RESEARCH ARTICLE Mass Spectrometry www.proteomics-journal.com Quantitative Proteomic Analysis of Low Linolenic Acid Transgenic Soybean Reveals Perturbations of Fatty Acid Metabolic Pathways Nazrul Islam, Philip D. Bates, K. M. Maria John, Hari B. Krishnan, Zhanyuan J. Zhang, Devanand L. Luthria, and Savithiry S. Natarajan* modifications, gene manipulations To understand the effect of fatty acid desaturase gene (GmFAD3) silencing on for oil composition were intensively perturbation of fatty acid (FA) metabolic pathways, the changes are compared investigated[1,2] due to consumer health in protein profiling in control and low linolenic acid transgenic soybeans using awareness and increased demand for tandem mass tag based mass spectrometry. Protein profiling of the transgenic vegetable oil. Soybean oil constitutes about 60% of total world seed oil (http:// line unveiled changes in several key enzymes of FA metabolism. This includes www.soystats.com) and most of it is used enzymes of lower abundance; fabH, fabF, and thioestrase associated with FA for human consumption. initiation, elongation, and desaturation processes and LOX1 5, ACOX, The fatty acid composition of the ACAA1, MFP2 associated with β-oxidation of α-linolenic acids pathways. In soybean oil is approximately 13% of addition, the GmFAD3 silencing results in a significant reduction in one of the palmitic acid (16:0), 4% stearic acid (18:0), 20% oleic acid (18:1), 55% linoleic major allergens, Gly m 4 (C6T3L5). These results are important for exploring acid (18:2), and 7–10% linolenic acid how plants adjust in their biological processes when certain changes are (18:3).[3] The palmitic and stearic acids induced in the genetic makeup. A complete understanding of these processes are saturated fatty acids, and the re- will aid researchers to alter genes for developing value-added soybeans. maining are unsaturated fatty acids. The lower concentration of polyunsaturated fatty acids (18:3) is always desirable as it reduces the shelf life due to oxidation, 1. Introduction which causes an unpleasant odor. Because of the demand from the end users, “The Better Bean Initiative” (BBI) was launched in Soybean has multifarious usages for human and animal 2000 by the United Soybean Board (USB) to improve the compo- consumption that has made it a prime target for genetic ma- sition of soybean.[4] As per BBI, the targeted composition of fatty nipulation. Such manipulations have been more prominent acids in soybean oil includes a higher level of oleic and lower lev- since the inception of genome sequences in 2010. Among these els of linolenic and saturated fatty acids.[2] To improve better shelf life and stability of soybean oil at higher temperatures, we targeted on fatty acid desaturase (FAD3) gene, Dr. N. Islam, Dr. S. S. Natarajan α Soybean Genomics and Improvement Laboratory responsible for -linolenic acid synthesis controlled by the three USDA-ARS active members: GmFAD3A, GmFAD3B,andGmFAD3C.[5] To Beltsville, MD 20705, USA reduce the linolenic acid level in soybean, the enzymes that cat- E-mail: [email protected] alyze the conversion of linoleic acids (18:2) to α-linolenic acids Dr.P.D.Bates (18:3) during fatty acid biosynthesis needed to be repressed. Us- Institute of Biological Chemistry Washington State University ing RNAi, we silenced the omega-3 FAD gene family in soybean Pullman, WA 99164, USA genome and found a lower amount of the α-linolenic acids in Dr.K.M.M.John,Dr.D.L.Luthria the seeds as compared to the control. We also confirmed that the Food Composition and Methods Development Laboratory silencing of these genes is inheritable.[5] BHNRC, USDA-ARS Transgenic approach is frequently targeted on single class Beltsville, MD 20705, USA of component/s. However, this targeted approach may result Dr. H. B. Krishnan Plant Genetics Research Unit in an increase or decrease of other traits, which may be desir- USDA-ARS, University of Missouri able/undesirable for end users. In addition, the expression of a Columbia, MO 65211, USA gene specifically in a polyploidy like soybean and wheat is not to- Dr. H. B. Krishnan, Dr. Z. J. Zhang tally independent; rather it is the product of interactions among Division of Plant Sciences the genes or diploid genomes in the polyploidy constitutions, as University of Missouri was reported in the case of wheat.[6] Krishnan et al.