Synthesis of the purine ring is a central metabolic function of all cells. The products, AMP and GMP, provide purine bases for DNA and RNA, as well as for a number of essential coenzymes (NAD, NADP, FAD, and coenzyme A) and signaling molecules (e.g. cAMP; Fig. 1 ). ATP serves as the energy source for many chemical reactions. In addition, in plants, the nucleotides are the precursors for purine alkaloids, and for the adenine moiety of cytokinin plant growth regulators (Fig. 1) . Despite the

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... tial functions for purines, salvage pathways, which retrieve the purine ring after nucleic acid or coenzyme breakdown, recycle nucleotides to meet day-to-day needs. Thus, the requirement to synthesize new purines in differentiated cells is small. It is only when DNA is replicated that de novo synthesis of the purine ring is required and so, although present in most tissues, the activity of the metabolic pathway is relatively low.