In Vitro Propagation of Humulus lupulus through the Induction of Axillary Bud Development

Humulus lupulus (hop) is a necessary material for beer brewing. Improved breeding cultivars of hops with enhanced tolerance to environmental stresses, such as drought and heat stress, accompanying climate change have been developed. However, a propagation system, which is needed for the proliferation of new cultivars, is not currently available for hops. In this study, we found that treatment of stem explants with 0.01-0.05 ppm gibberellic acid (GA₃) induced the development of axillary buds in the hop cultivar Kirin-2, resulting in the proliferation of shoot branching. Additionally, 0.01 ppm benzyl adenine (BA) enhanced the development of axillary buds formed in response to 0.05 ppm GA₃ in various hop cultivars, particularly Nugget. The development of axillary buds was strongly repressed by the application of 0.05 ppm BA at a concentration equal to the 0.05 ppm GA₃ concentration, which showed the possibility that a high concentration of cytokinin preferentially prevents the effect of GA₃ on the development of axillary buds in hops. These results indicated that combined treatment of stem explants with GA₃ and cytokinin at appropriate concentrations is effective for the propagation of proliferated hop cultivars through shoot branching.