Histology and Histochemistry of Somatic Embryogenesis

Abstract

The seminal reports of somatic embryogenesis in the umbellifers Oenanthe aquatica by Harry Waris in 1957 (Krikorian and Simola, Physiol Plant 105:348–355 (1999)) and carrot (Steward et al., Am J Bot 45:693–703 (1958)) paved the way for current studies on the mechanisms involved in the transition of somatic cells to the embryogenic state for many species (Fehér et al., Plant Cell Tiss Org 74:201–228, 2003; Elhiti and Stasolla, Plant embryo culture: methods and protocols, Humana Press, New York, 2011; Fehér, Biochim Biophys Acta 1849:385–402, 2015). Somatic embryogenesis has been a focal point of research in plant development. This process relies on somatic cell totipotency (i.e., the capacity to regenerate the entire plant from single somatic cells), and it has been long used in biotechnological breeding techniques as an efficient system for regenerating plants in a large-scale basis. Also, because it is a unique system which includes a large number of events—such as physiological reprogramming of explants as well as changes in the gene expression and cell division patterns, and in cell fate (Fehér, Acta Biol Szeged 52:53–56, 2008; Rose et al., Plant developmental biology-biotechnological perspectives. Springer, Heidelberg, 2010)—somatic embryogenesis has also become an appropriate method for studying the morphophysiological and molecular aspects of cell differentiation. The comprehension of the developmental events during the induction phase as well as the development of somatic embryos is essential to regulate each stage of the somatic embryogenesis developmental program efficiently. Additionally, it may be useful for the development of efficient protocols for somatic embryogenesis induction and validation in genetic transformation systems (Fehér et al., Plant Cell Tiss Org 74:201–228, 2003; Yang and Zhang, Crit Rev Plant Sci 29:36–57, 2010; Rocha and Dornelas, CAB Rev 8:1–17, 2013; Mahdavi-Darvari et al., Plant Cell Tiss Org 120:407–422, 2015). Anatomical and ultrastructural studies have contributed to the better understanding of the basic cellular mechanisms involved in the acquisition of competence and histodifferentiation of somatic embryos (Canhoto et al., Ann Bot 78:513–521, 1996; Verdeil et al., Trends Plant Sci 12:245–252, 2001; Moura et al., Plant Cell Tiss Org 95:175–184, 2008; Moura et al., Sci Agric 67:399–407, 2010 ; Almeida et al., Plant Cell Rep 31:1495–1515, 2012; Rocha et al., Protoplasma 249:747–758, 2012; Rocha et al., Plant Cell Tiss Org 120:1087–1098, 2015; Rocha et al., Protoplasma 111:69–78, 2016). In addition, histochemical methods have enabled the monitoring of the mobilization and synthesis of reserve compounds during embryogenic development. This way, the dynamic and fate of cells committed to the somatic embryogenesis can be supported by microscopy techniques. The formation of an embryogenic callus and the subsequent differentiation of somatic embryos can be analyzed over time, and the cytological changes that have occurred during these processes can also be of great value, by associating the observed cytological changes with the expression patterns of several genes from the initial explant through competence acquisition to the formation of somatic embryos. Somatic embryogenesis has been intensively studied over the past decades. A range of descriptive studies using light and electron microscopy has provided a detailed characterization of histocytological events underlying the progression from somatic cells to the formation of embryos. Here, we review recent studies that have advanced our understanding of the anatomical and ultrastructural changes that characterize the somatic embryogenesis developmental pathway.