DISCOVERY OF CYTOKININS AND THEIR ROLE IN MORPHOGENESIS

The initial success in obtaining unlimited growth of plant tissues was limited to the use of explants that contained meristematic cells. Continued cell division and bud formation were soon obtained when tobacco pith tissues that contained mature and differentiated cells were cultured on nutrient media containing adenine and high levels of phosphate (Skoog and Tsui 1951). However, cell divisions occurred only when the explant included vascular tissue (Jablonski and Skoog 1954). A variety of plant extracts, including coconut milk, (the beneficial effect of coconut milk, the liquid endosperm of immature coconuts, on plant tissue cultures was first shown by van Overbeek et al. 1941) were added to the nutrient medium in an attempt to replace vascular tissues and to identify the factors responsible for their beneficial effect. Among these, yeast extract was found to be most effective and its active component was shown to have purine-like properties. This finding led to the addition of DNA to the medium which greatly enhanced cell division activity (see also Vasil 1959). These investigations resulted in the isolation of kinetin from old samples of herring sperm DNA (Miller et al. 1955), and the understanding of the hormonal (auxin-cytokinin) regulation of shoot morphogenesis in plants (Skoog and Miller 1957). Later studies led to the isolation of naturally occurring as well as many synthetic cytokinins, the elucidation of their role in cell division and bud development, and their extensive use in the micropropagation industry related to their suppression of apical dominance resulting in the development of many axillary shoots.