The specificity of cytokinin signalling in XXXX is mediated by differing ligand affinities and expression profiles of the receptors.

@entity24619 @entity958 @entity24620 @entity3793 @entity24618 @entity24912 @entity22176 @entity30119 @entity258
@entity958 has three membrane-located cytokinin receptors ( @entity24618 , @entity24619 and @entity24620 / @entity24620 ), which are sensor @entity3793 kinases containing a ligand-binding CHASE domain. Despite their structural similarity the role of these receptors differs in planta. Here we have explored which parameters contribute to signal specification. In a bacterial assay, the CHASE domain of @entity24618 has a similar ligand binding spectrum as @entity24620 / @entity24620 . It shows the highest affinity for @entity22176 (iP) and @entity30119 (tZ) with an apparent K(D) of 1.4 and 4.0   nm, respectively. Real-time PCR analysis of cytokinin primary response genes in double mutants retaining only single receptors revealed that all receptors are activated in planta by cytokinin concentrations in the @entity258 . However, there are differences in sensitivity towards the principal cytokinins iP and tZ. The activation of the cytokinin-sensitive P( @entity24912 ) :GUS reporter gene in three different double mutants shows specific, but also overlapping, spatial domains of activity, which were for all receptors predominantly in the shoot apical meristems and root cap columella. @entity24618 and @entity24619 signal specifically in leaf parenchyma cells, @entity24619 in stomata cells, and @entity24620 / @entity24620 in the root vasculature. Promoter-swap experiments demonstrate that @entity24620 / @entity24620 can functionally replace @entity24618 but not @entity24619 . However, the cytoplasmic @entity24619 @entity3793 kinase (Hk) domain can be replaced by the @entity24620 / @entity24620 Hk domain, which suggests that functionality is mediated in this case by the extracytosolic domain. Together, the data show that both differential gene expression and ligand preference contribute to specify the receptor activity.
