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Tissue Localization of Cytokinin Dehydrogenase in Maize

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Tissue Localization of Cytokinin Dehydrogenase in Maize Plant Cell Physiol. 46(5): 716–728 (2005) doi:10.1093/pcp/pci074, available online at www.pcp.oupjournals.org JSPP © 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 Petr Galuszka 1, Jitka Frébortová 2, Lenka Luhová 3, Kristin D. Bilyeu 4, James T. English 5 and Ivo Frébort 1, 6 1 Division of Molecular Biology, Department of Biochemistry, Palacký University, Šlechtitelu 11, 783 71 Olomouc, Czech Republic 2 Laboratory of Growth Regulators, Palacký University/Institute of Experimental Botany of the Academy of Science, Šlechtitelu 11, 783 71 Olomouc, Czech Republic 3 Department of Biochemistry, Palacký University, Šlechtitelu 11, 783 71 Olomouc, Czech Republic 4 United States Department of Agriculture / Agricultural Research Service, University of Missouri, Colombia, MO 65211, USA 5 Department of Plant Microbiology and Pathology, University of Missouri, Columbia, MO 65211, USA ; The degradation of cytokinins in plants is controlled Introduction by the flavoprotein cytokinin dehydrogenase (EC 1.5.99.12). Cytokinin dehydrogenase from maize showed the ability to Cytokinins are plant hormones that contribute to the regu- use oxidation products of guaiacol, 4-methylcatechol, aceto- lation of numerous developmental processes including apical syringone and several other compounds as electron accep- dominance, flower and fruit development, leaf senescence and tors. These results led us to explore the cability for indirect seed germination (reviewed in Haberer and Kieber 2002). production of suitable electron acceptors by different qui- Diverse cytokinins have been characterized in a variety of plant none-generating enzymes. The results reported here re- species (Auer 1997). Differences among cytokinins are related vealed that the electron acceptors may be generated in vivo predominantly to structural features, including the isoprenoid from plant phenolics by other enzymatic systems such as side chain that is attached to an adenine moiety (Mok and Mok peroxidase and tyrosinase/laccase/catechol oxidase. Histo- 2001). The small size of active cytokinins is thought to con- chemical localization of cytokinin dehydrogenase by activ- tribute to their common occurrence in the intercellular or ity staining and immunochemistry using optical and apoplastic region of plant tissues (Corbesier et al. 2003). Here, confocal microscopy showed that cytokinin dehydrogenase they bind to cell surface receptors and initiate a signal trans- is most abundant in the aleurone layer of maize kernels and duction cascade that leads to activation of genes involved in in phloem cells of the seedling shoots. Cytokinin de- tissue development (Yamada et al. 2001, Hutchison and Kieber hydrogenase was confirmed to be present in the apoplast of 2002, Schmülling 2002). cells. Co-staining of enzyme activity for laccase, an enzyme Apoplastic cytokinins can also be transported into an adja- poised to function on the cell wall in the apoplast, in those cent cell by interaction with purine transporters (Bürkle et al. tissues suggests a possible cooperation of the enzymes in 2003); however, the fate of imported active cytokinins is uncer- cytokinin degradation. Additionally, the presence of pre- tain because no cytoplasmic receptors for the hormone are cursors for electron acceptors of cytokinin dehydrogenase was detected in phloem exudates collected from maize seed- known. It is possible that active transport into the cell is a lings, suggestive of an enzymatic capacity to control cytoki- mechanism to position cytokinins for further metabolism while nin flux through the vasculature. A putative metabolic preventing them from activating the signal transduction cas- connection between cytokinin degradation and conversion cade poised at the cell membrane. Cytoplasmic cytokinins can of plant phenolics by oxidases was proposed. be inactivated by glucosylation or metabolized by non-secreted cytokinin dehydrogenases (CKXs; historically categorized as Keywords: Cytokinin — Cytokinin dehydrogenase — Laccase cytokinin oxidases) (Mok and Mok 2001, Bilyeu et al. 2001). — Maize — Plant phenolics. Apoplastic concentrations of cytokinins reflect the balance of accumulation via local synthesis and transport, vs. cell uptake Abbreviations: BSA, bovine serum albumin; CKX, cytokinin or inactivation. Cytoplasmic concentrations of cytokinins dehydrogenase (EC 1.5.99.12); DCPIP, 2,6-dichlorophenol indo- depend on the rate of transport into the cell from the apoplast phenol; DMSO, dimethylsulfoxide; EST, expressed sequence tag; iP, vs. the rates of reversible and irreversible inactivation. Lack of 6 6 N -(2-isopentenyl)adenine; iPR, N -(2-isopentenyl)adenosine; PBS, clarity about the relative affinity of cytokinins for transporters, phosphate-buffered saline; Q , 2,3-dimethoxy-5-methyl-1,4-benzoqui- 0 receptors and metabolizing enzymes present in a tissue or cel- none; RT–PCR, reverse transcription–polymerase chain reaction; ZmCKX1, the CKX enzyme from maize, encoded by the gene Zmckx1 lular environment has hindered understanding of the mecha- (GenBank AF044603). nism of cytokinin signaling (Heyl and Schmülling 2003). 