The major maize seed storage proteins, zeins, are deficient in lysine and tryptophan content, which contribute to the poor nutritional quality of corn. Whether through the identification of mutations or genetic engineering, kernels with reduced levels of zein proteins have been shown to have increased levels of lysine and tryptophan. It has been hypothesized that these increases are due to the reduction of lysine-poor zeins and a pleiotropic increase in the lysine-rich non-zein proteins. By transforming maize with constructs expressing chimeric double-stranded RNA, kernels derived from stable transgenic plants displayed significant declines in the accumulation of both 19- and 22-kD alpha-zeins, which resulted in higher lysine and tryptophan content than previously reported for kernels with reduced zein levels. The observation that lysine and tryptophan content is correlated with the protein levels measured in transgenic maize kernels is consistent with the hypothesis that a pleiotropic increase in non-zein proteins is contributing to an improved amino acid balance. In addition, a large increase in accumulation of free amino acids, consisting predominantly of asparagine, aspartate and glutamate, was observed in the zein reduction kernels.

SummaryCorn is one of the major crops in the world, but its low lysine content is often problematic for animal consumption. While exogenous lysine supplementation is still the most common solution for today's feed corn, high-lysine corn has been developed through genetic research and biotechnology. Reducing the lysine-poor seed storage proteins, zeins, or expressing a deregulated lysine biosynthetic enzyme, CordapA, has shown increased total lysine or free lysine content in the grains of modified corn plants, respectively. Here, by combining these two approaches through genetic crosses, the total lysine content has more than doubled in F1 progeny. We also observe a synergy between the transgenic zein reduction and the enhanced lysine biosynthesis by CordapA expression. The zein reduction plants are found to accumulate higher levels of aspartate, asparagine and glutamate, and therefore, provide excess precursors for the enhanced lysine biosynthesis.