Phytochemical and morphological characterization of hop (Humulus lupulus L.) cones over five developmental stages using high performance liquid chromatography coupled to time-of-flight mass spectrometry, ultrahigh performance liquid chromatography photodiode array detection, and light microscopy techniques

Humulus lupulus L. Strobilus Photosynthetic Capacity and Carbon Assimilation

The economic value of Humulus lupulus L. (hop) is recognized, but the primary metabolism of the hop strobilus has not been quantified in response to elevated CO₂. The photosynthetic contribution of hop strobili to reproductive effort may be important for growth and crop yield. This component could be useful in hop breeding for enhanced performance in response to environmental signals. The objective of this study was to assess strobilus gas exchange, specifically the response to CO₂ and light. Hop strobili were measured under controlled environment conditions to assess the organ's contribution to carbon assimilation and lupulin gland filling during the maturation phase. Leaf defoliation and bract photosynthetic inhibition were deployed to investigate the glandular trichome lupulin carbon source. Strobilus-level physiological response parameters were extrapolated to estimate strobilus-specific carbon budgets under current and future atmospheric CO₂ conditions. Under ambient atmospheric CO₂, the strobilus carbon balance was 92% autonomous. Estimated strobilus carbon uptake increased by 21% from 415 to 600 µmol mol^-1 CO₂, 14% from 600 to 900 µmol mol^-1, and another 8%, 4%, and 3% from 900 to 1200, 1500, and 1800 µmol mol^-1, respectively. We show that photosynthetically active bracts are a major source of carbon assimilation and that leaf defoliation had no effect on lupulin production or strobilus photosynthesis, whereas individual bract photosynthesis was linked to lupulin production. In conclusion, hop strobili can self-generate enough carbon assimilation under elevated CO₂ conditions to function autonomously, and strobilus bracts are the primary carbon source for lupulin biosynthesis.

Determination of α- and β-acids in hops by liquid chromatography or electromigration techniques: A critical review

Hop plays an essential role in brewing beer and its study and analysis is of paramount importance. - and -acids are considered two of the most important hop components. While -acids are associated with the bitter flavor, -acids have antimicrobial effects. This work aims to critically review the published analytical methods for - and -acids determination in hops employing separation methods in liquid medium: liquid chromatography (LC) and capillary electrophoresis (CE). The types of hop samples, the optimized protocols to extract the hop acids, and the main instrumental conditions for both LC and CE techniques are highlighted and discussed. Specific and critical aspects of the - and β-acids separation by LC and CE and some challenges in this field are raised. Several key aspects discussed in this review may be of practical importance for brewers, whether in the microbrewery or industry and for researchers in the brewing field.

Assessment of the Genetic and Phytochemical Variability of Italian Wild Hop: A Route to Biodiversity Preservation

Background: Northern Italy has an enormous heritage of hop biodiversity that need to be exploited and studied. The preservation and valorization through the characterization of the existent biodiversity is a primary goal of the European Green Deal 2023–2030. The aim of this study was to acquire information on the biodiversity of Italian wild hops. Methods: Genetic characterization of sixty accessions was done resorting to Single Sequence Repeated (SSR) markers. Phytochemical characterization of wild hops was achieved using: (i) high-performance liquid chromatography with ultraviolet detection for bitter acids quantification, (ii) steam distillation for essential oils quantification and (iii) Gas Chromatography-Mass Spectrometry for the determination of the aromatic profile. Results: The eight SSR primers showed high Polymorphic Information Content (PIC), especially HlGA23. α-Acids reached values between 0 and 4.125. The essential oils analysis highlighted variability within the studied population, with some accessions characterized by important spicy fraction, and others by fruity and floral notes. Conclusions: The present study allowed the characterization of Italian wild hops and demonstrated an interesting biodiversity. Part of this biodiversity have been shown to be potentially suitable for use in brewing. Moreover, several genotypes could be used in breeding programs to obtain new more sustainable varieties.

