Many studies demonstrated prominent roles of protein kinases in the regulation of cell differentiation, growth and development in plant system. Kinases are also implicated in different signal transduction pathways in abiotic and biotic stresses. By identifying the substrates of kinases, wide assessment on how the intact plant cell regulates cell growth and responses to external stimuli can be disclosed. Thus, the main objectives of this study are to identify in vivo substrate of the isolated serine/threonine/tyrosine (STY) protein kinase from Arabidopsis thaliana and thus, to uncover the role of the kinase in plant system. This study involved the molecular cloning and gene expression of STY kinase in prokaryotic and eukaryotic systems. The studies of growth and plasmid stability of recombinant bacteria and yeast carrying STY kinase were also elucidated to assess growth behavior and stability of recombinant kinase in host system after prolonged culturing. To address the substrate of STY kinase, the study of protein-protein interaction via yeast two-hybrid system was performed to detect the positive interactors of the kinase and ultimately to identify the role of STY kinase in plant.