Spontaneous induction of tetraploidy in hop using adventitious shoot regeneration method

• Published in: Plant breeding 2007-08, Vol.126 (4), p.416-421
• Main Authors: Skof, S, Bohanec, B, Kastelec, D, Luthar, Z
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Biological and medical sciences; cytokinin; flow cytometry; Fundamental and applied biological sciences. Psychology; Genetics and breeding of economic plants; Humulus lupulus; regeneration; tetraploidy
• ISSN: 0179-9541; 1439-0523
• DOI: 10.1111/j.1439-0523.2007.01378.x

In the process of breeding triploid hop cultivars, diploid varieties need to be raised to the tetraploid level. Existing methods are predominantly based on treatment of the apices with antimitotic substances, in vivo or in vitro, resulting in a reported maximum of 26% tetraploids and a large proportion (over 60%) of mixoploids. This study was set-up to test whether an adventitious shoot regeneration protocol can be used as an alternative. In the first part of the experiments, regeneration media and a choice of explants (internodes, petioles and leaf discs) were optimized using three hop cultivars with putatively low, medium or high adventitious shoot regeneration capacity. On an optimized induction medium supplemented with 6 mg/l N⁶-(2-isopentenyl) adenine (2iP), a regeneration rate up to 17.7% was obtained from cultured internodes, even of the most recalcitrant 'Aurora', while the regeneration rate of the most responsive 'Tettnanger' was 56.9%. Flow cytometric analysis of ploidy level revealed a high frequency of tetraploid induction, the highest (58.6%) being in 'Savinjski golding'. In 616 regenerants of all three cultivars, 30.4% were found to be tetraploid, the others were diploid. Cytokinin content was found to be of minor importance in tetraploid induction. The high frequency of spontaneous tetraploid induction and, in particular, the complete absence of mixoploids makes this protocol an alternative to the currently established methods of chromosome doubling based on the use of antimitotic substances. The occurrence of a high proportion of tetraploid adventitious regenerants in hop is discussed in view of certain other biotechnological applications.