Cytokinins are plant hormons, which play a very important role in plant development and growth. Cytokinin signalling is mediated via a so-called multistep phosphorelay system (MCS) derived from bacterial two-component signalling. In Arabidopsis thaliana the signal is transferred from sensor histidin kinases (AHKs) to nuclear response regulators (ARRs) via histidin phosphotransfer proteins (AHPs). The protein-protein interactions (PPI) within this pathway are quite well described, but little is known about how the MCS interacts with other pathways in A. thaliana at the level of PPI. For this reason new potential interactors of AHP2 and AHP5 have been identified via tandem affinity purification. Two of them – PTAC17 (plastid transcriptionally active chromosome) and CNP (crooked neck protein) were selected for further analysis. Homozygous lines carrying mutations in genes coding both potential interactors were used to study their potential impact on cytokinin signalling and/or response by so called root-elongation test (RET). The co-immunoprecipitation (Co-IP) procedure has been optimized to confirm interaction between PTAC17 and AHPs and to find out the interaction specificity of PTAC17 towards all AHPs (AHP1-AHP6). To achieve this, a transgenic suspension culture overexpressing individual AHPs in fusion with GFP was prepared. PTAC17 was expressed using the in vitro transcription/translation system. Optimizations and results from Co-IP as well as quantification of CK response in ptac17 and cnp mutants via RET will be presented.