CHASE domain-containing receptors play an essential role in the cytokinin response of the moss XXXX .

@entity3793 @entity958 @entity17573 @entity19164 @entity3015
UNASSIGNED: 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 @entity17573 (Hedw.) B.S. encodes three classical cytokinin receptors, the CHASE domain-containing @entity3793 kinases, CHK1, CHK2, and CHK3. In a complementation assay with protoplasts of receptor-deficient @entity958 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 (  chk1,   chk2,   chk3), double (  chk1,2,   chk1,3,   chk2,3), and triple (  chk1,2,3) mutants. Mutants were characterized for their cytokinin response and differentiation capacities. While the wild type did not grow on high doses of @entity19164 (1   M @entity3015 ), the   chk1,2,3 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 @entity958 , 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.
