Nonzygotic embryogenesis *
An embryo is the earliest recognizable multicellular stage of an individual that occurs before it has developed the structures or organs characteristic of a given species. In most organisms, embryos are morphologically distinct entities that function as an intermediate stage in the transition between the gametophytic to sporophytic life cycle. The first demonstration that plants could produce nonzygotic embryos in vitro was published in 1958 by Steward. This chapter discusses the use of specific plant growth regulators (PGRs), which are required in most instances, with regard to embryogenic cell initiation. In addition to auxin-like PGRs, cytokinins are required to induce embryogenesis in many dicotyledonous species. In a few instances, only a cytokinin is required to cause embryogenic cultures to develop. The most commonly used cytokinin is benzyladenine (BA), but others such as thidiazuron (TDZ) and kinetin, and the natural cytokinin, zeatin, also are utilized.