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Impact of overexpression of cytosolic isoform of O-acetylserine sulfhydrylase on soybean nodulation and nodule metabolome

Hari B. Krishnan, Bo Song, Nathan W. Oehrle, Jeffrey C. Cameron, Joseph M. Jez
Scientific reports 2018 v.8 no.2367 pp. 1-14
acid hydrolysis, antioxidants, aspartic acid, biosynthesis, carbon, catalytic activity, cysteine, cysteine synthase, energy, energy metabolism, fatty acids, flavonoids, gene overexpression, glutamic acid, glutathione, high performance liquid chromatography, metabolites, metabolome, nitrogen fixation, nodulation, plant growth, serine, soybeans, sulfur, transmission electron microscopy, tricarboxylic acid cycle, ultrastructure
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.