Pantothenate Synthetase is Essential but Not Limiting for Pantothenate Biosynthesis in Arabidopsis

Pantothenate (vitamin B5) is the universal precursor for coenzyme A (CoA), an essential cofactor that is required in the metabolism of carbohydrates and fatty acids. The final step of bacterial and eukaryotic pantothenate biosynthesis is catalyzed by pantothenate synthetase (PTS), which is encoded by a single gene in Arabidopsis thaliana (AtPTS). There was debate whether PTS represents the only mode of pantothenate production because previous biochemical evidence pointed to an additional pantothenate pathway in plants. Here we show that insertional mutant alleles of AtPTS confer a recessive embryo-lethal phenotype with mutant embryos arrested at the preglobular stage. Exogenous pantothenate was required for normal seed development and germination and also facilitated the remaining life cycle. Complementation of the mutant phenotype was likewise achieved by heterologous expression of E. coli PTS (panC). The panC transgene increased the total PTS activity in leaves by up to 500-fold but did not affect the steady-state level of pantothenate, indicating that PTS is essential but not limiting for pantothenate production. The auxotrophic AtPTS knockout phenotype suggests that the embryo and possibly all other tissues are autonomous for the biosynthesis of pantothenate. This view is consistent with the near-ubiquitous expression of AtPTS as judged by promoter:beta-glucuronidase analysis. Given the high demand for CoA during storage oil accumulation, we analyzed transcript and metabolite patterns of CoA biosynthesis in seeds. The data indicate that the pantothenate and CoA contents follow distinct developmental programs and that both transcriptional and posttranslational control mechanisms are important for CoA homeostasis.