Abstract
The soybean aphid (Aphis glycines Matsumura) is one of the most important insect pests of soybeans in the North-central region of the US. It has been hypothesized that aphids avoid effective defenses by inhibition of jasmonate-regulated plant responses. Given the role fatty acids play in jasmonate-induced plant defenses, we analyzed the fatty acid profile of soybean leaves and seeds from aphid-infested plants. Aphid infestation reduced levels of polyunsaturated fatty acids in leaves with a concomitant increase in palmitic acid. In seeds, a reduction in polyunsaturated fatty acids was associated with an increase in stearic acid and oleic acid. Soybean plants challenged with the brown stem rot fungus or with soybean cyst nematodes did not present changes in fatty acid levels in leaves or seeds, indicating that the changes induced by aphids are not a general response to pests. One of the polyunsaturated fatty acids, linolenic acid, is the precursor of jasmonate; thus, these changes in fatty acid metabolism may be examples of “metabolic hijacking” by the aphid to avoid the induction of effective defenses. Based on the changes in fatty acid levels observed in seeds and leaves, we hypothesize that aphids potentially induce interference in the fatty acid desaturation pathway, likely reducing FAD2 and FAD6 activity that leads to a reduction in polyunsaturated fatty acids. Our data support the idea that aphids block jasmonate-dependent defenses by reduction of the hormone precursor.
Highlights
Plants deploy biochemical and molecular strategies to deter feeding by insect herbivores
There were six different treatments: 1) aphid infestation where the population was left to develop without limit (SBA:unlimited); 2) aphid infestation where the population was left to grow to 250 aphids per plant and sprayed with insecticide to simulate recommended management practices (SBA:250); 3) infestation of soybean plants with only the soybean cyst nematode via the soil (SCN); 4) infection with only brown stem rot fungus via the soil (BSR); 5) a combination of the soybean aphid (SBA):unlimited, SCN, and BSR treatments (SBA:SCN:BSR); and 6) control, where plants were left untreated and free of all pests
Certain defense responses against aphids depend on fatty acids or derived molecules; no studies on the effect of SBA on soybean fatty acids have been reported to date
Summary
Plants deploy biochemical and molecular strategies to deter feeding by insect herbivores. The plant hormones salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) coordinate the deployment of biochemical defense against pathogens, insect pests and abiotic stresses [1]. SA is mostly involved in defense against biotrophs and hemibiotrophs [2], while JA and ET activate effective defenses against necrotrophs [3]. The response to insect herbivore attacks is mostly dependent on JA. Once plant tissues are damaged by chewing insects, linolenic acid is released from intracellular membrane lipids of the affected tissues.
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