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.

Next-generation technologies generate an overwhelming amount of gene sequence data. Efficient annotation tools are required to make these data amenable to functional genomics analyses. The Mercator pipeline automatically assigns functional terms to protein or nucleotide sequences. It uses the MapMan 'BIN' ontology, which is tailored for functional annotation of plant 'omics' data. The classification procedure performs parallel sequence searches against reference databases, compiles the results and computes the most likely MapMan BINs for each query. In the current version, the pipeline relies on manually curated reference classifications originating from the three reference organisms (Arabidopsis, Chlamydomonas, rice), various other plant species that have a reviewed SwissProt annotation, and more than 2000 protein domain and family profiles at InterPro, CDD and KOG. Functional annotations predicted by Mercator achieve accuracies above 90% when benchmarked against manual annotation. In addition to mapping files for direct use in the visualization software MapMan, Mercator provides graphical overview charts, detailed annotation information in a convenient web browser interface and a MapMan-to-GO translation table to export results as GO terms. Mercator is available free of charge via http://mapman.gabipd.org/web/guest/app/ Mercator.

Sink strength of growing potato tubers is believed to be limited by sucrose metabolism and/or starch synthesis. Sucrose synthase (Susy) is most likely responsible for the entire sucrose cleavage in sink tubers, rather than invertases. To investigate the unique role of sucrose synthase with respect to sucrose metabolism and sink strength in growing potato tubers, transgenic potato plants were created expressing Susy antisense RNA corresponding to the T-type sucrose synthase isoform. Although the constitutive 35S CaMV promoter was used to drive the expression of the antisense RNA the inhibition of Susy activity was tuber-specific, indicating that independent Susy isoforms are responsible for Susy activity in different potato organs. The inhibition of Susy leads to no change in sucrose content, a strong accumulation of reducing sugars and an inhibition of starch accumulation in developing potato tubers. The increase in hexoses is paralleled by a 40-fold increase in invertase activities but no considerable changes in hexokinase activities. The reduction in starch accumulation is not due to an inhibition of the major starch biosynthetic enzymes. The changes in carbohydrate accumulation are accompanied by a decrease in total tuber dry weight and a reduction of soluble tuber proteins. The reduced protein accumulation is mainly due to a decrease in the major storage proteins patatin, the 22 kDa proteins and the proteinase inhibitors. The lowered accumulation of storage proteins is not a consequence of the availability of the free amino acid pool in potato tubers. Altogether these data are in agreement with the assumption that sucrose synthase is the major determinant of potato tuber sink strength. Contradictory to the hypothesis that the sink strength of growing potato tubers is inversely correlated with the tuber number per plant, no increase in tuber number per plant was found in Susy antisense plants.

). † These authors contributed equally to the work. SummaryThe properties and expression patterns of the six isoforms of sucrose synthase in Arabidopsis are described, and their functions are explored through analysis of T-DNA insertion mutants. The isoforms have generally similar kinetic properties. Although there is variation in sensitivity to substrate inhibition by fructose this is unlikely to be of major physiological significance. No two isoforms have the same spatial and temporal expression patterns. Some are highly expressed in specific locations, whereas others are more generally expressed. More than one isoform is expressed in all organs examined. Mutant plants lacking individual isoforms have no obvious growth phenotypes, and are not significantly different from wild-type plants in starch, sugar and cellulose content, seed weight or seed composition under the growth conditions employed. Double mutants lacking the pairs of similar isoforms sus2 and sus3, and sus5 and sus6, are also not significantly different in these respects from wild-type plants. These results are surprising in the light of the marked phenotypes observed when individual isoforms are eliminated in crop plants including pea, maize, potato and cotton. A sus1/sus4 double mutant grows normally in well-aerated conditions, but shows marked growth retardation and accumulation of sugars when roots are subjected to hypoxia. The sucrose synthase activity in roots of this mutant is 3% or less of wild-type activity. Thus under well-aerated conditions sucrose mobilization in the root can proceed almost entirely via invertases without obvious detriment to the plant, but under hypoxia there is a specific requirement for sucrose synthase activity.

SummaryAntisense expression of a full length cDNA encoding plastid aldolase led to decreased expression of aldolase at the transcript and protein level in several 'antisense' potato transformants. To quantify the inhibition, activity was compared in corresponding leaves down a plant and in plants of different ages. Aldolase activity was decreased by 32-43%, 56-71%, 79-83% and 91-97% in A-70, A-3, A-51 and A-2. Separation on a Q-Sepharose-FF column showed the decrease was due to inhibition of plastid aldolase. The transformants showed a small increase of Rubisco activity, a small decrease of phosphoribulokinase activity, and larger but subproportional decreases of sedoheptulose-1,7-biphosphatase and plastid fructose-1,6-bisphosphatase activity. Ambient photosynthesis was inhibited by 10%, 40%, 66% and 85% in A-70, A-3, A-51 and A-2. The transformants contained increased triose phosphates, and very low ribulose-1,5-bisphosphate and glycerate-3-phosphate. Chlorophyll fluorescence indicated that photosystem II was more reduced and thylakoid energization was increased. Starch synthesis was decreased by 16% and 36% in A-70 and A-3, whereas sucrose synthesis was less strongly inhibited. Plant growth was not significantly altered in A-70, was decreased by 41% in A-3, and was severely inhibited in plants with under 20% of wild-type aldolase activity. Although plastid aldolase catalyses a readily reversible reaction, possesses no known regulatory properties, and would appear irrelevant for the control of metabolism and growth, small changes in its activity have marked consequences for photosynthesis, carbon partitioning and growth.

