Induction and selection of potentially useful mutants in banana.

Induced mutation techniques may be useful for generating the genetic variation needed to improve banana, a crop in which sexual reproduction is very limited. Embryogenic cell suspensions (ECS) offer a means of producing useful mutants, but the system remains to be fully optimised. Meristem cultures, traditionally used in mutagenesis experiments in banana, lead to chimerism, due to their multicellular origin, hampering the development of homohistont mutants. ECS, in contrast, allow large cell populations to be treated under controlled conditions, and chimerism can be avoided if somatic embryos arise from single cells. Colchicine treatment of ECS followed by flow cytometry of regenerated plants showed that no plants were mixoploid (chimeric). In experiments to optimise conditions for generating mutants from ECS, flow cytometric analysis of cell cycle kinetics proved more informative than analysis of growth curves for determining the optimal timing of irradiation. ECS fresh weight gain and regeneration capacity indicated that the optimal irradiation dose ranges from 50 to 75 Gy for ‘Williams’ (AAA, Cavendish subgroup) and ‘Three Hand Planty’ (AAB, plantain subgroup). Somaclonal variation, probably associated with chromosome instability, can occur among plantlets produced from ECS and could interfere with this approach to mutagenesis. Variable DNA ploidy was detected in ECS and in regenerated plants. The random nature of mutation calls for the screening of thousands of individuals. An early mass-screening method that relies on infiltration of juglone, a toxic metabolite of Mycosphaerella fijiensis, causal agent of black leaf streak, was developed. Fifteen putative mutants out of approximately 4000 regenerated plants regenerated were found to tolerate juglone and may have resistance to M. fijiensis. This research should increase the efficacy of mutation induction in Musa and could revolutionise the use of mutagenesis, not only to obtain new and useful mutants, but also to identify important genes and their function. INTRODUCTION Bananas and plantains (Musa spp.) are a staple food for millions of people and rank among the top five food commodities. Only 13% of the total production, mainly dessert bananas, is produced for export. Hence, the fruit is of vital importance for food