OPDA isomerase GST16 is involved in phytohormone detoxification and insect development

12-Oxophytodienoic acid (OPDA), a well-known phytohormone of the jasmonate family, has a reactive α,β-unsaturated carbonyl structure which easily adds cellular nucleophiles (Michael addition), making OPDA potentially toxic for herbivores. The glutathione S-transferase GST16 inactivates 12-OPDA in the insect gut by isomerization to inactive iso-OPDA. Quantitative tissue expression analysis showed that HarmGST16 transcripts were present in most larval tissues, including those of the midgut, fatbody and Malpighian tubules. Activity assays confirmed the presence of an active enzyme. Interestingly, feeding different diets to Helicoverpa armigera influenced gst16 expression levels in various tissues, and larvae fed wild-type tobacco leaves had reduced gst16 mRNA levels. The temporal expression of HarmGST16 during larval development was high in the second instar and reduced during the third, fourth and fifth instars. Plant-mediated RNA interference silencing of HarmGST16 retarded larval growth of H. armigera. Injecting cis-OPDA into the hemolymph of larvae caused premature pupation. This result, as well as the finding that GST16 influenced the growth of insects, suggests that GST16 may play an important role in larval development.