The mechanism for biosynthesis and molecular regulation of the aphid alarm pheromone (AAP) is still a mystery. Previous studies indicated that the biosynthesis of AAP was directly affected by the terpenoid backbone biosynthesis pathway, and several pathways involved in nutritional metabolism providing the bricks for AAP biosynthesis were up-regulated in response to simulated stimulation. This suggests that AAP biosynthesis might be regulated by complex metabolic pathways. Here the molecular responses of the bird cherry-oat aphid Rhopalosiphum padi to starvation stress were investigated, and the molecular pathways were further analyzed by using RNA interference (RNAi) and protein inhibitor, combined with gas chromatography-mass spectrometry analysis of (E)-beta-farnesene (E beta F), the major component of the alarm pheromone in R. padi. The results showed that the nutritional stress significantly reduced the weight of aphid and the quantity of E beta F, and meanwhile dramatically up-regulated the insulin receptor genes (InsR1/2) and down-regulated the downstream genes encoding the kinases PI3K and Akt, key enzymes in the glycolysis pathway (HK, A6PFK, PK) and the isoprenoid pathway (ACSS, HMGR, FPPS1, FPPS2, GGPPS, DPPS). PI3K inhibitor LY294002 treatment and RNAi-mediated knockdown of InsR1/2 significantly reduced the expression level of downstream genes and the quantity of E beta F. Furthermore, knockdown of PK, the rate-limiting enzyme in the glycolysis pathway, down-regulated the genes in the isoprenoid pathway and the production of E beta F; knockdown of the genes encoding isoprenyl diphosphate enzymes revealed that FPPS1 and FPPS2 were both required for E beta F biosynthesis. Our data suggested that AAP is synthesized via glycolysis and isoprenoid pathways under regulation by the insulin signaling pathway.
