Overexpression of serine acetyltransferase in maize leaves increases seed-specific methionine-rich zeins

Maize kernels do not contain enough of the essential sulphur-amino acid methionine (Met) to serve as a complete diet for animals, even though maize has the genetic capacity to store Met in kernels. Prior studies indicated that the availability of the sulphur (S)-amino acids may limit their incorporation into seed storage proteins. Serine acetyltransferase (SAT) is a key control point for S-assimilation leading to Cys and Met biosynthesis, and SAT overexpression is known to enhance S-assimilation without negative impact on plant growth. Therefore, we overexpressed Arabidopsis thaliana AtSAT1 in maize under control of the leaf bundle sheath cell-specific rbcS1 promoter to determine the impact on seed storage protein expression. The transgenic events exhibited up to 12-fold higher SAT activity without negative impact on growth. S-assimilation was increased in the leaves of SAT overexpressing plants, followed by higher levels of storage protein mRNA and storage proteins, particularly the 10-kDa δ-zein, during endosperm development. This zein is known to impact the level of Met stored in kernels. The elite event with the highest expression of AtSAT1 showed 1.40-fold increase in kernel Met. When fed to chickens, transgenic AtSAT1 kernels significantly increased growth rate compared with the parent maize line. The result demonstrates the efficacy of increasing maize nutritional value by SAT overexpression without apparent yield loss. Maternal overexpression of SAT in vegetative tissues was necessary for high-Met zein accumulation. Moreover, SAT overcomes the shortage of S-amino acids that limits the expression and accumulation of high-Met zeins during kernel development.