Tuber Metabolism Research Articles

UV-C irradiation accelerated the deposition of suberin and lignin at wounds by activating phenylpropanoid metabolism in potato tubers

UV-C could directly inactivate pathogens, and reduce postharvest diseases of fruits and vegetables by inducing resistance. To date, no studies have been carried out to verify whether UV-C may accelerate the wound healing of potato tubers. The present results demonstrated that UV-C irradiation up-regulated StPAL, St4CL, and StCAD expression and activated phenylalanine ammonia-lyase (PAL), 4-coumarate: coenzyme A ligase (4CL), and cinnamyl alcohol dehydrogenase (CAD). Meanwhile, UV-C irradiation elevated the levels of four phenolic acids (p-coumaric acid, caffeic acid, sinapic acid, and ferulic acid) and three lignin monomers (coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol), and accelerated the suberin polyphenolic (SPP) and lignin accumulation at wounds, thereby decreasing weight loss and disease index. The weight loss and disease index in the UV-C irradiated tubers were 36.48% and 56.25% lower than that of the control on the 14 d of healing, respectively. Taken together, UV-C irradiation promoted the process of wound healing in potato tubers by stimulating phenylpropanoid metabolism as well as accelerating the SPP and lignin accumulation at wounds. Therefore, UV-C irradiation could be employed as a simple, safe, and low-cost handling technology for accelerating wound healing of potato tubers.

Interference Expression of StMSD Inhibited the Deposition of Suberin and Lignin at Wounds of Potato Tubers by Reducing the Production of H2O2.

Superoxide dismutase (SOD) actively participates in the wound stress of plants. However, whether StMSD mediates the generation of H2O2 and the deposition of suberin polyphenolic and lignin at potato tuber wounds is elusive. In this study, we developed the StMSD interference expression of potato plants and tubers by Agrobacterium tumefaciens-mediated transformation. The StSOD expression showed a marked downregulation in StMSD-interference tubers, especially StCSD2 and StCSD3. The content of O2•− exhibited a noticeable increase together with the inhibition in H2O2 accumulation. Moreover, the gene expression levels of StPAL (phenylalanine ammonia-lyase) and StC4H (cinnamate-4-hydroxylase) were downregulated in StMSD-interference tubers, and less suberin polyphenolic and lignin depositions at the wounds were observed. Taken together, the interference expression of StMSD can result in less suberin polyphenolic and lignin deposition by inhibiting the disproportionation of O2•− to H2O2 and restraining phenylpropanoid metabolism in tubers.

Nucleases are upregulated in potato tubers afflicted with zebra chip disease.

The defense response of potato tubers afflicted with zebra chip disease involves oxidatively mediated upregulation of nucleases that likely modulate localized programmed cell death to restrict the phloem-mobile, CLso bacterial pathogen to the vasculature. Zebra chip (ZC) is a bacterial disease of potato (Solanum tuberosum L.) caused by Candidatus Liberibacter solanacearum (CLso). Tubers from infected plants develop characteristic brown discoloration of the vasculature, a result of localized programmed cell death (PCD). We examined the potential contribution of nucleases in the response of tubers to CLso infection. Specific activities of the major isozymes of dsDNase, ssDNase, and RNase were substantially upregulated in tubers from CLso-infected plants, despite their significantly lower soluble protein content. However, ZC disease had no effect on nuclease isozyme profiles. Activities of the predominant nuclease isoforms from healthy and CLso-infected tubers had similar pH optima, thermotolerance, and responses to metallic co-factors. Nuclease activities were heat stable to 60°C and resistant to precipitation with 70% (v/v) isopropanol, which constitute effective techniques for partial purification. DNase and RNase isozyme activities were highest at pH 7.2-8.5 and 6.8-7.2, respectively, and profiles were similar for tubers from CLso-infected and non-infected plants. RNase activities were mostly insensitive to inhibition by EDTA, except at pH 8.5 and above. DNase activities were inhibited by EDTA but less sensitive to inhibition at high pH than the RNases. The EDTA-mediated inhibition of DNase (ds/ss) activities was restored with ZnSO4, but not Ca+2 or Mg+2. By contrast, ZnSO4 inhibited the activities of RNases. DTT and CuSO4 inhibited the activities of all three nucleases. These results suggest that activation of tuber nucleases is dependent on the oxidation of sulfhydryl groups to disulfide and/or oxidation of Zn to Zn+2. In light of previous published results that established extensive CLso-induced upregulation of oxidative stress metabolism in tubers, we propose a model to show how increased nuclease activity could result from a glutathione-mediated oxidation of nuclease sulfhydryl groups in diseased tubers. DNases and RNases are likely an integral part of the hypersensitive response and may modulate PCD to isolate the pathogen to the vascular tissues of tubers.

