Regulation of Somatic Embryogenesis in Higher Plants

Abstract

Somatic embryogenesis is the developmental process by which somatic cells undergo restructuring to generate embryogenic cells. These cells then go through a series of morphological and biochemical changes that result in the formation of a somatic or non-zygotic embryo capable of regenerating plants. Somatic embryogenesis represents a unique developmental pathway that includes a number of characteristic events: dedifferentiation of cells, activation of cell division, and reprogramming of their physiology, metabolism, and gene expression patterns. Valuable studies have elucidated the developmental processes pertaining to the control parameters of somatic embryogenesis. These studies have emphasized the transitional state from somatic to embryogenic cells, identified differentially expressed genes in nonembryogenic and embryogenic calli, isolated varieties of genes that are likely involved in the embryogenic pathway, compared specific gene products that accumulate during different stages of somatic embryogenesis, and discovered molecular or protein markers for somatic embryogenesis. An understanding of embryogenic pathway initiation, and origin of somatic embryos (unicellular or multicellular) is critical to scientific and biotechnological applications. Cell tracking has been successfully applied to determine the fate of embryogenic cells. Identification of hormone-inducible genes has yielded clues to the control of gene expression during embryogenic development. Characterization of genes taking part in signal transduction pathways, such as somatic embryogenesis receptor-like kinases, has generated great interest in the switching of several signal cascades during somatic embryogenesis, and has identified how genes encoding transcription factors such as BBM, LEC1, and LEC2 could be used to regulate somatic embryo development. Protein markers are useful probes for defining embryogenic potential and for marking different phases in plant development. This review provides a systematic and comprehensive analysis of current advances in the development of somatic embryogenesis, as well as the cellular and molecular mechanism of somatic embryogenesis in higher plants. The developmental pathway, differential gene expression, and extracellular protein markers during somatic embryogenesis are especially emphasized.