Susan R. Verrall

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.

Detection and Quantification of Spongospora subterranea f. sp. subterranea in Soils and on Tubers Using Specific PCR Primers

PCR-based methods were developed for the detection and quantification of the potato pathogen Spongospora subterranea f. sp. subterranea (S. subterranea) in peel, tuber washings and soil. A partial sequence was obtained for S. subterranea ribosomal DNA and specific PCR primers (Sps1 and Sps2) were chosen from the internal transcribed spacer regions. These primers amplified a 391 bp product from S. subterranea DNA but did not amplify DNA from potato or a range of soil-borne microbes, including related species. Diluted S. subterranea DNA was detected at a concentration equivalent to 25×10−5 cystosori or 1 zoospore per PCR. Amplification was detected from peel and washings of infected and apparently healthy tubers, but not from peel of Scottish classified seed potatoes or axenically micropropagated potatoes. A rapid method for extracting S. subterranea DNA from soils was developed. This yielded DNA pure enough for PCR within 3 h and facilitated the detection of 1–5 cystosori per gram of soil. A PCR quantification technique was developed involving comparison of product ratios obtained after co-amplification of S. subterranea DNA along with an internal standard (competitor DNA fragment). This quantitative technique was also adapted for use in soil. PCR detection of S. subterranea in soil was considerably more sensitive than previously reported immunoassays and was quicker and easier than conventional bait plant bioassays. Such an assay could be useful for developing disease risk assessments for field soils and seed potato stocks and for future studies on the ecology and control of S. subterranea.

Effect of volatiles from bacteria and yeast on the growth and pigmentation of sapstain fungi

Sapstain fungi affect the appearance of wood due to colonisation by pigmented hyphae but without producing significant strength losses. This is due to the production of melanin in the fungal cell walls of the staining fungi. Any biological control strategy targeted against this type of deterioration would therefore be considered successful if it inhibited either fungal growth or pigment production. Previous work has established that specific bacterial and yeast isolates selected on the basis of agar screening studies could significantly reduce levels of staining in wood block tests. This paper presents the results of a study to examine the role of volatile organic compounds (VOCs) produced by four bacterial and three yeast isolates on the growth and pigment production by a range of five sapstain fungi on three media types. VOCs from three of the four bacterial strains tested completely inhibited growth of the five target sapstain fungi but only when the antagonists were grown on tryptone soya media. When antagonists were grown on either malt agar or a low nutrient medium levels of inhibition were either significantly reduced or non-existent. Yeast antagonists generally produced lower levels of growth inhibition than the bacteria but a Williopsis mrakii isolate gave 100% inhibition of three of the five sapstain fungi. Production of inhibitory VOCs was highly dependent on the specific antagonist as well as its growth substrate and all five sapstain fungi showed varying sensitivities to the VOCs produced. Not all fungi were inhibited, growth of O. piliferum and A. pullulans being stimulated by the VOCs from antagonists but only when grown under low nutrient conditions. In some instances, where growth was only slightly reduced, the level of pigmentation of the sapstain colony was significantly reduced compared with corresponding controls. The implications of this work for the biological control of sapstain fungi are discussed.

Phytochemical Diversity in Tubers of Potato Cultivars and Landraces Using a GC-MS Metabolomics Approach

Phytochemical diversity with respect to a range of polar (including amino acids, organic acids, sugars, and sugar alcohols) and nonpolar (including fatty acids, alkanols, and sterols) metabolites was examined within tubers from a total of 29 genetically diverse potato cultivars and Chilean landraces using a metabolomics approach by gas chromatography-mass spectrometry. From principal component analysis of the polar and nonpolar metabolite data there was insufficient variation to differentiate the majority of cultivars and landraces. Analysis of all polar metabolite profiles revealed separation of two cultivars (Glenna and Morag) from the other cultivars and landraces and a separate cluster of one landrace line, largely due to higher levels of sugars. Pentland Javelin was distinct in containing high levels of many amino acids. The two Solanum tuberosum group phureja cultivars (Inca Sun and Mayan Gold) were not particularly similar and were not separated from the S. tuberosum group tuberosum cultivars. Analysis of the nonpolar metabolite data revealed partial separation of two landrace lines and, on the basis of some minor fatty acids, Mayan Gold was distinct. The differences in metabolite profiles are considered in terms of the taxonomy and breeding history of the cultivars and possible influences from other factors such as developmental stage of the tuber. With a view to exploring biosynthetic links between metabolites, a pairwise correlation analysis was performed on all metabolites. The significance of high correlations between many amino acids and between several nonpolar metabolites is discussed.

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.

