Developmental biology of somatic embryogenesis

Somatic embryogenesis (SE) is a remarkable developmental process enabling nonzygotic plant cells to form embryos and, ultimately, fertile plants. It is an expression of totipotency. This chapter initially considers the genotypic component and the progenitor stem cells where SE is induced to form the initial asymmetric division of the somatic embryogenesis program. These cells are part of a stem cell niche dependent on the surrounding cells. Recent evidence is discussed that, before the SE pathway can be initiated, a GA-modulated pathway that represses inappropriate embryogenesis needs to be derepressed. The current understanding of how stress and hormones induce the activation of specific SE genes is examined. Important stress components are reactive oxygen species and the signalling of stress-related hormones. The action of the key developmental hormones auxin and cytokinin in relation to developmental genes is considered and, based on current understanding, a model is presented for the mechanism of SE. While there are many SE applications in contemporary biotechnology, understanding the reprogramming process associated with SE remains an important question for developmental biology.