Chilling or Chemical Induction of Dormancy Release in Blackcurrant (Ribes nigrum) Buds is Associated with Characteristic Shifts in Metabolite Profiles.

This study reveals striking differences in the content and composition of hydrophilic and lipophilic compounds in blackcurrant buds (Ribes nigrum L., cv. Ben Klibreck) resulting from winter-chill or chemical dormancy release following treatment with ERGER, a biostimulant used to promote uniform bud break. Buds exposed to high winter chill exhibited widespread shifts in metabolite profiles relative to buds that experience winter chill by growth under plastic. Specifically, extensive chilling resulted in significant reductions in storage lipids and phospholipids, and increases in galactolipids relative to buds that experienced lower chill. Similarly, buds exposed to greater chill exhibited higher levels of many amino acids and dipeptides, and nucleotides and nucleotide phosphates than those exposed to lower chilling hours. Low chill buds (IN) subjected to ERGER treatment exhibited shifts in metabolite profiles similar to those resembling high chill buds that were evident as soon as three days after treatment. We hypothesise that chilling induces a metabolic shift which primes bud outgrowth by mobilising lipophilic energy reserves, enhancing phosphate availability by switching from membrane phospholipids to galactolipids and enhancing the availability of free amino acids for de novo protein synthesis by increasing protein turnover. Our results additionally suggest that ERGER acts at least in part by priming metabolism for bud outgrowth. Finally, the metabolic differences presented highlight the potential for developing biochemical markers for dormancy status providing an alternative to time consuming forcing experiments.

WHIRLY1 functions in the nucleus to regulate barley leaf development and associated metabolite profiles.

The WHIRLY (WHY) DNA/RNA binding proteins fulfil multiple but poorly characterised functions in leaf development. Here, we show that WHY1 transcript levels were highest in the bases of 7-day old barley leaves. Immunogold labelling revealed that the WHY1 protein was more abundant in the nuclei than the proplastids of the leaf bases. To identify transcripts associated with leaf development we conducted hierarchical clustering of differentially abundant transcripts along the developmental gradient of wild-type leaves. Similarly, metabolite profiling was employed to identify metabolites exhibiting a developmental gradient. A comparative analysis of transcripts and metabolites in barley lines (W1-1 and W1-7) lacking WHY1, which show delayed greening compared with the wild type revealed that the transcript profile of leaf development was largely unchanged in W1-1 and W1-7 leaves. However, there were differences in levels of several transcripts encoding transcription factors associated with chloroplast development. These include a barley homologue of the Arabidopsis GATA transcription factor that regulates stomatal development, greening and chloroplast development, NAC1; two transcripts with similarity to Arabidopsis GLK1 and two transcripts encoding ARF transcriptions factors with functions in leaf morphogenesis and development. Chloroplast proteins were less abundant in the W1-1 and W1-7 leaves than the wild type. The levels of tricarboxylic acid cycle metabolites and GABA were significantly lower in WHY1 knockdown leaves than the wild type. This study provides evidence that WHY1 is localised in the nuclei of leaf bases, contributing the regulation of nuclear-encoded transcripts that regulate chloroplast development.

Sulforaphane-enriched extracts from glucoraphanin-rich broccoli exert antimicrobial activity against gut pathogens in vitro and innovative cooking methods increase in vivo intestinal delivery of sulforaphane.