Vitamin C and the abscisic acid-insensitive 4 transcription factor are important determinants of aphid resistance in arabidopsis

AIMS Aphids, like other insects, are probably unable to synthesize vitamin C (ascorbic acid), which is therefore an essential dietary nutrient that has to be obtained from the host plant. Plant responses to aphids involve hormones such as salicylic acid (SA), jasmonic acid (JA), and abscisic acid (ABA), but hormone/redox interactions remain poorly characterized. We therefore investigated hormone/redox signaling in the response of Arabidopsis thaliana to infestation by the aphid Myzus persicae, focusing on the interactions between ascorbic acid and ABA, together with the influence of altered ascorbate and ABA signaling on the SA- and JA-dependent pathways. RESULTS Whole-genome microarray analysis revealed highly dynamic transcriptional responses to aphid infestation with extensive differences between transcript profiles of infested and systemic leaves, revealing aphid-dependent effects on the suites of transcripts involved in the redox, SA, and ABA responses. Central roles for ascorbate, ABA-insensitive 4 (ABI4), and oxidative signal-inducible 1 in plant resistance to aphids were demonstrated by altered fecundity on respective mutants. However, ABA had a negative effect on aphid resistance, as did ABI4 or redox-responsive transcription factor 1. The decrease in aphid fecundity observed in mutants defective in ascorbate accumulation (vtc2) was absent from abi4vtc2 double mutants that are also deficient in ABA signaling (abi4). Aphid-dependent transcriptome responses reveal a role for ascorbate-regulated receptor-like kinases in plant defenses against aphids. INNOVATION Vitamin C deficiency enhances plant resistance to aphids through redox signaling pathways rather than dietary requirements. CONCLUSION ABI4 is a linchpin of redox regulation of the innate immune response to aphids.

Identification of volatile organic compounds (VOCs) from bacteria and yeast causing growth inhibition of sapstain fungi

Abstract This paper describes an experiment to identify volatile organic compounds (VOCs) from a range of three bacteria and one yeast strain that had previously been shown to be inhibitory to selected sapstain fungi. The bacteria and yeast were cultured on two media, malt extract (ME) and tryptone soya (TS) and the VOCs trapped on chromatographic adsorbant before being analysed by Integrated Thermal Desorption—GC-MS. Since sapstain fungi were only inhibited by VOCs produced on the TS media, it was possible to use Principle Component Analysis to highlight the individual VOCs that are most likely to be responsible for the inhibition. A number of ketones together with dimethyl disulphide and dimethyl trisulphide were highlighted. The importance of VOC production by organisms during the biological control of sapstain is discussed.

A Metabolomics Study of Cultivated Potato (Solanum tuberosum) Groups Andigena, Phureja, Stenotomum, and Tuberosum Using Gas Chromatography−Mass Spectrometry

Phytochemical diversity was examined by gas chromatography-mass spectrometry in tubers of genotypes belonging to groups Andigena, Phureja, Stenotomum, and Tuberosum of the potato, Solanum tuberosum. Polar extracts (mainly amino acids, organic acids, sugars, and sugar alcohols) and nonpolar extracts (mainly fatty acids, fatty alcohols, and sterols) were examined. There was a large range in levels of metabolites, including those such as asparagine, fructose, and glucose, that are important to tuber quality, offering considerable scope for selecting germplasm for breeding programmes. There were significant differences in the levels of many metabolites among the groups. The metabolite profiles of genotypes belonging to Phureja and Stenotomum were similar and different from those of Tuberosum and the majority of Andigena genotypes. There was some agreement with the phylogeny of the groups in that Stenotomum is believed to be the ancestor of Phureja and they are both distinct from Tuberosum. Andigena genotypes could be partially distinguished according to geographical origin, Bolivian genotypes being particularly distinct from those from Ecuador. Biosynthetic links between metabolites were explored by performing pairwise correlations of all metabolites. The significance of some expected and unexpected strong correlations between many amino acids (e.g., between isoleucine, lysine, valine, and other amino acids) and between several nonpolar metabolites (e.g., between many fatty acids) is discussed. For polar metabolites, correlation analysis gave essentially similar results irrespective of whether the whole data set, only Andigena genotypes, or only Phureja genotypes were used. In contrast, for the nonpolar metabolites, Andigena only and Phureja only data sets resulted in weaker and stronger correlations, respectively, compared to the whole data set, and may suggest differences in the biochemistry of the two groups, although the interpretation should be viewed with some caution.

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.

Phenolic compounds in blackcurrant (Ribes nigrum L.) leaves relative to leaf position and harvest date

Blackcurrant leaves are an essential source of phenolic compounds and this study investigated their variation relative to leaf positions and harvest date. The phenolic content varied between harvest dates, although leaf position on the shoot and interactions also played an important role. The contents of quercetin-malonyl-glucoside, kaempferol-malonyl-glucoside isomer and kaempferol-malonyl-glucoside were higher than that of the other identified phenolic compounds, whereas epigallocatechin was the lowest for all investigated leaf positions and harvest dates. The content of several of the compounds was highest in June, while quercetin-glucoside, kaempferol-glucoside and total phenols, increased towards the end of the season. Leaf position influenced the content of myricetin-malonyl-glucoside, myricetin-malonyl-glucoside isomer, quercetin-malonyl-glucoside and kaempferol-glucoside at the end of the season. Knowledge relating to the influence of ontogenetic and harvest time on the content of specific phenolic compounds might contribute in tailoring functional foods or pharmaceutical products using blackcurrant leaves as natural ingredients.