A draft phased assembly of the diploid Cascade hop (Humulus lupulus) genome

Hop (Humulus lupulus L. var Lupulus) is a diploid, dioecious plant with a history of cultivation spanning more than one thousand years. Hop cones are valued for their use in brewing and contain compounds of therapeutic interest including xanthohumol. Efforts to determine how biochemical pathways responsible for desirable traits are regulated have been challenged by the large (2.8 Gb), repetitive, and heterozygous genome of hop. We present a draft haplotype-phased assembly of the Cascade cultivar genome. Our draft assembly and annotation of the Cascade genome is the most extensive representation of the hop genome to date. PacBio long-read sequences from hop were assembled with FALCON and partially phased with FALCON-Unzip. Comparative analysis of haplotype sequences provides insight into selective pressures that have driven evolution in hop. We discovered genes with greater sequence divergence enriched for stress-response, growth, and flowering functions in the draft phased assembly. With improved resolution of long terminal retrotransposons (LTRs) due to long-read sequencing, we found that hop is over 70% repetitive. We identified a homolog of cannabidiolic acid synthase (CBDAS) that is expressed in multiple tissues. The approaches we developed to analyze the draft phased assembly serve to deepen our understanding of the genomic landscape of hop and may have broader applicability to the study of other large, complex genomes.

Downy mildew resistance is genetically mediated by prophylactic production of phenylpropanoids in hop

Downy mildew in hop (Humulus lupulus L.) is caused by Pseudoperonospora humuli and generates significant losses in quality and yield. To identify the biochemical processes that confer natural downy mildew resistance (DMR), a metabolome- and genome-wide association study was performed. Inoculation of a high density genotyped F1 hop population (n = 192) with the obligate biotrophic oomycete P. humuli led to variation in both the levels of thousands of specialized metabolites and DMR. We observed that metabolites of almost all major phytochemical classes were induced 48 hr after inoculation. But only a small number of metabolites were found to be correlated with DMR and these were enriched with phenylpropanoids. These metabolites were also correlated with DMR when measured from the non-infected control set. A genome-wide association study revealed co-localization of the major DMR loci and the phenylpropanoid pathway markers indicating that the major contribution to resistance is mediated by these metabolites in a heritable manner. The application of three putative prophylactic phenylpropanoids led to a reduced degree of leaf infection in susceptible genotypes, confirming their protective activity either directly or as precursors of active compounds.

Wine or Beer? Comparison, Changes and Improvement of Polyphenolic Compounds during Technological Phases

Wine and beer are nowadays the most popular alcoholic beverages, and the benefits of their moderate consumption have been extensively supported by the scientific community. The main source of wine and beer's antioxidant behavior are the phenolic substances. Phenolic compounds in wine and beer also influence final product quality, in terms of color, flavor, fragrance, stability, and clarity. Change in the quantity and quality of phenolic compounds in wine and beer depends on many parameters, beginning with the used raw material, its place of origin, environmental growing conditions, and on all the applied technological processes and the storage of the final product. This review represents current knowledge of phenolic compounds, comparing qualitative and quantitative profiles in wine and beer, changes of these compounds through all phases of wine and beer production are discussed, as well as the possibilities for increasing their content. Analytical methods and their importance for phenolic compound determination have also been pointed out. The observed data showed wine as the beverage with a more potent biological activity, due to a higher content of phenolic compounds. However, both of them contain, partly similar and different, phenolic compounds, and recommendations have to consider the drinking pattern, consumed quantity, and individual preferences. Furthermore, novel technologies have been developing rapidly in order to improve the polyphenolic content and antioxidant activity of these two beverages, particularly in the brewing industry.