Optimal light intensity and quality increased the saffron daughter corm yield by inhibiting the degradation of reserves in mother corms during the reproductive stage

Saffron (Crocus sativus L.) two-segment (TS) cropping system, with mother corms flowering in a controlled environment first and then growing daughter corms in the field, can greatly increase stigma yield and quality, but may decrease daughter corm yield (mainly due to depletion of nutrient reserves in mother corms). Light plays crucial roles in regulation of nutrient reserve metabolism in tubers, corms, and other storage organs. However, how the metabolism of mother corm reserves, which determines the daughter corm yield, responds to light intensity and quality during the reproductive stage remains unclear. Plants grew under five light intensities (56, 200, 400, 600, and 800 μmol m−2 s−1) and three combinations of red (R) to blue (B) light (4R1B, 3R1B, and 3R2B) at 200 μmol m−2 s−1 during the reproductive stage. After flowering, the plants were transplanted to the field. Leaf growth, net photosynthesis, carbohydrate and nutrient content, stigma yield and quality, and daughter corm yield were assessed. The stigma yield and quality decreased with increasing light intensity, whereas the carbohydrate and total N, P, and K content in the mother corm showed the opposite trend. The light treatment of 200(3R2B) not only maintained the stigma yield (up to 482.4 mg (10 corms)−1) at a highest level, but also improved the apocarotenoid content (up to 33.6%) in stigma, and carbohydrate and nutrient content in mother corm. Compared with 56 μmol m−2 s−1, light 200 (3R2B) decreased the length and weight of leaves, ratio of leaf to corm weight, and starch and nutrient decomposition in corms, but increased the rate of photomorphogenesis, stomatal opening, chlorophyll concentration (up to 72.9%), net photosynthesis (up to 448.6%), and leaf sucrose concentration (up to 156.7%). A decreased demand for leaf development and increased carbon capture inhibited the decomposition of carbohydrates and nutrients in the mother corm. The high sucrose concentration in leaves served not only as a carbon substrate for leaf growth, but also as a signal for inhibiting the translocation of carbon from corm to leaves and then the decomposition of starch in corm. The increased reserves in mother corms and improved stomatal opening combined to result in large leaf area, well-developed vascular tissue, and then high rate of leaf photosynthesis in the field, which provided assimilates for achieving yield potential and a high proportion of large-size corms. This is the first report on optimizing light intensity and quality during the reproductive stage to partially eliminate daughter corm yield decreases in the TS cropping system. Furthermore, this work also contributed to understanding the metabolism of nutrient reserves and leaf growth in saffron as regulated by light intensity and quality.

