Drought stress in soybean: Impacts on soybean aphid populations (Aphis glycines Matsumura) and Soybean mosaic virus infection

Soybean (Glycine max) is an important crop due to its versatility as a food source as well as various applications in industries such as biofuel and textiles. Among various environmental stresses that impact soybean productivity, drought stress is a major concern causing reductions in yield of up to 40%. In addition to a direct impact on yield, drought stress promotes outbreaks of insect pests and pathogens. The soybean aphid (Aphis glycines) is a major pest that not only reduces yields by direct feeding, but also due to the transmission of various economically important viruses such as Soybean mosaic virus (SMV). There are no published reports on the impact of drought stress on soybean aphid population growth rate and disease development. The objectives of our study were to: (1) determine the effects of drought stress on aphid populations and SMV disease development and (2) determine plant molecular mechanisms underlying the interaction. Soybean plants were subjected to varying levels of drought stress (25, 50, 75, and 100% volumetric water content) and subsequently exposed to uninfected aphids, and SMV infection by viruliferous aphid feeding and mechanical inoculation. Drought stress and water saturated conditions negatively impacted aphid populations. Gene expression analysis revealed significant interaction between phytohormones involved in biotic stress (Salicylic acid and Jasmonic acid) and abiotic stress (Abscisic acid) responses. Artificial feeding assays using phloem exudates collected from drought stressed and aphid-infested plants revealed a major impact on plant nutritional status. Our results suggest that during drought stress a complex interplay between metabolic pathways and defense responses influence the outcomes between soybean aphid, SMV, and the host plant.