Raffinose in chloroplasts is synthesized in the cytosol and transported across the chloroplast envelope

Metabolite-mediated adaptation of crops to drought and the acquisition of tolerance

Drought is one of the major and growing threats to agriculture productivity and food security. Metabolites are involved in the regulation of plant responses to various environmental stresses, including drought stress. The complex drought tolerance can be ascribed to several simple metabolic traits. These traits could then be used for detecting the genetic architecture of drought tolerance. Plant metabolomes show dynamic differences when drought occurs during different developmental stages or upon different levels of drought stress. Here, we reviewed the major and most recent findings regarding the metabolite-mediated plant drought response. Recent progress in the development of drought-tolerant agents is also discussed. We provide an updated schematic overview of metabolome-driven solutions for increasing crop drought tolerance and thereby addressing an impending agricultural challenge.

Analysis of Raffinose Metabolism-Related Sugar Components in Seeds

Raffinose family oligosaccharides (RFOs) are synthesized from sucrose and subsequent addition of galactose moieties which was provided by galactinol. Galactinol is synthesized from UDP-galactose and myo-inositol. RFOs accumulate at late stage of seed development and play important roles in seed longevity. RFOs are major components in seeds of many plant species. Here, we document a methodology for extraction and quantitative analysis of raffinose metabolism-related soluble sugars or the derivative alcohols in plant seeds. This protocol, based on high-performance liquid chromatography (HPLC), achieves the efficient separation and accurate quantification of sucrose, myo-inositol, galactinol, and raffinose within 25 min of retention time.

Raffinose degradation-related gene GhAGAL3 was screened out responding to salinity stress through expression patterns of GhAGALs family genes

A-galactosidases (AGALs), the oligosaccharide (RFO) catabolic genes of the raffinose family, play crucial roles in plant growth and development and in adversity stress. They can break down the non-reducing terminal galactose residues of glycolipids and sugar chains. In this study, the whole genome of AGALs was analyzed. Bioinformatics analysis was conducted to analyze members of the AGAL family in Gossypium hirsutum, Gossypium arboreum, Gossypium barbadense, and Gossypium raimondii. Meanwhile, RT-qPCR was carried out to analyze the expression patterns of AGAL family members in different tissues of terrestrial cotton. It was found that a series of environmental factors stimulated the expression of the GhAGAL3 gene. The function of GhAGAL3 was verified through virus-induced gene silencing (VIGS). As a result, GhAGAL3 gene silencing resulted in milder wilting of seedlings than the controls, and a significant increase in the raffinose content in cotton, indicating that GhAGAL3 responded to NaCl stress. The increase in raffinose content improved the tolerance of cotton. Findings in this study lay an important foundation for further research on the role of the GhAGAL3 gene family in the molecular mechanism of abiotic stress resistance in cotton.

Identification and Alternative Splicing Profile of the Raffinose synthase Gene in Grass Species

Raffinose synthase (Rafs) is an important enzyme in the synthesis pathway of raffinose from sucrose and galactinol in higher plants and is involved in the regulation of seed development and plant responses to abiotic stresses. In this study, we analyzed the Rafs families and profiled their alternative splicing patterns at the genome-wide scale from 10 grass species representing crops and grasses. A total of 73 Rafs genes were identified from grass species such as rice, maize, foxtail millet, and switchgrass. These Rafs genes were assigned to six groups based the phylogenetic analysis. We compared the gene structures, protein domains, and expression patterns of Rafs genes, and also unraveled the alternative transcripts of them. In addition, different conserved sequences were observed at these putative splice sites among grass species. The subcellular localization of PvRafs5 suggested that the Rafs gene was expressed in the cytoplasm or cell membrane. Our findings provide comprehensive knowledge of the Rafs families in terms of genes and proteins, which will facilitate further functional characterization in grass species in response to abiotic stress.