PURPOSE: Studies on broccoli (Brassica oleracea var. italica) indicate beneficial effects against a range of chronic diseases, commonly attributed to their bioactive phytochemicals. Sulforaphane, the bioactive form of glucoraphanin, is formed by the action of the indigenous enzyme myrosinase. This study explored the role that digestion and cooking practices play in bioactivity and bioavailability, especially the rarely considered dose delivered to the colon. METHODS: The antimicrobial activity of sulforaphane extracts from raw, cooked broccoli and cooked broccoli plus mustard seeds (as a source myrosinase) was assessed. The persistence of broccoli phytochemicals in the upper gastrointestinal tract was analysed in the ileal fluid of 11 ileostomates fed, in a cross-over design, broccoli soup prepared with and without mustard seeds. RESULTS: The raw broccoli had no antimicrobial activity, except against Bacillus cereus, but cooked broccoli (with and without mustard seeds) showed considerable antimicrobial activity against various tested pathogens. The recovery of sulforaphane in ileal fluids post soup consumption was < 1% but the addition of mustard seeds increased colon-available sulforaphane sixfold. However, when sulforaphane was extracted from the ileal fluid with the highest sulforaphane content and tested against Escherichia coli K12, no inhibitory effects were observed. Analysis of glucosinolates composition in ileal fluids revealed noticeable inter-individual differences, with six "responding" participants showing increases in glucosinolates after broccoli soup consumption. CONCLUSIONS: Sulforaphane-rich broccoli extracts caused potent antimicrobial effects in vitro, and the consumption of sulforaphane-enriched broccoli soup may inhibit bacterial growth in the stomach and upper small intestine, but not in the terminal ileum or the colon.

Redox Control of Aphid Resistance through Altered Cell Wall Composition and Nutritional Quality.

The mechanisms underpinning plant perception of phloem-feeding insects, particularly aphids, remain poorly characterized. Therefore, the role of apoplastic redox state in controlling aphid infestation was explored using transgenic tobacco (Nicotiana tabacum) plants that have either high (PAO) or low (TAO) ascorbate oxidase (AO) activities relative to the wild type. Only a small number of leaf transcripts and metabolites were changed in response to genotype, and cell wall composition was largely unaffected. Aphid fecundity was decreased significantly in TAO plants compared with other lines. Leaf sugar levels were increased and maximum extractable AO activities were decreased in response to aphids in all genotypes. Transcripts encoding the Respiratory Burst Oxidase Homolog F, signaling components involved in ethylene and other hormone-mediated pathways, photosynthetic electron transport components, sugar, amino acid, and cell wall metabolism, were increased significantly in the TAO plants in response to aphid perception relative to other lines. The levels of galactosylated xyloglucan were decreased significantly in response to aphid feeding in all the lines, the effect being the least in the TAO plants. Similarly, all lines exhibited increases in tightly bound (1→4)-ß-galactan. Taken together, these findings identify AO-dependent mechanisms that limit aphid infestation.

The redox state of the apoplast influences the acclimation of photosynthesis and leaf metabolism to changing irradiance.

The redox state of the apoplast is largely determined by ascorbate oxidase (AO) activity. The influence of AO activity on leaf acclimation to changing irradiance was explored in wild-type (WT) and transgenic tobacco (Nicotiana tobaccum) lines containing either high [pumpkin AO (PAO)] or low [tobacco AO (TAO)] AO activity at low [low light (LL); 250 µmol m-2  s-1 ] and high [high light (HL); 1600 µmol m-2  s-1 ] irradiance and following the transition from HL to LL. AO activities changed over the photoperiod, particularly in the PAO plants. AO activity had little effect on leaf ascorbate, which was significantly higher under HL than under LL. Apoplastic ascorbate/dehydroascorbate (DHA) ratios and threonate levels were modified by AO activity. Despite decreased levels of transcripts encoding ascorbate synthesis enzymes, leaf ascorbate increased over the first photoperiod following the transition from HL to LL, to much higher levels than LL-grown plants. Photosynthesis rates were significantly higher in the TAO leaves than in WT or PAO plants grown under HL but not under LL. Sub-sets of amino acids and fatty acids were lower in TAO and WT leaves than in the PAO plants under HL, and following the transition to LL. Light acclimation processes are therefore influenced by the apoplastic as well as chloroplastic redox state.

WHIRLY1 Functions in the Control of Responses to Nitrogen Deficiency But Not Aphid Infestation in Barley.

