Enzyme of the plastid non-mevalonate pathway for isoprenoid biosynthesis that catalyzes the formation of 4-diphosphocytidyl-2-C-methyl-D-erythritol from CTP and 2-C-methyl-D-erythritol 4-phosphate (MEP). Is essential for chloroplast development and required for pigments and gibberellins biosynthesis. 2-C-methyl-D-erythritol 4-phosphate + CTP + H(+) = 4-CDP-2-C-methyl-D-erythritol + diphosphate Divalent metal cations. Mg(2+), Ni(2+) and Mn(2+) are the most effective. Co(2+) and Ca(2+) are only minimally effective. Isoprenoid biosynthesis; isopentenyl diphosphate biosynthesis via DXP pathway; isopentenyl diphosphate from 1-deoxy-D-xylulose 5-phosphate: step 2/6. Expressed in leaves, stems and flowers, but not in roots. Circadian-regulated with a peak in the late period of the light phase. Albino phenotype and seedling lethal when homozygous. The phenotype is caused by an early arrest in chloroplast differentiation. Belongs to the IspD/TarI cytidylyltransferase family. IspD subfamily.