Humulus lupulus, the common hop or hops, is a species of flowering plant in the hemp family Cannabaceae, native to Europe, western Asia and North America. It is a perennial, herbaceous climbing plant which sends up new shoots in early spring and dies back to a cold-hardy rhizome in autumn. It is dioecious (separate male and female plants).
Hops are sometimes described as vine plants rather than vines because they have stiff downward facing hairs that provide stability and allow them to climb. These shoots allow H. lupulus to grow anywhere from 4.6 to 6.1 metres (15 to 20 ft). Hops have fragrant, wind-pollinated flowers that attract butterflies.
The female cone-shaped fruits from H. lupulus are used by breweries to preserve and flavor beer, and so H. lupulus is widely cultivated for use by the brewing industry. The fragrant flower cones, known as hops, impart a bitter flavor, and also have aromatic and preservative qualities.H. lupulus contains myrcene, humulene, xanthohumol, myrcenol, linalool, tannins, and resin.
The genus name Humulus is a medieval name that was at some point Latinized after being borrowed from a Germanic source exhibiting the h•m•l consonant cluster, as in Middle Low German homele.
According to Soviet Iranist V. Abaev this could be a word of Sarmatian origin which is present in the modern Ossetian language (Ossetian: Хуымæллæг) and derives from proto-Iranian hauma-arayka, an Aryan haoma.
From Sarmatian dialects this word spread across Eurasia, thus creating a group of related words in Turkic, Finno-Ugric, Slavic and Germanic languages (see Russian: хмель, Chuvash хăмла, Finnish humala, Hungarian komló, Mordovian комла, Avar хомеллег).
The specific epithet lupulus is Latin for "small wolf". The name refers to the plant's tendency to strangle other plants, mainly osiers or basket willows (Salix viminalis), like a wolf does a sheep. Hops could be seen growing over these willows so often that it was named the willow-wolf.
Humulus lupulus is a perennial herbaceous plant up to 10 meters tall, living up to 20 years. It has simple leaves with 3-5 deep lobes that can be opposite or alternate . The staminate (male) flowers do not have petals, while the pistillate (female) flowers’ petals completely cover the fruit. The cones found on female plants are called strobili. The fruit of H. lupulus is an achene, meaning that the fruit is dry and does not split open at maturity. The achene is surrounded by tepals and lupulin-secreting glands are concentrated on the fruit.
Humulus lupulus grows best in the latitude range of 38°-51° in full sun with moderate amounts of rainfall. It uses the longer summer days as a cue for when to flower,  which is usually around July/ August.
Humulus lupulus can cause dermatitis to some who handle them. It is estimated that about 1 in 30 people are affected by this.
The five varieties of this species (Humulus lupulus) are:
- H. l. var. lupulus – Europe, western Asia
- H. l. var. cordifolius – eastern Asia
- H. l. var. lupuloides (syn. H. americanus) – eastern North America
- H. l. var. neomexicanus - western North American.
- H. l. var. pubescens – midwestern and eastern North America 
Many cultivars are found in the list of hop varieties. A yellow-leafed ornamental cultivar, Humulus lupulus 'Aureus', is cultivated for garden use. It is also known as golden hop, and holds the Royal Horticultural Society's Award of Garden Merit (AGM).
Use in brewing
The chemical compounds found in H. lupulus are main components in flavoring and bittering beer. Some other compounds help with creating foam in beer. Chemicals such as linalool and aldehydes contribute to the flavor of beer. The main components of bitterness in beer are iso-alpha acids, with many other compounds contributing to the overall bitterness of beer. Until the Middle Ages, Myrica gale was the most common plant used for brewing beer.H. lupulus took off as a flavoring agent for beer because it contains preserving agents, making the beer viable for longer.
