Analysis of the raffinose family oligosaccharide pathway in pea seeds with contrasting carbohydrate composition

Integration of transcriptome and metabolome reveals the accumulation of related metabolites and gene regulation networks during quinoa seed development

Quinoa seeds are gluten- and cholesterol-free, contain all amino acids required by the human body, have a high protein content, provide endocrine regulation, protein supplementation, and cardiovascular protection effects. However, metabolite accumulation and transcriptional regulatory networks in quinoa seed development are not well understood. Four key stages of seed development in Dianli-3260 and Dianli-557 were thus analyzed and 849 metabolites were identified, among which sugars, amino acids, and lipids were key for developmental processes, and their accumulation showed a gradual decrease. Transcriptome analysis identified 40,345 genes, of which 20,917 were differential between the M and F phases, including 8279 and 12,638 up- and down-regulated genes, respectively. Grain development processes were mainly enriched in galactose metabolism, pentose and glucuronate interconversions, the biosynthesis of amino acids, and carbon metabolism pathways, in which raffinose, phosphoenolpyruvate, series and other metabolites are significantly enriched, gene-LOC110689372, Gene-LOC110710556 and gene-LOC110714584 are significantly expressed, and these metabolites and genes play an important role in carbohydrate metabolism, lipid and Amino acid synthesis of quinoa. This study provides a theoretical basis to expand our understanding of the molecular and metabolic development of quinoa grains.

Optimizing raffinose family oligosaccharides content in plants: A tightrope walk

Plants synthesize various compounds for their growth, metabolism, and stress mitigation, and one such group of compounds is the raffinose family of oligosaccharides (RFOs). RFOs are non-reducing oligosaccharides having galactose residues attached to a sucrose moiety. They act as carbohydrate reserves in plants, assisting in seed germination, desiccation tolerance, and biotic/abiotic stress tolerance. Although legumes are among the richest sources of dietary proteins, the direct consumption of legumes is hindered by an excess of RFOs in the edible parts of the plant, which causes flatulence in humans and monogastric animals. These opposing characteristics make RFOs manipulation a complicated tradeoff. An in-depth knowledge of the chemical composition, distribution pattern, tissue mobilization, and metabolism is required to optimize the levels of RFOs. The most recent developments in our understanding of RFOs distribution, physiological function, genetic regulation of their biosynthesis, transport, and degradation in food crops have been covered in this review. Additionally, we have suggested a few strategies that can sustainably reduce RFOs in order to solve the flatulence issue in animals. The comprehensive information in this review can be a tool for researchers to precisely control the level of RFOs in crops and create low antinutrient, nutritious food with wider consumer acceptability.

Spatio-temporal expression pattern of Raffinose Synthase genes determine the levels of Raffinose Family Oligosaccharides in peanut (Arachis hypogaea L.) seed

Raffinose family oligosaccharides (RFOs) are known to have important physiological functions in plants. However, the presence of RFOs in legumes causes flatulence, hence are considered antinutrients. To reduce the RFOs content to a desirable limit without compromising normal plant development and functioning, the identification of important regulatory genes associated with the biosynthetic pathway is a prerequisite. In the present study, through comparative RNA sequencing in contrasting genotypes for seed RFOs content at different seed maturity stages, differentially expressed genes (DEGs) associated with the pathway were identified. The DEGs exhibited spatio-temporal expression patterns with high RFOs variety showing early induction of RFOs biosynthetic genes and low RFOs variety showing a late expression at seed maturity. Selective and seed-specific differential expression of raffinose synthase genes (AhRS14 and AhRS6) suggested their regulatory role in RFOs accumulation in peanut seeds, thereby serving as promising targets in low RFOs peanut breeding programs. Despite stachyose being the major seed RFOs fraction, differential expression of raffinose synthase genes indicated the complex metabolic regulation of this pathway. The transcriptomic resource and the genes identified in this study could be studied further to develop low RFOs varieties, thus improving the overall nutritional quality of peanuts.

An alternative pathway to plant cold tolerance in the absence of vacuolar invertase activity

To cope with cold stress, plants have developed antioxidation strategies combined with osmoprotection by sugars. In potato (Solanum tuberosum) tubers, which are swollen stems, exposure to cold stress induces starch degradation and sucrose synthesis. Vacuolar acid invertase (VInv) activity is a significant part of the cold-induced sweetening (CIS) response, by rapidly cleaving sucrose into hexoses and increasing osmoprotection. To discover alternative plant tissue pathways for coping with cold stress, we produced VInv-knockout lines in two cultivars. Genome editing of VInv in 'Désirée' and 'Brooke' was done using stable and transient expression of CRISPR/Cas9 components, respectively. After storage at 4°C, sugar analysis indicated that the knockout lines showed low levels of CIS and maintained low acid invertase activity in storage. Surprisingly, the tuber parenchyma of vinv lines exhibited significantly reduced lipid peroxidation and reduced H2 O2 levels. Furthermore, whole plants of vinv lines exposed to cold stress without irrigation showed normal vigor, in contrast to WT plants, which wilted. Transcriptome analysis of vinv lines revealed upregulation of an osmoprotectant pathway and ethylene-related genes during cold temperature exposure. Accordingly, higher expression of antioxidant-related genes was detected after exposure to short and long cold storage. Sugar measurements showed an elevation of an alternative pathway in the absence of VInv activity, raising the raffinose pathway with increasing levels of myo-inositol content as a cold tolerance response.

Genome-wide association mapping of seed oligosaccharides in chickpea

Chickpea (Cicer arietinum L.) is one of the major pulse crops, rich in protein, and widely consumed all over the world. Most legumes, including chickpeas, possess noticeable amounts of raffinose family oligosaccharides (RFOs) in their seeds. RFOs are seed oligosaccharides abundant in nature, which are non-digestible by humans and animals and cause flatulence and severe abdominal discomforts. So, this study aims to identify genetic factors associated with seed oligosaccharides in chickpea using the mini-core panel. We have quantified the RFOs (raffinose and stachyose), ciceritol, and sucrose contents in chickpea using high-performance liquid chromatography. A wide range of variations for the seed oligosaccharides was observed between the accessions: 0.16 to 15.13 mg g^-1 raffinose, 2.77 to 59.43 mg g^-1 stachyose, 4.36 to 90.65 mg g^-1 ciceritol, and 3.57 to 54.12 mg g^-1 for sucrose. Kabuli types showed desirable sugar profiles with high sucrose, whereas desi types had high concentrations RFOs. In total, 48 single nucleotide polymorphisms (SNPs) were identified for all the targeted sugar types, and nine genes (Ca_06204, Ca_04353, and Ca_20828: Phosphatidylinositol N-acetylglucosaminyltransferase; Ca_17399 and Ca_22050: Remorin proteins; Ca_11152: Protein-serine/threonine phosphatase; Ca_10185, Ca_14209, and Ca_27229: UDP-glucose dehydrogenase) were identified as potential candidate genes for sugar metabolism and transport in chickpea. The accessions with low RFOs and high sucrose contents may be utilized in breeding specialty chickpeas. The identified candidate genes could be exploited in marker-assisted breeding, genomic selection, and genetic engineering to improve the sugar profiles in legumes and other crop species.

SK, Sheoran S, K. UB, K. BN, Kumar S, Singh AN, Singh HV. Seed Longevity in Legumes: Deeper Insights Into Mechanisms and Molecular Perspectives

Sustainable agricultural production largely depends upon the viability and longevity of high-quality seeds during storage. Legumes are considered as rich source of dietary protein that helps to ensure nutritional security, but associated with poor seed longevity that hinders their performance and productivity in farmer's fields. Seed longevity is the key determinant to assure proper seed plant value and crop yield. Thus, maintenance of seed longevity during storage is of prime concern and a pre-requisite for enhancing crop productivity of legumes. Seed longevity is significantly correlated with other seed quality parameters such as germination, vigor, viability and seed coat permeability that affect crop growth and development, consequently distressing crop yield. Therefore, information on genetic basis and regulatory networks associated with seed longevity, as well as molecular dissection of traits linked to longevity could help in developing crop varieties with good storability. Keeping this in view, the present review focuses towards highlighting the molecular basis of seed longevity, with special emphasis on candidate genes and proteins associated with seed longevity and their interplay with other quality parameters. Further, an attempt was made to provide information on 3D structures of various genetic loci (genes/proteins) associated to seed longevity that could facilitate in understanding the interactions taking place within the seed at molecular level. This review compiles and provides information on genetic and genomic approaches for the identification of molecular pathways and key players involved in the maintenance of seed longevity in legumes, in a holistic manner. Finally, a hypothetical fast-forward breeding pipeline has been provided, that could assist the breeders to successfully develop varieties with improved seed longevity in legumes.