WHIRLY1 is largely targeted to plastids, where it is a major constituent of the nucleoids. To explore WHIRLY1 functions in barley (Hordeum vulgare), RNA interference-knockdown lines (W1-1, W1-7, and W1-9) that have very low levels of HvWHIRLY1 transcripts were characterized in plants grown under optimal and stress conditions. The WHIRLY1-1 (W1-1), W1-7, and W1-9 plants were phenotypically similar to the wild type but produced fewer tillers and seeds. Photosynthesis rates were similar in all lines, but W1-1, W1-7, and W1-9 leaves had significantly more chlorophyll and less sucrose than the wild type. Transcripts encoding specific subsets of chloroplast-localized proteins, such as ribosomal proteins, subunits of the RNA polymerase, and thylakoid nicotinamide adenine dinucleotide (reduced) and cytochrome b6/f complexes, were much more abundant in the W1-7 leaves than the wild type. Although susceptibility of aphid (Myzus persicae) infestation was similar in all lines, the WHIRLY1-deficient plants showed altered responses to nitrogen deficiency, maintaining higher photosynthetic CO2 assimilation rates than the wild type under limiting nitrogen. Although all lines showed globally similar low nitrogen-dependent changes in transcripts and metabolites, the increased abundance of FAR-RED IMPAIRED RESPONSE1-like transcripts in nitrogen-deficient W1-7 leaves infers that WHIRLY1 has a role in communication between plastid and nuclear genes encoding photosynthetic proteins during abiotic stress.

Nontargeted LC-MSn Profiling of Compounds in Ileal Fluids That Decrease after Raspberry Intake Identifies Consistent Alterations in Bile Acid Composition.

Ileostomy studies provide a unique insight into the digestion of foods, allowing identification of physiologically relevant dietary phytochemicals and their metabolites that are important to gut health. We previously reported an increase of components, including novel triterpenoids, in ileal fluids of 11 ileostomates following consumption of raspberries using nontargeted LC-MSn techniques in combination with data deconvolution software. The current study focused on components that consistently decreased postsupplementation. After data deconvolution, 32 components were identified that met exclusion parameters of m/z signals and which decreased significantly in ileal fluids from eight of 11 participants post-raspberry supplementation. Two-thirds of these components were identified putatively from their MS properties. Consistent decreases were observed in components that possibly reflected "washing out" of presupplementation intake of common foods/drinks including (poly)phenol metabolites. Metabolites associated with fat metabolism such as hydroxylated fatty acids and cholate-type bile acids were specifically reduced. However, more directed re-examination of the data revealed that although some cholates were consistently reduced, the more polar glyco- and tauro-linked bile acid derivatives increased consistently, by as much as 100-fold over presupplementation levels. The possible reasons for these substantial alterations in bile acid composition in ileal fluids in response to raspberry intake are discussed.

Systematic analysis of phloem-feeding insect-induced transcriptional reprogramming in Arabidopsis highlights common features and reveals distinct responses to specialist and generalist insects.

Phloem-feeding insects (PFIs), of which aphids are the largest group, are major agricultural pests causing extensive damage to crop plants. In contrast to chewing insects, the nature of the plant response to PFIs remains poorly characterized. Scrutiny of the literature concerning transcriptional responses of model and crop plant species to PFIs reveals surprisingly little consensus with respect to the transcripts showing altered abundance following infestation. Nevertheless, core features of the transcriptional response to PFIs can be defined in Arabidopsis thaliana. This comparison of the PFI-associated transcriptional response observed in A. thaliana infested by the generalists Myzus persicae and Bemisia tabaci with the specialist Brevicoryne brassicae highlights the importance of calcium-dependent and receptor kinase-associated signalling. We discuss these findings within the context of the complex cross-talk between the different hormones regulating basal immune response mechanisms in plants. We identify PFI-responsive genes, highlighting the importance of cell wall-associated kinases in plant-PFI interactions, as well as the significant role of kinases containing the domain of unknown function 26. A common feature of plant-PFI interaction is enhanced abundance of transcripts encoding WRKY transcription factors. However, significant divergence was observed with respect to secondary metabolism dependent upon the insect attacker. Transcripts encoding enzymes and proteins associated with glucosinolate metabolism were decreased following attack by the generalist M. persicae but not by the specialist B. brassicae. This analysis provides a comprehensive overview of the molecular patterns associated with the plant response to PFIs and suggests that plants recognize and respond to perturbations in the cell wall occurring during PFI infestation.