Hop Polyphenols in Relation to Verticillium Wilt Resistance and Their Antifungal Activity

(1) Background: Verticillium wilt (VW) of hop is a devastating disease caused by the soil-borne fungi Verticillium nonalfalfae and Verticillium dahliae. As suggested by quantitative trait locus (QTL) mapping and RNA-Seq analyses, the underlying molecular mechanisms of resistance in hop are complex, consisting of preformed and induced defense responses, including the synthesis of various phenolic compounds. (2) Methods: We determined the total polyphenolic content at two phenological stages in roots and stems of 14 hop varieties differing in VW resistance, examined the changes in the total polyphenols of VW resistant variety Wye Target (WT) and susceptible Celeia (CE) on infection with V. nonalfalfae, and assessed the antifungal activity of six commercial phenolic compounds and total polyphenolic extracts from roots and stems of VW resistant WT and susceptible CE on the growth of two different V. nonalfalfae hop pathotypes. (3) Results: Generally, total polyphenols were higher in roots than stems and increased with maturation of the hop. Before flowering, the majority of VW resistant varieties had a significantly higher content of total polyphenols in stems than susceptible varieties. At the symptomatic stage of VW disease, total polyphenols decreased in VW resistant WT and susceptible CE plants in both roots and stems. The antifungal activity of total polyphenolic extracts against V. nonalfalfae was higher in hop extracts from stems than those from roots. Among the tested phenolic compounds, only p-coumaric acid and tyrosol markedly restricted fungal growth. (4) Conclusions: Although the correlation between VW resistance and total polyphenols content is not straightforward, higher levels of total polyphenols in the stems of the majority of VW resistant hop varieties at early phenological stages probably contribute to fast and efficient activation of signaling pathways, leading to successful defense against V. nonalfalfae infection.

Identification of tandem repeat families from long-read sequences of Humulus lupulus

Hop (Humulus lupulus L.) is known for its use as a bittering agent in beer and has a rich history of cultivation, beginning in Europe and now spanning the globe. There are five wild varieties worldwide, which may have been introgressed with cultivated varieties. As a dioecious species, its obligate outcrossing, non-Mendelian inheritance, and genomic structural variability have confounded directed breeding efforts. Consequently, understanding the hop genome represents a considerable challenge, requiring additional resources. In order to facilitate investigations into the transmission genetics of hop, we report here a tandem repeat discovery pipeline developed using k-mer filtering and dot plot analysis of PacBio long-read sequences from the hop cultivar Apollo. From this we identified 17 new and distinct tandem repeat sequence families, which represent candidates for FISH probe development. For two of these candidates, HuluTR120 and HuluTR225, we produced oligonucleotide FISH probes from conserved regions of and demonstrated their utility by staining meiotic chromosomes from wild hop, var. neomexicanus to address, for example, questions about hop transmission genetics. Collectively, these tandem repeat sequence families represent new resources suitable for development of additional cytogenomic tools for hop research.

Hop (Humulus lupulus L.) terroir has large effect on a glycosylated green leaf volatile but not on other aroma glycosides

Genetics and environment both influence the content of hop (Humulus lupulus L.) aroma compounds. The effects of these two factors on aroma glycosides, which can change the aroma profile of beer over time, were examined in a preliminary study. Twenty-three hop cultivars were grown in the northwestern United States in two locations with distinct terroirs. UPLC-MS/MS analysis of hop cone extracts revealed that growing location had a large effect on hexyl glucoside levels but only a negligible effect on levels of linalyl, raspberry ketone, and 2-phenylethyl glucoside, which were mostly affected by genetic differences. The large terroir effect on hexyl glucoside, which releases a green leaf volatile with a grassy aroma when hydrolyzed, but not on the other aroma glucosides, which have more desirable aromas when hydrolyzed, could have an impact on beer aroma profiles.

Discrimination of geographical origin of hop (Humulus lupulus L.) using geochemical elements combined with statistical analysis