A proteomic analysis of Arabidopsis ribosomal phosphoprotein P1A mutant

Ribosomal proteins are involved in the regulation of plant growth and development. However, the regulatory processes of most ribosomal proteins remain unclear. In this study, Arabidopsis plants with the mutation in ribosomal phosphoprotein P1A (RPP1A) produce larger and heavier seeds than wild-type plants. A comparative quantitative label-free proteomic analysis revealed that a total of 215 proteins were differentially accumulated between the young siliques of the wild type and rpp1a mutant. Knockout of RPP1A significantly reduced the abundance of proteins involved in carboxylic acid metabolism and lipid biosynthesis. Consistent with this, a metabolic analysis showed that the organic acids in the tricarboxylic acid cycle and the carbohydrates in the pentose phosphate pathway were severely reduced in the mature rpp1a mutant seeds. In contrast, the abundance of proteins related to seed maturation, especially seed storage proteins, was markedly increased during seed development. Indeed, seed storage proteins were accumulated in the mature rpp1a mutant seeds, and the seed nitrogen and sulfur contents were also increased. These results indicate that more carbon intermediates probably enter the nitrogen flow for the enhanced synthesis of seed storage proteins, which might subsequently contribute to the enlarged seed size in the rpp1a mutant. SIGNIFICANCE: Ribosomes are responsible for protein synthesis and are generally perceived as the housekeeping components in the cells. In this study, the knockout of RPP1A leads to an increased seed size through repressing carbon metabolism and lipid biosynthesis, and increasing the synthesis of seed storage proteins. Meanwhile, the abundance of seed storage proteins and the nitrogen and sulfur concentrations were increased in the mature rpp1a mutant seeds. The results provide a novel insight into the genetic regulatory networks for the control of seed size and seed storage protein accumulation, and this knowledge may facilitate the improvement of crop seed size.

Mobile forms of carbon in trees: metabolism and transport

Plants constitute 80% of the biomass on earth, and almost two-thirds of this biomass is found in wood. Wood formation is a carbon (C)-demanding process and relies on C transport from photosynthetic tissues. Thus, understanding the transport process is of major interest for understanding terrestrial biomass formation. Here, we review the molecules and mechanisms used to transport and allocate C in trees. Sucrose is the major form in which C is transported in plants, and it is found in the phloem sap of all tree species investigated so far. However, in several tree species, sucrose is accompanied by other molecules, notably polyols and the raffinose family of oligosaccharides. We describe the molecules that constitute each of these transport groups, and their distribution across different tree species. Furthermore, we detail the metabolic reactions for their synthesis, the mechanisms by which trees load and unload these compounds in and out of the vascular system, and how they are radially transported in the trunk and finally catabolized during wood formation. We also address a particular C recirculation process between phloem and xylem that occurs in trees during the annual cycle of growth and dormancy. A search of possible evolutionary drivers behind the diversity of C-carrying molecules in trees reveals no consistent differences in C transport mechanisms between angiosperm and gymnosperm trees. Furthermore, the distribution of C forms across species suggests that climate-related environmental factors will not explain the diversity of C transport forms. However, the consideration of C-transport mechanisms in relation to tree-rhizosphere coevolution deserves further attention. To conclude the review, we identify possible future lines of research in this field.

Sequential expression of raffinose synthase and stachyose synthase corresponds to successive accumulation of raffinose, stachyose and verbascose in developing seeds of Lens culinaris Medik

Raffinose, stachyose and verbascose form the three major members of the raffinose family oligosaccharides (RFO) accumulated during seed development. Raffinose synthase (RS; EC 2.4.1.82) and stachyose synthase (STS; EC 2.4.1.67) have been associated with raffinose and stachyose synthesis, but the precise mechanism for verbascose synthesis is not well understood. In this study, full-length RS (2.7 kb) and STS (2.6 kb) clones were isolated by screening a cDNA library prepared from developing lentil seeds (18, 20, 22 and 24 days after flowering [DAF]) to understand the roles of RS and STS in RFO accumulation in developing lentil seeds. The nucleotide sequences of RS and STS genes were similar to those reported for Pisum sativum. Patterns of transcript accumulation, enzyme activities and RFO concentrations were also comparable to P. sativum. However, during lentil seed development raffinose, stachyose and verbascose accumulation corresponded to transcript accumulation for RS and STS, with peak transcript abundance occurring at about 22-24 DAF, generally followed by a sequential increase in raffinose, stachyose and verbascose concentrations followed by a steady level thereafter. Enzyme activities for RS, STS and verbascose synthase (VS) also indicated a sudden increase at around 24-26 DAF, but with an abrupt decline again coinciding with the subsequent steady state increase in the RFO. Galactan:galactan galactosyl transferase (GGT), the galactinol-independent pathway enzyme, however, exhibited steady increase in activity from 24 DAF onwards before abruptly decreasing at 34 DAF. Although GGT activity was detected, isolation of a GGT sequence from the cDNA library was not successful.

Stachyose triggers apoptotic like cell death in drought sensitive but not resilient plants

Programmed cell death (PCD) is one of the most intensively researched fields in modern mammalian biology with roles in cancer, aging, diabetes and numerous neurodegenerative diseases. It is becoming increasingly clear that PCD also plays significant roles in plant defence and responses to the environment. Given their unique ability to tolerate desiccation (cells remain viable even after they've lost 95% of their water), resurrection plants make ideal models to study the regulation of plant PCD pathways. Previously, we showed that the Australian resurrection plant, Tripogon loliiformis, suppresses plant PCD, via trehalose-mediated activation of autophagy pathways, during drying. In the present study, we created a full-length T. loliiformis cDNA library, performed a large-scale Agrobacterium screen for improved salinity tolerance and identified Stachyose synthase (TlStach) as a potential candidate for improving stress tolerance. Tripogon loliiformis shoots accumulate stachyose synthase transcripts and stachyose during drying. Attempts to generate transgenic plants expressing TlStach failed and were consistent with previous reports in mammals that demonstrated stachyose-mediated induction of apoptosis. Using a combination of transcriptomics, metabolomics and cell death assays (TUNNEL and DNA laddering), we investigated whether stachyose induces apoptotic-like cell death in T. loliiformis. We show that stachyose triggers the formation of the hallmarks of plant apoptotic-like cell death in the desiccation sensitive Nicotiana benthamiana but not the resilient T. loliiformis. These findings suggest that T. loliiformis suppresses stachyose-mediated apoptotic-like cell death and provides insights on the role of sugar metabolism and plant PCD pathways. A better understanding of how resilient plants regulate sugar metabolism and PCD pathways may facilitate future targeting of plant metabolic pathways for increased stress tolerance.