[7] successfully Columbia, MO 65211, USA introduced a leginsulin gene (cysteine-rich protein) in soybean. DOI: 10.1002/pmic.201800379 However, the total amount of sulfur-containing amino acids did Proteomics 2019, 19, 1800379 1800379 (1 of 11) C 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.proteomics-journal.com not change as compared to the control. Similarly, efforts have been made to improve rice grain protein quality by inserting a Significance Statement [8] sunflower seed albumin gene encoding a sulfur-rich protein. Soybean has been a vital source of human food and animal A proteomic analysis demonstrated the total amount of sulfur feed due to its protein and oil content. Recently, there has content in the transgenic seeds did not improve because of the been a significant interest in modifying the fatty acid com- [9] competition of sulfur with the endogenous sulfur-rich proteins. position of the soybean oil due to its nutritional, health, and Therefore, any disruption in the constituents of a genome either processing significance. Classical breeding and genetic ma- by gene silencing or introducing a foreign gene needs to be in- nipulation using biotechnical approaches have been used to vestigated thoroughly to elucidate and understand the underlying improve the quantity and quality of soybean oil. In the present metabolomic mechanism. study, we investigated the impact of silencing FAD3 gene in Several successful attempts have been made to improve soy- soybean using mass spectrometry approaches to identify bean seed oil compositions by silencing genes that are responsi- changes in protein profile. Because of the complexity of the α ble for encoding fatty acids such as GmFAD3 which encodes - seed proteome, we adopted a high-throughput and sensitive [5,10] linolenic acid content in soybean seed oil. In addition, in pre- tandem mass tag (TMT) technique that can quantify more vious studies, limited information is available on how silencing proteins to understand the mechanism of genetic manipula- of GmFAD3 disrupts the metabolic pathways of FA biosynthe- tion. The results demonstrated that the fatty acid desaturase sis. In the present study, we investigated the impact of silencing gene (GmFAD3) silencing affected the key enzymes in FA FAD3 gene in soybean using mass spectrometry approaches to metabolism. A complete understanding of these processes identify changes in protein and fatty acid profiles in transgenic will aid researchers to alter genes for developing value-added soybeans. Because of the complexity of the seed proteome, we soybeans and help address potential biosafety issue of geneti- adopted a high-throughput and sensitive tandem mass tag (TMT) cally modified soybean. technique that can quantify more proteins as compared to classi- cal gel separation techniques. extracted twice with hexane (5 mL). The extraction was performed under an ultrasonic bath (power 600 W) for a period of 15 min. 2. Experimental Section The extracts were centrifuged at 5000 rpm for 10 min and the su- 2.1. Chemicals pernatant was collected and evaporated to dryness under a slow stream of nitrogen gas. The concentrated soybean oil was resus- The TMT 6plex was purchased from Thermo Scientific pended with 2 mL hexane and from that 1 mL was separated (https://www.thermofisher.com). For fatty acid methyl esters and evaporated to dryness for the preparation of FAMEs deriva- (FAMEs) analyses, the HPLC-grade chemicals (methanol, tives by transesterification of extracted soybean oil. The derivati- and acetonitrile) were purchased from Burdick and Jackson zation was performed using 5 mL of acidified methanol (10 mL (Muskegon, MI, USA). FAMEs standards were purchased from of acetyl chloride to 90 mL of cold methanol). The mixture kept NcChek prep (www.nu-chekprep.com/). All extracts were filtered at ambient temperature overnight with continuous stirring, and through PVDF (0.4 μmfilter). then 3 mL of water was added. The FAMEs were extracted with 2 mL of hexane. The hexane layer was separated and analyzed with GC.[11] 2.2. Plant Materials Soybean cultivar, Jack (control), and four independent FAD3- 2.4. Protein Extraction silenced transgenic lines (FAD-A, FAD-B, FAD-C,andFAD- E) derived from the Jack were grown in 13 L pots. The Protein extraction from soybean was performed using a phe- [12] plants were grown in PRO-MIX (Premier Horticulture, Que- nol extraction protocol as described by Hurkman. Initially, bec, Canada) medium and fertilized with Osmocot 14-14- 200 mg of the ground soybean from each line were defatted us- [13] 14 (Hummert International, Earth City, USA) in a green- ing hexane. The residue was extracted with approximately 1 house (University of Missouri, MO) with supplemental light mL of the buffer containing sucrose (0.7 m), tris (0.5 m), EDTA (intensity 50–90 klux). Greenhouse settings were 16-h day (50 mm), KCl (0.1 m), DTT (25 mm), and PMSF (2 mm). The mix- length with 30/18 °C day/night temperatures. Dry seeds ture was incubated for 30 min at room temperature with shaking. were harvested by hand and stored in cold seed storage The mixture was centrifuged at 8000 g for 30 min and the su- room. pernatant was separated and an equal amount of water saturated phenol was added to it and the mixture was mixed well for 10 min. The phenol phase which contains the proteins were collected af- ter centrifugation at 4 °C for 30 min. The proteins (phenol phase) 2.3.
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