Dissection of Dynamic Transcriptome Landscape of Leaf, Bract, and Lupulin Gland in Hop (Humulus lupulus L.)

Abstract

The hop plant (Humulus lupulus L.) produces several valuable secondary metabolites, such as prenylflavonoid, bitter acids, and essential oils. These compounds are biosynthesized in glandular trichomes (lupulin glands) endowed with pharmacological properties and widely implicated in the beer brewing industry. The present study is an attempt to generate exhaustive information of transcriptome dynamics and gene regulatory mechanisms involved in biosynthesis and regulation of these compounds, developmental changes including trichome development at three development stages, namely leaf, bract, and mature lupulin glands. Using high-throughput RNA-Seq technology, a total of 61.13, 50.01, and 20.18 Mb clean reads in the leaf, bract, and lupulin gland libraries, respectively, were obtained and assembled into 43,550 unigenes. The putative functions were assigned to 30,996 transcripts (71.17%) based on basic local alignment search tool similarity searches against public sequence databases, including GO, KEGG, NR, and COG families, which indicated that genes are principally involved in fundamental cellular and molecular functions, and biosynthesis of secondary metabolites. The expression levels of all unigenes were analyzed in leaf, bract, and lupulin glands tissues of hop. The expression profile of transcript encoding enzymes of BCAA metabolism, MEP, and shikimate pathway was most up-regulated in lupulin glands compared with leaves and bracts. Similarly, the expression levels of the transcription factors and structural genes that directly encode enzymes involved in xanthohumol, bitter acids, and terpenoids biosynthesis pathway were found to be significantly enhanced in lupulin glands, suggesting that production of these metabolites increases after the leaf development. In addition, numerous genes involved in primary metabolism, lipid metabolism, photosynthesis, generation of precursor metabolites/energy, protein modification, transporter activity, and cell wall component biogenesis were differentially regulated in three developmental stages, suggesting their involvement in the dynamics of the lupulin gland development. The identification of differentially regulated trichome-related genes provided a new foundation for molecular research on trichome development and differentiation in hop. In conclusion, the reported results provide directions for future functional genomics studies for genetic engineering or molecular breeding for augmentation of secondary metabolite content in hop.

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

“…There is increasing interest among growers and industry partners to better understand the response of hops to HT and LW stress, and to breed varieties with improved tolerance to abiotic stress. There are a number of genomic resources now available to assist breeding for H. lupulus, including the genome sequence 40,47,48 , a proteome 49 , and transcriptomes 13,14 . This study adds the transcriptome sequence under a combination of HT stress and LW stress, as well as a transcriptome from hop root tissue.…”

“…Therefore, we used leaf and stem tissue and www.nature.com/scientificreports/ their associated lupulin glands as proxies for cone lupulin glands. The bitter acids are found in leaf tissue in small amounts 12,13 . Clark et al 14 and Mishra et al 13 found many genes for the bitter acid pathway expressed in leaf tissue, though some, including VPS and BCAT2, have higher expression in lupulin glands extracted from cones.…”

“…The bitter acids are found in leaf tissue in small amounts 12,13 . Clark et al 14 and Mishra et al 13 found many genes for the bitter acid pathway expressed in leaf tissue, though some, including VPS and BCAT2, have higher expression in lupulin glands extracted from cones. Some genes appear to be expressed only in cone lupulin glands, such as the prenyltransferase genes HlPT1L and HlPT2 and BCAT1 14,22 , and consequently these were not detected in our study.…”

Cannabis Glandular Trichomes: A Cellular Metabolite Factory

“…Trichome differentiation mechanisms have been investigated in Arabidopsis thaliana , with transcription factor (TFs) groups playing key roles in the transcriptional networks for trichome production and patterns ( Tian et al, 2017 ). While genomic studies are available for other economic plants, including Humulus lupulus which belongs to the Cannabaceae family ( Matoušek et al, 2016 ; Mishra et al, 2020 ), similar studies for cannabis are lacking despite their potentially important impacts for precise cannabis trichome control. Taking advantage of the genetic libraries available for related species with similar resin secretions will help guide these much-need studies ( Braich et al, 2019 ; Zager et al, 2019 ; Liu et al, 2021 ).…”

Developmental regulation of lupulin gland-associated genes in aromatic and bitter hops (Humulus lupulus L.)

“…Hop ( Humulus lupulus L.) is a diploid, dioecious, perennial climbing plant belonging to the Cannabaceae family. Female plants are cultivated for the commercial production of inflorescences (cones), which are mainly used in the brewing industry but also commonly used in the production of pharmaceuticals and cosmetics [ 1 ]. In cones, flavour-active secondary metabolites are biosynthesized and accumulate in glandular trichomes and lupulin glands.…”