Manipulating the Sulfur Composition of Seeds

Abstract

The amino acid composition of seeds is frequently sub-optimal for human and animal nutrition. We have attempted to modify the quantities of stored sulfur amino acids in seeds using three complementary strategies. Firstly, a transgene encoding a naturally sulfur amino acid-rich sunflower seed albumin (SSA), was expressed under control of a seed-specific promoter in narrow leaf lupin, chickpea, and field pea. In all cases, this strategy was successful in significantly increasing total sulfur amino acid concentrations of the mature seeds compared to non-transgenic controls. However, the abundance of some endogenous sulfur-rich seed proteins decreased in the transgenic seeds, indicating that the new storage sink competed to some extent with endogenous proteins for a limiting supply of sulfur amino acids in developing seeds. High levels of several enzymes of the sulfur amino acid biosynthetic pathway were detected in developing lupin and chickpea seeds, supporting the view that developing legume seeds are active in sulfur amino acid biosynthesis. A second strategy, of increasing sulfur supply to the plants, had limited success in further increasing seed sulfur amino acid content. A third strategy was used in an attempt to supplement the sulfur amino acid content of narrow leaf lupin seeds. A gene encoding a cysteine feedback-insensitive, plant serine acetyltransferase (SAT) was expressed under the control of a strong, seed-specific promoter in transgenic lupins. Transgenic developing seeds had greatly increased SAT activity, and some of the highest in planta concentrations of free cysteine yet reported. However, the free methionine concentrations of the developing seeds were unchanged by SAT over-expression, and the mature seeds contained no more total (protein-bound) sulfur amino acids than non-transgenic controls. The results confirmed the activity of the cysteine biosynthetic pathway in developing lupin seeds, but indicated that biosynthesis of cysteine was not limiting for accumulation of free methionine in developing seeds, or for accumulation of sulfur-rich seed storage proteins in mature seeds.