Two distinct phases 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 biosynthetic-secretory phase.