Nitrogen deficiency in barley (Hordeum vulgare) seedlings induces molecular and metabolic adjustments that trigger aphid resistance.

Agricultural nitrous oxide (N2O) pollution resulting from the use of synthetic fertilizers represents a significant contribution to anthropogenic greenhouse gas emissions, providing a rationale for reduced use of nitrogen (N) fertilizers. Nitrogen limitation results in extensive systems rebalancing that remodels metabolism and defence processes. To analyse the regulation underpinning these responses, barley (Horedeum vulgare) seedlings were grown for 7 d under N-deficient conditions until net photosynthesis was 50% lower than in N-replete controls. Although shoot growth was decreased there was no evidence for the induction of oxidative stress despite lower total concentrations of N-containing antioxidants. Nitrogen-deficient barley leaves were rich in amino acids, sugars and tricarboxylic acid cycle intermediates. In contrast to N-replete leaves one-day-old nymphs of the green peach aphid (Myzus persicae) failed to reach adulthood when transferred to N-deficient barley leaves. Transcripts encoding cell, sugar and nutrient signalling, protein degradation and secondary metabolism were over-represented in N-deficient leaves while those associated with hormone metabolism were similar under both nutrient regimes with the exception of mRNAs encoding proteins involved in auxin metabolism and responses. Significant similarities were observed between the N-limited barley leaf transcriptome and that of aphid-infested Arabidopsis leaves. These findings not only highlight significant similarities between biotic and abiotic stress signalling cascades but also identify potential targets for increasing aphid resistance with implications for the development of sustainable agriculture.

Impacts on the metabolome of down-regulating polyphenol oxidase in potato tubers.

Tubers of potato (Solanum tuberosum L. cv. Estima) genetically modified to reduce polyphenol oxidase (PPO) activity and enzymatic discolouration were assessed for changes in the metabolome using Liquid Chromatography-Mass Spectrometry (LC-MS) and Gas Chromatography (GC)-MS. Metabolome changes induced over a 48 hour (h) period by tuber wounding (sliced transverse sections) were also assessed using two PPO antisense lines (asPPO) and a wild-type (WT) control. Data were analysed using Principal Components Analysis and Analysis of Variance to assess differences between genotypes and temporal changes post-tuber wounding (by slicing). The levels of 15 metabolites (out of a total of 134 that were detected) differed between the WT and asPPO lines in mature tubers at harvest. A considerably higher number (63) of these metabolites changed significantly over a 48 h period following tuber wounding. For individual metabolites the magnitude of the differences between the WT and asPPO lines at harvest were small compared with the impacts of tuber wounding on metabolite levels. Some of the observed metabolite changes are explicable in terms of pathways known to be affected by wound responses. Whilst some statistically significant interactions (11 metabolites) were observed between line and time after wounding, very few profiles were consistent when comparing the WT with both asPPO lines, and the underlying metabolites appeared to be random in terms of the pathways they occupy. Overall, mechanical damage to tubers has a considerably greater impact on the metabolite profile than any potential unintended effects resulting from the down-regulation of PPO gene expression.

Physiological, biochemical and molecular responses of the potato (Solanum tuberosum L.) plant to moderately elevated temperature.

Although significant work has been undertaken regarding the response of model and crop plants to heat shock during the acclimatory phase, few studies have examined the steady-state response to the mild heat stress encountered in temperate agriculture. In the present work, we therefore exposed tuberizing potato plants to mildly elevated temperatures (30/20 °C, day/night) for up to 5 weeks and compared tuber yield, physiological and biochemical responses, and leaf and tuber metabolomes and transcriptomes with plants grown under optimal conditions (22/16 °C). Growth at elevated temperature reduced tuber yield despite an increase in net foliar photosynthesis. This was associated with major shifts in leaf and tuber metabolite profiles, a significant decrease in leaf glutathione redox state and decreased starch synthesis in tubers. Furthermore, growth at elevated temperature had a profound impact on leaf and tuber transcript expression with large numbers of transcripts displaying a rhythmic oscillation at the higher growth temperature. RT-PCR revealed perturbation in the expression of circadian clock transcripts including StSP6A, previously identified as a tuberization signal. Our data indicate that potato plants grown at moderately elevated temperatures do not exhibit classic symptoms of abiotic stress but that tuber development responds via a diversity of biochemical and molecular signals.