Diphenyleneiodonium delays the deposition of suberin polyphenolic and lignin at wounds of potato tubers by inhibiting Strbohs transcription and NADPH oxidase activity

Diphenyleneiodonium (DPI) is a specific and effective inhibitor of NADPH oxidase (NOX). However, there are no reports on whether DPI affects the transcription of Strbohs family members, and the accumulation of suberin polyphenolic (SPP) and lignin of potato tubers during healing. In this study, DPI treatment was shown to exert an inhibitory effect on the expression of Strbohs at the wound tissue, especially StrbohA, StrbohB, and StrbohE. DPI clearly reduced the activity of NOX and superoxide dismutase, and decreased O2− and H2O2 accumulation. DPI also decreased the activity of phenylalanine ammonia-lyase, cinnamate-4-hydroxylase, 4-coumarate-CoA ligase, and cinnamyl alcohol dehydrogenase as well as the contents of total phenolics and lignin. Moreover, DPI caused the lower deposition amount of SPP and lignin at wounds of potato tubers. Above results suggested that DPI led to the less SPP and lignin deposition at the wound surfaces by decreasing H2O2 production via Strbohs / NOX and suppressing phenylpropanoid metabolism in potato tubers.

Ca2+ applications affect the phenylpropanoid metabolism in potato tubers induced by T-2 toxin

The dry rot of potato caused by Fusarium sulphureum is the main postharvest disease that causes serious economic loss and leads to mycotoxin contamination. Therefore, it is necessary to find a strategy to increase the plant’s resistance against disease. T-2 toxin at low concentration can act as an elicitor to induce resistance against disease. Calcium ion plays a vital role in induced resistance by responding the environmental stress in plant. In this study, the effect of Ca2+ on the development of dry rot, weight loss rate, the suberin polyphenolic (SPP) and suberin polyaliphatic (SPA) accumulation and their participation in phenylpropanoid metabolism were investigated in treated potato. The results showed that lesion diameter and weight loss rate significantly increased after ethylenebis (oxyethylenenitrilo) tetraacetic acid (EGTA) treatment, but the addition of exogenous Ca2+ noticeably inhibited the increase. The accumulations of SPP and SPA, enzymatic activities, gene expressions, as well as the contents of total phenols and lignin and their major substrates involved in phenylpropanoid metabolism were decreased after EGTA treatment, these indicators were recovered after the addition of exogenous Ca2+. Therefore, the results suggested that Ca2+ treatment induced resistance against dry rot, inhibited weight loss rate and accelerated the accumulation of SPP and SPA by activating phenylpropanoid metabolism in treated potato.

Studying the effects of Potassium Phosphite (1%) on physiological changes in potato (Solanum tubrusum L.) tuber under late blight stress

Phosphite-based fungicides are a group of chemicals that are directly harmless to the environment and have a direct fungicide effect, while also indirectly stimulating the plant immune response. Potato late blight is one of the most destructive potatoes diseases globally, causing many morphological and physiological changes in potato leaves and tubers. In current experiment, the potato plants with three-time spray of potassium phosphite (KPhi) on potato leaves, following three months in storage, and inoculated with late blight were investigated. This study examined the possible role of KPhi in delays of late blight invasion and physiological responses including the reactive oxygen species (ROS) metabolism in potato tubers after challenging with late blight pathogen. Samples were collected at 0, 24, 48, 72 and 96 hours of time points for evaluations. Antioxidant enzymes such as superoxidase dismutase (SOD), peroxidase (POD) and catalase (CAT), in addition content of metabolites such as phytoalexin and phenolic after infection with pathogen was investigated. Meanwhile, KPhi treatment resulted in plant tolerance with improved resistance, increased antioxidant enzymes activities and non-enzymatic compounds compare to untreated plants under similar stress. The results of this study indicate that the role of KPhi reduces the adverse effects of pathogens, suppress potato late blight and improves antioxidant enzymes activities and non-enzymatic compounds in potato tubers.

Morphological and metabolic profiling of a tropical-adapted potato association panel subjected to water recovery treatment reveals new insights into plant vigor.

