Rapid defense responses in maize leaves induced by Spodoptera exigua caterpillar feeding

Insects such as the beet armyworm (Spodoptera exigua) cause extensive damage to maize (Zea mays). Maize plants respond by triggering defense signaling, changes in gene expression, and biosynthesis of specialized metabolites. Leaves of maize inbred line B73, which has an available genome sequence, were infested with S. exigua for 1 to 24 h, followed by comparisons of the transcript and metabolite profiles with those of uninfested controls. The most extensive gene expression responses occurred rapidly, within 4-6 h after caterpillar infestation. However, both gene expression and metabolite profiles were altered within 1 h and continued to change during the entire 24 h experiment. The defensive functions of three caterpillar-induced genes were examined using available Dissociation transposon insertions in maize inbred line W22. Whereas mutations in the benzoxazinoid biosynthesis pathway (Bx1 and Bx2) significantly improved caterpillar growth, the knockout of a 13-lipoxygenase (Lox8) involved in jasmonic acid biosynthesis did not. Interestingly, 9-lipoxygenases, which lead to the production of maize death acids, were more strongly induced by caterpillar feeding than 13-lipoxygenases, suggesting an as yet unknown function in maize defense against herbivory. Together, these results provide a comprehensive view of the dynamic transcriptomic and metabolomic responses of maize leaves to caterpillar feeding.