The present invention provides a method enhancing the polyisoprenoid biosynthesis pathway. Also provided are an isoprenoid-producing plant having an overall enhanced pathway of polyisoprenoid biosynthesis, and a method of producing a polyisoprenoid using the isoprenoid-producing plant. The present invention relates to a method of regulating the expression of hydroxymethylglutaryl-CoA reductase by a bZIP transcription factor.