Abstract
BackgroundTheobroma cacao, the chocolate tree, is an important economic crop in East Africa, South East Asia, and South and Central America. Propagation of elite varieties has been achieved through somatic embryogenesis (SE) but low efficiencies and genotype dependence still presents a significant limitation for its propagation at commercial scales. Manipulation of transcription factors has been used to enhance the formation of SEs in several other plant species. This work describes the use of the transcription factor Baby Boom (BBM) to promote the transition of somatic cacao cells from the vegetative to embryonic state.ResultsAn ortholog of the Arabidopsis thaliana BBM gene (AtBBM) was characterized in T. cacao (TcBBM). TcBBM expression was observed throughout embryo development and was expressed at higher levels during SE as compared to zygotic embryogenesis (ZE). TcBBM overexpression in A. thaliana and T. cacao led to phenotypes associated with SE that did not require exogenous hormones. While transient ectopic expression of TcBBM provided only moderate enhancements in embryogenic potential, constitutive overexpression dramatically increased SE proliferation but also appeared to inhibit subsequent development.ConclusionOur work provides validation that TcBBM is an ortholog to AtBBM and has a specific role in both somatic and zygotic embryogenesis. Furthermore, our studies revealed that TcBBM transcript levels could serve as a biomarker for embryogenesis in cacao tissue. Results from transient expression of TcBBM provide confirmation that transcription factors can be used to enhance SE without compromising plant development and avoiding GMO plant production. This strategy could compliment a hormone-based method of reprogramming somatic cells and lead to more precise manipulation of SE at the regulatory level of transcription factors. The technology would benefit the propagation of elite varieties with low regeneration potential as well as the production of transgenic plants, which similarly requires somatic cell reprogramming.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0479-4) contains supplementary material, which is available to authorized users.
Highlights
Theobroma cacao, the chocolate tree, is an important economic crop in East Africa, South East Asia, and South and Central America
Identification of Baby Boom (BBM) T. cacao homolog To identify a candidate for a T. cacao BBM homologue, a tBlastN analysis was performed against the T. cacao genome [24] using the Arabidopsis BBM (AT5G17430) protein sequence [13] as a query
To test whether TcBBM expression could be used as a biomarker for cacao SE initiation, we studied its gene expression levels throughout the process of primary and secondary somatic embryogenesis (Figure 3A) (A set of descriptive terms used to describe the cacao SE system are listed in Additional file 2)
Summary
Theobroma cacao, the chocolate tree, is an important economic crop in East Africa, South East Asia, and South and Central America. Theobroma cacao, the chocolate tree, is the basis for an 83 billion dollar a year retail chocolate industry and is a critical component of numerous economies in West Africa, South East Asia, South and Central America. This industry is predicting a shortage of cocoa (fermented and dried cacao seeds) in the near future due to an increase in chocolate demand and the recent spread of devastating. The lack of understanding of the mechanisms that govern this dramatic reprogramming of somatic cells represents the greatest limitation to the rational improvement of this method for the propagation of many important species, and remains a critically important aspect of producing transgenic plants
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