Annals of Botany

Promoting the hop’s bitter harvest

A combination of beer and botany is one that will appeal to many of our readers. Some will have strong views about what makes a good beer and will name enthusiastically their favourite brews. It is widely known that hop (Humulus lupulus) is used as a source of the bitter flavours of beer; more specifically it is secondary metabolites such as bitter acids, produced by the female cone, that are the relevant compounds. Moreover, as pointed out by Castro et al. (Hobart and Bellerive, Tasmania; pp. 265–273) there is now an interest in the medicinal properties of some of these metabolites, providing a further motivation for study. The authors review evidence showing that metabolite profiles of different varieties are very constant and thus can be used in varietal identification. What then controls the differences between varieties? In this paper, the authors have focused on the enzyme valerophenone synthase (VPS), which catalyses the first step in the pathway leading specifically to the bitter acids. Two main approaches were used. In the first, the VPS genes from 13 hop varieties were sequenced, revealing nine single-nucleotide polymorphisms (SNPs). Seven of these were synonymous (did not result in an amino change in the protein); the two non-synonymous polymorphisms did not correlate with bitter acid content. The second approach used real-time quantitative PCR to investigate VPS expression in one low-acid and three high-acid varieties. As an aside, we note that they used a novel internal control, poly-ubiquitin, for this. There was a clear correlation between the level of VPS gene expression in the developing cones and the accumulation of bitter acids: expression was ten-fold lower in the low-acid variety than in the high-acid varieties. Differences in expression levels are often mediated by differences in the interactions between regulatory molecules and gene promoters. Detailed analysis of VPS gene promoters is thus an obvious next step.