S04 - Session P6 - In-vitro regeneration of immature zygotic embryos of a dryland East African tree Melia volkensii for accelerated breeding and improvement

Authors: Titus Magomere *, Priscilla Kimani, Lydia Wamalwa, Kahiu Ngugi, Joseph Machua, Jackson Mulatya, Stefaan Werbrouck

Melia volkensii is an East African tree species that can be felled in 15 years and produces mahogany-type wood. The tree has great potential in commercial plantations in the arid and semi-arid regions of Kenya. The current breeding programme focuses on growth rate, growth habit and shape. However, the breeding process is slowed down by the long juvenile period and slow seed development. This study evaluated early embryo sequencing as a means to circumvent the slow seed maturation. Sixty days after flowering, 150 immature Melia fruits were harvested from each of ten clones from a clonal seed orchard. After superficial sterilisation of the fruits, first the seeds and then the embryos were isolated. Nine replicate embryos per treatment were transferred to Murashige & Skoog (MS) medium supplemented with: (1) Hormone-free, (2) BAP 0.75, 1.0, 1.5 mg/l, (3) Thidiazuron 0.25, 0.5, 1.0 mg/l, (4) IAA 2.5, 5.0, 10 mg/l and (5) 1.0 mg/l TDZ + 5.0 mg/l IAA. The cultures were incubated at 25+-1 ºC and evaluated every 21 days and transferred to fresh medium. After 147 days, data were recorded on survival, development of callus, somatic embryos and shoots. The average survival ranged from 25.8% to 61%. Embryos on hormone-free media developed thick fleshy shoots. All treatments developed callus, albeit to varying degrees. Media with TDZ induced best somatic embryos that subsequently developed into microshoots. In one immature embryo (C38), even 64 microshoots were counted. Microshoots could also develop from non-embryogenic callus on media containing 1.5 mg/l BAP. All these micro-shoots developed further on hormone-free MS media. This study shows that early embryo rescue can not only shorten the breeding process, but also produce many clones in one fell swoop, allowing their value to be tested immediately in different environments.