Method to obtain an increased threonine and/or methionine content in plants

Several essential amino acids, including threonine and methionine, are present in too low amounts in plant derived food and feed to meet nutritional requirements. Threonine is considered to be the second and third most limiting amino acid in, respectively, pig and broiler feeds composed of corn and soybean meal. In corn, the sulfur amino acids are the third most limiting amino acids, after lysine and tryptophan, for the dietary requirements of many animals. Thus, an increase in threonine or sulfur amino acid content of either corn or soybean would improve the nutritional quality of the mixtures and reduce the need for supplementation through addition of expensive threonine or methionine.
A higher threonine content that normally found in wild type plants is also interesting as it might increase resistance toward certain
pathogens.
In this approach the focus was especially directed to the DHDPS enzyme, which is the first enzyme of the lysine-specific branch of the biosynthetic pathway of the aspartate-derived amino acids.
A new specific DHDPS isogene and corresponding polypeptide has been isolated from plants and found to have a profound effect on DHDPS activity in the model plant Arabidopsis thaliana being transformed with the nucleic acid fragment encoding the DHDPS isogene. Surprisingly, expression in A. thaliana resulted in a significant decrease of the DHDPS activity and a significant increase in the threonine content. This effect was specific for the DHDPS isogene and was not observed when other DHDPS genes are overexpressed. The effect is not due to silencing of endogenous DHDPS genes. Expression of the isogene in E. coli showed that the specific activity of the encoded DHDPS enzyme is strongly reduced. Together our experiments showed an unexpected dominant inhibitory effect of the novel DHDPS that can be exploited to drastically change amino acid composition.