Glucocorticoid receptor-regulated TcLEC2 expression triggers somatic embryogenesis in Theobroma cacao leaf tissue

PLoS One. 2018 Nov 26;13(11):e0207666. doi: 10.1371/journal.pone.0207666. eCollection 2018.

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Cacao / drug effects
  • Cacao / growth & development
  • Cacao / metabolism*
  • Dexamethasone / pharmacology
  • Gene Expression Regulation, Plant / drug effects
  • Plant Leaves / drug effects
  • Plant Leaves / metabolism
  • Plant Proteins / genetics
  • Plant Proteins / metabolism*
  • Plant Somatic Embryogenesis Techniques
  • Plants, Genetically Modified / growth & development
  • Plants, Genetically Modified / metabolism
  • RNA, Plant / metabolism
  • Receptors, Glucocorticoid / genetics*
  • Receptors, Glucocorticoid / metabolism
  • Transcription Factors / genetics
  • Transcription Factors / metabolism*

Substances

  • Plant Proteins
  • RNA, Plant
  • Receptors, Glucocorticoid
  • Transcription Factors
  • Dexamethasone

Grants and funding

This work was supported by The Pennsylvania State University College of Agricultural Sciences, the Huck Institutes of the Life Sciences, the Penn State Endowed Program in Molecular Biology of Cacao, NSF Plant Genome Research Award 1546863 to MG, and by the USDA National Institute of Food and Agriculture, Federal Appropriations under Project PEN04569 and Accession number 1003147 to MG. 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.