Beer is a popular alcoholic beverage worldwide, traditionally made from water, barley and hop (Humulus lupulus L.) strobili. The strobili contain lupulin glands whose components (mostly bitter acids and polyphenols) confer unique and locally different flavours to beer types. It is therefore relevant for brewers and consumers to precisely know the geographical origin of hop plants used for high-quality beer. Hop plants belonging to the variety Hallertau Perle, grown in two locations, Cavalese and Imèr, of the Trentino Region (Italy) were analysed to establish a direct relationship between the chemical elements detected in soil and in plant parts. Chemical elements were determined by X-ray fluorescence and inductively coupled plasma mass spectrometry in soil, leaf and strobili samples from Cavalese and Imèr. The data from the two areas were compared by a nonparametric test (Mann-Whitney) and multivariate statistics (principal component analysis and partial least squares discriminant analysis). The geochemical characterization and the statistical analyses showed different concentrations of major and trace elements in soil and plant parts from the two areas. A reliable correlation could be established between some elements in soil and strobili samples, that is Nb, Fe, Rb and Zr for Cavalese and Mg, Ni, Zn and Zr for Imèr. These elements could therefore be used as geochemical fingerprints to identify the geographical origin of strobili from the two study areas, an approach useful to verify the origin of hop plants for the production of high-quality beer.

Assessing Flux Distribution Associated with Metabolic Specialization of Glandular Trichomes

Many aromatic plants accumulate mixtures of secondary (or specialized) metabolites in anatomical structures called glandular trichomes (GTs). Different GT types may also synthesize different mixtures of secreted metabolites, and this contributes to the enormous chemical diversity reported to occur across species. Over the past two decades, significant progress has been made in characterizing the genes and enzymes that are responsible for the unique metabolic capabilities of GTs in different lineages of flowering plants. Less is known about the processes that regulate flux distribution through precursor pathways toward metabolic end-products. We discuss here the results from a meta-analysis of genome-scale models that were developed to capture the unique metabolic capabilities of different GT types.

Phenolic Substances in Beer: Structural Diversity, Reactive Potential and Relevance for Brewing Process and Beer Quality

For the past 100 years, polyphenol research has played a central role in brewing science. The class of phenolic substances comprises simple compounds built of 1 phenolic group as well as monomeric and oligomeric flavonoid compounds. As potential anti- or prooxidants, flavor precursors, flavoring agents and as interaction partners with other beer constituents, they influence important beer quality characteristics: flavor, color, colloidal, and flavor stability. The reactive potential of polyphenols is defined by their basic chemical structure, hydroxylation and substitution patterns and degree of polymerization. The quantitative and qualitative profile of phenolic substances in beer is determined by raw material choice. During the malting and brewing process, phenolic compounds undergo changes as they are extracted or enzymatically released, are subjected to heat-induced chemical reactions or are precipitated with or adsorbed to hot and cold trub, yeast cells and stabilization agents. This review presents the current state of knowledge of the composition of phenolic compounds in beer and brewing raw materials with a special focus on their fate from raw materials throughout the malting and brewing process to the final beer. Due to high-performance analytical techniques, new insights have been gained on the structure and function of phenolic substance groups, which have hitherto received little attention. This paper presents important information and current studies on the potential of phenolics to interact with other beer constituents and thus influence quality parameters. The structural features which determine the reactive potential of phenolic substances are discussed.

Insecticidal effects of extracts of Humulus lupulus (hops) L. cones and its principal component, xanthohumol

Insecticidal effects of the dichloromethane, ethyl acetate, acetone, ethanol and methanol extracts of Humulus lupulus (hops) L. cones and its principal components, xanthohumol was investigated on five stored pests, Sitophilus granarius (L.), Sitophilus oryzae (L.), Acanthoscelides obtectus (Say.), Tribolium castaneum (Herbst) and Lasioderma serricorne (F.). The mortality of adults of the insects treated with 2, 5, 5, 10 and 20 mg ml̠-1 concentrations of the extracts and xanthuhumol was counted after 24, 48, 72, 96 and 120 h. In order to determine the toxic effects of the substances tested against all tested insects, durations for 50% mortality of the adults, and LD50 values were also determined in the first 48 h by probit analysis. Our results also showed that xanthohumol was more toxic against the pests in comparison with the extracts applications. LD50 values for xanthohumol were found to be low dose as compared with the extracts. Xanthohumol was more toxic against S. granarius (L.) with 6.8 µg of LD50 value. Among the extracts, methanol extract was less effective than other extracts against all tested insects. The ethyl acetate extract of H. lupulus cones was the most effective extract against the tested pests. The quantitative amounts of xanthohumol in the extracts were determined using a high-performance liquid chromatography. The quantitative data indicated that amount of xanthohumol in the extracts increased with increase of polarity of the solvents used from methanol to dichloromethane. The methanol extract contained the high amount of xanthohumol with 5.74 g/100 g extract (0.46 g/100 g plant sample).

