This indicates that plant cyanases contribute to cyanate tolerance and are involved in decomposition of cyanate in the plant. BLAST-P searches of the NCBI databases indicated that putative cyanases are produced in some plants. Amino acid sequences of these plant cyanases were highly conserved as are the known cyanases in prokaryotes and fungi. Reconstruction of phylogenetic relationships among the known cyanases provided evidence for a common evolutionary origin for plant cyanases. It is suggested that the conserved cyanases are derived from an ancient gene, which makes it possible to study inter- and intra-specific relationships using cyanase as a phylogenetic marker. The high resolution crystal structure of E. coli cyanase explains the structural and kinetic properties of the enzyme: the active enzyme is a homodecamer composed of five inactive dimmers, and catalytic residues were identified. In our study, homology modelling showed the monomer structures of AtCYN and OsCYN were similar to that of EcCYN. And the similar active homodecamer of plant cyanases was confirmed. Analysis of AtCYN mutants E94L and S117A confirmed the conserved catalytic residues, and indicated that not only the glutamate but also the serine contributes to the formation of the active homodecamer, which was not mentioned in the previous studies of EcCYN. And the difference may be explained by the different structure near the serine. The cyanases shared higher sequence identity and some similar properties, but we also found different properties between the both plant cyanases. Although the assay conditions differ in pH and concentration of substrate, Km NaHCO3 values of both plant cyanases are approximate to that of the characterized PpCYN and SmCYN. However, the characterized cyanases differ in Km KCNO values. These data suggested these cyanases have similar binding affinity to bicarbonate but different binding affinity to cyanate, although the substrates bicarbonate and cyanate share same binding sites of the enzymes. The characterised cyanases are sensitive to the change in pH. AtCYN was pH-sensitive, and its optimum pH value was 7.7, which is similar to EcCYN and SmCYN. Differently, AtCYN lost enzyme activity at low pH, which suggested the enzyme was destabilized in acidic environment.