Two distinct phase of glandular trichome development in hop (Humulus lupulus L.)

Abstract

Hop bitter resins and essential oils are biosynthesized and accumulated in lupulin glands of hop (Humulus lupulus L.). Valerophenone synthase (VPS) is involved in the first steps of biosynthesis of bitter resins, and the VPS gene is specifically expressed in lupulin glands on bracteoles and leaves. In this study, spatial and temporal expression of the VPS gene was studied using in-situ hybridization. No gene signals were observed in the early stage of lupulin gland development, but strong expression was observed when the cuticle was slightly detached from the glandular head cells. Expression levels were remarkably reduced with accumulation of hop bitter resins and essential oils in the sub-cuticular space. The findings suggest that development of lupulin glands is strictly divided into a growth phase and biosyntheticsecretory phase.

Number and size of lupulin glands, glandular trichomes of hop (Humulus lupulus L.), play a key role in contents of bitter acids and polyphenols in hop cone

“…Generally, alpha/beta ratios have been slightly higher in hop cones than lupulin glands (Likens et al ., ; Xu et al ., ). That is mainly caused by time of harvest or sampling because α‐acid contents in lupulin glands (Likens et al ., ; Hirosawa et al ., ; De Keukeleire et al ., ; Sugiyama et al ., ), volume of lupulin glands (Srečec et al ., ) and hop cone (Kavalier et al ., ) have been increasing during hop cone maturation. The contents of α‐ and β‐acids correlated in hop cones with r = 0.646 but negatively with r = −0.429 in lupulin glands within used hop genotypes.…”

“…Glandular trichomes are most abundant in the inflorescence but also present on vegetative leaves (Wang et al ., ; Kavalier et al ., ). Two types of glandular trichomes have been described in hops: peltate lupulin glands, which are large and contain 100–200 cells, and bulbous glands, which are much smaller, containing eight cells at maturity (Sugiyama et al ., ). Lupulin gland development can be classified into two main stages: the growth stage and the biosynthetic‐secretory stage.…”

“…The resulting mature lupulin gland is a biconal structure, filled with secretions contained by the cuticle and subcuticular wall (Kim & Mahlberg, ). Developmental stages of bulbous glands are difficult to identify due to their small size and rapid development, and have not been as well studied (Sugiyama et al ., ). Coinciding with the development of lupulin glands, terpenophenolics accumulate over the progression of cone development (De Keukeleire et al ., ).…”

Quantitative trait loci in hop (Humulus lupulus L.) reveal complex genetic architecture underlying variation in sex, yield and cone chemistry

“…The influence of polyphenols in beer brewing are not thoroughly understood, but several polyphenol compounds have been found to have potential pharmaceutical applications, particularly 8-prenylnarigenin as a phytoestrogen [52] and xanthohumol as a cancer chemopreventative agent [53]. Secondary metabolites accumulate in high concentrations in lupulin glands, which are peltate glandular trichomes found in great density on the bracteoles in hop inflorescences (cones) [54,55]. There is evidence to show that the lupulin glands may also be involved in the biosynthesis of the secondary metabolites [56].…”

Morphogenesis, volume and number of hop (Humulus lupulus L.) glandular trichomes, and their influence on alpha-acid accumulation in fresh bracts of hop cones

“…Moreover, they described how secretions contributed to the thickening of the cuticle and created a network of striae distributed throughout the cuticle. The development of lupulin glands is strictly divided into a growth phase and biosynthetic-secretory phase (SUGIYAMA et al 2006). Activity of VPS gene (valerophenone synthase), which is involved in the first steps of bitter resin (a-acid) biosynthesis, was strong when the cuticle was slightly detached from the glandular head cells.…”