Abstract
Theobroma cacao, the source of cocoa, is a crop of particular importance in many developing countries. Availability of elite planting material is a limiting factor for increasing productivity of Theobroma cacao; therefore, the development of new strategies for clonal propagation is essential to improve farmers’ incomes and to meet increasing global demand for cocoa. To develop a more efficient embryogenesis system for cacao, tissue was transformed with a transgene encoding a fusion of Leafy Cotyledon 2 (TcLEC2) to a glucocorticoid receptor domain (GR) to control nuclear localization of the protein. Upon application of the glucocorticoid dexamethasone (dex), downstream targets of LEC2 involved in seed-development were up-regulated and somatic embryos (SEs) were successfully regenerated from TcLEC2-GR transgenic flower and leaf tissue in large numbers. Immature SEs regenerated from TcLEC2-GR leaves were smaller in size than immature SEs from floral tissue, suggesting a different ontogenetic origin. Additionally, exposure of TcLEC2-GR floral explants to dex increased the number of SEs compared to floral explants from control, non-transgenic trees or from TcLEC2-GR floral explants not treated with dex. Testing different durations of exposure to dex indicated that a three-day treatment produced optimal embryo regeneration. Leaf derived SEs were successfully grown to maturity, converted into plants, and established in the greenhouse, demonstrating that these embryos are fully developmentally competent. In summary, we demonstrate that regulating TcLEC2 activity offers a powerful new strategy for optimizing somatic embryogenesis pipelines for cacao.
Highlights
As the source of cocoa beans, Theobroma cacao is an important tropical tree crop and a major export from many developing countries
Our prior work demonstrated that constitutive overexpression of TcLEC2 in cacao tissues resulted in an uncontrolled embryogenic chain reaction [24]
We decided to regulate the Somatic embryogenesis in cacao leaves effects of TcLEC2 overexpression by fusing the CDS to the glucocorticoid receptor (GR)
Summary
As the source of cocoa beans, Theobroma cacao is an important tropical tree crop and a major export from many developing countries. 5–6 million farmers grow cacao, the centerpiece of the multi-billion-dollar chocolate industry. The majority of cocoa comes from lowinput, small-holder farms [1]. Production on these farms is significantly hampered by disease. Pairwise Plants and Essenlix Corporation provided support in the form of salaries for authors ASF and YZ respectively, but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section
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