Antisense inhibition of threonine synthase leads to high methionine content in transgenic potato plants

Methionine (Met) and threonine (Thr) are members of the aspartate family of amino acids. In plants, their biosynthetic pathways diverge at the level o f O-phosphohomo-serine (Ser). The enzymes cystathionine gamma-synthase and Thr synthase (TS) compete for the common substrate O-phosphohomo-Ser with t he notable feature that plant TS is activated through S-adenosyl-Met, a met abolite derived from Met. To investigate the regulation of this branch poin t, we engineered TS antisense potato (Solanum tuberosum cv Desiree) plants using the constitutive cauliflower mosaic virus 35S promoter. In leaf tissu es, these transgenics exhibit a reduction of TS activity down to 6% of wild -type levels. Thr levels are reduced to 45% wild-type controls, whereas Met levels increase up to 239-fold depending on the transgenic line and enviro nmental conditions. Increased levels of homo-Ser and homo-cysteine indicate increased carbon allocation into the aspartate pathway. In contrast to fin dings in Arabidopsis, increased Met content has no detectable effect on mRN A or protein levels or on the enzymatic activity of cystathionine gamma-syn thase in potato. Tubers of TS antisense potato plants contain a Met level i ncreased by a factor of 30 and no reduction in Thr. These plants offer a ma jor biotechnological advance toward the development of crop plants with imp roved nutritional quality.