Impact of drought and salt stress on galactinol and raffinose family oligosaccharides in common bean (Phaseolus vulgaris)

Due to climate change, farmers will face more extreme weather conditions and hence will need crops that are better adapted to these challenges. The raffinose family oligosaccharides (RFOs) could play a role in the tolerance of crops towards abiotic stress. To investigate this, we determined for the first time the importance of galactinol and RFOs in the roots and leaves of common bean under drought and salt stress conditions. Initially, the physiological characteristics of common bean under agronomically relevant abiotic stress conditions were investigated by measuring the growth rate, transpiration rate, chlorophyll concentration and membrane stability, allowing to establish relevant sampling points. Subsequently, the differential gene expression profiles of the galactinol and RFO biosynthetic genes and the amount of galactinol and RFO molecules were measured in the primary leaves and roots of Phaseolus vulgaris cv. CIAP7247F at these sampling points, using RT-qPCR and HPAEC-PAD, respectively. Under drought stress, the genes galactinol synthase 1, galactinol synthase 3 and stachyose synthase were significantly upregulated in the leaves and had a high transcript level in comparison with the other galactinol and RFO biosynthetic genes. This was in accordance with the significantly higher amount of galactinol and raffinose detected in the leaves. Under salt stress, raffinose was also present in a significantly higher quantity in the leaves. In the roots, transcript levels of the RFO biosynthetic genes were generally low and no galactinol, raffinose or stachyose could be detected. These results suggest that in the leaves, both galactinol and raffinose could play a role in the protection of common bean against abiotic stresses. Especially, the isoform galactinol synthase 3 could have a specific role during drought stress and forms an interesting candidate to improve the abiotic stress resistance of common bean or other plant species.

Nutrient availability regulates Deschampsia antarctica photosynthetic and stress tolerance performance in Antarctica

Deschampsia antarctica is one of the only two native vascular plants in Antarctica, mostly located in the ice-free areas of the Peninsula's coast and adjacent islands. This region is characterized by a short growing season, frequent extreme climatic events, and soils with reduced nutrient availability. However, it is unknown whether its photosynthetic and stress tolerance mechanisms are affected by the availability of nutrients to deal with this particular environment. We studied the photosynthetic, primary metabolic, and stress tolerance performance of D. antarctica plants growing on three close sites (<500 m) with contrasting soil nutrient conditions. Plants from all sites showed similar photosynthetic rates, but mesophyll conductance and photobiochemistry were more limiting (~25%) in plants growing on low-nutrient availability soils. Additionally, these plants showed higher stress levels and larger investments in photoprotection and carbon pools, most probably driven by the need to stabilize proteins and membranes, and remodel cell walls. In contrast, when nutrients were readily available, plants shifted their carbon investment towards amino acids related to osmoprotection, growth, antioxidants, and polyamines, leading to vigorous plants without appreciable levels of stress. Taken together, these findings demonstrate that D. antarctica displays differential physiological performances to cope with adverse conditions depending on resource availability, allowing it to maximize stress tolerance without jeopardizing photosynthetic capacity.

Raffinose positively regulates maize drought tolerance by reducing leaf transpiration

Drought stress is one of the major constraints of global crop production. Raffinose, a non-reducing trisaccharide, has been considered to regulate positively the plant drought stress tolerance; however, evidence that augmenting raffinose production in leaves results in enhanced plant drought stress tolerance is lacking. The biochemical mechanism through which raffinose might act to mitigate plant drought stress remains unidentified. ZmRAFS encodes Zea mays RAFFINOSE SYNTHASE, a key enzyme that transfers galactose from the galactoside galactinol to sucrose for raffinose production. Overexpression of ZmRAFS in maize increased the RAFS protein and the raffinose content and decreased the water loss of leaves and enhanced plant drought stress tolerance. The biomass of the ZmRAFS overexpressing plants was similar to that of non-transgenic control plants when grown under optimal conditions, but was significantly greater than that of non-transgenic plants when grown under drought stress conditions. In contrast, the percentage of water loss of the detached leaves from two independent zmrafs mutant lines, incapable of synthesizing raffinose, was greater than that from null segregant controls and this phenomenon was partially rescued by supplementation of raffinose to detached zmrafs leaves. In addition, while there were differences in water loss among different maize lines, there was no difference in stomata density or aperture. Taken together, our work demonstrated that overexpression of the ZmRAFS gene in maize, in contrast to Arabidopsis, increased the raffinose content in leaves, assisted the leaf to retain water, and enhanced the plant drought stress tolerance without causing a detectable growth penalty.

