Transgenic soybean plants overexpressing O-acetylserine sulfhydrylase accumulate enhanced levels of cysteine and Bowman–Birk protease inhibitor in seeds

Abstract

Soybeans provide an excellent source of protein in animal feed. Soybean protein quality can be enhanced by increasing the concentration of sulfur-containing amino acids. Previous attempts to increase the concentration of sulfur-containing amino acids through the expression of heterologous proteins have met with limited success. Here, we report a successful strategy to increase the cysteine content of soybean seed through the overexpression of a key sulfur assimilatory enzyme. We have generated several transgenic soybean plants that overexpress a cytosolic isoform of O-acetylserine sulfhydrylase (OASS). These transgenic soybean plants exhibit a four- to tenfold increase in OASS activity when compared with non-transformed wild-type. The OASS activity in the transgenic soybeans was significantly higher at all the stages of seed development. Unlike the non-transformed soybean plants, there was no marked decrease in the OASS activity even at later stages of seed development. Overexpression of cytosolic OASS resulted in a 58-74% increase in protein-bound cysteine levels compared with non-transformed wild-type soybean seeds. A 22-32% increase in the free cysteine levels was also observed in transgenic soybeans overexpressing OASS. Furthermore, these transgenic soybean plants showed a marked increase in the accumulation of Bowman-Birk protease inhibitor, a cysteine-rich protein. The overall increase in soybean total cysteine content (both free and protein-bound) satisfies the recommended levels required for the optimal growth of monogastric animals.

Identification of new QTLs for seed mineral, cysteine, and methionine concentrations in soybean [Glycine max (L.) Merr.]

“…Soybean has a fully sequenced genome (Schmutz et al 2010) and results from soybean studies may be translated to other legumes due to extensive synteny (Bordat et al 2011;Lucas et al 2011;McClean et al 2010;Severin et al 2011). Soybean seed has approximately 40% protein in commercial cultivars (Kim et al 2012). However, soybean protein has an unbalanced amino acid composition for monogastric diets with respect to the sulfur (S)-containing amino acids cysteine (Cys) and methionine (Met) (Kim et al 2012;Panthee et al 2006a, b).…”

“…Soybean seed has approximately 40% protein in commercial cultivars (Kim et al 2012). However, soybean protein has an unbalanced amino acid composition for monogastric diets with respect to the sulfur (S)-containing amino acids cysteine (Cys) and methionine (Met) (Kim et al 2012;Panthee et al 2006a, b). Methionine is an essential amino acid, whereas cysteine is considered semi-essential, because farm animals and humans can convert methionine to cysteine (Kim et al 2012).…”

“…However, soybean protein has an unbalanced amino acid composition for monogastric diets with respect to the sulfur (S)-containing amino acids cysteine (Cys) and methionine (Met) (Kim et al 2012;Panthee et al 2006a, b). Methionine is an essential amino acid, whereas cysteine is considered semi-essential, because farm animals and humans can convert methionine to cysteine (Kim et al 2012). The ratios of total nitrogen (N) to total S (N:S ratio), is an indicator of proportional methionine and cysteine concentrations in soybean seeds (Sexton et al 1998).…”

Advancements in Transgenic Soy: From Field to Bedside

“…The physical synthesis of cysteine is carried out by the enzyme O-acetylserinesulfhydrylase (OASS). In an attempt to increase sulfur containing amino acids in soybean, Kim et al, overexpressed cytosolic OASS and found that transgenic seeds contained elevated levels of both protein bound cysteine (58-74%) and free cysteine (22-32%) [55]. Another approach used to increase sulfur amino acid content in soybean is the use of a maize zein gene which gives rise to several species of insoluble proteins containing high levels of methionine.…”

Comparative Studies Involving Transgenic and Non-Transgenic Soybean: What is Going On?

“…The sulfur assimilatory pathway in soybean metabolism can be metabolized into molecules that protect plants against oxidative stress. The genetic manipulation of the cytosolic isoform of O-acetylserine sulfhydrylase (OASS), an enzyme involved in the sulfur assimilatory pathway, resulted in high levels of thiols and increased tolerance of plants to metal toxicity [44].…”