Day length dependent restructuring of the leaf transcriptome and metabolome in potato genotypes with contrasting tuberization phenotypes.

Recent advances have defined some of the components of photoperiodic signalling that lead to tuberization in potato including orthologues of FLOWERING LOCUS T (StSP6A) and CYCLING DOF FACTOR (StCDF1). The aim of the current study is to investigate the molecular basis of permissive tuber initiation under long days in Solanum tuberosum Neo-Tuberosum by comparative analysis with an obligate short-day S. tuberosum ssp. Andigena accession. We show that the Neo-Tuberosum accession, but not the Andigena, contains alleles that encode StCDF1 proteins modified in the C-terminal region, likely to evade long day inhibition of StSP6A expression. We also identify an allele of StSP6A from the Neo-Tuberosum accession, absent in the Andigena, which is expressed under long days. Other leaf transcripts and metabolites that show different abundances in tuberizing and non-tuberizing samples were identified adding detail to tuberization-associated processes. Overall, the data presented in this study highlight the subtle interplay between components of the clock-CONSTANS-StSP6A axis which collectively may interact to fine-tune the timing of tuberization.

Qualitative phytochemical analysis and in vitro investigation of the immunomodulatory properties of honeys produced in Kazakhstan.

Honey is known to have antimicrobial, immunomodulatory and wound healing properties. The biological properties of honey have been attributed to phytochemicals derived from their source plants and research has focused on identifying the bioactive phytochemicals with therapeutic potential. In this study, we determined the ability of 5 honeys from Kazakhstan and manuka honey to stimulate TNF-α and TGF-ß production by human keratinocytes. TNF-α and TGF-ß levels increased over time in honey treated and untreated keratinocytes, whereas cells treated with sugar solutions that matched those of the honeys had reduced levels of both cytokines. This suggests that the non-sugar phytochemical components of the honeys may have prevented this decrease. Analysis by LC-MS confirmed that the honeys contained a diverse range of phytochemicals. Some phytochemicals e.g. pinobanksin and vanillin were present at different levels across the honey types, whereas other components, e.g. dicarboxylic acids and their glycosides, were abundant in all honeys.

Restraining Quiescence Release-Related Ageing in Plant Cells: A Case Study in Carrot.

The blackening of cut carrots causes substantial economic losses to the food industry. Blackening was not observed in carrots that had been stored underground for less than a year, but the susceptibility to blackening increased with the age of the carrots that were stored underground for longer periods. Samples of black, border, and orange tissues from processed carrot batons and slices, prepared under industry standard conditions, were analyzed to identify the molecular and metabolic mechanisms underpinning processing-induced blackening. The black tissues showed substantial molecular and metabolic rewiring and large changes in the cell wall structure, with a decreased abundance of xyloglucan, pectins (homogalacturonan, rhamnogalacturonan-I, galactan and arabinan), and higher levels of lignin and other phenolic compounds when compared to orange tissues. Metabolite profiling analysis showed that there was a major shift from primary to secondary metabolism in the black tissues, which were depleted in sugars, amino acids, and tricarboxylic acid (TCA) cycle intermediates but were rich in phenolic compounds. These findings suggest that processing triggers a release from quiescence. Transcripts encoding proteins associated with secondary metabolism were less abundant in the black tissues, but there were no increases in transcripts associated with oxidative stress responses, programmed cell death, or senescence. We conclude that restraining quiescence release alters cell wall metabolism and composition, particularly regarding pectin composition, in a manner that increases susceptibility to blackening upon processing.

Comparison of two headspace sampling techniques for the analysis of off-flavour volatiles from oat based products.

