Gel electrophoretic analysis of the proteinase inhibitor proteins induced in tomato leaves by airborne methyl jasmonate (EE Farmer, CA Ryan [1990] Proc Natl Acad Sci USA 87: 7713-7716) revealed the new appearance of inhibitors I and II and two other, higher molecular mass proteins (63.5 and 87 kilodaltons). Northern analysis of methyl jasmonate-induced inhibitors I and II mRNAs in tomato (Lycopersicon esculentum) leaves, and of alfalfa trypsin inhibitor (a Bowman-Birk family inhibitor) mRNA in alfalfa (Medicago sativa) leaves, indicated that nascent inhibitor mRNAs were regulated in a manner similar to wounding, that is, at the transcriptional level. In tobacco (Nicotiana tabacum), transformed with a fused gene composed of the 5' and 3' regions of a wound-inducible potato inhibitor II and a chloramphenicol acetyl transferase (CAT) gene coding region, CAT activity was induced in leaves by methyl jasmonate, consistent with a transcriptional regulation of the inhibitor II gene. In tomato leaves, inhibitor I and II mRNAs and proteins accumulated in leaves distal to those exposed to methyl jasmonate or jasmonic acid to similar levels as in exposed leaves. We suggest that in response to wound signals generated by insect or pathogen attacks, linolenic acid is released into the cytoplasm from plant cell membrane lipids and is rapidly converted in cells to jasmonic acid (or perhaps a closely related derivative such as methyl jasmonate), which serves as a signal to regulate the expression of proteinase inhibitor genes.