Using basic tools of molecular biology to study interactions between cotton and its herbivores

Background/Question/Methods

Aphids generally induce plant defenses regulated by salicylic acid (SA), which is involved in plants' response to pathogen attack. Jasmonic acid (JA), on the other hand, regulates plant response to chewing herbivores and cell-content feeders. While in several model plants SA and JA display negative cross-talk, numerous non-model plants seem to exhibit a wide range of interactions between the phytohormones. It seems that the SA- and JA- regulated defense pathways interact with each other and the nature of the interactions is highly variable. Based on our previous findings, we hypothesize that in addition to SA-related defenses, aphid feeding induces transcription of JA-regulated genes, thus inducing cotton defenses against a more damaging chewing herbivore. To test this, we exposed cotton plants (Gossypium hirsutum ) to aphids (Aphis gossypii) and beet armyworm (BAW; Spodoptera exigua ) for 2, 24, and 48 h. We used quantitative PCR to compare expression profiles of several genes involved in plant defense: PR-10 (SA), chitinase (SA), trypsin protease inhibitor (PI; JA), cysteine  PI (JA) and peroxidase (reactive oxygen species, ROS).

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. Moreover, BAW induced expression of JA-regulated genes, thereby confirming their role in cotton plants’ induced response to BAW herbivory. Our results suggest that aphids elicit transcription of genes coding for proteins involved in cotton defense against chewing herbivores. Aphid herbivory could have positive ecological consequences for cotton if aphids affect the level and timing of production of defensive compounds targeting chewing herbivores. Further, low levels of aphid herbivory in agricultural fields may have indirect environmental and economic benefits through reduced need for chemical suppression of aphids and increased plant biomass and yield.