Gut bacteria mediated adaptation of diamondback moth, Plutella xylostella, to secondary metabolites of host plants

Abstract

The diamondback moth (DBM), Plutella xylostella, has successfully adapted to the potent chemical defenses of Brassicaceae plants that deter most other herbivores. Gut bacteria are increasingly recognized as key to the biology of many species but their role in DBM adaptation to plant defense compounds is not well known. In this study, the secondary metabolites of radish seedlings, rich in flavonoids, were identified by liquid chromatography-mass spectrometry. These secondary metabolites reduced the larval growth of DBM lacking gut bacteria. The effect was rapidly eclipsed by the re-introduction of gut microbiota, which was dominated by Enterobacter (Proteobacteria). Similarly, while treatment with the flavonoid kaempferol adversely affected growth and extended the development time, these were alleviated by the re-introduction of Enterobacter sp. EbPXG5 (EbPXG5) to the DBM gut. EbPXG5 not only degrades kaempferol both in vitro and DBM gut, but is also shown to colonize the gut epithelium, forming a protective biofilm. Genomic sequencing of EbPXG5 showed that metabolic genes were the most abundant, especially those involved in xenobiotic degradation, and the metabolism of terpenoids and polyketides, which could participate in the degradation of plant secondary metabolites such as kaempferol. Overall, our results showed that EbPXG5 is a bacterium common in the gut of DBM larvae and has the in vitro and in vivo capacity to detoxify a major secondary metabolite that is produced in brassica plants as a defense against herbivores. This insect-bacterial association may be an important contributor to the status of DBM as a major pest of brassica crops worldwide. In this study, we identify an important role of gut bacteria in mediating the adaptation of diamondback moth (DBM) to plant secondary metabolites. We demonstrate that kaempferol’s presence in radish seedlings greatly reduces the fitness of DBM with depleted gut biota. Reinstatement of gut biota, particularly Enterobacter sp. EbPXG5, improved insect performance by degrading kaempferol. This bacterium was common in the larval gut of DBM, lining the epithelium as a protective film. Our work highlights the role of symbiotic bacteria in insect herbivore adaptation to plant defenses and provides a practical and mechanistic framework for developing a more comprehensive understanding of insect-gut microbe-host plant co-evolution.