The Arabidopsis thaliana histidine kinase 1 (AHK1) is part of the multistep phosphorelay system in plants and involved in the adjustment of the plant to osmotic stress. The adjustment might be achieved by an AHK1-dependent transition from the multistep phosphorelay system to Ser/Thr/Tyr phosphorylation. This work shows results of the phosphoproteomic analysis of an ahk1 knock down mutant in comparison to the wildtype, an idea, how these differentially phosphorylated proteins can be connected to AHK1-dependent signaling and different phenotypes of ahk1 knock down mutants which confirm these hypotheses.