Changes in water loss and cell wall metabolism during postharvest withering of tobacco (Nicotiana tabacum L.) leaves using tandem mass tag-based quantitative proteomics approach

Withering is an important biological process accompanied by dehydration and cell wall metabolism in postharvest plant organs during curing/processing and storage. However, dynamics involved in cell wall metabolism and resultant water loss during withering in postharvest tobacco leaves is not well-documented. Here, tandem mass tag (TMT)-based quantitative proteomic analysis in postharvest tobacco leaves (cultivar K326) under different withering conditions was performed. In total, 11,556 proteins were detected, among which 496 differentially abundant proteins (DAPs) were identified. To elucidate the withering mechanism of tobacco leaves, 27 DAPs associated with cell wall metabolism were screened. In particular, pectin acetylesterases, glucan endo-1,3-beta-glucosidases, xyloglucan endotransglucosylase/hydrolase, alpha-xylosidase 1-like, probable galactinol-sucrose galactosyltransferases, endochitinase A, chitotriosidase-1-like and expansin were the key proteins responsible for the withering of postharvest tobacco leaves. These DAPs were mainly involved in pectin metabolism, cellulose, hemicellulose and galactose metabolism, amino sugar and nucleotide sugar metabolism as well as cell wall expansion. Furthermore, relative water content and softness values were significantly and positively correlated. Thus, dehydration and cell wall metabolism were crucial for tobacco leaf withering under different conditions. Nine candidate DAPs were confirmed by parallel reaction monitoring (PRM) technique. These results provide new insights into the withering mechanism underlying postharvest physiological regulatory networks in plants/crops.

CRISPR/Cas9-Mediated Knockout of Galactinol Synthase-Encoding Genes Reduces Raffinose Family Oligosaccharide Levels in Soybean Seeds

Raffinose family oligosaccharides (RFOs) are major soluble carbohydrates in soybean seeds that cannot be digested by human and other monogastric animals. Hence, a major goal is to reduce RFO levels to improve the nutritional quality of soybean. In this study, we utilized a dual gRNAs CRISPR/Cas9 system to induce knockouts in two soybean galactinol synthase (GOLS) genes, GmGOLS1A and its homeolog GmGOLS1B. Genotyping of T0 plants showed that the construct design was efficient in inducing various deletions in the target sites or sequences spanning the two target sites of both GmGOLS1A and GmGOLS1B genes. A subset of induced alleles was successfully transferred to progeny and, at the T2 generation, we identified null segregants of single and double mutant genotypes without off-target induced mutations. The seed carbohydrate analysis of double mutant lines showed a reduction in the total RFO content of soybean seed from 64.7 mg/g dry weight to 41.95 mg/g dry weight, a 35.2% decrease. On average, the stachyose content, the most predominant RFO in soybean seeds, decreased by 35.4% in double mutant soybean, while the raffinose content increased by 41.7%. A slight decrease in verbascose content was also observed in mutant lines. Aside from changes in soluble carbohydrate content, some mutant lines also exhibited increased protein and fat contents. Otherwise, no difference in seed weight, seed germination, plant development and morphology was observed in the mutants. Our findings indicate that GmGOLS1A and GmGOLS1B contribute to the soybean oligosaccharide profile through RFO biosynthesis pathways, and are promising targets for future investigation, as well as crop improvement efforts. Our results also demonstrate the potential in using elite soybean cultivars for transformation and targeted genome editing.

NMR Based Metabolomic Analysis of Health Promoting Phytochemicals in Lentils

Lentils are a high-protein plant food and a valuable source of human nutrition, particularly in the Indian subcontinent. However, beyond sustenance, there is evidence that the consumption of lentils (and legumes in general) is associated with decreased risk of diseases, such as diabetes and cardiovascular disease. Lentils contain health-promoting phytochemicals, such as trigonelline and various polyphenolics. Fourteen lentil genotypes were grown at three locations to explore the variation in phytochemical composition in hulls and cotyledons. Significant differences were measured between genotypes and environments, with some genotypes more affected by environment than others. However, there was a strong genetic effect which indicated that future breeding programs could breed for lentils that product more of these health-promoting phytochemicals.

Physiological and Proteomic Responses of Mulberry Trees (Morus alba. L.) to Combined Salt and Drought Stress

Intensive investigations have been conducted on the effect of sole drought or salinity stress on the growth of plants. However, there is relatively little knowledge on how plants, particularly woody species, respond to a combination of these two stresses although these stresses can simultaneously occur in the field. In this study, mulberry, an economically important resource for traditional medicine, and the sole food of domesticated silkworms was subjected to a combination of salt and drought stress and analyzed by physiological methods and TMT-based proteomics. Stressed mulberry exhibited significant alteration in physiological parameters, including root/shoot ratio, chlorophyll fluorescence, total carbon, and ion reallocation. A total of 577 and 270 differentially expressed proteins (DEPs) were identified from the stressed leaves and roots, respectively. Through KEGG analysis, these DEPs were assigned to multiple pathways, including carbon metabolism, photosynthesis, redox, secondary metabolism, and hormone metabolism. Among these pathways, the sucrose related metabolic pathway was distinctly enriched in both stressed leaves and roots, indicating an important contribution in mulberry under stress condition. The results provide a comprehensive understanding of the adaptive mechanism of mulberry in response to salt and drought stress, which will facilitate further studies on innovations in terms of crop performance.

Functional characterization of galactinol synthase and raffinose synthase in desiccation tolerance acquisition in developing Arabidopsis seeds

Raffinose family oligosaccharides (RFOs) accumulate during seed development, and have been thought to be associated with the acquisition of desiccation tolerance (DT) by seeds. Here, comprehensive approaches were adopted to evaluate the changes of DT in developing Arabidopsis seeds of wild type, overexpression (OX-AtGS1/GS2/RS5), and mutant lines by manipulating the expression levels of the GALACTINOL SYNTHASE (GS) and RAFFINOSE SYNTHASE (RS) genes. Our results indicate that seeds of the double mutant (gs1, gs2) and rs5 delayed the timing of DT acquisition as compared to wild type. Subsequent detection confirmed that seeds from OX-AtGS1/GS2 plants with high levels of galactinol, raffinose, and stachyose, and OX-AtRS5 plants possess more raffinose and stachyose but less galactinol compared to wild type. These lines all showed greater germination percentage and shorter time to 50% germination after desiccation treatment at 11 and 15 days after flower (DAF). Further analysis revealed that the role of RFOs is time limited and mainly affects the middle stage (9-16 DAF) of seed development by enhancing seed viability and the ratio of GSH to GSSH in cells, but there is no significant difference in DT of mature seeds. In addition, RFOs could reduce damage to seeds caused by oxidative stress. We conclude that GALACTINOL SYNTHASE and RAFFINOSE SYNTHASE play important roles in DT acquisition during Arabidopsis seed development, and that galactinol and RFOs are crucial protective compounds in the response of seeds to desiccation stress.

NMR Metabolomics Defining Genetic Variation in Pea Seed Metabolites

Nuclear magnetic resonance (NMR) spectroscopy profiling was used to provide an unbiased assessment of changes to the metabolite composition of seeds and to define genetic variation for a range of pea seed metabolites. Mature seeds from recombinant inbred lines, derived from three mapping populations for which there is substantial genetic marker linkage information, were grown in two environments/years and analyzed by non-targeted NMR. Adaptive binning of the NMR metabolite data, followed by analysis of quantitative variation among lines for individual bins, identified the main genomic regions determining this metabolic variability and the variability for selected compounds was investigated. Analysis by t-tests identified a set of bins with highly significant associations to genetic map regions, based on probability (p) values that were appreciably lower than those determined for randomized data. The correlation between bins showing high mean absolute deviation and those showing low p-values for marker association provided an indication of the extent to which the genetics of bin variation might be explained by one or a few loci. Variation in compounds related to aromatic amino acids, branched-chain amino acids, sucrose-derived metabolites, secondary metabolites and some unidentified compounds was associated with one or more genetic loci. The combined analysis shows that there are multiple loci throughout the genome that together impact on the abundance of many compounds through a network of interactions, where individual loci may affect more than one compound and vice versa. This work therefore provides a framework for the genetic analysis of the seed metabolome, and the use of genetic marker data in the breeding and selection of seeds for specific seed quality traits and compounds that have high commercial value.