Grafting has become an imperative for intensive vegetable production since chlorofluorocarbon-based soil fumigants were banned from use on grounds of environmental protection. Compelled by this development, research into rootstock–scion interaction has broadened the potential applications of grafting in the vegetable industry beyond aspects of soil phytopathology. Grafting has been increasingly tapped for cultivation under adverse environs posing abiotic and biotic stresses to vegetable crops, thus enabling expansion of commercial production onto otherwise under-exploited land. Vigorous rootstocks have been employed not only in the open field but also under protected cultivation where increase in productivity improves distribution of infrastructural and energy costs. Applications of grafting have expanded mainly in two families: the Cucurbitaceae and the Solanaceae, both of which comprise major vegetable crops. As the main drives behind the expansion of vegetable grafting have been the resistance to soilborne pathogens, tolerance to abiotic stresses and increase in yields, rootstock selection and breeding have accordingly conformed to the prevailing demand for improving productivity, arguably at the expense of fruit quality. It is, however, compelling to assess the qualitative implications of this growing agronomic practice for human nutrition. Problems of impaired vegetable fruit quality have not infrequently been associated with the practice of grafting. Accordingly, the aim of the current review is to reassess how the practice of grafting and the prevalence of particular types of commercial rootstocks influence vegetable fruit quality and, partly, storability. Physical, sensorial and bioactive aspects of quality are examined with respect to grafting for watermelon, melon, cucumber, tomato, eggplant, and pepper. The physiological mechanisms at play which mediate rootstock effects on scion performance are discussed in interpreting the implications of grafting for the configuration of vegetable fruit physicochemical quality and nutritive value.

Only a few environmental factors have such a pronounced effect on plant growth and development as ultraviolet light (UV). Concerns have arisen due to increased UV-B radiation reaching the Earth’s surface as a result of stratospheric ozone depletion. Ecologically relevant low to moderate UV-B doses (0.3–1 kJ m–2 d–1) were applied to sprouts of the important vegetable crop Brassica oleracea var. italica (broccoli), and eco-physiological responses such as accumulation of non-volatile secondary metabolites were related to transcriptional responses with Agilent One-Color Gene Expression Microarray analysis using the 2×204 k format Brassica microarray. UV-B radiation effects have usually been linked to increases in phenolic compounds. As expected, the flavonoids kaempferol and quercetin accumulated in broccoli sprouts (the aerial part of the seedlings) 24 h after UV-B treatment. A new finding is the specific UV-B-mediated induction of glucosinolates (GS), especially of 4-methylsulfinylbutyl GS and 4-methoxy-indol-3-ylmethyl GS, while carotenoids and Chl levels remained unaffected. Accumulation of defensive GS metabolites was accompanied by increased expression of genes associated with salicylate and jasmonic acid signaling defense pathways and up-regulation of genes responsive to fungal and bacterial pathogens. Concomitantly, plant pre-exposure to moderate UV-B doses had negative effects on the performance of the caterpillar Pieris brassicae (L.) and on the population growth of the aphid Myzus persicae (Sulzer). Moreover, insect-specific induction of GS in broccoli sprouts was affected by UV-B pre-treatment.

Cold storage of potato (Solanum tuberosum L.) tubers is known to cause accumulation of reducing sugars. Hexose accumulation has been shown to be cultivar-dependent and proposed to be the result of sucrose hydrolysis via invertase. To study whether hexose accumulation is indeed related to the amount of invertase activities, two different approaches were used: (i) neutral and acidic invertase activities as well as soluble sugars were measured in cold-stored tubers of 24 potato cultivars differing in the cold-induced accumulation of reducing sugars and (ii) antisense potato plants with reduced soluble acid invertase activities were created and the soluble sugar accumulation in cold-stored tubers was studied. The cold-induced hexose accumulation in tubers from the different potato cultivars varied strongly (up to eightfold). Large differences were also detected with respect to soluble acid (50-fold) and neutral (5-fold) invertase activities among the different cultivars. Although there was almost no correlation between the total amount of invertase activity and the accumulation of reducing sugars there was a striking correlation between the hexose/sucrose ratio and the extractable soluble invertase activity. To exclude the possibility that other cultivar-specific features could account for the obtained results, the antisense approach was used to decrease the amount of soluble acid invertase activity in a uniform genetic background. To this end the cDNA of a cold-inducible soluble acid invertase (EMBL nucleic-acid database accession no. X70368) was cloned from the cultivar Desirée, and transgenic potato plants were created expressing this cDNA in the antisense orientation under control of the constitutive 35S cauliflower mosaic virus promotor. Analysis of the harvested and cold-stored tubers showed that inhibition of the soluble acid invertase activity leads to a decreased hexose and an increased sucrose content compared with controls. As was already found for the different potato cultivars the hexose/sucrose ratio decreased with decreasing invertase activities but the total amount of soluble sugars did not significantly change. From these data we conclude that invertases do not control the total amount of soluble sugars in cold-stored potato tubers but are involved in the regulation of the ratio of hexose to sucrose.