Aphids are generally thought to induce plant defenses regulated by salicylic acid (SA), and not jasmonic acid (JA), which regulates plant response to chewing herbivores. However, the two pathways interact with each other and the nature of the interactions varies depending on the plant and herbivore. In previous experiments, we demonstrated that beet armyworm (BAW; Spodoptera exigua) avoided cotton plants (Gossypium hirsutum) exposed to aphids (Aphis gossypii). BAW performance was reduced on cotton with aphids, and moderate aphid densities resulted in increased concentration of defense proteins, trypsin inhibitor, chitinase and peroxidase. Based on our previous findings, we hypothesize that in addition to SA-related defenses, aphid feeding induces transcription of JA-regulated genes, thus priming cotton defenses against a more damaging chewing herbivore. To test this, we exposed cotton plants to low numbers of aphids (~ 30 per leaf) for 2, 24, 48 and 120 hrs, and compared the expression of PR-10 (SA), chitinase (SA), trypsin protease inhibitor (PI; JA), cystein PI (JA) and peroxidase (reactive oxygen species, ROS) through quantitative PCR.
Results/Conclusions
We found transcription of all the genes to be induced by aphid herbivory. While expression of genes in SA pathways was higher at all time points, aphids also elicited induction of both JA-regulated genes, trypsin and cysteine PI’s after aphids remained on the plants for 24 hrs. Cysteine PI continued to be expressed at higher levels after 48 and 120 hrs of aphid feeding, trypsin PI was overexpressed only at 24 hrs. Our results suggest that aphids elicit transcription of genes coding for proteins involved in plant defense against chewing herbivores. Low levels of aphid herbivory could have positive ecological consequences for cotton if they induce the production of defensive compounds that negatively impact more damaging herbivores. It will also have important environmental and economic repercussions through reduced pesticide use against aphids and other herbivores and increased plant yield.