Modulation of jasmonate-isoleucine catabolism by virus-induced gene silencing improves insect resistance in maize (Zea mays)

Plant defenses against insect herbivores are regulated by the jasmonic acid (JA) signaling pathway. Jasmonoyl-L-isoleucine (JA-Ile), which is produced by the conjugation of JA and isoleucine in response to insect herbivory, is a master regulator of induced defense responses. Due to the cost of induced defenses, plants have several ways to optimize JA-regulated defense signaling. One of those mechanisms is the deactivation of JA-Ile by deconjugation and oxidation. In Arabidopsis, regulation of JA-Ile turnover and the effect of impaired JA-Ile catabolism is well studied. However, it remains unknown how manipulation of JA catabolism impacts resistance to chewing and sap-feeding herbivores in maize. In this study, we modified JA defense signaling by silencing expression of JA catabolic genes using virus-induced gene silencing (VIGS) and determined its effect on insect resistance. Initially, we used a sugarcane mosaic virus (SCMV) vector to investigate whether maize gene expression can be reduced by VIGS. The fragments of three genes, phytoene desaturase, lesion mimic 22, and iojap, were cloned into SCMV and used to demonstrate systemic gene expression silencing in maize inbred line P39. Next, we investigated the effect of expression silencing ZmJIH1 and ZmCYP94B1, which encode the deconjugation and oxidation of JA-Ile, respectively, on insect resistance.  Knockdown of those genes reduced the growth of Spodoptera frugiperda (fall armyworm) caterpillars and the reproduction of Rhopalosiphum maidis (corn leaf aphid). These results indicate that alteration of JA metabolism can be employed to improve insect resistance in maize.