SUMMARYPotato (Solanum tuberosum L.) is one of the world's most important crops, but it is facing major challenges due to climatic changes. To investigate the effects of intermittent drought on the natural variability of plant morphology and tuber metabolism in a novel potato association panel comprising 258 varieties we performed an augmented block design field study under normal irrigation and under water‐deficit and recovery conditions in Ica, Peru. All potato genotypes were profiled for 45 morphological traits and 42 central metabolites via nuclear magnetic resonance. Statistical tests and norm of reaction analysis revealed that the observed variations were trait specific, that is, genotypic versus environmental. Principal component analysis showed a separation of samples as a result of conditional changes. To explore the relational ties between morphological traits and metabolites, correlation‐based network analysis was employed, constructing one network for normal irrigation and one network for water‐recovery samples. Community detection and difference network analysis highlighted the differences between the two networks, revealing a significant correlational link between fumarate and plant vigor. A genome‐wide association study was performed for each metabolic trait. Eleven single nucleotide polymorphism (SNP) markers were associated with fumarate. Gene Ontology analysis of quantitative trait loci regions associated with fumarate revealed an enrichment of genes regulating metabolic processes. Three of the 11 SNPs were located within genes, coding for a protein of unknown function, a RING domain protein and a zinc finger protein ZAT2. Our findings have important implications for future potato breeding regimes, especially in countries suffering from climate change.

Deciphering source and sink responses of potato plants (Solanum tuberosum L.) to elevated temperatures.

Potato is an important staple food with increasing popularity worldwide. Elevated temperatures significantly impair tuber yield and quality. Breeding heat-tolerant cultivars is therefore an urgent need to ensure sustainable potato production in the future. An integrated approach combining physiology, biochemistry, and molecular biology was undertaken to contribute to a better understanding of heat effects on source- (leaves) and sink-organs (tubers) in a heat-susceptible cultivar. An experimental set-up was designed allowing tissue-specific heat application. Elevated day and night (29°C/27°C) temperatures impaired photosynthesis and assimilate production. Biomass allocation shifted away from tubers towards leaves indicating reduced sink strength of developing tubers. Reduced sink strength of tubers was paralleled by decreased sucrose synthase activity and expression under elevated temperatures. Heat-mediated inhibition of tuber growth coincided with a decreased expression of the phloem-mobile tuberization signal SP6A in leaves. SP6A expression and photosynthesis were also affected, when only the belowground space was heated, and leaves were kept under control conditions. By contrast, the negative effects on tuber metabolism were attenuated, when only the shoot was subjected to elevated temperatures. This, together with transcriptional changes discussed, indicated a bidirectional communication between leaves and tubers to adjust the source capacity and/or sink strength to environmental conditions.

Influence of Organic Farming on the Potato Transcriptome

Organic agriculture sparks a lively debate on its potential health and environmental benefits. Comparative studies often investigate the response of crops to organic farming through targeted approaches and within a limited experimental work. To clarify this issue, the transcriptomic profile of a cultivar of the potato grown for two years under organic and conventional farming was compared with the profile of an experimental clone grown in the same location of Southern Italy for one year. Transcriptomic raw data were obtained through Potato Oligo Chip Initiative (POCI) microarrays and were processed using unsupervised coupling multivariate statistical analysis and bioinformatics (MapMan software). One-hundred-forty-four genes showed the same expression in both years, and 113 showed the same expression in both genotypes. Their functional characterization revealed the strong involvement of the farming system in metabolism associated with the nutritional aspects of organic tubers (e.g., phenylpropanoid, flavonoid, glycoalcaloid, asparagine, ascorbic acid). Moreover, further investigation showed that eight of 42,034 features exhibited the same trend of expression irrespective of the year and genotype, making them possible candidates as markers of traceability. This paper raises the issue regarding the choice of genotype in organic management and the relevance of assessing seasonal conditions effects when studying the effects of organic cultivation on tuber metabolism.

