Nucleotide metabolism operates in all living organisms, embodies an evolutionarily ancient and indispensable complex of metabolic pathways and is of utmost importance for plant metabolism and development. In plants, nucleotides can be synthesized de novo from 5-phosphoribosyl-1-pyrophosphate and simple molecules (e.g., CO(2), amino acids, and tetrahydrofolate), or be derived from preformed nucleosides and nucleobases via salvage reactions. Nucleotides are degraded to simple metabolites, and this process permits the recycling of phosphate, nitrogen, and carbon into central metabolic pools. Despite extensive biochemical knowledge about purine and pyrimidine metabolism, comprehensive studies of the regulation of this metabolism in plants are only starting to emerge. Here we review progress in molecular aspects and recent studies on the regulation and manipulation of nucleotide metabolism in plants.

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Using a transgene-based screening, we previously isolated several Arabidopsis mutants defective in protein import into chloroplasts. Positional cloning of one of the loci, CIA1, revealed that CIA1 encodes Gln phosphoribosyl pyrophosphate amidotransferase 2 (ATase2), one of the three ATase isozymes responsible for the first committed step of de novo purine biosynthesis. The cia1 mutant had normal green cotyledons but small and albino/pale-green mosaic leaves. Adding AMP, but not cytokinin or NADH, to plant liquid cultures partially complemented the mutant phenotypes. Both ATase1 and ATase2 were localized to chloroplasts. Overexpression of ATase1 fully complemented the ATase2-deficient phenotypes. A T-DNA insertion knockout mutant of the ATase1 gene was also obtained. The mutant was indistinguishable from the wild type. A double mutant of cia1/ATase1-knockout had the same phenotype as cia1, suggesting at least partial gene redundancy between ATase1 and ATase2. Characterizations of the cia1 mutant revealed that mutant leaves had slightly smaller cell size but only half the cell number of wild-type leaves. This phenotype confirms the role of de novo purine biosynthesis in cell division. Chloroplasts isolated from the cia1 mutant imported proteins at an efficiency less than 50% that of wild-type chloroplasts. Adding ATP and GTP to isolated mutant chloroplasts could not restore the import efficiency. We conclude that de novo purine biosynthesis is not only important for cell division, but also for chloroplast biogenesis.De novo biosynthesis of the purine ring is essential for plant growth and development. The major products, AMP and GMP, are the building blocks for DNA and RNA. AMP, when converted into ATP, is the major energy source for multiple cellular processes. Several important coenzymes, e.g. NAD and FAD, are also derived from the same pathway. In nodules of N-fixing tropical legumes, such as soybean (Glycine max) and cowpea (Vigna unguiculata), the pathway also plays a dominant role in primary nitrogen metabolism. The activity of enzymes in the purine biosynthesis pathway is enhanced considerably in nodules compared to other tissues. Therefore, most studies of purine biosynthesis in plants have used these legume nodules as materials and focused on the function of purine biosynthesis in nitrogen assimilation (for review, see Smith and Atkins, 2002). Studies on the role of purine biosynthesis in normal plant physiology or in non-N-fixing plants have been relatively few.The location of purine biosynthesis within plant cells is still in dispute. The plant enzymes in the pathway are similar to those in Escherichia coli, except that each plant enzyme has an N-terminal extension that is presumed to function as an organelle-targeting signal (Smith and Atkins, 2002). Fractionation studies of nodules indicate that the pathway is located within plastids (Boland and Schubert, 1983;Shelp et al., 1983). However, recent reports indicate that the pathway is present in both mitochondria and plastids (Atkins et al., 1997) or in...