Submitted to: Plant And Cell Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/15/2005
Publication Date: 3/3/2005
Citation: Frebortova, J., Galuszka, P., Luhova, L., Bilyeu, K.D., English, J., Frebort, I. 2005. Tissue localization of cytokinin dehydrogenase in maize:possible involvement of quinone species generated from plant phenolics by other enzymatic systems in the catalytic reaction. Plant And Cell Physiology. 46(5):716-728. Interpretive Summary: Plant hormones have an impact on nearly every developmental stage in the life cycle of a plant. Plants produce a variety of enzymes that regulate the biosynthesis, perception, and metabolism of hormones. Cytokinin hormones have been shown to participate in the signal for plant cells to divide. In order to control the local concentration and thus the effect of cytokinins, plants can locally upregulate cytokinin degrading enzymes known as cytokinin dehydrogenases. The plant cytokinin dehydrogenases appear to belong to a diverse super family of enzymes that perform a variety of reactions, often using oxygen as a cofactor, but cytokinin dehydrogenase functions poorly with oxygen as a cofactor. The objective of this work was to investigate the ability of other plant enzymes to produce cofactors that participate in cytokinin dehydrogenase reactions and examine the overlap of expression of cytokinin dehydrogenase and any putative cofactor-generating enzymes. The results indicate that cytokinin dehydrogenase can utilize a number of cofactors generated from other plant enzymes. In particular, the enzyme laccase was capable of providing cofactors which participated in cytokinin dehydrogenase reactions. Laccase and cytokinin dehydrogenase were found to be present in the same tissues and presumbably in the same subcellular compartment. These results will benefit plant breeders and basic plant scientists by improving our understanding of the connections between plant hormone regulation and agronomic traits such as improved flower set and seed filling.
Technical Abstract: The degradation of cytokinins in plants is controlled by the flavoprotein cytokinin dehydrogenase (EC18.104.22.168). Cytokinin dehydrogenase from maize showed the capability of using oxidation products of guaiacol, 4-methylcatechol, caffeic acid, cetosyringone, some flavonoids 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.