Carbon Disulphide (CS2) Promotes Sprouting and Affects the Metabolism of Harvested Minitubers Grown from True Potato Seed

Chemical agents may be used to overcome dormancy in potato tubers so as to permit early planting. In this paper, the effect of carbon disulphide on sprouting and related metabolic processes in potato tubers grown from true potato seed (TPS) was investigated. Carbon disulphide was applied to potato tubers derived from TPS, at a rate of 20 ml m−3 container volume for 72 h. It accelerated tuber sprouting and increased tuber weight loss. It also caused an increase in the respiration rate, as well as ethylene emission from the tubers during the first 6 days after treatment. The respiration rate of carbon disulphide-treated tubers remained at a higher level than that of untreated controls for 35 days after treatment. In addition, carbon disulphide increased the concentrations of the soluble sugars sucrose and fructose, but not glucose, in the tuber tissues (buds, parenchyma). These findings suggest that carbon disulphide releases potato minitubers from dormancy and that dormancy breakage is associated with changes in tuber metabolism prior to the visible sprouting.

Potato tuber expression of Arabidopsis WRINKLED1 increase triacylglycerol and membrane lipids while affecting central carbohydrate metabolism.

SummaryTuber and root crops virtually exclusively accumulate storage products in the form of carbohydrates. An exception is yellow nutsedge (Cyperus esculentus) in which tubers have the capacity to store starch and triacylglycerols (TAG) in roughly equal amounts. This suggests that a tuber crop can efficiently handle accumulation of energy dense oil. From a nutritional as well as economic aspect, it would be of interest to utilize the high yield capacity of tuber or root crops for oil accumulation similar to yellow nutsedge. The transcription factor WRINKLED1 from Arabidopsis thaliana, which in seed embryos induce fatty acid synthesis, has been shown to be a major factor for oil accumulation. WRINKLED1 was expressed in potato (Solanum tuberosum) tubers to explore whether this factor could impact tuber metabolism. This study shows that a WRINKLED1 transcription factor could induce triacylglycerol accumulation in tubers of transformed potato plants grown in field (up to 12 nmol TAG/mg dry weight, 1% of dry weight) together with a large increase in polar membrane lipids. The changes in metabolism further affected starch accumulation and composition concomitant with massive increases in sugar content.

Control of potato tuber dormancy and sprouting by expression of sense and antisense genes of pyrophosphatase in potato

Inorganic pyrophosphate (PPi) is an enzyme involved in sugar metabolism in potato tubers. In our previous study, we isolated an inorganic pyrophosphatase (PPase) gene from potato and obtained the transgenic potato plants transformed with the sense and antisense PPase genes respectively. In the present experiment, the physiological indexes, tuber dormancy, and sprouting characteristics of the transgenic potatoes were analyzed and evaluated. The result showed that the PPase activity and the inorganic phosphate content of tubers were lower in the antisense transgenic plant lines but were higher in the sense transgenic plant lines, compared with wild-type tubers. Soluble sugars, such as glucose, fructose and sucrose increased in transgenic plants that had overexpression of the sense PPase gene, but decreased in the antisense transgenic plant lines, compared with wild-type tubers. Tuber sprouting time of the antisense transgenic plants were delayed for 2 and 3 weeks and reached the 100 % sprouting rate only after 14 and 16 weeks storage compared with the wild-type when tubers are stored under 25 and 4 °C, respectively. In contrast, tuber sprouting time of the sense transgenic plants was earlier by approximately 2 weeks than that of wild-type tubers under these storage temperatures.

Temperature-dependent regulation of sugar metabolism in wild-type and low-invertase transgenic chipping potatoes during and after cooling for low-temperature storage

