Transformed potato (Solanum tuberosum L.) plants expressing the antisense-fragment of M21334 gene were estimated. In transgenic plants the decrease of anionic isoperoxidase pI - 3.5 activity was detected. So, the data testify that M21334 gene encodes this isoperoxidase. Decrease of lignin accumulation and dramatic decline of resistance of transgenic potato plants to the late blight agent Phytophthora infestans emphasize an importance of isoperoxidase pI - 3.5 in defense reaction against late blight.