Sugar metabolism in the desiccation tolerant grass Oropetium thomaeum in response to environmental stresses

Oropetium thomaeum is a desiccation tolerant grass and acquisition of desiccation tolerance is correlated with changes in carbohydrate metabolism. Here we address the question whether the changes in carbohydrate metabolism are specific to the dehydration process or whether other environmental factors such as high temperature, low temperature, hypoxia, salinity or exogenous ABA application trigger the same or different changes in the sugar metabolism. Fifteen different sugar metabolites were identified by GC/MS, including erythritol, arabinose, fructose, galactose, glucose, myo-inositol, sedoheptulose, sucrose, trehalose, galactinol, maltose, raffinose, manninotriose and stachyose. Together with starch, these sugars were placed into the pathways of sucrose metabolism and raffinose family oligosaccharides (RFOs) metabolism, as well as into the group of rare sugars. By comparing the changes of sugars under various stresses, we concluded that the changes in the sugar metabolism are both convergent and divergent in response to different stresses. Except for the general response to stress, such as starch degradation, the changes of specific sugar metabolites reflect a stress-specific response of O. thomaeum. Erythritol seems to be specific for dehydration, myo-inositol for salt stress and trehalose for hypoxia stress. Similar as dehydration, low temperature, salt stress and ABA application resulted in the accumulation of sucrose and RFOs in O. thomaeum, which indicates that these stresses share high similarity with dehydration. Thus it is proposed that sucrose and RFOs have a general protective role under these stresses. In contrast sucrose and RFOs did not accumulate in response to high temperature or hypoxia whose effects tend to be consumptive and destructive. The accumulation of galactose, melibiose and manninotriose demonstrate that RFOs are degraded under stress. The accumulation of these sugar metabolites might result from the reaction of RFOs and stress-produced hydroxyl radicals, which supports a possible role of RFOs in stress defense. In addition, ABA application led to substantial synthesis of stachyose which occurs only in response to dehydration, indicating that stachyose synthesis is possibly closely related to ABA in O. thomaeum.

Preparation and identification of oligosaccharides in lotus seeds and determination of their distribution in different parts of lotus

Three fractions (I-III) were separated from crude oligosaccharides of lotus seeds by fast protein liquid chromatography with final purity of 97.6, 96.3, and 96.8%, respectively. The fractions were identified as sucrose, raffinose, and stachyose by using TLC, HPLC with charged aerosol detector (CAD), LC-MS, and methylation analysis. Subsequently sucrose and raffinose family oligosaccharides (RFOs) with degree of polymerization (DP) 3-5 (raffinose, stachyose, and verbascose) have been quantified by HPLC-CAD for the first time. All calibration curves for investigated analytes showed good linear regression (R2  > 0.9952). Their limit of detection and limit of quantity were in the ranges 0.14-0.28 and 0.36-0.48 μg/mL, respectively. The recoveries ranged from 96.6 to 103.4%. The contents of sucrose and RFOs DP3-DP5 were different in lotus seeds and other parts of lotus samples, but similar in their own variety. Additionally, the distribution of RFOs in different parts of lotus were also compared and the results indicated that RFOs might be mainly synthesized in lotus seeds. This work is helpful for understanding the way of biosynthesis of RFOs in lotus as well as quality control of plants containing RFOs.

Comparative transcriptome and metabolome analysis suggests bottlenecks that limit seed and oil yields in transgenic Camelina sativa expressing diacylglycerol acyltransferase 1 and glycerol-3-phosphate dehydrogenase

BACKGROUND

Camelina sativa has attracted much interest as alternative renewable resources for biodiesel, other oil-based industrial products and a source for edible oils. Its unique oil attributes attract research to engineering new varieties of improved oil quantity and quality. The overexpression of enzymes catalyzing the synthesis of the glycerol backbone and the sequential conjugation of fatty acids into this backbone is a promising approach for increasing the levels of triacylglycerol (TAG). In a previous study, we co-expressed the diacylglycerol acyltransferase (DGAT1) and glycerol-3-phosphate dehydrogenase (GPD1), involved in TAG metabolism, in Camelina seeds. Transgenic plants exhibited a higher-percentage seed oil content, a greater seed mass, and overall improved seed and oil yields relative to wild-type plants. To further increase seed oil content in Camelina, we utilized metabolite profiling, in conjunction with transcriptome profiling during seed development to examine potential rate-limiting step(s) in the production of building blocks for TAG biosynthesis.

RESULTS

Transcriptomic analysis revealed approximately 2518 and 3136 transcripts differentially regulated at significant levels in DGAT1 and GPD1 transgenics, respectively. These transcripts were found to be involved in various functional categories, including alternative metabolic routes in fatty acid synthesis, TAG assembly, and TAG degradation. We quantified the relative contents of over 240 metabolites. Our results indicate major metabolic switches in transgenic seeds associated with significant changes in the levels of glycerolipids, amino acids, sugars, and organic acids, especially the TCA cycle and glycolysis intermediates.

CONCLUSIONS

From the transcriptomic and metabolomic analysis of DGAT1, GPD1 and DGAT1 + GPD1 expressing lines of C. sativa, we conclude that TAG production is limited by (1) utilization of fixed carbon from the source tissues supported by the increase in glycolysis pathway metabolites and decreased transcripts levels of transcription factors controlling fatty acids synthesis; (2) TAG accumulation is limited by the activity of lipases/hydrolases that hydrolyze TAG pool supported by the increase in free fatty acids and monoacylglycerols. This comparative transcriptomics and metabolomics approach is useful in understanding the regulation of TAG biosynthesis, identifying bottlenecks, and the corresponding genes controlling these pathways identified as limitations, for generating Camelina varieties with improved seed and oil yields.

Genome-Wide Identification, Evolutionary and Expression Analyses of the GALACTINOL SYNTHASE Gene Family in Rapeseed and Tobacco

Galactinol synthase (GolS) is a key enzyme in raffinose family oligosaccharide (RFO) biosynthesis. The finding that GolS accumulates in plants exposed to abiotic stresses indicates RFOs function in environmental adaptation. However, the evolutionary relationships and biological functions of GolS family in rapeseed (Brassica napus) and tobacco (Nicotiana tabacum) remain unclear. In this study, we identified 20 BnGolS and 9 NtGolS genes. Subcellular localization predictions showed that most of the proteins are localized to the cytoplasm. Phylogenetic analysis identified a lost event of an ancient GolS copy in the Solanaceae and an ancient duplication event leading to evolution of GolS4/7 in the Brassicaceae. The three-dimensional structures of two GolS proteins were conserved, with an important DxD motif for binding to UDP-galactose (uridine diphosphate-galactose) and inositol. Expression profile analysis indicated that BnGolS and NtGolS genes were expressed in most tissues and highly expressed in one or two specific tissues. Hormone treatments strongly induced the expression of most BnGolS genes and homologous genes in the same subfamilies exhibited divergent-induced expression. Our study provides a comprehensive evolutionary analysis of GolS genes among the Brassicaceae and Solanaceae as well as an insight into the biological function of GolS genes in hormone response in plants.

Angiosperm Plant Desiccation Tolerance: Hints from Transcriptomics and Genome Sequencing

Desiccation tolerance (DT) in angiosperms is present in the small group of resurrection plants and in seeds. DT requires the presence of protective proteins, specific carbohydrates, restructuring of membrane lipids, and regulatory mechanisms directing a dedicated gene expression program. Many components are common to resurrection plants and seeds; however, some are specific for resurrection plants. Understanding how each component contributes to DT is challenging. Recent transcriptome analyses and genome sequencing indicate that increased expression is essential of genes encoding protective components, recently evolved, species-specific genes and non-protein-coding RNAs. Modification and reshuffling of existing cis-regulatory promoter elements seems to play a role in the rewiring of regulatory networks required for increased expression of DT-related genes in resurrection species.

Genetic Variation Controlling Wrinkled Seed Phenotypes in Pisum: How Lucky Was Mendel?