Interaction of Basal Foliage Removal and Late-Season Fungicide Applications in Management of Hop Powdery Mildew

Canopy management is an important aspect of control of powdery mildew diseases and may influence the intensity of fungicide applications required to suppress disease. In hop, powdery mildew (caused by Podosphaera macularis) is most damaging to cones when infection occurs during bloom and the juvenile stages of cone development. Experiments were conducted over 3 years to evaluate whether fungicide applications could be ceased after the most susceptible stages of cone development (late July) without unduly affecting crop yield and quality when disease pressure was moderated with varying levels of basal foliage removal. In experimental plots of 'Galena' hop, the incidence of leaves with powdery mildew was similar whether fungicides were ceased in late July or made in late August. Disease levels on leaves were unaffected by the intensity of basal foliage removal, whereas the intensity of basal foliage removal interacted with the duration of fungicide applications to affect disease levels on cones. Similar experiments conducted in large plots of 'Tomahawk' hop in a commercial hop yard similarly found no significant impact on disease levels on leaves from either the duration of fungicide applications or intensity of basal foliage removal. In contrast, on cones, application of fungicides into August had a modest, suppressive effect on powdery mildew. There was also some evidence that the level of powdery mildew on cones associated with fungicide treatment was influenced by the intensity of basal foliage removal. When fungicide applications ceased in late July, there was a progressive decrease in the incidence of cones with powdery mildew with increasing intensity of basal foliage removal. Removing basal foliage two to three times allowed fungicide applications to be terminated in late July rather than late August without diminishing disease control on cones, yield, or cone quality factors. Thus, this study further establishes that fungicide applications made during the early stages of hop cone development have the strongest effect on suppression of powdery mildew on cones. The additive effect of fungicide applications targeted to the periods of greatest cone susceptibility and canopy management to reduce disease favorability may obviate the need for fungicide applications later in the season. This appears to be a viable strategy in mature hop yards of certain cultivars when disease pressure is not excessively high.

Meta-Analysis Reveals a Critical Period for Management of Powdery Mildew on Hop Cones

Results of 28 field trials conducted over a 12-year period investigating management of hop powdery mildew caused by Podosphaera macularis were quantitatively summarized by meta-analysis to compare product efficacy and use patterns by mode of action as defined by Fungicide Resistance Action Committee (FRAC) groups. Availability of original observations enabled individual participant data meta-analysis. Differences in control of powdery mildew on leaves and cones were apparent among fungicide FRAC groups when individual products were evaluated over the course of a growing season. FRAC groups 13, 3, and U13 provided the most efficacious control of powdery mildew on leaves. Percent disease control on cones was influenced by midseason foliar disease and fungicide mode-of-action. FRAC 13 provided significantly better disease control on cones than all other groups except U13, 3, and premixes of 7 with 11. Disease control on leaves was similar when a rotational program of fungicides was used, independent of the modes of action, but improved on cones if FRAC groups 13 and 3 were both included compared with programs consisting of FRAC groups 11 and 3, 11 and 5, or 3 and 5. Disease control on cones was improved from 32 to 52%, on average, when the fungicide quinoxyfen (FRAC 13) was applied at least once during the early stages of cone development, defined in this analysis as 20 July to 10 August, as compared with all other treatments. Efficacy of disease control on cones by quinoxyfen was moderated by and interacted with the incidence of leaves with powdery mildew. Disease control on cones was further improved if two applications of quinoxyfen were made during this period. Collectively, these findings suggest that disease control during juvenile stages of cone development largely influences the success of fungicide programs and point to the critical importance of focusing management efforts during this stage of development, independent of what actual management strategy is employed.

