Secreted proteins as modulators of plant embryogenesis (original) (raw)

Progress in Plant Growth Regulation, 1992

Abstract

In all plant species investigated it is possible to induce somatic cells of most plant organs to proliferate in liquid synthetic media supplemented with auxins alone or with auxins and cytokinins. Initially in carrot, and later in many other plant species, it was shown that some of these proliferating cells are capable of producing somatic embryos able to develop into fertile flowering plants. The strategy most commonly employed is to expose expiant tissue to a high concentration of a stable and persistent auxin such as 2,4-D. After a period of proliferation of the released non-embryogenic expiant cells in the presence of 2,4-D, a subpopulation of single cells and clusters of cells can be distinguished that have acquired embryogenic potential. From these single embryogenic cells (Komamine et al. 1990) or from clusters of embryogenic cells designated proembryogenic masses (Halperin, 1966), somatic embryos can develop after simple culture manipulations. These usually involve enrichment for proembryogenic masses, followed by a 10 to 100 fold reduction in cell density together with a decrease in the concentration or the complete removal of exogenous auxins. Most embryogenic carrot cultures are maintained over many subcultures with 2,4-D either as the sole growth regulator, or in the presence of both 2,4-D and cytokinin. Therefore, cells that have embryogenic potential may either be continuously formed from non-embryogenic cells, or constitute an independent self-propagating subpopulation. In carrot cultures, depletion of the population of embryogenic cells eventually leads to loss of the embryogenic potential. In other culture systems such as alfalfa, non-embryogenic cells can be maintained in media containing NAA, while a short exposure to a high concentration of 2,4-D is sufficient for these cells to acquire embryogenic potential (Bogre et al. 1990). Although many variations in the source of the original explant material and in culture conditions have been reported, the basic scheme always involves an induction phase in the presence of 2,4-D, during which proliferating cells become embryogenic, and an expression phase in the absence of 2,4-D, during which the actual somatic embryos develop.

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