Biotechnology and in vitro mutagenesis for banana improvement

Banana is the second most commonly grown fruit crop in Malaysia. Overall banana production has decreased due to the increasing threat of Fusarium wilt disease, high labour costs and marketing issues. This program was initiated to improve banana cultivars by induced mutations and biotechnology, especially to produce mutant varieties with improved traits such as Fusarium tolerance, short stature plants, early fruiting and high bunch weight. Banana shoot-tip cultures were most suitable for micropropagation for large-scale plant production. Commercial companies have adopted this method, and they produce 1.3 million plants annually, with approximately 0.5% somaclonal variation. However, the cost of production of in vitro plants could be reduced by low-cost micropropagation. Somaclonal variation has been effective in banana for the selection of useful somaclones, e.g. early flowering and tolerance to Fusarium wilt. In Novaria, an early flowering mutant, 7% of the plants survived in the Fusarium 'hot spot' for 3 years. Consequently, somaclonal variation is being used as a strategy to select useful mutants. Since bananas are mostly sterile polyploids, highly heterozygous, and propagated vegetatively, genetic improvement by cross-breeding is an insurmountable task. On the other hand, mutation techniques are highly suitable for banana improvement. Pisang Berrangan (AAA), a popular dessert banana variety, was gamma-irradiated at several dosages (20-60 Gy). The highest percentage of variants, such as changes in leaf coloration and leaf texture, leaf deformation, stunted growth etc., was obtained with 45 Gy treatment. The double-tray system was developed for the selection of mutants tolerant to Fusarium wilt. Among selected plants showing tolerance to Fusarium wilt, none survived field evaluation under Fusarium 'hot spot' conditions. Molecular characterization with RAPD of resistant and susceptible banana types showed random variation for different markers. However, four primers showed bands specific to either resistant or susceptible seed progenies, but could not provide information on the degree of co-dominance.