One of the traits studied by Mendel in pea (Pisum sativum L.) was the wrinkled-seeded phenotype, and the molecular basis for a mutation underlying this phenotype was discovered in the 1990s. Although the starch-branching enzyme gene mutation identified at the genetic locus r is most likely to be that in seeds available to Mendel in the mid-1800s, it has remained an open question as to whether or not additional natural mutations in this gene exist within Pisum germplasm collections. Here, we explore this question and show that all but two wrinkled-seeded variants in one such collection correspond to either the mutant allele described previously for the r locus or a mutation at a second genetic locus, rb, affecting the gene encoding the large subunit of Adenosine diphosphoglucose (ADP-glucose) pyrophosphorylase; the molecular basis for the rb mutation is described here. The genetic basis for the phenotype of one (JI 2110) of the two lines which are neither r nor rb has been studied in crosses with a round-seeded variant (JI 281); for which extensive genetic marker data were expected. In marked contrast to the trait studied by Mendel and the rb phenotype; the data suggest that the wrinkled-seeded phenotype in JI 2110 is maternally determined, controlled by two genetic loci, and the extent to which it is manifested is very sensitive to the environment. Metabolite analysis of the cotyledons of JI 2110 revealed a profile for sucrose and sucrose-derived compounds that was more similar to that of wild-type round-seeded, than that of wrinkled-seeded r, pea lines. However, the metabolite profile of the seed coat (testa) of JI 2110 was distinct from that of other round-seeded genotypes tested which, together with analysis of recombinant inbred progeny lines, suggests an explanation for the seed phenotype.

Differential expression of two galactinol synthase isoforms LcGolS1 and LcGolS2 in developing lentil (Lens culinaris Medik. cv CDC Redberry) seeds

Galactinol synthase (GS, EC 2.4.1.123) catalyzes the transfer of a galactosyl residue from UDP-galactose to myo-inositol to synthesize galactinol, a precursor for raffinose family oligosaccharides (RFO) biosynthesis. Screening, a cDNA library constructed with RNA isolated from developing lentil seeds, with partial GS genes resulted in identification of cDNA clones for two isoforms of GS, LcGolS1 (1336 bp, ORF-1002 bp, 334 amino acids) and LcGolS2 (1324bp, ORF-975bp, 325 amino acids) with predicted molecular weights of 38.7 kDa and 37.6 kDa, respectively. During lentil seed development, LcGolS1 transcripts showed higher accumulation during 26-32 days after flowering (DAF) corresponding to seed desiccation, while LcGolS2 showed maximum accumulation at 24 DAF, prior to increase in LcGolS1 transcripts. GS enzyme activity was maximum at 26 and 28 DAF and corresponded to galactinol accumulation, which also increased rapidly at 22 DAF with maximum accumulation at 26 DAF. Substrates for GS activity, myo-inositol and glucose/galactose were present in high concentrations during early stages of seed development but gradually decreased from 20 DAF to 32 DAF when galactinol concentration increased coinciding with increased GS enzyme activity.

Differentially expressed galactinol synthase(s) in chickpea are implicated in seed vigor and longevity by limiting the age induced ROS accumulation

Galactinol synthase (GolS) catalyzes the first and rate limiting step of Raffinose Family Oligosaccharide (RFO) biosynthetic pathway, which is a highly specialized metabolic event in plants. Increased accumulation of galactinol and RFOs in seeds have been reported in few plant species, however their precise role in seed vigor and longevity remain elusive. In present study, we have shown that galactinol synthase activity as well as galactinol and raffinose content progressively increase as seed development proceeds and become highly abundant in pod and mature dry seeds, which gradually decline as seed germination progresses in chickpea (Cicer arietinum). Furthermore, artificial aging also stimulates galactinol synthase activity and consequent galactinol and raffinose accumulation in seed. Molecular analysis revealed that GolS in chickpea are encoded by two divergent genes (CaGolS1 and CaGolS2) which potentially encode five CaGolS isoforms through alternative splicing. Biochemical analysis showed that only two isoforms (CaGolS1 and CaGolS2) are biochemically active with similar yet distinct biochemical properties. CaGolS1 and CaGolS2 are differentially regulated in different organs, during seed development and germination however exhibit similar subcellular localization. Furthermore, seed-specific overexpression of CaGolS1 and CaGolS2 in Arabidopsis results improved seed vigor and longevity through limiting the age induced excess ROS and consequent lipid peroxidation.

Galactinol synthase enzyme activity influences raffinose family oligosaccharides (RFO) accumulation in developing chickpea (Cicer arietinum L.) seeds

To understand raffinose family oligosaccharides (RFO) metabolism in chickpea (Cicer arietinum L.) seeds, RFO accumulation and corresponding biosynthetic enzymes activities were determined during seed development of chickpea genotypes with contrasting RFO concentrations. RFO concentration in mature seeds was found as a facilitator rather than a regulating step of seed germination. In mature seeds, raffinose concentrations ranged from 0.38 to 0.68 and 0.75 to 0.99 g/100 g, whereas stachyose concentrations varied from 0.79 to 1.26 and 1.70 to 1.87 g/100 g indicating significant differences between low and high RFO genotypes, respectively. Chickpea genotypes with high RFO concentration accumulated higher concentrations of myo-inositol and sucrose during early seed developmental stages suggesting that initial substrate concentrations may influence RFO concentration in mature seeds. High RFO genotypes showed about two to three-fold higher activity for all RFO biosynthetic enzymes compared to those with low RFO concentrations. RFO biosynthetic enzymes activities correspond with accumulation of individual RFO during seed development.

Galactinol as marker for seed longevity

Reduced seed longevity or storability is a major problem in seed storage and contributes to increased costs in crop production. Here we investigated whether seed galactinol contents could be predictive for seed storability behavior in Arabidopsis, cabbage and tomato. The analyses revealed a positive correlation between galactinol content and seed longevity in the three species tested, which indicates that this correlation is conserved in the Brassicaceae and beyond. Quantitative trait loci (QTL) mapping in tomato revealed a co-locating QTL for galactinol content and seed longevity on chromosome 2. A candidate for this QTL is the GALACTINOL SYNTHASE gene (Solyc02g084980.2.1) that is located in the QTL interval. GALACTINOL SYNTHASE is a key enzyme of the raffinose family oligosaccharide (RFO) pathway. To investigate the role of enzymes in the RFO pathway in more detail, we applied a reverse genetics approach using T-DNA knock-out lines in genes encoding enzymes of this pathway (GALACTINOL SYNTHASE 1, GALACTINOL SYNTHASE 2, RAFFINOSE SYNTHASE, STACHYOSE SYNTHASE and ALPHA-GALACTOSIDASE) and overexpressors of the cucumber GALACTINOL SYNTHASE 2 gene in Arabidopsis. The galactinol synthase 2 mutant and the galactinol synthase 1 galactinol synthase 2 double mutant contained the lowest seed galactinol content which coincided with lower seed longevity. These results show that galactinol content of mature dry seed can be used as a biomarker for seed longevity in Brassicaceae and tomato.

Soluble carbohydrate content variation in Sanionia uncinata and Polytrichastrum alpinum, two Antarctic mosses with contrasting desiccation capacities

BACKGROUND

Cryptogamic vegetation dominates the ice-free areas along the Antarctic Peninsula. The two mosses Sanionia uncinata and Polytrichastrum alpinum inhabit soils with contrasting water availability. Sanionia uncinata grows in soil with continuous water supply, while P. alpinum grows in sandy, non-flooded soils. Desiccation and rehydration experiments were carried out to test for differences in the rate of water loss and uptake, with non-structural carbohydrates analysed to test their role in these processes.

RESULTS

Individual plants of S. uncinata lost water 60 % faster than P. alpinum; however, clumps of S. uncinata took longer to dry than those of P. alpinum (11 vs. 5 h, respectively). In contrast, rehydration took less than 10 min for both mosses. Total non-structural carbohydrate content was higher in P. alpinum than in S. uncinata, but sugar levels changed more in P. alpinum during desiccation and rehydration (60-50 %) when compared to S. uncinata. We report the presence of galactinol (a precursor of the raffinose family) for the first time in P. alpinum. Galactinol was present at higher amounts than all other non-structural sugars.

