism. Of the genes involved in bitter acid production, the critical gene valerophenone synthase experienced substantial and significant reductions in expression levels across stress treatments, suggesting stress-induced lability in this gene may be at least partially responsible for declines in bitter acid content measured previously. We also identified a number of transcripts with homology to genes known to improve abiotic stress tolerance in other plants that may be useful as markers for breeding improved abiotic stress tolerance in hop, and we provide the first transcriptome from hop root tissue.