Two different headspace sampling techniques were compared for analysis of aroma volatiles from freshly produced and aged plain oatcakes. Solid phase microextraction (SPME) using a Carboxen-Polydimethylsiloxane (PDMS) fibre and entrainment on Tenax TA within an adsorbent tube were used for collection of volatiles. The effects of variation in the sampling method were also considered using SPME. The data obtained using both techniques were processed by multivariate statistical analysis (PCA). Both techniques showed similar capacities to discriminate between the samples at different ages. Discrimination between fresh and rancid samples could be made on the basis of changes in the relative abundances of 14-15 of the constituents in the volatile profiles. A significant effect on the detection level of volatile compounds was observed when samples were crushed and analysed by SPME-GC-MS, in comparison to undisturbed product. The applicability and cost effectiveness of both methods were considered.

Pectin engineering to modify product quality in potato.

Although processed potato tuber texture is an important trait that influences consumer preference, a detailed understanding of tuber textural properties at the molecular level is lacking. Previous work has identified tuber pectin methyl esterase (PME) activity as a potential factor impacting on textural properties, and the expression of a gene encoding an isoform of PME (PEST1) was associated with cooked tuber textural properties. In this study, a transgenic approach was undertaken to investigate further the impact of the PEST1 gene. Antisense and over-expressing potato lines were generated. In over-expressing lines, tuber PME activity was enhanced by up to 2.3-fold; whereas in antisense lines, PME activity was decreased by up to 62%. PME isoform analysis indicated that the PEST1 gene encoded one isoform of PME. Analysis of cell walls from tubers from the over-expressing lines indicated that the changes in PME activity resulted in a decrease in pectin methylation. Analysis of processed tuber texture demonstrated that the reduced level of pectin methylation in the over-expressing transgenic lines was associated with a firmer processed texture. Thus, there is a clear link between PME activity, pectin methylation and processed tuber textural properties.

Identification of components in Kazakhstan honeys that correlate with antimicrobial activity against wound and skin infecting microorganisms.

BACKGROUND: Antimicrobial drug resistance is a major public health threat that can render infections including wound and skin infections untreatable. The discovery of new antimicrobials is critical. Approaches to discover novel antimicrobial therapies have included investigating the antimicrobial activity of natural sources such as honey. In this study, the anti-microbial activity and chemical composition of 12 honeys from Kazakhstan and medical grade manuka honey were investigated. METHODS: Agar well diffusion and broth culture assays were used to determine anti-microbial activity against a range of skin and wound infecting micro-organisms. Folin-Ciocalteu method was used to determine the total phenol content of the honeys and non-targeted liquid chromatography analysis was performed to identify components that correlated with antimicrobial activity. RESULTS: In the well diffusion assay, the most susceptible micro-organisms were a clinical isolate of Methicillin resistant Staphylococcus aureus (MRSA) and Enterococcus faecalis (ATCC 19433). Buckwheat & multi-floral honey from Kazakhstan demonstrated the highest antimicrobial activity against these two micro-organisms. Kazakhstan honeys with a buckwheat floral source, and manuka honey had the highest total phenol content. Non-targeted liquid chromatography analysis identified components that correlated with anti-microbial activity as hydroxyphenyl acetic acid, p-coumaric acid, (1H)-quinolinone, and abscisic acid. CONCLUSIONS: The Kazakhstan honeys selected in this study demonstrated antimicrobial activity against wound and skin infecting micro-organisms. Compounds identified as correlating with antimicrobial activity could be considered as potential bioactive agents for the treatment of wound and skin infections.

Flavonoid profiling and transcriptome analysis reveals new gene-metabolite correlations in tubers of Solanum tuberosum L.

Anthocyanin content of potato tubers is a trait that is attracting increasing attention as the potential nutritional benefits of this class of compound become apparent. However, our understanding of potato tuber anthocyanin accumulation is not complete. The aim of this study was to use a potato microarray to investigate gene expression patterns associated with the accumulation of purple tuber anthocyanins. The advanced potato selections, CO97216-3P/PW and CO97227-2P/PW, developed by conventional breeding procedures, produced tubers with incomplete expression of tuber flesh pigmentation. This feature permits sampling pigmented and non-pigmented tissues from the same tubers, in essence, isolating the factors responsible for pigmentation from confounding genetic, environmental, and developmental effects. An examination of the transcriptome, coupled with metabolite data from purple pigmented sectors and from non-pigmented sectors of the same tuber, was undertaken to identify these genes whose expression correlated with elevated or altered polyphenol composition. Combined with a similar study using eight other conventional cultivars and advanced selections with different pigmentation, it was possible to produce a refined list of only 27 genes that were consistently differentially expressed in purple tuber tissues compared with white. Within this list are several new candidate genes that are likely to impact on tuber anthocyanin accumulation, including a gene encoding a novel single domain MYB transcription factor.

