Date of Award
Master of Science
Biochemistry, Biophysics and Molecular Biology
Soybean aphids, Aphis glycines, are a specialized phloem-feeding pest of soybeans. They are known to employ different mechanisms to bypass plant defenses and reduce soybean plant yields of up to 50% a year. Once attacked or sensing distress, soybeans induce the oxylipin pathway, a biosynthetic pathway that produces key plant hormones. Among these hormones is jasmonic acid (JA), which is produced from linolenic acid (18:3). Jasmonic acid can signal the production of protease inhibitors as one of the products that obstructs the attackers’ ability to digest protein. Previous studies show that levels of three key fatty acid precursors (palmitic, 16:0; linoleic, 18:2; linolenic, 18:3) to JA biosynthesis are changed by aphids. We are interested in determining if the change in these fatty acid levels induced by soybean aphids is only a localized response or whether the changes are also triggered systemically. We found that accumulation of 16:0 and a decrease in levels of 18:3 only occurs at the local site of infestation. We also studied the expression levels of FAD6, a gene encoding for a fatty acid desaturase catalyzing the dehydration of oleate (18:1) to 18:2, and found that FAD6 expression levels are also decreased only locally. In addition to the oxylipin pathway, we are also interested in the abscisic acid (ABA) pathway, which is involved in pathogen and abiotic induced stress and is proposed to be antagonistic to the JA pathway in soybean. We used expression of the SCOF-1 gene as a marker for ABA signaling, and found SCOF-1 to be induced both locally and systemically in the presence of aphids.
We also performed a lipidomics analysis on soybean tissues that were infested with soybean aphids for 7 days. We found significant changes in several lipid species including digalactosyldiacylglycerol (DGDG), monogalactosyldiacylglycerol (MGDG), phosphatidylglycerol (PG), lysophosphatidylethanolamine (lysoPE), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), and phosphatidic acid (PA). The changes of highest interest are presented in significant increases of PA and PE, where not only individual species, but also total PA and PE content were affected. To complement the lipidomics analysis, we mined transcriptome data from a previous experiment carried out in the same conditions, looking for changes in expression of genes related to lipid metabolism. Transcriptome data showed an increase in the expression of several lipid metabolism related genes including phospholipase A, phospholipase C, and phospholipase D. We also found several important genes that regulate oxylipins biosynthesis to be upregulated upon aphid feeding. Finally, we found several lipid-binding genes to be induced and some to be repressed by aphid feeding. Together, our results suggest that phospholipids play a role in the soybean response to aphid feeding.
Nguyen, Khoi, "Local vs. systemic modifications of soybean (Glycine max) defense signals in response to soybean aphid (Aphis glycines Matsumura) infestation" (2015). Graduate Theses and Dissertations. 14603.