Regulation of sugar metabolism in potato tubers stored at low temperatures has significant ramifications for producers and consumers of potato chips and French fries. Low-temperature storage reduces losses due to sprouting and disease but induces undesirable accumulation of the reducing sugars glucose and fructose. These sugars react with free amino acids during frying to produce dark-colored and bitter-tasting products that have elevated levels of acrylamide, a suspected human carcinogen. Elevated amounts of tuber glucose and fructose are produced when vacuolar acid invertase (VInv) hydrolyses sucrose derived from starch degradation. In this study, wild-type tubers and tubers in which VInv expression was reduced by RNA interference were used to study the temperature-dependent regulation of sugar accumulation and expression of carbohydrate metabolism genes during and after the cooling of tubers to 9 or 3°C. It was found that transcriptional control of genes central to carbohydrate metabolism occurred in three overlapping phases. First, early in cooling, slight increases in VInv and β-amylase expression occurred. Second, at lower temperatures, large decreases in AGPase and GBSS expression were observed. Finally, when tubers reached 3–5°C, large increases in β-amylase expression were observed, coinciding with large increases in sucrose in all genotypes and large increases in glucose and fructose in tubers with high VInv expression. Suppression of VInv expression did not alter the expression of β-amylase, AGPase, GBSS, or other genes, indicating that there was little or no feedback from reducing sugars to transcriptional control of carbohydrate metabolism. Invertase activity of WT tubers stored at 9°C was comparable to that of WT tubers stored at 3°C, but reducing sugars were markedly higher at the lower temperature. Taken together, these data support a model in which reducing sugar accumulation is controlled both by temperature-regulated changes in sucrose accumulation, resulting from increased starch degradation by β-amylase and decreased starch resynthesis by AGPase and GBSS, and by differences in VInv activity that persist throughout storage.

Differential effect of ethylene supplementation and inhibition on abscisic acid metabolism of potato (Solanum tuberosum L.) tubers during storage

Ethylene is used as a commercial potato sprout suppressant, but the role of ethylene in potato dormancy is unclear and contradictory. Two potato cultivars ‘Estima’ and ‘Marfona’ were treated with air or continuous ethylene (10μLL−1), transferred from air to ethylene or vice versa at first indication of sprouting (10% peeping of the control tubers), over three seasons. The ethylene binding inhibitor, 1-methylcyclopropene, was applied (1μLL−1 for 24h) either at harvest and/or at first indication of sprouting. Tubers were stored at 6°C for 30 weeks and assessed for sprout length and number, and non-structural carbohydrate content. Abscisic acid (ABA) metabolism in tubers throughout storage was monitored. Ethylene applied from harvest or the first indication of sprouting reduced sprout growth significantly for both cultivars while ethylene induced sugar accumulation was prevented by 1-MCP application prior to ethylene storage. Multiple applications of 1-MCP showed no additional benefit to ethylene induced sugar accumulation. A rise in ABA content in flesh tissue was observed in response to ethylene application while 1-MCP blocked the ethylene induced rise in ABA caused by exogenous and endogenous ethylene. The increase in ABA caused by ethylene may delay dormancy break yet 1-MCP blocked this rise in ABA with no apparent effect on sprout behaviour. Sucrose is a prerequisite signalling molecule for hormonal dormancy control. 1-MCP reduced the ethylene induced rise in sucrose content therefore although 1-MCP reduced ABA, the reduction in sucrose content may still have delayed sprout initiation. However, the level of sucrose in the 1-MCP treated tubers did not decrease below that of the control. Taken together, ABA and sucrose may play a role in the sprout suppressing ability of exogenously applied ethylene, but it is clear that other factors are involved.

Effect of Storage Temperature on Carbohydrate Metabolism and Development of Cold-Induced Sweetening in Indian Potato (Solanum TuberosumL.) Varieties

