Opaque-2 induced zein reduction and lysine increase suggest a route to quality protein sweet corn

The zeins make up over 60 percent of the kernel total proteins in maize subspecies including sweet corn. However, zeins are devoid of the essential amino acid lysine rendering the protein content of sweet corn incomplete. The opaque-2 mutation reduces the accumulation of the zeins in maize endosperm, subsequently increasing lysine-containing non-zeins. In this study, the objective was to introduce the opaque-2 mutation from Quality Protein Maize (QPM) varieties (o2 donor) into sugary-1 and shrunken-2 sweet corn varieties to produce high-lysine sweet corn, namely quality protein sweet corn (QPS). To do this, two QPM varieties were crossed to sweet corn parents. The F₁ plants were self-pollinated to produce F₂ seeds which visibly segregated for opaque-2 and sweet corn phenotypes. Sweet corn kernels were advanced through the inbred phase by selecting for the opaque-2 mutation at kernel maturity, sweetness and flavor, and texture phenotypes at the prime eating stage (20 days after pollination; DAP). Six QPS lines were selected for o2 uniformity, satisfactory sweetness and flavor, and tender texture. These QPS lines were then used to produce 12 F₁ QPS hybrids. The analysis of lysine and other amino acid profiles in six QPS inbred and 12 QPS hybrid lines revealed that all inbreds and hybrids had increased protein-bound lysine, and some lines had higher free lysine compared to the corresponding wild-type sweet corn parents. Furthermore, biochemical assays revealed that QPS sugar and starch contents were similar to parental sweet corn. This study shows proof of concept for the ability of the opaque-2 mutation to improve sweet corn protein quality without incurring penalties on its quality traits. This can ultimately contribute to enhanced nutrition and profitability of this important vegetable crop.