Development of partial ontogenic resistance to powdery mildew in hop cones and its management implications

Knowledge of processes leading to crop damage is central to devising rational approaches to disease management. Multiple experiments established that infection of hop cones by Podosphaera macularis was most severe if inoculation occurred within 15 to 21 days after bloom. This period of infection was associated with the most pronounced reductions in alpha acids, cone color, and accelerated maturation of cones. Susceptibility of cones to powdery mildew decreased progressively after the transition from bloom to cone development, although complete immunity to the disease failed to develop. Maturation of cone tissues was associated with multiple significant affects on the pathogen manifested as reduced germination of conidia, diminished frequency of penetration of bracts, lengthening of the latent period, and decreased sporulation. Cones challenged with P. macularis in juvenile developmental stages also led to greater frequency of colonization by a complex of saprophytic, secondary fungi. Since no developmental stage of cones was immune to powdery mildew, the incidence of powdery mildew continued to increase over time and exceeded 86% by late summer. In field experiments with a moderately susceptible cultivar, the incidence of cones with powdery mildew was statistically similar when fungicide applications were made season-long or targeted only to the juvenile stages of cone development. These studies establish that partial ontogenic resistance develops in hop cones and may influence multiple phases of the infection process and pathogen reproduction. The results further reinforce the concept that the efficacy of a fungicide program may depend largely on timing of a small number of sprays during a relatively brief period of cone development. However in practice, targeting fungicide and other management tactics to periods of enhanced juvenile susceptibility may be complicated by a high degree of asynchrony in cone development and other factors that are situation-dependent.

Characterization of the formation of branched short-chain fatty acid:CoAs for bitter acid biosynthesis in hop glandular trichomes

Bitter acids, known for their use as beer flavoring and for their diverse biological activities, are predominantly formed in hop (Humulus lupulus) glandular trichomes. Branched short-chain acyl-CoAs (e.g. isobutyryl-CoA, isovaleryl-CoA and 2-methylbutyryl-CoA), derived from the degradation of branched-chain amino acids (BCAAs), are essential building blocks for the biosynthesis of bitter acids in hops. However, little is known regarding what components are needed to produce and maintain the pool of branched short-chain acyl-CoAs in hop trichomes. Here, we present several lines of evidence that both CoA ligases and thioesterases are likely involved in bitter acid biosynthesis. Recombinant HlCCL2 (carboxyl CoA ligase) protein had high specific activity for isovaleric acid as a substrate (K cat /K m = 4100 s(-1) M(-1)), whereas recombinant HlCCL4 specifically utilized isobutyric acid (Kcat/K m = 1800 s(-1) M(-1)) and 2-methylbutyric acid (Kcat/K m = 6900 s(-1) M(-1)) as substrates. Both HlCCLs, like hop valerophenone synthase (HlVPS), were expressed strongly in glandular trichomes and localized to the cytoplasm. Co-expression of HlCCL2 and HlCCL4 with HlVPS in yeast led to significant production of acylphloroglucinols (the direct precursors for bitter acid biosynthesis), which further confirmed the biochemical function of these two HlCCLs in vivo. Functional identification of a thioesterase that catalyzed the reverse reaction of CCLs in mitochondria, together with the comprehensive analysis of genes involved BCAA catabolism, supported the idea that cytosolic CoA ligases are required for linking BCAA degradation and bitter acid biosynthesis in glandular trichomes. The evolution and other possible physiological roles of branched short-chain fatty acid:CoA ligases in planta are also discussed.

Transcriptome analysis of bitter acid biosynthesis and precursor pathways in hop (Humulus lupulus)

BACKGROUND

Bitter acids (e.g. humulone) are prenylated polyketides synthesized in lupulin glands of the hop plant (Humulus lupulus) which are important contributors to the bitter flavour and stability of beer. Bitter acids are formed from acyl-CoA precursors derived from branched-chain amino acid (BCAA) degradation and C5 prenyl diphosphates from the methyl-D-erythritol 4-phosphate (MEP) pathway. We used RNA sequencing (RNA-seq) to obtain the transcriptomes of isolated lupulin glands, cones with glands removed and leaves from high α-acid hop cultivars, and analyzed these datasets for genes involved in bitter acid biosynthesis including the supply of major precursors. We also measured the levels of BCAAs, acyl-CoA intermediates, and bitter acids in glands, cones and leaves.