This study investigated the changes in carbohydrate metabolism in tubers of 11 Indian potato varieties stored at room temperature, 15C and 4C for 150 days to understand the development of cold-induced sweetening (CIS). Low-temperature storage negligibly influenced starch and maltose contents of the tubers but induced a significant increase of reducing sugars, total soluble sugars, fructose, glucose and hexoses : sucrose ratio, and a decrease of sucrose content was noticeable at 4C. A strong positive correlation was found between reducing sugars and total soluble sugars, and between fructose and glucose. The activity of β-amylase was considerably increased by storage at low temperature, and it weakly correlated with starch content. Also, the absence of maltose accumulation with increased β-amylase activity was observed. Acid invertase activity drastically rose at low temperature and strongly paralleled reducing sugars, glucose, fructose and hexose : sucrose ratio. The K. Jyoti variety was designated as CIS-tolerant and the K. Badshah variety as CIS-susceptible. Practical Applications Development of cold-induced sweetening (CIS) is important for basic research and for potato-processing industry. This work allowed us to group 11 Indian potato varieties into low-sugar-forming and high-sugar-forming groups, to identify varieties suitable for processing immediately after harvest or short time storage and to identify varieties with high starch content suitable for starch extraction. Hence, it provides capital information to the industry about varieties with good starch yield, which can be cold-stored without drastic sugar increase, and to breeders for searching genes of resistance to CIS in Indian potatoes. This study also demonstrated that during CIS development in these varieties, acid invertase is the key enzyme, β-amylase is not the main enzyme of starch degradation and there is possible significant activity of maltase in potato tubers. These observations pave the way through biotechnology work to develop new potato varieties which can cope with this postharvest problem.

Sugar metabolism, chip color, invertase activity, and gene expression during long-term cold storage of potato (Solanum tuberosum) tubers from wild-type and vacuolar invertase silencing lines of Katahdin.

BackgroundStoring potato tubers at low temperatures minimizes sprouting and disease but can cause an accumulation of reducing sugars in a process called cold-induced sweetening. Tubers with increased amounts of reducing sugars produce dark-colored, bitter-tasting fried products with elevated amounts of acrylamide, a possible carcinogen. Vacuolar invertase (VInv), which converts sucrose produced by starch breakdown to glucose and fructose, is the key determinant of reducing sugar accumulation during cold-induced sweetening. In this study, wild-type tubers and tubers in which VInv expression was reduced by RNA interference were used to investigate time- and temperature-dependent changes in sugar contents, chip color, and expression of VInv and other genes involved in starch metabolism in tubers during long-term cold storage.ResultsVInv activities and tuber reducing sugar contents were much lower, and tuber sucrose contents were much higher, in transgenic than in wild-type tubers stored at 3-9°C for up to eight months. Large differences in VInv mRNA accumulation were not observed at later times in storage, especially at temperatures below 9°C, so differences in invertase activity were likely established early in the storage period and maintained by stability of the invertase protein. Sugar contents, chip color, and expression of several of the studied genes, including AGPase and GBSS, were affected by storage temperature in both wild-type and transgenic tubers. Though transcript accumulation for other sugar-metabolism genes was affected by storage temperature and duration, it was essentially unaffected by invertase silencing and altered sugar contents. Differences in stem- and bud-end sugar contents in wild-type and transgenic tubers suggested different compartmentalization of sucrose at the two ends of stored tubers.ConclusionsVInv silencing significantly reduced cold-induced sweetening in stored potato tubers, likely by means of differential VInv expression early in storage. Transgenic tubers retained sensitivity to storage temperature, and accumulated greater amounts of sucrose, glucose and fructose at 3°C than at 7-9°C. At each storage temperature, suppression of VInv expression and large differences in tuber sugar contents had no effect on expression of AGPase and GBSS, genes involved in starch metabolism, suggesting that transcription of these genes is not regulated by tuber sugar content.

On functional module detection in metabolic networks.

Functional modules of metabolic networks are essential for understanding the metabolism of an organism as a whole. With the vast amount of experimental data and the construction of complex and large-scale, often genome-wide, models, the computer-aided identification of functional modules becomes more and more important. Since steady states play a key role in biology, many methods have been developed in that context, for example, elementary flux modes, extreme pathways, transition invariants and place invariants. Metabolic networks can be studied also from the point of view of graph theory, and algorithms for graph decomposition have been applied for the identification of functional modules. A prominent and currently intensively discussed field of methods in graph theory addresses the Q-modularity. In this paper, we recall known concepts of module detection based on the steady-state assumption, focusing on transition-invariants (elementary modes) and their computation as minimal solutions of systems of Diophantine equations. We present the Fourier-Motzkin algorithm in detail. Afterwards, we introduce the Q-modularity as an example for a useful non-steady-state method and its application to metabolic networks. To illustrate and discuss the concepts of invariants and Q-modularity, we apply a part of the central carbon metabolism in potato tubers (Solanum tuberosum) as running example. The intention of the paper is to give a compact presentation of known steady-state concepts from a graph-theoretical viewpoint in the context of network decomposition and reduction and to introduce the application of Q-modularity to metabolic Petri net models.

