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Published on March 20, 2003; 10.1104/pp.102.015933

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Received October 10, 2002
Returned for revision November 6, 2002
Accepted January 3, 2003

Functional Analysis of Cystathionine {gamma}-Synthase in Genetically Engineered Potato Plants

Oliver Kreft , Rainer Hoefgen , and Holger Hesse *

Max-Planck-Institut für Molekulare Pflanzenphysiologie, Department of Professor Dr. Lothar Willmitzer, Am Mühlenberg 1, 14476 Golm, Germany

* Corresponding author; email: hesse{at}mpimp-golm.mpg.de.

In plants, metabolic pathways leading to methionine (Met) and threonine diverge at the level of their common substrate, O-phosphohomoserine (OPHS). To investigate the regulation of this branch point, we engineered transgenic potato (Solanum tuberosum) plants affected in cystathionine {gamma}-synthase (CgS), the enzyme utilizing OPHS for the Met pathway. Plants overexpressing potato CgS exhibited either: (a) high transgene RNA levels and 2.7-fold elevated CgS activities but unchanged soluble Met levels, or (b) decreased transcript amounts and enzyme activities (down to 7% of wild-type levels). In leaf tissues, these cosuppression lines revealed a significant reduction of soluble Met and an accumulation of OPHS. Plants expressing CgS antisense constructs exhibited reductions in enzyme activity to as low as 19% of wild type. The metabolite contents of these lines were similar to those of the CgS cosuppression lines. Surprisingly, neither increased nor decreased CgS activity led to visible phenotypic alterations or significant changes in protein-bound Met levels in transgenic potato plants, indicating that metabolic flux to Met synthesis was not greatly affected. Furthermore, in vitro feeding experiments revealed that potato CgS is not subject to feedback regulation by Met, as reported for Arabidopsis. In conclusion, our results demonstrate that potato CgS catalyzes a near-equilibrium reaction and, more importantly, does not display features of a pathway-regulating enzyme. These results are inconsistent with the current hypothesis that CgS exerts major Met metabolic flux control in higher plants.




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