Inosine monophosphate dehydrogenases are key players for functional embryo and seedling development in Arabidopsis
In plants, nucleotide de novo synthesis is required throughout development to energize biochemical reactions, provide DNA for cell division and RNAs for gene expression and translation. Inosine Monophosphate Dehydrogenases (IMPDH) are key enzymes in guanylate synthesis. We identify IMPDH2 from Arabidopsis as the major isoform, being enzymatically active in the plant cytosol and catalyzing the key enzymatic step in CTP biosynthesis. BIFC analysis suggests that IMPDH 1 nd 2 interact with thenmselves and with Cytidine Triphosphate Synthetase (CTPS) isoforms, by this connecting purone and pyrimidine metabolsim. Nucleotide quantification identifies IMPDH2 as bottleneck in guanylate biosynthesis, leading to nucleotide imbalance and limitation, causing reductions in ribosomal RNAs by reducing Target of Rapamycin (TOR) activity. Impaired photosynthesis and growth are consequences thereof. In contrast, overexpression of IMPDH supported growth and in strong lines lead to altered organ development presumably due to altered auxin metabolism during embryo development Overall design: To analyze the lack of IMPDH2 on global gene expression , Col-0 (wild-type control) plants were compared to IMPDH2 T-DNA insertion line imphh2-0 at 4 and 8 days after germination. Plants were grown on soil at 120 µmol photons m-2 s-1, and samples were harvested after 3 hours of light  in three biological replicates.