It is intended to provide a method of transferring a gene into a plant material via a bacterium belonging to the genus Agrobacterium which comprises: 1) treating the plant material; and 2) infecting the plant material with the agrobacterium; characterized in that, in the above steps 1) and/or 2), use is made of a medium having a metal salt containing copper ion at an elevated concentration. It is also intended to provide a method of constructing a transformant plant characterized by using the gene transfer method as described above.
