|SONG, BO - Northeast Agricultural University, China|
|CAMERON, JEFFREY - Washington University|
|JEZ, JOSEPH - Washington University|
Submitted to: Scientific Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2018
Publication Date: 2/5/2018
Publication URL: http://handle.nal.usda.gov/10113/5934978
Citation: Krishnan, H.B., Song, B., Oehrle, N.W., Cameron, J.C., Jez, J.M. 2018. Impact of overexpression of cytosolic isoform of O-acetylserine sulfhydrylase on soybean nodulation and nodule metabolome. Scientific Reports. 8:2367. https://doi.org/10.1038/s41598-018-20919-8.
Interpretive Summary: Soybean is a rich source of protein. Unfortunately, soybean proteins contain low amounts of two important amino acids, methionine and cysteine, that are vital for optimal growth of humans and animals. Previously we have generated transgenic soybeans overexpressing OASS, a key enzyme involved in the synthesis of cysteine. OASS overproducing transgenic soybean seed showed a 74% increase in protein-bound cysteine content. However, the growth of these transgenic plants was negatively impacted by overexpression of OASS. In this study, we have examined the causative reason for the growth inhibition. Biochemical and ultrastructral studies revealed significant changes in the root nodule structure (a nitrogen fixation structure that feeds nitrogen to the plant) and a moderate decease in the supply of energy metabolites to them. The information obtained from this study will help biotechnologists to overcome the growth inhibition by judicially manipulating the sulfur-assimilatory enzyme expression levels. This will enable us to improve the overall quality of soybean seed proteins without the associated adverse effects. Superior quality soy proteins can be utilized to meet the nutritional requirements of the multitude of malnourished people around the world.
Technical Abstract: Nitrogen-fixing nodules, which are also major sites of sulfur assimilation, contribute significantly to the sulfur needs of the whole soybean plants. Nodules are the predominant sites for cysteine accumulation and the activity of O-acetylserine sulfhydrylase (OASS; also known as O-acetylserine(thiol)lyase), which catalyzes the formation of cysteine, is central to the sulfur assimilation process in plants. In this study, we examined the impact of overexpressing OASS on soybean nodulation and nodule metabolome. Overexpression of OASS did not affect the nodule number, but negatively impacted plant growth, which was reflected by reduction in shoot weight. Transmission electron microscopy observations revealed ultrastructure alterations in OASS overexpressing nodules that are typically associated with nodule senescence. HPLC measurement of antioxidant metabolites demonstrated that levels of cysteine, glutathione, and homoglutathione nearly doubled in OASS overexpressing nodules when compared to control non-transgenic nodules. Metabolite profiling by LCMS and GCMS demonstrated that several metabolites related to serine, aspartate, glutamate, and branched-chain amino acid pathways were significantly elevated in OASS overexpressing nodules. Striking differences were also observed in the flavonoid levels between the OASS overexpressing and control soybean nodules. Our results suggest that OASS overexpressing plants compensate for the increase in carbon requirement for sulfur assimilation by reducing the biosynthesis of some amino acids, and by replenishing the TCA cycle through fatty acid hydrolysis. These data may indicate that in OASS overexpressing soybean nodules there is a moderate decease in the supply of energy metabolites to the nodule, which is then compensated by the degradation of cellular components to meet the needs of the nodule energy metabolism.