Bacteria associated with Russian wheat aphid (Diuraphis noxia) enhance aphid virulence to wheat

Phenotypic responses to biotic stresses are often studied as the interactions between two species; however, in the phytobiome, these responses frequently result from complex interactions involving several organisms. Here, we show that variation in chlorosis caused by Russian wheat aphid (Diuraphis noxia) feeding is determined, in part, by aphid-associated bacteria. Proteomic analysis of fluids injected into a sterile medium by the aphid during feeding indicate that 99% of the proteins are of bacterial origin. Of these, the greatest proportion are produced by bacteria in the order Enterobacteriales. Bacteria from five genera in four families that have the capacity to produce these proteins were isolated directly from aphids as well as from wheat leaves only after D. noxia feeding. By themselves or in combination, these bacteria were not virulent to wheat, even at high inoculum levels. Metagenomic analysis showed that the same five D. noxia-associated genera dominated the non-Buchnera component of the aphid microbiome, and that representation of these genera was reduced in aphids from colonies established after isolation of newborn nymphs from their mothers prior to feeding (isolated aphids). Isolation or treatment with antibiotics reduced bacterial numbers, and these aphids caused less feeding damage on wheat than non-isolated or non-antibiotic-treated aphids. Our data show that bacterial proteins are a significant component of Russian wheat aphid saliva, that the bacteria producing these proteins are associated with aphids and plants fed upon by aphids, and that these aphid-associated bacteria facilitate aphid virulence to wheat.