cytokinin - Chapter 6 Plant Growth Regulators II:...

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Chapter 6 Plant Growth Regulators II: Cytokinins, their Analogues and Antagonists 1. BIOLOGICAL EFFECTS Hormones in plants differ from most of those in animals by having pleiotropic effects; that is, they are involved in the control of a wide range of developmental processes. At the same time the effect of a hormone on any developmental process depends on the species. For example, ethylene inhibits growth in dicotyledons and most monocotyledons but is promotory in deepwater rice and other hydrophytes. Moreover, two or more hormones can interact synergistically or antagonistically in many circumstances. Equally, any given hormone may affect the biosynthesis or metabolism of another, thus affecting endogenous levels. The issue is further complicated by the fact that environmental factors - e.g. light, water status, wounding, pathogens - may modify responses and indeed hormone levels themselves. The reason for this appears to be that hormones (and growth regulators) and environmental factors share many components in their transduction chains (i.e. the very early events which occur after the signal - abiotic or biotic - is perceived by the plant tissue). These transduction chains interact to produce an integrated response. 2. PROPERTIES AND DISCOVERY OF CYTOKININS 2.1. BIOLOGICAL ACTIVITY Cytokinins comprise a separate class of growth substances and growth regulators. They produce various effects when applied to intact plants. They particularly stimulate protein synthesis and participate in cell cycle control. It is perhaps for this reason that they can promote the maturation of chloroplasts and delay the senescence of detached leaves. Cytokinin application to a single site in the plant (e.g. to one leaf) causes the treated organ to become an active sink for amino acids, which then migrate to the organ from surrounding sites. The effect of cytokinins is most noticeable in tissue cultures where they are used, often together with auxins, to stimulate cell division and control morphogenesis. Added to shoot culture media, these compounds overcome apical dominance and release lateral buds from dormancy. 2.2. DISCOVERY As in the case of auxins, there are both naturally- occurring compounds and their synthetic analogues. The first cytokinin to be discovered, kinetin (1), was isolated in Professor Skoog's laboratory at the University of Wisconsin, following experiments to promote continuing growth of the callus which formed on tobacco stem sections on nutrient media. Cells of the explants initially proliferated quickly but although the addition of IAA increased the amount of callus produced, growth soon stopped and was not resumed even if pieces of the newly-formed tissue were subcultured onto a fresh medium. Cell division and callus growth did continue however, if either coconut milk or yeast extract were added to the medium and so attempts were made to isolate the active principle. As chromatography of ethanol - soluble fractions
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This note was uploaded on 07/23/2011 for the course HOS 6737c taught by Professor Moore during the Spring '09 term at University of Florida.

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cytokinin - Chapter 6 Plant Growth Regulators II:...

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