Antimicrobial proteins capable of isolation from seeds of Aralia or Impatiens show a wide range of antifungal activity and some antibacterial activity. DNA encoding the proteins may be isolated and incorporated into vectors. Plants transformed with this DNA may be produced. The proteins find commerical application as antifungal or antibacterial agents; transformed plants will show increased disease-resistance. The invention further provides a method of expressing polyproteins in transgenic plants using DNA constructs based on the structure of the gene encoding the Impatiens antimicrobial proteins.