In vitro regeneration of immature zygotic embryos of Melia Volkensii Gürke for accelerated breeding

Melia volkensii is a dryland mahogany tree species in East Africa that can be harvested for timber in 15 years and has potential for commercial planting in arid and semi-arid areas. The current breeding programme focusing on growth rate, growth habit and shape is delayed by the long juvenile period and slow seed development. This study evaluated early embryo regeneration as a way to circumvent slow seed maturation. Sixty days after pollination (DAP), zygotic embryos were isolated from immature fruits and transferred to Murashige & Skoog (MS) medium without plant growth regulator (PGR) or with different concentrations of 6-benzylaminopurine (BAP), thidiazuron (TDZ), indole-3-acetic acid (IAA) and a combination of TDZ and IAA. After 147 days, survival ranged from 2 to 44%. Embryos on PGR-free media developed thick fleshy shoots. All treatments developed callus to varying degrees and TDZ induced somatic embryos that subsequently developed into shoot clusters. Microshoots also developed from non-embryogenic callus on media containing 1.5 mg/l BAP and directly on media containing IAA. These shoots developed further on hormone-free media and were then rooted on media containing indole-3-butyric acid (IBA). Overall, seedlings were regenerated in 207 days after pollination, compared with the natural seed development period of nearly 390 days, yielding a potential time saving of 47% for progeny generation. This study demonstrates that early embryo regeneration can not only shorten the breeding process, but also rapidly produce many clones so that their value can be tested immediately in different environments.