Pathway Analysis of Sulfur Containing Amino Acids

To learn about the control mechanisms involved in the biosynthesis of sulfur-containing amino acids, we are isolating and studying genes involved and their promoters. Methionine is synthesised from cysteine and phosphohomoserine via the enzymes cystathionine gamma-synthase (CgS), cystathionine beta-lyase (CbL), and methionine synthase (MS); we have cloned and characterised these three genes in potato.

Nutritional Quality Improvement

Improvement in nutritional quality with respect to balanced amino acid composition is a major goal for plant breeders. For monogastric animals, the nutritional quality of food/feed is limited by the content of a few essential amino acids, especially lysine, threonine, cysteine, methionine, and tryptophane. Commercial feed production currently relies on chemically synthesised or fermenter-derived amino acid supplementation. In order to optimise crop quality through metabolic engineering, two approaches can be followed, termed as 'pull' and 'push' approaches, respectively: Enriched sink proteins can be overproduced and/or free essential amino acid content can be elevated. Our research aim of increasing metabolic fluxes towards free amino acid biosynthesis (push) will also be helpful in providing the basic knowledge necessary to achieving the applied goal of improving plant nutritional quality. By providing a tuber-targeted transgenic protein rich in methionine as a sink (pull), we aim to eventually, in collaboration with others, produce potatoes and other crop plants with improved nutritional quality.Our group coordinated an EU-funded project in the Framework Programme 5 of the EU Commission, termed OPTI-2. Project OPTI-2 was an international, multidisciplinary project funded by the EU focused on improving the levels of essential amino acids in crop plants. Project numbers are: QLRT 2000-00103; QLRT 2001-02928

G Galili and R Höfgen (2002) Metabolic engineering of amino acids and storage proteins in plants. Metabolic Engineering, 4, 3-11

R Höfgen and H Hesse (2000) Molecular Engineering of Sulfate Assimilation. In: Sulfur Nutrition and Sulfur Assimilation in Higher Plants: molecular, biochemical and physiological aspects. C Brunold, H Rennenberg, LJ De Kok, I Stulen, and JC Davidian, (eds), Paul Haupt, Bern, 2000, pp. 109-124.

Further reading

Tabe L. Hagan N. Higgins TJV. (2002) Plasticity of seed protein composition in response to nitrogen and sulfur availability. Current Opinion in Plant Biology. 5:212-217

Tabe L. Higgins TJV. (1998) Engineering plant protein composition for improved nutrition. Trends in Plant Science. 3:282-286

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