May catalyze the production of the insect juvenile hormone methyl farnesoate (MeFA) to trigger defense against insect herbivory. (2E,6E)-farnesoate + S-adenosyl-L-methionine = methyl (2E,6E)-farnesoate + S-adenosyl-L-homocysteine juvenile hormone III carboxylate + S-adenosyl-L-methionine = juvenile hormone III + S-adenosyl-L-homocysteine Binds 1 Mg(2+) ion per subunit. Activated by Mn(2+) ions. Strongly inhibited by Cu(2+), Zn(2+), Fe(3+) and Fe(2+) ions. Moderately inhibited by Na(+) and Ca(2+) ions. Rapidly degraded at temperatures above 40 degrees Celsius. kcat is 0.004 sec(-1) with farnesoic acid as substrate. Optimum pH is 8-8.5. Sesquiterpene biosynthesis. Homodimer. Mostly expressed in leaves and, at very low levels, in roots, stems, flowers and siliques. Induced in the presence of the herbivory P.xylostella larvae (PubMed:14617060). Accumulates slighty in response to wounding and to several defense responses-inducing compounds including salicylic acid (SA), jasmonic acid (MeJA) and alamethicin (Ala), an antibiotic peptide of fungal origin (PubMed:16165084). Belongs to the methyltransferase superfamily. SABATH family.