Elevated methionine content in soybean seed by overexpressing maize β-zein protein

Abstract

Soybean provides superior and readily available protein for human and livestock. However, nutritional value of soybean is limited due to the deficiency of an essential amino acid, methionine. To improve total methionine content of soybean, a methionine-rich seed storage protein, β-zein, was introduced into soybean cultivar ’Jack’ under the control of legumin B4 promoter or CaMV 35S promoter. Totally 4 T3 transgenic lines exhibited higher expression levels of foreign genes, and legumin B4 promoter directed a stronger accumulation of β-zein protein than CaMV 35S promoter. Compared to wild type plant, total methionine content in transgenic soybean seeds significantly increased by up to approximately 15%. Although the introduction of β-zein gene improved total methionine content, the level was negligible compared to native soybean storage proteins, implying that the inadequate soluble methionine is the limiting factor. Based on these observations, a new strategy for simultaneously increasing the “source” and “sink” of methionine metabolism is proposed to further improvement of total methionine content in soybean seed.