Profiles of the biosynthesis and metabolism of pyridine nucleotides in potatoes (Solanum tuberosum L.)

As part of a research program on nucleotide metabolism in potato tubers (Solanum tuberosum L.), profiles of pyridine (nicotinamide) metabolism were examined based on the in situ metabolic fate of radio-labelled precursors and the in vitro activities of enzymes. In potato tubers, [(3)H]quinolinic acid, which is an intermediate of de novo pyridine nucleotide synthesis, and [(14)C]nicotinamide, a catabolite of NAD, were utilised for pyridine nucleotide synthesis. The in situ tracer experiments and in vitro enzyme assays suggest the operation of multiple pyridine nucleotide cycles. In addition to the previously proposed cycle consisting of seven metabolites, we found a new cycle that includes newly discovered nicotinamide riboside deaminase which is also functional in potato tubers. This cycle bypasses nicotinamide and nicotinic acid; it is NAD --> nicotinamide mononucleotide --> nicotinamide riboside --> nicotinic acid riboside --> nicotinic acid mononucleotide --> nicotinic acid adenine dinucleotide --> NAD. Degradation of the pyridine ring was extremely low in potato tubers. Nicotinic acid glucoside is formed from nicotinic acid in potato tubers. Comparative studies of [carboxyl-(14)C]nicotinic acid metabolism indicate that nicotinic acid is converted to nicotinic acid glucoside in all organs of potato plants. Trigonelline synthesis from [carboxyl-(14)C]nicotinic acid was also found. Conversion was greater in green parts of plants, such as leaves and stem, than in underground parts of potato plants. Nicotinic acid utilised for the biosynthesis of these conjugates seems to be derived not only from the pyridine nucleotide cycle, but also from the de novo synthesis of nicotinic acid mononucleotide.

Enhancing Sucrose Synthase Activity in Transgenic Potato (Solanum tuberosum L.) Tubers Results in Increased Levels of Starch, ADPglucose and UDPglucose and Total Yield

Sucrose synthase (SuSy) is a highly regulated cytosolic enzyme that catalyzes the conversion of sucrose and a nucleoside diphosphate into the corresponding nucleoside diphosphate glucose and fructose. To determine the impact of SuSy activity in starch metabolism and yield in potato (Solanum tuberosum L.) tubers we measured sugar levels and enzyme activities in tubers of SuSy-overexpressing potato plants grown in greenhouse and open field conditions. We also transcriptionally characterized tubers of SuSy-overexpressing and -antisensed potato plants. SuSy-overexpressing tubers exhibited a substantial increase in starch, UDPglucose and ADPglucose content when compared with controls. Tuber dry weight, starch content per plant and total yield of SuSy-overexpressing tubers increased significantly over those of control plants. In contrast, activities of enzymes directly involved in starch metabolism in SuSy-overexpressing tubers were normal when compared with controls. Transcriptomic analyses using POCI arrays and the MapMan software revealed that changes in SuSy activity affect the expression of genes involved in multiple biological processes, but not that of genes directly involved in starch metabolism. These analyses also revealed a reverse correlation between the expressions of acid invertase and SuSy-encoding genes, indicating that the balance between SuSy- and acid invertase-mediated sucrolytic pathways is a major determinant of starch accumulation in potato tubers. Results presented in this work show that SuSy strongly determines the intracellular levels of UDPglucose, ADPglucose and starch, and total yield in potato tubers. We also show that enhancement of SuSy activity represents a useful strategy for increasing starch accumulation and yield in potato tubers.