As the de novo pyrimidine biosynthetic pathway is energy consuming, plant cells reutilize pyrimidine bases and nucleosides derived from the preformed nucleotides (Figure 4). Of the bases, only uracil is directly reused via a specific phosphoribosyltransferase whereas the pyrimidine nucleosides, uridine, cytidine and deoxycytidine are exclusively salvaged to their respective nucleotides, UMP, CMP and dCMP. High activity of uridine/cytidine kinase and nucleoside phosphotransferase in plants may contribute the salvage of these nucleosides (Kanamori-Fukuda et al., 1981).