While the molecular basis for cytokinin action is quite well understood in flowering plants, little is known about the cytokinin signal transduction in early diverging land plants. The genome of the bryophyte Physcomitrella patens (Hedw.) B.S. encodes three classical cytokinin receptors, the CHASE domain-containing histidine kinases, CHK1, CHK2, and CHK3. In a complementation assay with protoplasts of receptor-deficient Arabidopsis thaliana as well as in cytokinin binding assays, we found evidence that CHK1 and CHK2 receptors can function in cytokinin perception. Using gene targeting, we generated a collection of CHK knockout mutants comprising single (Δchk1, Δchk2, Δchk3), double (Δchk1,2, Δchk1,3, Δchk2,3), and triple (Δchk1,2,3) mutants. Mutants were characterized for their cytokinin response and differentiation capacities. While the wild type did not grow on high doses of cytokinin (1 µM benzyladenine), the Δchk1,2,3 mutant exhibited normal protonema growth. Bud induction assays showed that all three cytokinin receptors contribute to the triggering of budding, albeit to different extents. Furthermore, while the triple mutant showed no response in this bioassay, the remaining mutants displayed budding responses in a diverse manner to different types and concentrations of cytokinins. Determination of cytokinin levels in mutants showed no drastic changes for any of the cytokinins; thus, in contrast to Arabidopsis, revealing only small impacts of cytokinin signaling on homeostasis. In summary, our study provides a first insight into the molecular action of cytokinin in an early diverging land plant and demonstrates that CHK receptors play an essential role in bud induction and gametophore development.

• Keywords
• Bryophyte, Physcomitrella patens, cytokinin, cytokinin receptor, evolution, moss, phytohormone, plant growth regulator, signaling, two-component system.,
• MeSH
• Biological Assay MeSH
• Butadienes pharmacology   MeSH
• Cytokinins pharmacology   MeSH
• Phenotype MeSH
• Adaptation, Physiological MeSH
• Gene Knockout Techniques MeSH
• Hemiterpenes pharmacology   MeSH
• Bryopsida drug effects   genetics   metabolism   MeSH
• Mutation genetics   MeSH
• Pentanes pharmacology   MeSH
• Receptors, Cell Surface chemistry   metabolism   MeSH
• Gene Expression Regulation, Plant MeSH
• Plant Proteins chemistry   metabolism   MeSH
• Protein Structure, Tertiary MeSH
• Structure-Activity Relationship MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Butadienes MeSH
• Cytokinins MeSH
• Hemiterpenes MeSH
• isoprene MeSH Browser
• Pentanes MeSH
• Receptors, Cell Surface MeSH
• Plant Proteins MeSH