The Many Ways of Somatic Embryo Initiation

Abstract

It is widely believed that all cells of a plant are totipotent and can regenerate the whole organism. This view is supported by uncountable experimental observations demonstrating the regrowth of whole plants from various explants, even from single cells. However, recent investigations have demonstrated that plant regeneration may proceed via transdifferentiation of meristems or root meristem-like callus tissues due to adult stem cells present all over the plants. These pathways do not start from single totipotent cells. There is a strong argument for plant cell totipotency, however, and that is somatic embryogenesis. During this process, differentiated somatic cells change their fate to develop into an embryo. Animal embryos can develop only from the totipotent zygote and its direct descendants (this cell state can also be artificially produced by injecting a somatic cell nucleus into an egg cell cytoplasm during cloning). Plant cells have to be induced to start somatic embryogenesis and not all of them are competent to respond properly to the induction. In conclusion, plant cells cannot be considered as totipotent per se, but some of them can regain totipotency under appropriate conditions. In addition, accumulating evidence supports the view that even somatic embryo development can follow various initial steps not necessarily requiring cellular totipotency. Although, there are experimental observations to support the progression of somatic embryogenesis through a zygote-like state in certain experimental systems, in other instances the reorganization of several cells into an embryo was described. The direct release/induction of the embryo development program in vegetative plant cells may represent a third pathway of somatic embryo development. In this chapter, a brief literature review is provided to support the above view on plant cell totipotency as well as on the various ways to start somatic embryogenesis.