Systemin is one of the many plant hormones that have been isolated during the past few decades. In contrast to the five major classical plant hormones, auxin, gibberellic acid, cytokinin, ethylene, and abscisic acid, which are relatively simple molecules, systemin is an 18-amino acid polypeptide. Systemin is readily transported from wound sites and is considered to be the primary systemic signal. Systemin, a polypeptide processed from a 200-amino precursor called prosystemin, is analogous to polypeptide hormones in animals.