Totipotency and embryogenesis in plant cell and tissue cultures.

Abstract

An important development in the field of plant cell and tissue culture has been the demonstration in the past decade of the totipotency of higher plant cells. Isolated single cells were first successfully grown on a nurse tissue separated by a filter paper and gave rise to a callus tissue. Later, completely isolated single cells of tobacco were grown in microchambers to form small clumps of cells which then could be differentiated to form adult tobacco plants. Indirect evidence of the totipotency of higher plant cells has also been provided in a number of other plants. Embryo-like structures (or embryoids) or whole plants, or both, have been obtained from such highly differentiated cells as the pollen grains (gametic and haploid), photosynthetic palisade cells in leaves, epidermal cells from the hypocytyl, and the triploid endosperm cells; all of these cell types perform very highly specialized functions in the plant. Plant protoplasts (cell wall is digested with enzymes) have also been cultured to give rise to normal adult plants. In many instances embryoids have been produced in vitro from several species of flowering plants which do not show such asexual activity in nature. These embryoids are normally indistinguishable morphologically from embryos produced by gametic fusion, often follow the same pattern of cell divisions and differentiation as the developing zygote, and are economically important as they provide clonal populations. Early work in this area emphasized the necessity of dissociating tissues into single cells and providing a nutritional environment identical to that of the zygote in the embryo sac (usually by supplementing the medium with liquid endosperm from coconuts), before the cells could be released morphogenetically to express their totipotency by forming embryoids. Much of the recent work, however, has shown that perfect development of embryoids can be obtained in completely synthetic media in callus tissues as well as in suspension cultures.