Submitted to: MicrobiologyOpen
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2019
Publication Date: 1/28/2020
Citation: Lawrence, S.D., Novak, N.G., Shao, J.Y., Ghosh, S.B., Blackburn, M.B. 2020. Cabbage looper (Trichoplusia ni) salivary glands contain unique bacterial flora in contrast with their alimentary canal, mandibular gland and Malpighian tubules . MicrobiologyOpen. 9(4):e994.
Interpretive Summary: Crop plants defend themselves against insect feeding by producing toxic or irritating compounds, and substances that interfere with the insect's digestion. The salivary gland secretions of insects have been shown to play a very significant role in triggering and modifying these defensive responses. Recently, it has also been shown that gut bacteria associated with the insect can also alter a plant's defensive responses to being fed upon. We examined the bacterial community of the salivary gland of cabbage loopers feeding on collards and found that it was substantially different from that of the gut. These results will guide further research on how salivary gland bacteria modify plant defenses against insects, and lead to methods for altering the plant microbiome to strengthen resistance to insect damage. These results will be of interest to researchers studying organic farming systems, and to plant breeders.
Technical Abstract: In recent years, several studies have examined the gut microbiome of lepidopteran larvae and how factors such as host plant affect it, and in turn, how gut bacteria affect host plant responses to herbivory. In addition, other studies have detailed how secretions of the labial, or salivary glands, can alter host plant defensive responses. We examined the gut microbiome of the cabbage looper (Trichoplusia ni) feeding on collards (Brassica oleracea), and separately analyzed the microbiomes of various organs that open directly into the alimentary canal, including the labial glands, mandibular glands, and the Malpighian tubules. In this study, the gut microbiome of T. ni was found to be generally consistent with those of other lepidopteran larvae in prior studies. The greatest diversity of bacteria appeared in the Firmicutes, Actinobacteria, Proteobacteria, and Bacteriodetes. Well represented genera included Staphylococcus, Streptococcus, Corynebacterium, Pseudomonas, Diaphorobacteria, Methylobacterium, Flavobacterium, and Cloacibacterium. Across all organs, two Amplicon sequence variants (ASVs) associated with the genera Diaphorobacter and Cloacibacterium appeared to be most abundant. In terms of the most prevalent ASVs, the alimentary canal, Malpighian tubules, and mandibular glands appear to have similar complements of resident bacteria, with relatively few significant differences evident. However, aside from the Diaphorobacter and Cloacibacterium ASVs common to all the organs, the labial glands appeared to possess a distinctive complement of bacteria which was absent or poorly represented in the other organs. Among these were representatives of the Pseudomonas, Flavobacterium, Caulobacterium, Anaerococcus, and Methylobacterium. These results suggest that the labial glands present bacteria with different selective pressures than those occurring in the mandibular gland, Malpighian tubules and the alimentary canal. Given the documented effects that salivary gland secretions and the gut microbiome can exert on host plant defenses, the effects exerted by the bacteria inhabiting the labial glands themselves deserve further study.