Induction of direct somatic organogenesis in onion (Allium cepa L.) using a two-step flower or ovary culture

A novel method for direct organogenesis in onion (Allium cepa L.) resulting in the formation of multiple shoot structures induced on mature flower bud s or ovaries in a two-step culture procedure is described. Flowers were cul tured on an induction medium containing 2 mg/l 2,4-dichlorophenoxyacetic ac id (2,4-D) and 2 mg/l 6-benzylaminopurine (BAP). After 6 days (superior to 3 or 12 days), flowers or extracted ovaries were transferred to a different iation medium containing 2 mg/l thidiazuron (TDZ). Medium solidification wi th gellan gum was superior to agar or agar/gellan gum mixture. A similar re generation frequency was achieved at high (100 gf) and lower (50 g/l) sucro se content. Regeneration was obtained from all 12 cultivars or inbred lines examined, although the efficiency and the occurrence of hyperhydricity var ied depending on genotype and procedure used. Studies of plant growth regul ators revealed that in the induction medium, the auxin 2,4-D was superior t o 5 mg/l naphthaleneacetic acid or picloram, which partially or completely inhibited regeneration. Omitting cytokinin in the induction medium or subst itution of BAP with 2 mg/l 2iP lowered regeneration, while substitution wit h 1 mg/l TDZ was equally effective. In the differentiation medium, lower co ncentrations of TDZ (1 and 0.5 mg/l) or substitution of TDZ with 5 mg/l BAP were equally or less effective.