To produce crystal of cytokinin biosynthesis enzyme suitable for analyzing stereostructure, elucidate a three dimensional structure of the cytokinin biosynthesis enzyme and screen a compound inhibiting the cytokinin synthesis based on the structure.

The method for producing crystal of the cytokinin biosynthesis enzyme comprises a process in which the cytokinin biosynthesis enzyme is co-expressed with chaperone protein in E. coli cells, a process for purifying the cytokinin biosynthesis enzyme from a crushed solution of the E. coli in the absence of magnesium ions and a process for crystallizing the obtained protein solution of the cytokinin biosynthesis enzyme using a pH4.5-6.5 buffer solution containing magnesium ions and a precipitant.