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cytokinins - In Vitro Cell Dev Biol.Plant 36:102107...

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In Vitro Cell. Dev. Biol.—Plant 36:102–107, March–April 2000 q 2000 Society for In Vitro Biology 1054-5476/99 $10.00 1 0.00 CYTOKININS: BIOSYNTHESIS, METABOLISM AND PERCEPTION MACHTELD C. MOK * , RUTH C. MARTIN, and DAVID W. S. MOK Department of Horticulture and Center for Gene Research and Biotechnology, Oregon State University, Corvallis, Oregon 97331 (Accepted 30 November 1999; editor T. A. Thorpe) Summary Cytokinins are essential hormones for plant growth and development. They are also of vital importance for in vitro manipulations of plant cells and tissues. The biological activities and chemistry of cytokinins are well defined but very little is known about their mode of action and it is only recently that cytokinin genes have been identified in plants. This review summarizes the current status of knowledge on cytokinin biosynthesis, metabolism and signal transduction, with an emphasis on genes encoding metabolic enzymes and putative receptors, and genes rapidly induced by cytokinins. Key words: plant hormone; phytohormone; growth regulator; zeatin; thidiazuron. Introduction Cytokinins were discovered as factors promoting cell division in tobacco tissue cultures (Miller et al., 1955). Since then, cytokinins have been shown to regulate a host of additional developmental events, such as de novo bud formation, release of buds from apical dominance, leaf expansion, delay of senescence, promotion of seed germination and chloroplast formation (Mok, 1994). Naturally occurring cytokinins are derivatives of N 6 -( D 2 -isopentenyl)adenine (i 6 Ade) and cytokinins with a hydroxylated side-chain, such as zeatin ( trans -zeatin), are major constituents in plants. Synthetic cytokinins include adenine derivatives, such as kinetin, as well as compounds structurally unrelated to natural cytokinins, such as certain phenylureas (Shaw, 1994; Shudo, 1994). Although cyto- kinins are known to have pronounced effects on plant development and there is a wealth of knowledge on cytokinin chemistry, little is known about their mode of action. It is possible that severe impairment of cytokinin action is lethal and therefore strategies of isolating mutants, which have been essential in elucidating signal transduction pathways of other plant hormones, notably ethylene, have not been successful. The rarity of cytokinin mutants may also explain the slow progress in our understanding of cytokinin biosynthesis in plants, as plant genes controlling cytokinin biosynthesis have yet to be identified. Nevertheless, genes involved in cytokinin perception and genes encoding cytokinin metabolic enzymes have recently been cloned, providing useful tools for further molecular analyses. This review summarizes key findings on cytokinin biosynthesis, metabolism and perception, with an emphasis on newly discovered genes.
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