RESULTS

Transcripts encoding all the enzymes of BCAA metabolism were significantly more abundant in lupulin glands, indicating that BCAA biosynthesis and subsequent degradation occurs in these specialized cells. Branched-chain acyl-CoAs and bitter acids were present at higher levels in glands compared with leaves and cones. RNA-seq analysis showed the gland-specific expression of the MEP pathway, enzymes of sucrose degradation and several transcription factors that may regulate bitter acid biosynthesis in glands. Two branched-chain aminotransferase (BCAT) enzymes, HlBCAT1 and HlBCAT2, were abundant, with gene expression quantification by RNA-seq and qRT-PCR indicating that HlBCAT1 was specific to glands while HlBCAT2 was present in glands, cones and leaves. Recombinant HlBCAT1 and HlBCAT2 catalyzed forward (biosynthetic) and reverse (catabolic) reactions with similar kinetic parameters. HlBCAT1 is targeted to mitochondria where it likely plays a role in BCAA catabolism. HlBCAT2 is a plastidial enzyme likely involved in BCAA biosynthesis. Phylogenetic analysis of the hop BCATs and those from other plants showed that they group into distinct biosynthetic (plastidial) and catabolic (mitochondrial) clades.

CONCLUSIONS

Our analysis of the hop transcriptome significantly expands the genomic resources available for this agriculturally-important crop. This study provides evidence for the lupulin gland-specific biosynthesis of BCAAs and prenyl diphosphates to provide precursors for the production of bitter acids. The biosynthetic pathway leading to BCAAs in lupulin glands involves the plastidial enzyme, HlBCAT2. The mitochondrial enzyme HlBCAT1 degrades BCAAs as the first step in the catabolic pathway leading to branched chain-acyl-CoAs.

Flavonoid production in transgenic hop (Humulus lupulus L.) altered by PAP1/MYB75 from Arabidopsis thaliana L

Hop is an important source of secondary metabolites, such as flavonoids. Some of these are pharmacologically active. Nevertheless, the concentration of some classes as flavonoids in wild-type plants is rather low. To enhance the production in hop, it would be interesting to modify the regulation of genes in the flavonoid biosynthetic pathway. For this purpose, the regulatory factor PAP1/AtMYB75 from Arabidopsis thaliana L. was introduced into hop plants cv. Tettnanger by Agrobacterium-mediated genetic transformation. Twenty kanamycin-resistant transgenic plants were obtained. It was shown that PAP1/AtMYB75 was stably incorporated and expressed in the hop genome. In comparison to the wild-type plants, the color of female flowers and cones of transgenic plants was reddish to pink. Chemical analysis revealed higher levels of anthocyanins, rutin, isoquercitin, kaempferol-glucoside, kaempferol-glucoside-malonate, desmethylxanthohumol, xanthohumol, α-acids and β-acids in transgenic plants compared to wild-type plants.

Increase in cone biomass and terpenophenolics in hops ( Humulus lupulus L.) by treatment with prohexadione-calcium

Humulus lupulus L. (hop), a specialty crop bred for flavor characteristics of the inflorescence, is an essential ingredient in beer. Hop inflorescences, commonly known as hop cones, contain terpenophenolic compounds, which are important for beer flavoring and of interest in biomedical research. Hop breeders focus their efforts on increasing cone biomass and terpenophenolic content. As an alternative to traditional breeding, hops were treated with prohexadione-calcium (Pro-Ca), a growth inhibitor previously shown to have positive agronomic effects in several crops. Application of Pro-Ca to hop plants during cone maturation induced increases in cone biomass production by 1.5-19.6% and increased terpenophenolic content by 9.1-87.3%; however, some treatments also induced significant decreases in terpenophenolic content. Induced changes in cone biomass production and terpenophenolic accumulation were most dependent on cultivar and the developmental stage at which plants were treated.