Abstract
BackgroundThe Arabidopsis thaliana LEC2 gene encodes a B3 domain transcription factor, which plays critical roles during both zygotic and somatic embryogenesis. LEC2 exerts significant impacts on determining embryogenic potential and various metabolic processes through a complicated genetic regulatory network.ResultsAn ortholog of the Arabidopsis Leafy Cotyledon 2 gene (AtLEC2) was characterized in Theobroma cacao (TcLEC2). TcLEC2 encodes a B3 domain transcription factor preferentially expressed during early and late zygotic embryo development. The expression of TcLEC2 was higher in dedifferentiated cells competent for somatic embryogenesis (embryogenic calli), compared to non-embryogenic calli. Transient overexpression of TcLEC2 in immature zygotic embryos resulted in changes in gene expression profiles and fatty acid composition. Ectopic expression of TcLEC2 in cacao leaves changed the expression levels of several seed related genes. The overexpression of TcLEC2 in cacao explants greatly increased the frequency of regeneration of stably transformed somatic embryos. TcLEC2 overexpressing cotyledon explants exhibited a very high level of embryogenic competency and when cultured on hormone free medium, exhibited an iterative embryogenic chain-reaction.ConclusionsOur study revealed essential roles of TcLEC2 during both zygotic and somatic embryo development. Collectively, our evidence supports the conclusion that TcLEC2 is a functional ortholog of AtLEC2 and that it is involved in similar genetic regulatory networks during cacao somatic embryogenesis. To our knowledge, this is the first detailed report of the functional analysis of a LEC2 ortholog in a species other then Arabidopsis. TcLEC2 could potentially be used as a biomarker for the improvement of the SE process and screen for elite varieties in cacao germplasm.
Highlights
The Arabidopsis thaliana LEC2 gene encodes a B3 domain transcription factor, which plays critical roles during both zygotic and somatic embryogenesis
In the Arabidopsis genome, 87 genes were previously annotated as B3 domain containing genes that were further classified into five different families: auxin response factor (ARF), abscisic acid-insensitive3 (ABI3) of which AtLEC2 is a member, high level expression of sugar inducible (HSI), related to ABA INSENSITIVE 3 (ABI3)/VP1 (RAV) and reproductive meristem (REM) [31]
In order to identify a putative ortholog of AtLEC2 in cacao, the full-length amino acid sequence of Arabidopsis AtLEC2 was blasted against the predicted proteome of the Belizean Criollo genotype (B97-61/B2) using blastp algorithm with E-value cut-off of 1e−5 [33], which resulted in identification of 13 possible candidate genes (Additional file 1)
Summary
The Arabidopsis thaliana LEC2 gene encodes a B3 domain transcription factor, which plays critical roles during both zygotic and somatic embryogenesis. The tropical tree Theobroma cacao L. is cultivated as a cash crop in many developing countries and provides the main ingredients for chocolate production. Cacao trees are generally highly heterozygous and when propagated by seed, only a small fraction of individuals are high producing [2,3,4]. Vegetative propagation systems provide a means to avoid the issue of trait variation, through cloning of the top elite individual genotypes. In vitro somatic embryogenesis (SE) tissue culture offers an approach to speed up the development and deployment of genetically improved genotypes because of its potentially very high multiplication rate and scalability. A deeper understanding of the genes and mechanisms involved in regulating the SE process in cacao could potentially lead to improvement of SE methods for commercial plant production. To characterize the mechanisms regulating embryogenesis, we have chosen a translational biology approach, leveraging the knowledge gained from the model plant Arabidopsis
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