Identification, Characterization and Expression Profiling of the RS Gene Family during the Withering Process of White Tea in the Tea Plant (Camellia sinensis) Reveal the Transcriptional Regulation of CsRS8

Raffinose synthetase (RS) is a key enzyme in the process of raffinose (Raf) synthesis and is involved in plant development and stress responses through regulating Raf content. As a sweetener, Raf makes an important contribution to the sweet taste of white tea. However, studies on the identification, analysis and transcriptional regulation of CsRSs (Camellia sinensis RS genes) are still lacking. In this study, nine CsRSs were identified from the tea plant (Camellia sinensis) genome database. The CsRSs were classified into five groups in the phylogenetic tree. Expression level analysis showed that the CsRSs varied in different parts of the tea plant. Transcriptome data showed that CsRSs could respond to persistent drought and cold acclimation. Except for CsRS5 and CsRS9, the expression pattern of all CsRSs increased at 12 h and decreased at 30 h during the withering process of white tea, consistent with the change trend of the Raf content. Furthermore, combining yeast one-hybrid assays with expression analysis, we found that CsDBB could potentially regulate the expression of CsRS8. Our results provide a new perspective for further research into the characterization of CsRS genes and the formation of the white tea flavour.

Transcriptional regulation of the raffinose family oligosaccharides pathway in Sorghum bicolor reveals potential roles in leaf sucrose transport and stem sucrose accumulation

Bioenergy sorghum hybrids are being developed with enhanced drought tolerance and high levels of stem sugars. Raffinose family oligosaccharides (RFOs) contribute to plant environmental stress tolerance, sugar storage, transport, and signaling. To better understand the role of RFOs in sorghum, genes involved in myo-inositol and RFO metabolism were identified and relative transcript abundance analyzed during development. Genes involved in RFO biosynthesis (SbMIPS1, SbInsPase, SbGolS1, SbRS) were more highly expressed in leaves compared to stems and roots, with peak expression early in the morning in leaves. SbGolS, SbRS, SbAGA1 and SbAGA2 were also expressed at high levels in the leaf collar and leaf sheath. In leaf blades, genes involved in myo-inositol biosynthesis (SbMIPS1, SbInsPase) were expressed in bundle sheath cells, whereas genes involved in galactinol and raffinose synthesis (SbGolS1, SbRS) were expressed in mesophyll cells. Furthermore, SbAGA1 and SbAGA2, genes that encode neutral-alkaline alpha-galactosidases that hydrolyze raffinose, were differentially expressed in minor vein bundle sheath cells and major vein and mid-rib vascular and xylem parenchyma. This suggests that raffinose synthesized from sucrose and galactinol in mesophyll cells diffuses into vascular bundles where hydrolysis releases sucrose for long distance phloem transport. Increased expression (>20-fold) of SbAGA1 and SbAGA2 in stem storage pith parenchyma of sweet sorghum between floral initiation and grain maturity, and higher expression in sweet sorghum compared to grain sorghum, indicates these genes may play a key role in non-structural carbohydrate accumulation in stems.

Comprehensive analysis of the GALACTINOL SYNTHASE (GolS) gene family in citrus and the function of CsGolS6 in stress tolerance

Galactinol synthase (GolS) catalyzes the first and rate-limiting step in the synthesis of raffinose family of oligosaccharides (RFOs), which serve as storage and transport sugars, signal transducers, compatible solutes and antioxidants in higher plants. The present work aimed to assess the potential functions of citrus GolS in mechanisms of stress response and tolerance. By homology searches, eight GolS genes were found in the genomes of Citrus sinensis and C. clementina. Phylogenetic analysis showed that there is a GolS ortholog in C. clementina for each C. sinensis GolS, which have evolved differently from those of Arabidopsis thaliana. Transcriptional analysis indicated that most C. sinensis GolS (CsGolS) genes show a low-level tissue-specific and stress-inducible expression in response to drought and salt stress treatments, as well as to ‘Candidatus Liberibacter asiaticus’ infection. CsGolS6 overexpression resulted in improved tobacco tolerance to drought and salt stresses, contributing to an increased mesophyll cell expansion, photosynthesis and plant growth. Primary metabolite profiling revealed no significant changes in endogenous galactinol, but different extents of reduction of raffinose in the transgenic plants. On the other hand, a significant increase in the levels of metabolites with antioxidant properties, such as ascorbate, dehydroascorbate, alfa-tocopherol and spermidine, was observed in the transgenic plants. These results bring evidence that CsGolS6 is a potential candidate for improving stress tolerance in citrus and other plants.

