Cytokinins

The cell-division-promoting activity of cytokinins was first noticed by Haberlandt in 1913, in coconut milk (liquid endosperm), but the compound itself was only identified in 1954, when Skoog and Miller found that autoclaved herring-sperm DNA stimulated cell division in tobacco pith tissue and named the responsible substance kinetin - a compound that does not actually occur naturally in plants. Lethan coined the general term 'cytokinin' in 1963, and in 1964 Lethan and Miller isolated the first natural cytokinin, zeatin, from unripe maize grains; isopentenyl adenine (IPA) is the most widespread naturally occurring cytokinin in plants today. Cytokinins form mainly in the root apex, shoot apex, buds and young fruits, and are chemically derivatives of the purine base adenine. Their activity is classically measured using the neem cotyledon bioassay, where cotyledon enlargement in cytokinin solution (versus plain water) indicates activity, and they are distributed mainly via the xylem, concentrated more in roots than auxin or gibberellin are. Physiologically, cytokinins promote cell division (particularly alongside auxin), induce cell enlargement together with IAA and gibberellins, can break dormancy in light-sensitive seeds such as tobacco, promote lateral bud growth in the presence of the apical bud (opposing auxin's apical dom …