Organogenesis in Vitro as a Developmental Process

Abstract Plants have a remarkable propensity for asexual or vegetative propagation, and it is not surprising that this capacity extends to plant cells or tissues cultured in vitro (24). The “regenerative response” in vitro includes somatic embryogenesis as well as organogenesis (formation of shoots or roots from cultured tissue). It may be useful to make a further distinction, subdividing the organogenic response. In many instances, explants have the capacity to give rise to shoots, roots, or floral structures when cultured on a medium that supplies mineral salts, vitamins, and a carbon source, but is devoid of plant hormones. These processes may be termed “adventive organogenesis”. Exogenously supplied plant hormones often can facilitate these processes, but are not absolutely required for organogenesis to occur. In these instances, the immediate precursors of the new organs are cells in the explant itself (5, 6). The other type of organogenesis is not adventive and involves a “dedifferentiation” of the explant, elaboration of callus tissue along the cut edges of the explant, and the induction of new organs from this newly formed callus tissue. Exogenously supplied phytohormone not only controls the process but is required for organogenesis to occur. This paper is concerned with the specifics of this last type of organogenesis; we suspect that the general principles derived from this work apply to all organogenesis in vitro. As much as organogenesis in vitro reflects the mechanisms of organogenesis in vivo, these general principles will also apply to the development in whole plants ex vitro.