6 Corresponding author: E-mail, [email protected]; Fax, +420-585634933. 716 Localization of cytokinin dehydrogenase in maize 717 Over the past decade, many genes have been identified included members of the metabolic resource pool in tissues that affect cytokinin synthesis, transport, metabolism and their where the protein is expressed. function in growth regulation. In Arabidopsis, for instance, it The uniqueness or redundancy of CKX gene family mem- appears that a gene family of 20 or fewer members controls bers can be deciphered only after mechanistic details of their these processes (Heyl and Schmülling 2003, Kakimoto 2003, individual function have been characterized in native tissue and Popelková et al. 2004). The role of individual members of cellular environments. As a first step, we describe in this report these families is beginning to be uncovered, although compre- details of the spatial distribution of ZmCKX1 in various organ hensive understanding of genetic redundancy is lacking. tissues. We also describe ZmCKX1 activity as affected by Despite advances in gene discovery, mechanistic details of molecular components of the local metabolic resource pool that cytokinin metabolism and function are also lacking. We have can serve as electron acceptors directly or after oxidative modi- investigated one metabolic component, that of irreversible fication. cytokinin inactivation. This is an important process involved in maintenance of optimal hormone concentration. CKXs are the Results only known enzymes capable of irreversible degradation of cytokinins. They inactivate the hormone by removal of the iso- Localization of active cytokinin dehydrogenase in maize prenoid side chain of zeatin, N6-(2-isopentenyl)adenine (iP) Comparisons of CKX activity among plant tissues are and their ribosides to produce adenine or adenosine and the commonly made on the basis of relative activity in tissue corresponding aldehydes (Galuszka et al. 2000). Inactivation of homogenates. Recent studies have shown that a variety of cyto- cytokinins by CKX may be coordinated with specific plant plasmic and cell wall-associated constituents can serve as elec- developmental processes. In maize, for instance, zeatin riboside tron acceptors for ZmCKX1, but with variable efficiency levels increase transiently in developing kernels, but they (Galuszka et al. 2001, Frébortová et al. 2004). The distribution decline during kernel maturation in conjunction with up-regu- of potential electron acceptors may vary among plant tissues lation of CKX (Dietrich et al. 1995). and cell types. In tissue preparations, cellular structure is CKX was first isolated from kernels of maize (Zea mays) destroyed, and diverse CKXs are blended along with meta- and designated as ZmCKX1 (Houba-Herin et al. 1999, Morris bolic constituents that may normally be compartmentalized. et al. 1999). The respective gene Zmckx1 was subsequently Consequently, estimation and comparison of CKX activity in cloned. Since then, other members of the CKX gene family partially purified, homogenized tissues can be confounded by have been identified in developing maize kernels (Bilyeu et al. differences in CKX concentration and proportional influences 2003). It soon became clear that other plants contain small of members of the mixed electron acceptor pool. families of CKX genes (Bilyeu et al. 2001, Werner et al. 2001, To standardize comparisons of CKX activity, we devel- Bilyeu et al. 2003, Galuszka et al. 2004, Popelková et al. 2004). oped an activity assay based on our previous characterization In Arabidopsis and rice, complete genome sequence informa- of the enzymatic reaction with saturating levels of the artificial tion has enabled CKX genes to be categorized based on the electron acceptor 2,6-dichlorophenol indophenol (DCPIP) presence or absence of an N-terminal signal peptide-coding (Frébort et al. 2002). Providing saturating amounts of a suita- sequence that presumably specifies secretion of the enzymes to ble CKX electron acceptor eliminates the dependence of the apoplast (Schmülling et al. 2003). enzyme activity on possibly limited amounts of naturally Recent studies by Brugière et al. (2003) have established occurring electron acceptors in crude enzyme preparations. that Zmckx1 is expressed in the vascular bundles
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