Caterpillar gut symbionts mediate the intersection of insect and plant immunity

The role of gut bacteria of caterpillar from field colonies has received scant attention especially in the studies of caterpillar induced plant defenses where most studies used lab colonies, which possess lower diversity and amount of gut bacteria. Here, we isolated bacteria colonies from field collected Helicoverpa zea. We reintroduced these bacteria to antibiotic-treated larvae via artificial diet.  We found Enterobacter  cloacae  classified by 16S rRNA, rpoB and hsp60 genes sequencing induced salivary glucose oxidase (GOX). Induced plant defences such as polyphenol oxidase and proteinase inhibitors were induced by feeding by caterpillars reinoculated with E. cloacae while the SA related pathogenesis gene Pr1 is suppressed compared with non-bacterial treatment. Interestingly, both saliva and regurgitant collected from E.cloacae treated caterpillars induced tomato defences. Indicating that elicitors in oral secretions such as GOX were responsible for inducing plant defenses. In addition, caterpillars inoculated with E.cloacae showed suppressed weight gain and suppression of hemolymph phenoloxidase activity, a major insect immune response. Our finding shows a direct role of caterpillar gut symbionts in regulating immunity of both caterpillars and plants and extend our understanding of tritrophic-interactions among the plant, insect and microbe.