CTP Synthase 2 From Is Required for Complete Embryo Development.

Pyrimidine synthesis is an essential pathway in all organisms. The final and rate-limiting step in the synthesis of the nucleotide cytidine triphosphate (CTP) is catalyzed by CTP synthase (CTPS), and harbors five isoforms. Single mutant lines defective in each one of the four isoforms do not show apparent phenotypical alterations in comparison to wild-type plants. However, lines that contain T-DNA insertions in the gene were unable to produce homozygous offspring. Here, we show that exhibits a distinct expression pattern throughout embryo development, and loss-of-function mutants are embryo lethal, as siliques from plants contained nearly 25% aborted seeds. This phenotype was rescued by complementation with under control of its endogenous promoter. CTPS2::GFP lines revealed expression only in the tip of columella cells in embryo root tips of the heart and later stages. Furthermore, expression in mature roots, most pronounced in the columella cells, shoots, and vasculature tissue of young seedlings, was observed. Filial generations of plants did not germinate properly, even under external cytidine supply. During embryo development, the expression pattern resembled the established auxin reporter DR5::GFP. Indeed, the cloned promoter region we used in this study possesses a repeat of an auxin response element, and auxin supply increased expression in a cell-type-specific manner. Thus, we conclude that CTPS2 is essential for CTP supply in developing embryos, and loss-of-function mutants in are embryo lethal.