© 2004 Oxford University Press
Short Communication |
Two Cytokinin Receptors of Arabidopsis thaliana, CRE1/AHK4 and AHK3, Differ in their Ligand Specificity in a Bacterial Assay
Spíchal1,2
1 Free University of Berlin, Institute of Biology/Applied Genetics, Albrecht-Thaer-Weg 6, D-14195 Berlin, Germany
2 Institute of Experimental Botany, CAS, and Laboratory of Growth Regulators, Palack
University, 
lechtitelu 11, CZ-78371 Olomouc, Czech Republic
3 Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya 35, 127276 Moscow, Russia
4 Laboratory of Molecular Microbiology, School of Agriculture, Nagoya University, Chikusa-ku, Nagoya, 464-8601 Japan
Abstract
Strains of Escherichia coli that express two different cytokinin receptors of Arabidopsis thaliana, CRE1/AHK4 and AHK3, were used to study the relative sensitivity of these receptors to various cytokinins. Both receptors were most sensitive to the bases of the isoprenoid-type cytokinins trans-zeatin and isopentenyladenine but differed significantly in the recognition of other cytokinin compounds. In particular, CRE1/AHK4 recognized at 1 µm concentration only trans-zeatin while AHK3 recognized cis-zeatin and dihydrozeatin as well, although with a lower sensitivity. Similarly, CRE1/AHK4 was not activated by cytokinin ribosides and ribotides, but AHK3 was. Comparisons using the ARR5::GUS fusion gene as a cytokinin reporter in Arabidopsis showed similar relative degrees of responses in planta, except that cytokinins with aromatic side chains showed much higher activities than in the bacterial assay. These results indicate that the diverse cytokinin compounds might have specific functions in the numerous cytokinin-regulated processes, which may depend in turn on different receptors and their associated signalling pathways. The importance of precise control of local concentrations of defined cytokinin metabolites to regulate the respective downstream event is corroborated.
Footnotes
5 Corresponding author: E-mail, tschmue{at}zedat.fu-berlin.de; Fax, +49-30-8385-4345.
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