DNA sequence and expression variation of hop (Humulus lupulus) Valerophenone Synthase (VPS), a key gene in bitter acid biosynthesis

† Background The hop plant (Humulus lupulus) is a source of many secondary metabolites, with bitter acids essential in the beer brewing industry and others having potential applications for human health. This study investigated variation in DNA sequence and gene expression of valerophenone synthase (VPS), a key gene in the bitter acid bio-synthesis pathway of hop. † Methods Sequence variation was studied in 12 varieties, and expression was analysed in four of the 12 varieties in a series across the development of the hop cone. † Results Nine single nucleotide polymorphisms (SNPs) were detected in VPS, seven of which were synonymous. The two non-synonymous polymorphisms did not appear to be related to typical bitter acid profiles of the varieties studied. However, real-time quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analysis of VPS expression during hop cone development showed a clear link with the bitter acid content. The highest levels of VPS expression were observed in two triploid varieties, ‘Symphony’ and ‘Ember’, which typically have high bitter acid levels. †Conclusions In all hop varieties studied, VPS expression was lowest in the leaves and an increase in expression was consistently observed during the early stages of cone development