Abstract
BackgroundPotato 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 co-expressing a gene involved in methionine synthesis with a gene encoding a methionine-rich storage protein in potato plants.ResultsIn higher plants, cystathionine γ-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 (CgSΔ90), which is not regulated by methionine, in potato plants. To increase the incorporation of free methionine into a storage protein the CgSΔ90 was co-transformed with the methionine-rich 15-kD β-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 CgSΔ90 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-lyase (PAL), the enzyme catalysing the first step of anthocyanin synthesis, were decreased.ConclusionEctopic expression of CgSΔ90 increases the methionine content of tubers, however, results in phenotypic aberrations in potato. Co-expression of the 15-kD β-zein with CgSΔ90 results in elevation of protein-bound methionine content of tubers, but can not overcome the phenotypical changes caused by CgSΔ90 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 CgSΔ90 transgenic tubers suggesting that methionine synthesis and production of anthocyanins is linked.
Highlights
Potato is a staple food in the diet of the world's population and being used as animal feed
Effect of CgSΔ90 expression on phenylalanine ammonia lyase (PAL) mRNA levels We have found that anthocyanin content of the transgenic potato tubers expressing CgSΔ90 at high level was strongly reduced compared to the wild-type (Figure 4)
The high level of CgSΔ90 expression can alter the methionine content of tubers and the amount of other amino acids belonging to the aspartate family
Summary
Potato is a staple food in the diet of the world's population and being used as animal feed. The branch point intermediate of methionine synthesis is O-phospho-homoserine (OPHS), which is a common substrate for both threonine synthase (TS) and cystathionine γ-syn-. OPHS, O-phosphohomoserine; TS threonine synthase; CgS, cystathionine γsynthase; CbL, cystathione β-lyase; MS, methionine synthase. Transgenic tobacco, potato, and alfalfa plants over-expressing the Arabidopsis CgS had higher levels of soluble methionine [6,7,8]. Using this approach, the increase of methionine levels in tubers of transgenic potato lines was 6-fold compared to those in wild-type potato plants [7]
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