Pests and diseases
- Damson hop aphid (Phorodon humuli)
- Two spotted spider mite (Tetranychus urticae)
- Japanese Beetle (Popillia japonica)
- Comma butterfly (Polygonia c-album)
Taste and aroma
Hops are unique for containing secondary metabolites, flavonoids, oils, and polyphenols that impact the flavor of the products they are common in, such as beer. The bitter flavors in hops can be accounted for by acids composed of prenylated polyketides (a group of secondary metabolites), which highly impact the taste of hop-based products.  Multiple genes have been identified as factors in the expression of taste including O-methyltransferase 1, geranyl diphosphate synthase, and chalcone synthase. Genomic analyses have shown evidence that the intervention of humans in the selection process of the hop over the thousands of years it has been cultivated have provided noticeable enhancements in aroma and bitterness as well as selection of varieties with high yield rates.
Relation to Cannabis sativa
The hop is within the same family of plants such as hemp and marijuana, called Cannabaceae.  The hop plant diverged from Cannabis sativa (C. sativa) over 20 million years ago and has evolved to be three times the physical size.  The hop and C. sativa are estimated to have approximately a 73% overlap in genomic content.  The overlap between enzymes includes polyketide synthases and prenyltransferases.  The hop and C. sativa also have significant overlap in the cannabidiolic acid synthase gene, which is expressed in the tissues of the leaves in both plants. 
Flowering, growth, and stress response
Predicted genes in homologous primary contigs have been identified as accounting for various traits expressed via variation in the growth, flowering, and stress responses in the plant. These homologous primary contigs correspond to regions with large amounts of sequence variation. Genes in the hop that contain higher rates of sequence divergence in homologous primary contigs (overlapping DNA sequences inherited by a common ancestor) have been attributed to the expression of flowering, growth and responses to (both antibiotic and biotic) stress in the plant. The responses to stress are thought to manifest in the distinct differences and difficulties in the cultivation processes between geographically popular varieties of the hop plant.  Outside environmental stress, such as changes in temperature and water availability has also been shown to significantly alter the transcriptome and incite reductions in genes known to be involved in the synthesis of secondary metabolites (including bitter acids), which are organic compounds produced that do not impact development or reproduction of hops. Environmental stress has also been shown to reduce expression of the valerophenone synthase gene, which is known to be an essential genetic component in the regulation of bitter acid production. This shows that impacts of outside stress on Humulus lupulus likely has a direct implication of the expression of the bitter flavor that remains an essential component of the popularity of the plant. 
Future research possibilities
Because of the growing understanding regarding the hop's overlap in gene structures with cannabidiolic acid synthase, the precursor structure to Cannabidiol, there is a gap in general understanding about potential unknown compounds and benefits in hops. As the understanding of the health benefits available in Cannabidiol increases, there is a growing demand to further investigate the overlap between cannabidiolic acid synthase and Humulus lupulus. 
The genome of Humulus lupulus is relatively large and has been shown to be a similar size to the human genome. The complexity of the hop genome has made it difficult to understand and identify unknown genetic properties, however with the growing availability of accessible sequencing, there is room for more advanced understanding of the plant.  Because of the growing concern of climate change, and the assumption that there will be an increase of heat waves, it is likely that growing large yields of hops could become more difficult. This could result in changes to the transcriptome of the hop, or result in a decrease of certain varieties, leaving less room for further research. 
- Humulus lupulus contains xanthohumol, which is converted by large intestine bacteria into the phytoestrogen 8-prenylnaringenin, which may have a relative binding affinity to estrogen receptors  as well as potentiating effects on GABAA receptor activity
- Humulus lupulus extract is antimicrobial, an activity which has been exploited in the manufacture of natural deodorant.
- Spent H. lupulus extract has also been shown to have antimicrobial and anti-biofilm activities, raising the possibility this waste product of the brewing industry could be developed for medical applications.
- Extracts of the bitter alpha-acids present in H. lupulus have been shown to decrease nocturnal activity, acting as a sleep aide, in certain concentrations.
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