Physiological and molecular mechanisms of drought stress in soybean: Impacts on soybean aphid populations and virus infection

Soybean (Glycine max) is an important crop due to its versatility as a food source, and applications in industries such as biofuel and textiles. Drought stress is a major environmental concern causing reduced yields, as well as, promoting outbreaks of insects and pathogens.  Soybean aphid (Aphis glycines Matsumura) is an invasive pest that not only reduces yields by feeding, but also transmits economically- important viruses such as Soybean mosaic virus (SMV). The objectives of our study were to: (1) determine effects of drought stress on aphid populations and SMV infection and (2) determine plant physiological and molecular mechanisms underlying the interaction. Soybean plants were subjected to varying levels of soil moisture (drought, well-watered, and saturated) and exposed to uninfected aphids and viruliferous aphids. Plants subjected to well-watered conditions harbored highest aphid populations, whereas aphid populations were lowest on saturated plants.  Drought stress did not affect viruliferous aphid populations, but virus replication was significantly reduced.  Artificial feeding assays using phloem exudates from drought stressed plants indicated a major impact on plant nutritional status.   Gene expression analysis revealed that drought stress caused up-regulation of phytohormones involved in abiotic (abscisic acid and ethylene) and biotic stress (jasmonic acid and salicyclic acid).  Our results suggest that during drought stress a complex interplay between metabolic pathways and defense responses influence outcomes between soybean aphids, SMV, and host plants.