Localization of Cytokinin Biosynthetic Sites in Pea Plants and Carrot Roots

Abstract

ABSTRACE MATERIALS AND METHODSThe biosynthesis of cytokinins was examined in pea (Pisum sativum L.) plant organs and carrot (Daucus carota L.) root tissues. When pea roots, stems, and leaves were grown separately for three weeks on a culture medium containing 18-"4Cadenine 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 three days in a liquid culture medium containing 18-'Cladenine without cytokinin demonstrates that radioactive cytokinins were synthesized in the cambium but not in the noncambium tissue preparatioa. 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 N'4-A2-isopentyl)adenine-5'-monophosphate, 644-hydroxy-3-methyl-2-butenyl-amino)-9-,-ribofuranosylpurine-5'-monophosphate, N'-Q52-isopeatenyl)adenosine, 6-(4-hydroxy-3-methyl-2-butenylamino)-9-ft-ribofuraosylpurine, N'-(A2-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.There is evidence that cytokinins exist in the roots (15,16) and that cytokinins from xylem exudate may come from the roots (9,17 2Abbreviations: Ade, adenine; Ado, adenosine; i6Ade, N6-(A2-isopentenyl)adenine; i6Ado, N6-(A2-isopentenyl)adenosine; io6Ade, 6-(4-hydroxy-3-methyl-2-butenylamino) purine; io6Ado, 6-(4-hydroxy-3-methyl-2-butenylamino)-9-,-ribofuranosylpurine. imbibed for 16 h with running tap water, and surface-sterilized for 10 min with 0.5% NaOCl. The seeds were thoroughly rinsed with sterile distilled H20, and three seeds were grown on 50 ml of a basic culture medium (8)

Hormonal Control of Parthenocarpic Ovary Growth by the Apical Shoot in Pea1

“…However, the observation that decapitation induces parthenocarpy in the absence of roots (the main purported source of cytokinins; Chen et al, 1985), as far as the mature leaves were present in the plant (Table I), does not support the idea of cytokinins being involved in stimulating fruit growth on decapitated plants. The possibility cannot be discarded that mature leaves and/or the stem, organs also capable of cytokinin biosynthesis (Chen et al, 1985), are a source of these hormones.…”

Auxin Acts in Xylem-Associated or Medullary Cells to Mediate Apical Dominance

“…However, because direct regulation of the IPT gene by auxin, rather than crude decapitation, has not been demonstrated, a direct link between the two cannot be made. Chen et al (1985) have demonstrated biochemically that, in roots, cambial tissue is responsible for cytokinin synthesis, but the site of synthesis in stems has not been reported. Identification of this site by in situ localization of the putative IPT gene or biochemical assay, and correlation with the site of auxin action described here, provide further support for a role for auxin in the regulation of cytokinin biosynthesis.…”

Control of Outgrowth and Dormancy in Axillary Buds

“…It is thought that auxin acts to control the concentration of cytokinin derived from the roots (Bangerth, 1994). Cytokinins are produced not only in the root, however, but also in the shoot (Chen et al, 1985). Cytokinin concentrations in bean xylem exudate of bean after decapitation increase within 16 h and gradually return to basal levels (Bangerth, 1994), and those in chickpea axillary buds after decapitation increase 7-fold by 6 h and 25-fold by 24 h (Turnbull et al, 1997), suggesting that cytokinins are necessary to initiate outgrowth of axillary buds.…”