CONCLUSIONS

Individual plants of S. uncinata were not able to retain water for long periods but by growing and forming carpets, this species can retain water the longest. In contrast individual P. alpinum plants required more time to lose water than S. uncinata, but as moss cushions they suffered desiccation faster than the later. On the other hand, both species rehydrated very quickly. We found that when both mosses lost 50 % of their water, carbohydrates content remained stable and the plants did not accumulate non-structural carbohydrates during the desiccation prosses as usually occurs in vascular plants. The raffinose family oligosaccarides decreased during desiccation, and increased during rehydration, suggesting they function as osmoprotectors.

Molecular cloning of AtRS4, a seed specific multifunctional RFO synthase/galactosylhydrolase in Arabidopsis thaliana

Stachyose is among the raffinose family oligosaccharides (RFOs) one of the major water-soluble carbohydrates next to sucrose in seeds of a number of plant species. Especially in leguminous seeds, e.g. chickpea, stachyose is reported as the major component. In contrast to their ambiguous potential as essential source of carbon for germination, RFOs are indigestible for humans and can contribute to diverse abdominal disorders. In the genome of Arabidopsis thaliana, six putative raffinose synthase genes are reported, whereas little is known about these putative raffinose synthases and their biochemical characteristics or their contribution to the RFO physiology in A. thaliana. In this paper, we report on the molecular cloning, functional expression in Escherichia coli and purification of recombinant AtRS4 from A. thaliana and the biochemical characterisation of the putative stachyose synthase (AtSTS, At4g01970) as a raffinose and high affinity stachyose synthase (Km for raffinose 259.2 ± 21.15 μM) as well as stachyose and galactinol specific galactosylhydrolase. A T-DNA insertional mutant in the AtRS4 gene was isolated. Only semi-quantitative PCR from WT siliques showed a specific transcriptional AtRS4 PCR product. Metabolite measurements in seeds of ΔAtRS4 mutant plants revealed a total loss of stachyose in ΔAtRS4 mutant seeds. We conclude that AtRS4 is the only stachyose synthase in the genome of A. thaliana that AtRS4 represents a key regulation mechanism in the RFO physiology of A. thaliana due to its multifunctional enzyme activity and that AtRS4 is possibly the second seed specific raffinose synthase beside AtRS5, which is responsible for Raf accumulation under abiotic stress.

Virus-induced alterations in primary metabolism modulate susceptibility to Tobacco rattle virus in Arabidopsis

During compatible virus infections, plants respond by reprogramming gene expression and metabolite content. While gene expression studies are profuse, our knowledge of the metabolic changes that occur in the presence of the virus is limited. Here, we combine gene expression and metabolite profiling in Arabidopsis (Arabidopsis thaliana) infected with Tobacco rattle virus (TRV) in order to investigate the influence of primary metabolism on virus infection. Our results revealed that primary metabolism is reconfigured in many ways during TRV infection, as reflected by significant changes in the levels of sugars and amino acids. Multivariate data analysis revealed that these alterations were particularly conspicuous at the time points of maximal accumulation of TRV, although infection time was the dominant source of variance during the process. Furthermore, TRV caused changes in lipid and fatty acid composition in infected leaves. We found that several Arabidopsis mutants deficient in branched-chain amino acid catabolism or fatty acid metabolism possessed altered susceptibility to TRV. Finally, we showed that increments in the putrescine content in TRV-infected plants correlated with enhanced tolerance to freezing stress in TRV-infected plants and that impairment of putrescine biosynthesis promoted virus multiplication. Our results thus provide an interesting overview for a better understanding of the relationship between primary metabolism and virus infection.

Dehydration induces expression of GALACTINOL SYNTHASE and RAFFINOSE SYNTHASE in seedlings of pea (Pisum sativum L.)

The exposition of 7-day-old pea seedlings to dehydration induced sudden changes in the concentration of monosaccharides and sucrose in epicotyl and roots tissues. During 24h of dehydration, the concentration of glucose and, to a lesser extent, fructose in seedling tissues decreased. The accumulation of sucrose was observed in roots after 4h and in epicotyls after 8h of stress. Epicotyls and roots also began to accumulate galactinol and raffinose after 8h of stress, when small changes in the water content of tissues occurred. The accumulation of galactinol and raffinose progressed parallel to water withdrawal from tissues, but after seedling rehydration both galactosides disappeared. The synthesis of galactinol and raffinose by an early induction (during the first hour of treatment) of galactinol synthase (PsGolS) and raffinose synthase (PsRS) gene expression as well as a later increase in the activity of both enzymes was noted. Signals possibly triggering the induction of PsGolS and PsRS gene expression and accumulation of galactinol and raffinose in seedlings are discussed.

Comparison of phenolic metabolism and primary metabolism between green 'Anjou' pear and its bud mutation, red 'Anjou'

Green 'Anjou' pear and its bud mutation, red 'Anjou' were compared to understand their differences in phenolic metabolism and its effect on primary metabolism. In the flesh of the two cultivars, no difference was detected in the concentration of any phenolic compound, the transcript level of MYB10 or the transcript levels or activities of key enzymes involved in anthocyanin synthesis. Compared with green 'Anjou', the shaded peel of red 'Anjou' had higher anthocyanin concentrations, higher transcript levels of MYB10 and higher activity of UDP-glucose:flavonoid 3-O-glycosyltransferase (UFGT), suggesting that MYB10 regulates UFGT to control anthocyanin synthesis in red 'Anjou' peel. In the sun-exposed peel, activities of phenylalanine ammonia lyase, dihydroflavonol reductase, flavonol synthase and anthocyanidin synthase as well as UFGT were higher in red 'Anjou' than in green 'Anjou'. The peel of red 'Anjou' had higher activities of sorbitol dehydrogenase, raffinose synthase and sucrose synthase and higher levels of raffinose, myo-inositol and starch, indicating that sorbitol metabolism, raffinose synthesis and starch synthesis were upregulated in red 'Anjou'. The flesh of red 'Anjou' had higher concentrations of glucose, but lower activities of ATP-dependent phosphofructokinase, pyruvate kinase and glucose-6-phosphate dehydrogenase and lower dark respiration. The peel of red 'Anjou' had higher activities of glutaminase, asparagine synthetase and asparaginase, and higher concentrations of asparagine, aspartate, alanine, valine, threonine and isoleucine. The effects of anthocyanin synthesis on primary metabolism in fruit peel are discussed.

Responses of Populus trichocarpa galactinol synthase genes to abiotic stresses

Galactinol synthase (GolS; EC 2.4.1.123) is a member of the glycosyltransferase eight family that catalyzes the first step in the biosynthesis pathway of the raffinose family of oligosaccharides (RFOs). The accumulation of RFOs in response to abiotic stress indicates a role for RFOs in stress adaptation. To obtain information on the roles of RFOs in abiotic stress adaptation in trees, we investigated the expression patterns of nine Populus trichocarpa GolS (PtrGolS) genes with special reference to stress responses. PtrGolS genes were differentially expressed in different organs, and the expressions of PtrGolS4 and PtrGolS6 were relatively high in all tested organs. The expression levels of all PtrGolS genes, except PtrGolS9, changed in response to abiotic stress in gene- and stress-type-specific manners. Moreover, short- and long-term stress treatments revealed that induction of PtrGolS by salt stress is obvious only in the early period of treatment (within 24 h), whereas water-deficit stress treatments continued to upregulate PtrGolS gene expression after two days of treatment, in addition to induction within 24 h of treatment. Consistent with these expression patterns, the galactinol content in leaves increased after four days of drought stress, but not under salt stress. Our findings suggest divergent roles for PtrGolS genes in abiotic stress responses in poplars.

