POLYPEPTIDE SIGNALING FOR PLANT DEFENSIVE GENES EXHIBITS ANALOGIES TODEFENSE SIGNALING IN ANIMALS

The activation of plant defensive genes in leaves of tomato plants in response to herbivore damage or mechanical wounding is mediated by a m obile 18-amino acid polypeptide signal called systemin, Systemin is de rived from a larger, 200-amino acid precursor called prosystemin, simi lar to polypeptide hormones and soluble growth factors in animals. Sys temin activates a lipid-based signaling cascade, also analogous to sig naling systems found in animals, In plants, linolenic acid is released from membranes and is converted to the oxylipins phytodienoic acid an d jasmonic acid through the octadecanoid pathway. Plant oxylipins are structural analogs of animal prostaglandins which are derived from ara chidonic acid in response to various signals, including polypeptide fa ctors, Constitutive overexpression of the prosystemin gene in transgen ic tomato plants resulted in the overproduction of prosystemin and the abnormal release of systemin, conferring a constitutive overproductio n of several systemic wound-response proteins (SWRPs). The data indica te that systemin is a master signal for defense against attacking herb ivores, The same defensive proteins induced by wounding are synthesize d in response to oligosaccharide elicitors that are generated in leaf cells in response to pathogen attacks, Inhibitors of the octadecanoid pathway, and a mutation that interrupts this pathway, block the induct ion of SWRPs by wounding, systemin, and oligosaccharide elicitors, ind icating that the octadecanoid pathway is essential for the activation of defense genes by all of these signals, The tomato mutant line that is functionally deficient in the octadecanoid pathway is highly suscep tible to attacks by Manduca sexta larvae. The similarities between the defense signaling pathway in tomato leaves and those of the defense s ignaling pathways of macrophages and mast cells of animals suggests th at both the plant and animal pathways may have evolved from a common a ncestral origin.