Discerning intra-tuber differences in textural properties in cooked Solanum tuberosum group Tuberosum and group Phureja tubers.

BACKGROUND: The textural properties of potato tubers influence their acceptability and palatability and these properties differ between varieties, groups and progeny. The aim of this study was to compare the textural properties of cooked tubers of Solanum tuberosum group Phureja with those of group Tuberosum. RESULTS: To assess intra-tuber differences, the textural properties of seven cubes from defined positions along the longitudinal axis of tubers of four Tuberosum group cultivars and three Phureja group lines were tested after cooking using an amended wedge fracture method. Tuberosum group tubers gave consistently higher peak force and work done values during fracture than the Phureja group tubers. Moreover, the values for cubes 1-6 from any tuber were not significantly different and only cube 7, from the stem end, gave higher values. Therefore, the use of any of cubes 1-6 is a valid measurement of the tuber as a whole but the central cube 4 may be most conveniently located. The dry matter content of the cubes did not influence the textural properties of the cubes, which suggested that starch swelling is not the main driving force for textural differences. Total pectin methyl esterase (PME) activity was consistently higher in cubes of the Tuberosum group cultivars over the Phureja group lines. CONCLUSION: The method developed is valid and consistent for assessing textural differences within potato germplasm. The relationship between PME activity and enhanced resistance to fracture suggests that PME may modulate pectin cohesiveness, perhaps through increasing Ca(2+)-bridges, to provide greater resistance to fracture.

Physiological, Biochemical, and Transcriptional Responses to Single and Combined Abiotic Stress in Stress-Tolerant and Stress-Sensitive Potato Genotypes.

Potato production is often constrained by abiotic stresses such as drought and high temperatures which are often present in combination. In the present work, we aimed to identify key mechanisms and processes underlying single and combined abiotic stress tolerance by comparative analysis of tolerant and susceptible cultivars. Physiological data indicated that the cultivars Desiree and Unica were stress tolerant while Agria and Russett Burbank were stress susceptible. Abiotic stress caused a greater reduction of photosynthetic carbon assimilation in the susceptible cultivars which was associated with a lower leaf transpiration rate. Oxidative stress, as estimated by the accumulation of malondialdehyde was not induced by stress treatments in any of the genotypes with the exception of drought stress in Russett Burbank. Stress treatment resulted in increases in ascorbate peroxidase activity in all cultivars except Agria which increased catalase activity in response to stress. Transcript profiling highlighted a decrease in the abundance of transcripts encoding proteins associated with PSII light harvesting complex in stress tolerant cultivars. Furthermore, stress tolerant cultivars accumulated fewer transcripts encoding a type-1 metacaspase implicated in programmed cell death. Stress tolerant cultivars exhibited stronger expression of genes associated with plant growth and development, hormone metabolism and primary and secondary metabolism than stress susceptible cultivars. Metabolite profiling revealed accumulation of proline in all genotypes following drought stress that was partially suppressed in combined heat and drought. On the contrary, the sugar alcohols inositol and mannitol were strongly accumulated under heat and combined heat and drought stress while galactinol was most strongly accumulated under drought. Combined heat and drought also resulted in the accumulation of Valine, isoleucine, and lysine in all genotypes. These data indicate that single and multiple abiotic stress tolerance in potato is associated with a maintenance of CO2 assimilation and protection of PSII by a reduction of light harvesting capacity. The data further suggests that stress tolerant cultivars suppress cell death and maintain growth and development via fine tuning of hormone signaling, and primary and secondary metabolism. This study highlights potential targets for the development of stress tolerant potato cultivars.