Defensive duel: The role of fall armyworm "oral cues" in shaping sorghum defenses

Flavonoids are major plant secondary metabolites that provide defense against feeding by fall armyworm (FAW; Spodotera frugiperda), which is an important chewing insect pest on many crops, including sorghum (Sorghum bicolor). Previously, it has been shown that sorghum flavonoids are required for providing resistance to FAW. We demonstrate here the role of FAW oral cues in modulating sorghum flavonoid defenses. While feeding, chewing insects release two kinds of oral cues, which are oral secretions (OS)/regurgitant and saliva. Our results indicate that FAW OS induced the expression of genes related to flavonoid biosynthesis and total flavonoids, thereby enhancing sorghum’s defense against FAW larvae. Conversely, FAW saliva suppressed the flavonoid-based defenses, and promoted FAW caterpillar growth. Thus, we infer that different oral cues in FAW may have contrasting roles in altering sorghum defenses. These findings expand our understanding on the precise modes of action of caterpillar oral cues in modulating plant defenses and thus help design novel pest management strategies against FAW in this vital cereal crop.