Abstract: Disclosed is a method for producing a gene product by expressing a gene encoding said gene product in plant host cells, comprising the steps of: constructing a vector expressible in plant host cells, the vector comprising a promoter region derived from an .alpha.-amylase gene of a plant, and a gene encoding a desired gene product; transforming a compatible plant host cell with the vector; cultivating the resultant transformant host cell; subjecting the cultivated transformant host cell to a sugar-depleted or sugar-free condition to promote the expression of the gene under the control of the promoter region; and recovering the expressed gene product. Also disclosed is a method for producing a gene product method which comprises the steps of: constructing a vector expressible in plant host cells, the vector comprising a promoter region derived from an .alpha.