Bacteria associated with Russian wheat aphid honeydew initiate plant defense responses

Russian wheat aphid (RWA, Diuraphis noxia) is a serious pest that impacts small grains globally. Recently we determined that RWA-induced chlorosis on wheat is partially determined by aphid-associated bacteria. Aphids with high titers of bacteria induced gene expression and accumulation of salicylic acid (SA), a hormone involved in insect resistance, in wheat. High, sustained expression of SA biosynthetic genes was followed by downregulation of jasmonic acid (JA) biosynthetic genes; JA is associated with insect resistance. We hypothesize that aphid-associated bacteria contribute to aphid virulence by modulating the plants’ insect defense mechanisms. Although saliva from aphids contains culturable bacteria, scanning electron microscopy (SEM) studies did not detect bacteria in high numbers in the aphid salivary glands or foreguts. However, since the same genera of bacteria were detected in aphid honeydew as in saliva, we are exploring the role of honeydew in the induction of plant defense. To simplify the analysis, we tested barley, an alternate host for RWA with considerable genetic resources, and found that aphids with higher bacterial titers were more virulent to barley than aphids with low bacterial titers. Here, we will report on the impact of honeydew on plant defense response pathways in barley.