CTP-Synthase 2 from Arabidopsis thaliana is required for complete embryo development

Pyrimidine de novo synthesis is an essential pathway in all organisms. The final and rate limiting step in the synthesis of the nucleotide CTP is catalyzed by CTP-Synthase (CTPS) and Arabidopsis harbors five isoforms. Single knockouts of each of these do not show apparent phenotypical alterations with the exception of CTPS2. T-DNA insertion lines for this isoform were unable to produce homozygous offspring. Here we show that CTPS2 exhibits a distinct expression pattern throughout embryo development and loss of function mutants were embryo lethal, as siliques from +/ctps2 plants contained nearly 25 % aborted seeds. This phenotype was rescued by complementation with CTPS2 under control of its endogenous promoter. Reporter lines revealed CTPS2 expression only in the tip of columella cells in embryos of the heart and later stages. Furthermore CTPS2 expression in roots, most pronounced in the columella cells, shoots and vasculature tissue of young seedlings was observed. Filial generations of +/ctps2 plants did not germinate properly, even under external cytidine supply. During embryo development CTPS2 expression was similar to the well known auxin reporter DR5. Indeed, the cloned promoter region we used in this study possesses a repeat of an auxin response element. Thus, we conclude that CTPS2 is essential for CTP supply in the developing embryo and a knockout of CTPS2 is embryo lethal.