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 (GA(3)) 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 GA(3) 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 GA(3) concentration, which showed the possibility that a high concentration of cytokinin preferentially prevents the effect of GA(3) on the development of axillary buds in hops. These results indicated that combined treatment of stem explants with GA(3) and cytokinin at appropriate concentrations is effective for the propagation of proliferated hop cultivars through shoot branching.