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Overproduction of threonine by limitation of the activity of dihydrodipicolinate synthase

Aspartate semialdehyde (ASA) is a common precursor for lysine, threonine and methionine synthesis in plants. Dihydrodipicolinate synthase (DHDPS, commitment to lysine synthesis) and homoserine dehydrogenase (HSDH, leads to threonine and methionine) are sharing ASA as a common substrate. Thus ASA is expected to be the key point in controlling the partitioning of the carbon skeleton into these two branches of the pathway. Genetic and molecular studies we have performed on Arabidopsis thaliana revealed that two dhdps genes, 2 akthr genes encoding bifunctional AK-HSDH enzymes and a mhsdh gene could be involved in the control of this partitioning.

In addition, 3 aklys genes encoding monofunctional aspartate kinase (AK) have to be considered too as AK activity is determining the rate of ASA synthesis. Arabidopsis thaliana knockout mutants in dhdps1, dhdps2, aklys2, aklys3, akthr1 and mhsdh were isolated in order to determine the respective role of these enzymes in amino acid synthesis. The only significant modification in the content of amino acids we recorded in the analyses of all the lines concerned an increase of threonine content in the dhdps2 knockout line (up to five fold the wild type level in reproductive organs) while the level of lysine was only weakly affected. In the dhdps1 knockout mutant, lysine was decreased but threonine did not accumulate. The global AK specific activity of aklys2, aklys3 and akthr1 knockouts was comparable to the activity of the wild type plants.

The reduction of the HSDH activity in the akthr1 knockout did not impair the synthesis of threonine and methionine. We showed that the use of a DHDPS inhibitor allowed us to obtain wild type plants in which no DHDPS activity was left. This also led to a drastic accumulation of free threonine. In this study, we demonstrated that a molecular or chemically obtained limitation of the DHDPS activity leads to threonine overproduction and that any manipulation of the HSDH activity we tried did not affect significantly the synthesis of aspartate derived amino acids.

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