There is currently considerable interest in developing techniques for producing somatic embryos from conifers. Somatic embryos could be used for micropropagation of trees with desirable characteristics such as improved dimension increment, better quality wood and disease resistance. The availability of somatic embryos would also provide excellent experimental material for basic studies of conifer embryo development. Finally, embryogenic callus and suspension cultures should facilitate the development of a protoplast regeneration system to be used in future genetic manipulation studies. The use of embryogenic material has previously proved successful for developing protoplast systems with other recalcitrant species (e.g. Vasil and Vasil 1986).

Somatic embryogenesis in conifers

Somatic embryogenesis is the formation of an embryo from a cell other than a gamete or the product of gametic fusion This is not an artificial phenomenon, and is known in nature as a form of apomixis called adventitious embryony, first described by Strasburger in 1878 [1] (see Sharma and Thorpe, this volume) Although Steward [2] and Reinert [3] are generally given credit for the first descriptions of somatic embryogenesis in 1958, such credit might more properly belong to Levine [4], who in 1947 reported the recovery of carrot “seedlings” from tissues exposed to low levels of α-naphthaleneacetic acid, via a process whose description sounds very much like somatic embryogenesis

Morphogenic Aspects of Somatic Embryogenesis

This review will summarize molecular and genetic analyses aimed at identifying the mechanisms underlying the sequence of events during plant zygotic embryogenesis. These events are being studied in parallel with the histological and morphological analyses of somatic embryogenesis. The strength and limitations of somatic embryogenesis as a model system will be discussed briefly. The formation of the zygotic embryo has been described in some detail, but the molecular mechanisms controlling the differentiation of the various cell types are not understood. In recent years plant molecular and genetic studies have led to the identification and characterization of genes controlling the establishment of polarity, tissue differentiation and elaboration of patterns during embryo development. An investigation of the developmental basis of a number of mutant phenotypes has enabled the identification of gene activities promoting (1) asymmetric cell division and polarization leading to heterogeneous partitioning of the cytoplasmic determinants necessary for the initiation of embryogenesis (e.g. GNOM), (2) the determination of the apical-basal organization which is established independently of the differentiation of the tissues of the radial pattern elements (e.g. KNOLLE, FACKEL, ZWILLE), (3) the differentiation of meristems (e.g. SHOOT-MERISTEMLESS), and (4) the formation of a mature embryo characterized by the accumulation of LEA and storage proteins. The accumulation of these two types of proteins is controlled by ABA-dependent regulatory mechanisms as shown using both ABA-deficient and ABA-insensitive mutants (e.g. ABA, ABI3). Both types of embryogenesis have been studied by different techniques and common features have been identified between them. In spite of the relative difficulty of identifying the original cells involved in the developmental processes of somatic embryogenesis, common regulatory mechanisms are probably involved in the first stages up to the globular form. Signal molecules, such as growth regulators, have been shown to play a role during development of both types of embryos. The most promising method for identifying regulatory mechanisms responsible for the key events of embryogenesis will come from molecular and genetic analyses. The mutations already identified will shed light on the nature of the genes that affect developmental processes as well as elucidating the role of the various regulatory genes that control plant embryogenesis.

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Zygotic embryogenesis versus somatic embryogenesis

REVIEW ARTICLE Zygotic embryogenesis versus somatic embryogenesis

Biological effects due to weak magnetic field on plants.

Mesophyll protoplasts of Medicago sativa were exposed to low voltage electrical fields immediately following isolation. Several exposure times and voltages were utilized. At the lower doses, protoplast aggregation and subsequent embryogenesis were stimulated. A clone of ‘Rangelander’, which was directly-embryogenic (i.e. embryos were derived from single mesophyll protoplasts without an intervening callus phase), was induced to form embryos in all samples exposed to the lowest level electrical fields, while unexposed controls formed few or no embryos. A clone of ‘Regen S’, which was previously not directly-embryogenic, was induced to follow the ‘Rangelander’ pattern of development and to produce early globular embryos.

D. C. W. Brown

Papers

Stimulation of direct embryogenesis from mesophyll protoplasts of Medicago sativa.