In most insect species, juvenile hormones regulate critical physiological processes such as metamorphosis and reproduction. In insects, these sesquiterpenoids are synthesized by retrocerebral endocrine organs, the corpora allata, via the classical mevalonate (MVA) pathway. One of these compounds, juvenile hormone III (JH III), has also been identified in the sedge Cyperus iria. In higher plants, biosynthesis of the sesquiterpenoid backbone may proceed through two distinct pathways: the MVA pathway or the 2C-methyl erythritol 4-phosphate pathway or through a combination of both pathways. Cell suspension cultures of C. iria were used to elucidate the biosynthetic pathway of JH III in the plant. Enzyme inhibition and labeling studies conclusively demonstrated that the biosynthesis of the sesquiterpenoid backbone of JH III proceeds via the MVA pathway. Inhibitor and precursor feeding studies also suggest that later steps of JH III biosynthesis in C. iria are similar to the insect pathway and that the final enzymatic reaction in JH III biosynthesis is catalyzed by a cytochrome P 450 monooxygenase.Juvenile hormones are a group of structurally related sesquiterpenoids that regulate critical physiological processes including metamorphosis and reproduction in most insect species (Gilbert et al., 1996). Juvenile hormone III (JH III), methyl-10R,11-epoxy-3,7,11-trimethyl-2E,6E-dodecadienoate, has also been identified in the sedge Cyperus iria (Toong et al., 1988).The presence of an insect hormone and its high levels in C. iria throughout development suggest that this compound may play a role in protection of the plant against insect herbivory (Toong et al., 1988; Bede et al., 1999a). However, there is no evidence that JH III functions as a feeding deterrent. Therefore, any antiherbivory activity would arise from its ability to interfere with insect development. In two laboratory studies, contact of insects with C. iria effectively disrupted development (Toong et al., 1988;Schwartz et al., 1998). These effects were attributed to the JH III content of the plant. However, conclusive evidence that this hormone protects C. iria against insect herbivory has not yet been demonstrated. We have proposed alternatively that JH III may function as an allelopathic agent in C. iria (Bede and Tobe, 2000). This plant species is an extremely invasive weed, responsible for economic losses in many important crops such as rice (Oryza sativa; Holm et al., 1977; Catling, 1992). Treatment of seeds with JH III delays lettuce (Lactuca sativa) seed germination and inhibits shoot growth of rice seedlings (Bede and Tobe, 2000). Therefore, it is possible that this compound confers an ecological advantage to C. iria by leaching from the roots, in which the highest levels are localized, into the immediate environment and affecting the germination and growth of neighboring competing plant species.Temporal and spatial changes in JH III levels are observed in C. iria throughout development (Bede et al., 1999a). JH III levels increased in the immature se...