Hormonal Control of Somatic Embryo Development from Cultured Cells of Caraway

Abstract

The effects of abscisic acid, zeatin, and gibberellic acid on the development of somatic embryos from cultured cells of caraway (Carum carvi L.) were observed.Somatic embryos complete development on a basal medium without exogenous hormones, but some are subject to developmental abnormalities induding malformed cotyledons and accessory embryos. Both zeatin and gibberellic acid, especialy in combination, stimulate growth and increase the frequency of aberrant forms. Zeatin causes the formation of multiple shoots, leafy and abnormal cotyledons, and in the dark, enlarged hypocotybls; gibberelic acid effects root elongation, polycotyledony, and some caflus formation. In contrast, abscisic acid, at concentrations which do not inhibit embryo maturation, selectively suppresses abnormal proliferations. With abscisic acid, and especially in the dark, a high percentage of embryos complete development with two fleshy cotyledons on unelongated axes free of accessory embryos.In the light, zeatin eliminates abscisic acid inhibition while gibberellic acid only partially counters its effect, promoting elongated radicles and green rather than white cotyledons. In the dark, zeatin in combination with abscisic acid stimulates extensive callusing. Gibberellic acid does not reverse the effects of abscisic acid but rather enhances them and can counter the disruptive effects of zeatin.The results demonstrate that the balance between abscisic acid on the one hand and zeatin and gibberellic acid on the other can effectively control somatic embryo development and either disrupt or ensure normal maturation.Embryos can now be readily grown from cultured cells of certain angiosperms such as caraway (2) or carrot (3) following a simple sequential procedure. These somatic embryos are structurally comparable to zygotic embryos (19): they pass through the same globular, heart-shaped and torpedo stages as do seed embryos and give rise to axes with a root apex at one pole and cotyledons and a shoot apex at the other.In populations of somatic embryos, there is a range of aberrant forms in addition to dicotyledonous embryos: embryos with too few, too many, or malformed cotyledons or callused shoots; those with accessory embryos along the hypocotyl; and multiple embryos with many root-shoot axes arising from one morphogenetic unit. The frequency of occurrence of these unusual forms can vary from experiment to experiment or from culture to culture but is generally high enough to be in contrast to the uniformity seen in zygotic embryos where, with the exception of ISupported by a Barnard College Research Grant. differences in length, the frequency of abnormal forms is rather low. Why does the course of development proceed normally in zygotic and some somatic embryos while in others it is diverted? It has been suggested (3) that these changes may be related to imbalances in the culture medium or environment. If that is the case, modifications in these parameters may serve to reestablish a balance and consequently foster more normal embryogenesi...

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