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Plant and Cell Physiology Advance Access published online on March 3, 2005
Plant and Cell Physiology, doi:10.1093/pcp/pci074
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Plant and Cell Physiology 2005 © The Japanese Society of Plant Physiologists (JSPP); all rights reserved.
Received December 14, 2004
Accepted February 14, 2005

Regular Paper

Tissue Localization of Cytokinin Dehydrogenase in Maize: Possible Involvement of Quinone Species Generated from Plant Phenolics by Other Enzymatic Systems in the Catalytic Reaction

Petr Galuszka 1, Jitka Frébortová 2, Lenka Luhová 3, Kristin D. Bilyeu 4, James T. English 5, and Ivo Frébort 1*

1 Division of Molecular Biology, Department of Biochemistry, Palacky University, Slechtitelu 11, 783 71 Olomouc, Czech Republic
2 Laboratory of Growth Regulators, Palacky University/Institute of Experimental Botany of the Academy of Science, Slechtitelu 11, 783 71 Olomouc, Czech Republic
3 Department of Biochemistry, Palacky University, Slechtitelu 11, 783 71 Olomouc, Czech Republic
4 USDA/ARS, University of Missouri, Columbia, MO 65211, USA
5 Department of Plant Microbiology and Pathology, University of Missouri, Columbia, MO 65211, USA

* To whom correspondence should be addressed.
Ivo Frébort, E-mail: ivo.frebort{at}upol.cz


  Abstract

The degradation of cytokinins in plants is controlled by the flavoprotein cytokinin dehydrogenase (EC 1.5.99.12). Cytokinin dehydrogenase from maize showed the capability of using oxidation products of guaiacol, 4-methylcatechol, acetosyringone, and several other compounds as electron acceptors. These results led us to explore the ability of indirect production of suitable electron acceptors by different quinone-generating enzymes. The results reported here revealed that the electron acceptors may be generated in vivo from plant phenolics by other enzymatic systems such as peroxidase and tyrosinase/laccase/catechol oxidase. Histochemical localizations of cytokinin dehydrogenase by activity staining and immunochemistry using optical and confocal microscopy showed that cytokinin dehydrogenase is most abundant in the aleurone layer of maize kernels and in phloem cells of the seedling shoots. Cytokinin dehydrogenase was confirmed to be present in the apoplast of cells. Co-staining of enzyme activity for laccase, an enzyme poised to function on the cell wall in the apoplast, in those tissues suggests a possible cooperation of the enzymes in cytokinin degradation. Additionally, the presence of precursors for electron acceptors of cytokinin dehydrogenase was detected in phloem exudates collected from maize seedlings, suggestive of an enzymatic capacity to control cytokinin flux through the vasculature. A putative metabolic connection between cytokinin degradation and conversion of plant phenolics by oxidases was proposed.

Keywords: cytokinin; cytokinin dehydrogenase; laccase; maize; plant phenolics.
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