Dr. Jessica Hohenstein, Iowa State University, Department of Entomology
Abstract. The soybean aphid (Aphis glycines Matsumura) is an economically important insect pest of soybean [Glycine max (L.) Merr.] in the Midwest. Plant defenses mediated by the phytohormone jasmonic acid (JA) are effective against soybean aphids yet previous data suggests aphids may alter JA biosynthesis or block the JA response in the compatible interaction. At the same time, aphid feeding highly induced the abscisic acid (ABA) pathway. We tested the hypothesis that soybean aphids block JA biosynthesis and/or response and that the aphid-regulated block of JA defenses is achieved through decoy induction of the ABA pathway. Consistent with previous data, we showed that plants with previous exposure to aphids had attenuated JA-, wound-, and herbivore-induced JA responses compared to control plants. Following this, growth of Helicoverpa zea (Boddie) was facilitated by the aphid-regulated suppression of JA responses. Aphid-treated plants had increased levels of cis-JA but not biologically active JA-isoleucine suggesting JA was synthesized but not perceived. Additionally, aphid-treated plants had higher ABA content compared with control plants. Using chemical elicitor treatment and knockdown lines impaired in ABA biosynthesis (aba2-RNAi) or signaling (scof-1-RNAi), we showed that both endogenous and exogenous ABA suppressed wound-induced JA responses. Furthermore, aphid populations were significantly reduced in the absence of a functional ABA biosynthetic or signaling pathway. Moreover, the aphid-regulated attenuation of JA signaling was abolished in these lines. These results are consistent with the hypothesis that soybean aphids exploit the stress hormone abscisic acid (ABA) to suppress effective JA-mediated plant defenses