REGENERATION OF ROOTS, SHOOTS AND EMBRYOS - PHYSIOLOGICAL, BIOCHEMICAL AND MOLECULAR ASPECTS

When the proper stimuli are given, somatic plant cells may form advent itious embryos, roots or shoots. The three pathways of regeneration sh ow apparent similarities. They consist of three analogous phases: 1) d edifferentiation (during which the tissue becomes competent to respond to the organogenic/embryogenic stimulus), 2) induction (during which cells become determined to form either a root, a shoot or an embryo), and 3) realization (outgrowth to an organ or an embryo). The first pha se may involve a period of callus growth (indirect regeneration), but often cells present in the explant become competent without cell divis ion or without cell division at a large scale (direct regeneration). I n an explant, only very few cells show the organogenic/embryogenic res ponse. In direct regeneration, the three regenerative pathways start f rom cells in different tissues. This is most obvious when the differen t types of regeneration occur in the same explant. The hormonal trigge r for the dedifferentiation phase is a general one, probably auxin. Du ring the induction phase, each pathway requires specific hormonal trig gers. During the realization phase, hormones should be absent or at lo w concentration. The successive steps in the regeneration process coin cide with events on the molecular and biochemical levels, but so far n o coherent picture has emerged. In particular during the early stages of regeneration, research on these levels is hampered by a technical p roblem, viz., the very low proportion of cells that participate in the process of regeneration. New methods may overcome this problem.