A reliable and rapid method for soluble sugars and RFO analysis in chickpea using HPAEC-PAD and its comparison with HPLC-RI

A high performance anion exchange chromatography (HPAEC) coupled with pulsed amperometric detection (PAD) was optimised to separate with precision, accuracy and high reproducibility soluble sugars including oligosaccharides present in pulse meal samples. The optimised method within 20min separated myo-inositol, galactinol, glucose, fructose, sucrose, raffinose, stachyose and verbascose in chickpea seed meal extracts. Gradient method of eluting solvent (sodium hydroxide) resulted in higher sensitivity and rapid detection compared to similar analytical methods. Peaks asymmetry equivalent to one and resolution value ⩾1.5 support column's precision and accuracy for quantitative determinations of soluble sugars in complex mixtures. Intermediate precision determined as relative standard deviation (1.8-3.5%) for different soluble sugars confirms reproducibility of the optimised method. The developed method has superior sensitivity to detect even scarcely present verbascose in chickpea. It also quantifies myo-inositol and galactinol making it suitable both for RFO related genotype screening and biosynthetic studies.

An optimized method for NMR-based plant seed metabolomic analysis with maximized polar metabolite extraction efficiency, signal-to-noise ratio, and chemical shift consistency

An optimized method for NMR-based plant seed metabolomic analysis was established with extraction solvent, cell-breaking method and extract-to-buffer ratio.

Abiotic stress-induced accumulation of raffinose in Arabidopsis leaves is mediated by a single raffinose synthase (RS5, At5g40390)

BACKGROUND

The sucrosylgalactoside oligosaccharide raffinose (Raf, Suc-Gal1) accumulates in Arabidopsis leaves in response to a myriad of abiotic stresses. Whilst galactinol synthases (GolS), the first committed enzyme in Raf biosynthesis are well characterised in Arabidopsis, little is known of the second biosynthetic gene/enzyme raffinose synthase (RS). Conflicting reports suggest the existence of either one or six abiotic stress-inducible RSs (RS-1 to -6) occurring in Arabidopsis. Indirect evidence points to At5g40390 being responsible for low temperature-induced Raf accumulation in Arabidopsis leaves.

RESULTS

By heterologously expressing At5g40390 in E.coli, we demonstrate that crude extracts synthesise Raf in vitro, contrary to empty vector controls. Using two independent loss-of-function mutants for At5g40390 (rs 5-1 and 5-2), we confirm that this RS is indeed responsible for Raf accumulation during low temperature-acclimation (4°C), as previously reported. Surprisingly, leaves of mutant plants also fail to accumulate any Raf under diverse abiotic stresses including water-deficit, high salinity, heat shock, and methyl viologen-induced oxidative stress. Correlated to the lack of Raf under these abiotic stress conditions, both mutant plants lack the typical stress-induced RafS activity increase observed in the leaves of wild-type plants.

CONCLUSIONS

Collectively our findings point to a single abiotic stress-induced RS isoform (RS5, At5g40390) being responsible for Raf biosynthesis in Arabidopsis leaves. However, they do not support a single RS hypothesis since the seeds of both mutant plants still contained Raf, albeit at 0.5-fold lower concentration than seeds from wild-type plants, suggesting the existence of at least one other seed-specific RS. These results also unambiguously discount the existence of six stress-inducible RS isoforms suggested by recent reports.

Comparative transcriptomic analysis of developing cotton cotyledons and embryo axis

Background

As a by product of higher value cotton fibre, cotton seed has been increasingly recognised to have excellent potential as a source of additional food, feed, biofuel stock and even a renewable platform for the production of many diverse biological molecules for agriculture and industrial enterprises. The large size difference between cotyledon and embryo axis that make up a cotton seed results in the under-representation of embryo axis gene transcript levels in whole seed embryo samples. Therefore, the determination of gene transcript levels in the cotyledons and embryo axes separately should lead to a better understanding of metabolism in these two developmentally diverse tissues.

Results

A comparative study of transcriptome changes between cotton developing cotyledon and embryo axis has been carried out. 17,384 unigenes (20.74% of all the unigenes) were differentially expressed in the two adjacent embryo tissues, and among them, 7,727 unigenes (44.45%) were down-regulated and 9,657 unigenes (55.55%) were up-regulated in cotyledon.

Conclusions

Our study has provided a comprehensive dataset that documents the dynamics of the transcriptome at the mid-maturity of cotton seed development and in discrete seed tissues, including embryo axis and cotyledon tissues. The results showed that cotton seed is subject to many transcriptome variations in these two tissue types and the differential gene expression between cotton embryo axis and cotyledon uncovered in our study should provide an important starting point for understanding how gene activity is coordinated during seed development to make a seed. Further, the identification of genes involved in rapid metabolite accumulation stage of seed development will extend our understanding of the complex molecular and cellular events in these developmental processes and provide a foundation for future studies on the metabolism, embryo differentiation of cotton and other dicot oilseed crops.

Overexpression of TsGOLS2, a galactinol synthase, in Arabidopsis thaliana enhances tolerance to high salinity and osmotic stresses

Galactinol synthase (GOLS, EC 2.4.1.123), a key enzyme in the synthesis of raffinose family oligosaccharides (RFOs), catalyzes the condensation of UDP-galactose with myo-inositol to produce galactinol as the sole donor for the synthesis of RFOs. RFOs have been implicated in mitigating effects of environmental stresses on plants. TsGOLS2, was cloned from Thellungiella salsuginea with high homology to AtGOLS2. TsGOLS2 was up-regulated by several abiotic stresses. We overexpressed TsGOLS2 in Arabidopsis thaliana. The contents of galactinol, raffinose, and α-ketoglutaric acid were significantly increased in transgenic plants. Compared to wild type plants, salt-stressed transgenic A. thaliana exhibited higher germination rate, photosynthesis ability, and seedling growth. After being treated with osmotic stress by high concentration of sorbitol, transgenic plants retained high germination rates and grew well during early development. These results indicated that overexpression of TsGOLS2 in A. thaliana improved the tolerance of transgenic plants to high salinity and osmotic stress.

Molecular characterization and expression of three galactinol synthase genes that confer stress tolerance in Salvia miltiorrhiza

To adapt to changes in their growing environment, plants express several stress-responsive genes. For example, the products of galactinol synthase (Gols) genes play a key role in regulating the levels of raffinose family oligosaccharides and conferring resistance to stress. We cloned and characterized three Gols genes in Salvia miltiorrhiza. Their expression followed three distinct patterns. Compared with the control, SmGols1 was up-regulated by temperature changes but was suppressed by exposure to methyl jasmonate or short-term drought. This gene had the greatest abundance of transcripts and was assigned a general function of carbon storage. SmGols2 responded to all stress and hormone treatments, and transcripts were maintained at a high level. Finally, expression of SmGols3 was weaker than the other two genes, but was increased significantly under different treatments. Over the experimental period, its expression declined to normal levels in response to all treatments except exposure to 100 μM ABA, long-term drought, heat (42 °C), or chilling (8 °C). Based on our finding of cis-elements in the 5' flanking regions, we concluded that these genes seem to be regulated by several HSF transcription factors. We also targeted their 90-bp conserved sequences and used them for RNA interference analysis. Some were knocked down to various extents in our transgenic lines. Fluctuations in their malondialdehyde contents under different stress treatments, as well as the rate of water loss in transformed plants, suggested that lipid peroxidation was more likely to occur in the transgenics than in the control. These results indicate that SmGols genes could have a main function in responding to cold or heat. Therefore, we believe that it is important to investigate this mechanism for tolerance in S. miltiorrhiza and to examine how expression of these SmGols and other homologs are influenced by abiotic stresses.

Genome-wide identification of genes involved in raffinose metabolism in Maize

The raffinose family oligosaccharides (RFOs), such as raffinose and stachyose, are synthesized by a set of distinct galactosyltransferases, which sequentially add galactose units to sucrose. The accumulation of RFOs in plant cells are closely associated with the responses to environmental factors, such as cold, heat and drought stresses. Systematic analysis of genes involved in the raffinose metabolism has not been reported to date. Searching the recently available working draft of the maize genome, six kinds of enzyme genes were speculated, which should encode all the enzymes involved in the raffinose metabolism in maize. Expression patterns of some related putative genes were analyzed. The conserved domains and phylogenetic relationships among the deduced maize proteins and their homologs isolated from other plant species were revealed. It was discovered that some of the key enzymes, such as galactinol synthase (ZmGolS5, ZmGolS45 and ZmGolS37), raffinose synthase (ZmRS1, ZmRS2, ZmRS3 and ZmRS10), stachyose synthase (ZmRS8) and β-fructofuranosidase, are encoded by multiple gene members with different expression patterns. These results reveal the complexity of the raffinose metabolism and the existence of metabolic channels for diverse RFOs in maize and provide useful information for improving maize stress tolerance through genetic engineering.

