Chapter 8 - Cytokinins

Abstract

Cytokinins serve many important functions in plant development and morphogenesis. They are involved in the regulation of cell division; they interact with auxins in the control of apical dominance and lateral branching and the root-shoot ratio in intact plants and in tissue culture. They retard the senescence of leaves and promote the light-independent deetiolation response, including greening, of dark-grown seedlings. Several cytokinins occur naturally in plants. They have an adenine base and a five-carbon isopentenyl side chain. Among these, zeatin, specifically trans-zeatin, is the most abundant. The synthesis of cytokinins in higher plants has been unclear and controversial for a long time, but progress finally seems to be achieved with the cloning of genes encoding isopentenyl transferases (IPTs) in Arabidopsis. Genes encoding cytokinin oxidase and glycosylating enzymes are cloned from several plants leading to an increased understanding of regulation of cytokinin metabolism.Several synthetic cytokinins are known to have the adenine base and an aromatic ring in the side chain. Supplied to plant tissues, these cytokinins form ribosylated derivatives and glucosyl or amino acid substituents in the adenine ring, but they are more stable in plant tissues because they are not subject to side chain cleavage by cytokinin oxidase. Some synthetic compounds, which inhibit cytokinin promotion of cell division in tissue/ cell culture, probably as competitive inhibitors, are known.