Localization of Cytokinin Biosynthetic Sites in Pea Plants and Carrot Roots

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Abstract
The biosynthesis of cytokinins were examined in pea (P. sativum L.) plant organs and carrot (D. carota L.) root tissues. When pea roots, stems, and leaves were grown separately for 3 wk on a culture medium containing [8-14C]adenine without an exogenous supply of cytokinin and auxin, radioactive cytokinins were synthesized by each of these organs. Incubation of carrot root cambium and noncambium tissues for 3 days in a liquid culture medium containing [8-14C]adenine without cytokinin demonstrates that radioactive cytokinins were synthesized in the cambium but not in the noncambium tissue preparation. The radioactive cytokinins extracted from each of these tissues were analyzed by Sephadex LH-20 columns, reverse phase high pressure liquid chromatography, paper chromatography in various solvent systems, and paper electrophoresis. The main species of cytokinins detectable by these methods are N6-(.DELTA.2-isopentyl)adenine-5''-monophosphate, 6-(4-hydroxy-3-methyl-2-butenyl-amino)-9-.beta.-ribofuranosylpurine-5''-monophosphate, N6-(.DELTA.2-isopentenyl)adenosine, 6-(4-hydroxy-3-methyl-2-butenylamino)-9-.beta.-ribofuranosylpurine, N6-(.DELTA.2-isopentenyl)adenine, and 6-(4-hydroxy-3-methyl-2-butenylamino)purine. On the basis of the amounts of cytokinin synthesized per gram fresh tissues, these results indicate that the root is the major site, but not the only site, of cytokinin biosynthesis. Furthermore, cambium and possibly all actively dividing tissues are responsible for the synthesis of this group of plant hormones.