Expression of a GALACTINOL SYNTHASE gene is positively associated with desiccation tolerance of Brassica napus seeds during development

Desiccation tolerance of seeds is positively correlated with raffinose family oligosaccharides (RFOs). However, RFOs' role in desiccation tolerance is still a matter of controversy. The aim of this work was to monitor the accumulation of RFO during acquisition of desiccation tolerance in rapeseed (Brassica napus L.). Rapeseeds become desiccation tolerant at 21-24d after flowering (DAF), and the time was coincident with an accumulation of raffinose and stachyose. A gene encoding galactinol synthase (GolS; EC2.4.1.123), involved in RFO biosynthesis, was cloned and functionally characterized. Enzymatic properties of recombinant galactinol synthase were also determined. Accumulation of BnGOLS-1 mRNA in developing rapeseeds was concomitant with dry weight deposition and the acquisition of desiccation tolerance, and was concurrent with the formation of raffinose and stachyose. The physiological implications of BnGOLS-1 expression patterns in developing seeds are discussed in light of the hypothesized role of RFOs in seed desiccation tolerance.

Phytic acid and raffinose series oligosaccharides metabolism in developing chickpea seeds

Phytic acid and raffinose series oligosaccharides (RFOs) have anti-nutritional properties where phytic acid chelates minerals and reduces their bioavailability to humans and other animals, and RFOs cause flatulence. Both phytic acid and RFOs cannot be digested by monogastric animals and are released as pollutant-wastes. Efforts are being made to reduce the contents of these factors without affecting the viability of seeds. This will require a thorough understanding of their metabolism in different crops. Biosynthetic pathways of both metabolites though are interlinked but not well described. This study was made on metabolism of these two contents in developing chickpea (Cicer arietinum L cv GL 769) seeds. In this study, deposition of RFOs was found to occur before deposition of phytic acid. A decline in inorganic phosphorus and increase in phospholipid phosphorus and phytic acid was observed in seeds during development. Acid phosphatase was the major phosphatase in seed as well as podwall and its activity was highest at early stage of development, thereafter it decreased. Partitioning of (14) C label from (14) C-glucose and (14) C-sucrose into RFOs and phytic acid was studied in seeds in presence of inositol, galactose and iositol and galactose, which favored the view that galactinol synthase is not the key enzyme in RFOs synthesis.

Effect of abandonment and plant classification on carbohydrate reserves of meadow plants

We studied the effect of cessation of management on carbohydrate reserves of plants in meadows with different environmental characteristics and plant composition. We recorded storage carbohydrates and seasonal changes for 40 plant species. We asked whether there are differences in responses of carbohydrate reserves in forbs versus graminoids and in plants storing starch versus plants storing osmotically active carbohydrates. We analysed belowground organs before the meadows were mown and at the end of the vegetation season in mown versus recently abandoned plots. Whereas starch and fructans were widely distributed, raffinose family oligosaccharides were the main carbohydrate reserves of the Lamiaceae and Plantago lanceolata. Properties of carbohydrate reserves differed between forbs and graminoids but no difference was found between plants storing starch versus osmotically active carbohydrates. Graminoids had lower carbohydrate concentrations than forbs. We observed a positive effect of mowing on carbohydrate concentrations of graminoids in the dry, calcium-rich meadow and higher seasonal fluctuations of these values in the acid, wet meadow, suggesting that local factors and/or the species pool affect carbohydrate reserves. Despite local conditions, graminoids represent a distinct functional group in meadows from the point of view of their storage economy. We suggest that as well as growth, storage processes should also be considered for understanding the functioning of meadow plant communities.

A WRKY transcription factor participates in dehydration tolerance in Boea hygrometrica by binding to the W-box elements of the galactinol synthase (BhGolS1) promoter

Accumulation of compatible osmolytes, such as soluble sugars, in plants is an important osmoprotective mechanism. Sugars play a role in osmotic adjustment and are associated with stabilization of proteins and cell structures, reactive oxygen species scavenging, signaling functions or induction of adaptive pathways. Galactinol is the galactosyl donor for the synthesis of raffinose family oligosaccharides (RFOs) and its synthesis by galactinol synthase (GolS) is the first committed step of the RFOs biosynthetic pathway. GolS genes are induced by a variety of stresses in both stress-sensitive and tolerant-plant species; however, the mechanism of transcriptional regulation is not fully established. In this paper, we characterized a GolS gene (BhGolS1) that was dehydration and ABA-inducible in the resurrection plant Boea hygrometrica and conferred dehydration tolerance in a transgenic tobacco system. Four W-box cis-elements were identified in the BhGolS1 promoter and shown to be bound by an early dehydration and ABA-inducible WRKY gene (BhWRKY1). These data suggest a mechanism where BhWRKY1 is likely to function in an ABA-dependent signal pathway to regulate BhGolS1 expression, which leads to the accumulation of RFOs in desiccation-tolerant B. hygrometrica leaves.

Physiological roles of plant glycoside hydrolases

The functions of plant glycoside hydrolases and transglycosidases have been studied using different biochemical and molecular genetic approaches. These enzymes are involved in the metabolism of various carbohydrates containing compounds present in the plant tissues. The structural and functional diversity of the carbohydrates implies a vast spectrum of enzymes involved in their metabolism. Complete genome sequence of Arabidopsis and rice has allowed the classification of glycoside hydrolases in different families based on amino acid sequence data. The genomes of these plants contain 29 families of glycoside hydrolases. This review summarizes the current research on plant glycoside hydrolases concerning their principal functional roles, which were attributed to different families. The majority of these plant glycoside hydrolases are involved in cell wall polysaccharide metabolism. Other functions include their participation in the biosynthesis and remodulation of glycans, mobilization of energy, defence, symbiosis, signalling, secondary plant metabolism and metabolism of glycolipids.

Changes in the sugarcane metabolome with stem development. Are they related to sucrose accumulation?

Sucrose content increases with internode development down the stem of sugarcane. In an attempt to determine which other changes in metabolites may be linked to sucrose accumulation gas chromatography-mass spectrometry was used to obtain metabolic profiles from methanol/water extracts of four samples of different age down the stem of cultivar Q117. Extracts were derivatized with either N-methyl-N-(trimethylsilyl) trifluoracetamide (TMS) or N-methyl N-(tert-butyldimethylsilyl) trifluoroacetamide (TBS) separately in order to increase the number of metabolites that could be detected. This resulted in the measurement of 121 and 71 metabolites from the TMS and TBS derivatization, respectively. Fifty-five metabolites were identified using commercial and publicly available libraries. Statistical analysis of the metabolite profiles resulted in clustering of tissue types. Particular metabolites were correlated with the level of sucrose accumulation, which as expected increased down the stem. Metabolites, such as tricarboxylic acid cycle intermediates and amino acids, were more abundant in the M2 sample (meristem to internode 2) that was actively growing and decreased in an apparently coordinated developmentally programmed manner in more mature internodes down the stem. However, other metabolites such as trehalose and raffinose showed positive correlations with sucrose concentration. Here we discuss the technique used to measure metabolites in sugarcane and the changes in metabolite abundance down the sugarcane stem.

Genetic and genomic analysis of legume flowers and seeds

New tools, such as ordered mutant libraries, microarrays and sequence based comparative maps, are available for genetic and genomic studies of legumes that are being used to shed light on seed production, the objective of most arable farming. The new information and understanding brought by these tools are revealing the biological processes that underpin and impact on seed production.