The effects of enhanced methionine synthesis on amino acid and anthocyanin content of potato tubers

Background: Potato is a staple food in the diet of the world's population and also being used as animal feed. Compared to other crops, however, potato tubers are relatively poor in the essential amino acid, methionine. Our aim was to increase the methionine content of tubers by coexpressing a gene involved in methionine synthesis with a gene encoding a methionine-rich storage protein in potato plants. Results: In higher plants, cystathionine gamma-synthase (CgS) is the first enzyme specific to methionine biosynthesis. We attempted to increase the methionine content of tubers by expressing the deleted form of the Arabidopsis CgS (CgS(Delta 90)), which is not regulated by methionine, in potato plants. To increase the incorporation of free methionine into a storage protein the CgS(Delta 90) was cotransformed with the methionine-rich 15-kD beta-zein. Results demonstrated a 2- to 6-fold increase in the free methionine content and in the methionine content of the zein-containing protein fraction of the transgenic tubers. In addition, in line with higher methionine content, the amounts of soluble isoleucine and serine were also increased. However, all of the lines with high level of CgS(Delta 90) expression were phenotypically abnormal showing severe growth retardation, changes in leaf architecture and 40-to 60% reduction in tuber yield. Furthermore, the colour of the transgenic tubers was altered due to the reduced amounts of anthocyanin pigments. The mRNA levels of phenylalanine ammonia-yase (PAL), the enzyme catalysing the first step of anthocyanin synthesis, were decreased. Conclusion: Ectopic expression of CgS(Delta 90) increases the methionine content of tubers, however, results in phenotypic aberrations in potato. Co-expression of the 15-kD P-zein with CgS(Delta 90) results in elevation of protein-bound methionine content of tubers, but can not overcome the phenotypical changes caused by CgS(Delta 90) and can not significantly improve the nutritional value of tubers. The level of PAL mRNA and consequently the amount of anthocyanin pigments are reduced in the CgS(Delta 90) transgenic tubers suggesting that methionine synthesis and production of anthocyanins is linked.