Tonoplast Sucrose Trafficking Modulates Starch Utilization and Water Deficit Behavior in Poplar Leaves

Leaf osmotic adjustment by the active accrual of compatible organic solutes (e.g. sucrose) contributes to drought tolerance throughout the plant kingdom. In Populus tremula x alba, PtaSUT4 encodes a tonoplast sucrose-proton symporter, whose downregulation by chronic mild drought or transgenic manipulation is known to increase leaf sucrose and turgor. While this may constitute a single drought tolerance mechanism, we now report that other adjustments which can occur during a worsening water deficit are damped when PtaSUT4 is constitutively downregulated. Specifically, we report that starch use and leaf relative water content (RWC) dynamics were compromised when plants with constitutively downregulated PtaSUT4 were subjected to a water deficit. Leaf RWC decreased more in wild-type and vector control lines than in transgenic PtaSUT4-RNAi (RNA-interference) or CRISPR (clustered regularly interspersed short palindromic repeats) knockout (KO) lines. The control line RWC decrease was accompanied by increased PtaSUT4 transcript levels and a mobilization of sucrose from the mesophyll-enriched leaf lamina into the midvein. The findings suggest that changes in SUT4 expression can increase turgor or decrease RWC as different tolerance mechanisms to reduced water availability. Evidence is presented that PtaSUT4-mediated sucrose partitioning between the vacuole and the cytosol is important not only for overall sucrose abundance and turgor, but also for reactive oxygen species (ROS) and antioxidant dynamics. Interestingly, the reduced capacity for accelerated starch breakdown under worsening water-deficit conditions was correlated with reduced ROS in the RNAi and KO lines. A role for PtaSUT4 in the orchestration of ROS, antioxidant, starch utilization and RWC dynamics during water stress and its importance in trees especially, with their high hydraulic resistances, is considered.

Comparative transcriptomic analyses of citrus cold-resistant vs. sensitive rootstocks might suggest a relevant role of ABA signaling in triggering cold scion adaption

BACKGROUND

The citrus genus comprises a number of sensitive tropical and subtropical species to cold stress, which limits global citrus distribution to certain latitudes and causes major economic loss. We used RNA-Seq technology to analyze changes in the transcriptome of Valencia delta seedless orange in response to long-term cold stress grafted on two frequently used citrus rootstocks: Carrizo citrange (CAR), considered one of the most cold-tolerant accessions; C. macrophylla (MAC), a very sensitive one. Our objectives were to identify the genetic mechanism that produce the tolerant or sensitive phenotypes in citrus, as well as to gain insights of the rootstock-scion interactions that induce the cold tolerance or sensitivity in the scion.

RESULTS

Plants were kept at 1 ºC for 30 days. Samples were taken at 0, 15 and 30 days. The metabolomic analysis showed a significant increase in the concentration of free sugars and proline, which was higher for the CAR plants. Hormone quantification in roots showed a substantially increased ABA concentration during cold exposure in the CAR roots, which was not observed in MAC. Different approaches were followed to analyze gene expression. During the stress treatment, the 0-15-day comparison yielded the most DEGs. The functional characterization of DEGs showed enrichment in GO terms and KEGG pathways related to abiotic stress responses previously described in plant cold adaption. The DEGs analysis revealed that several key genes promoting cold adaption were up-regulated in the CAR plants, and those repressing it had higher expression levels in the MAC samples.

CONCLUSIONS

The metabolomic and transcriptomic study herein performed indicates that the mechanisms activated in plants shortly after cold exposure remain active in the long term. Both the hormone quantification and differential expression analysis suggest that ABA signaling might play a relevant role in promoting the cold hardiness or sensitiveness of Valencia sweet orange grafted onto Carrizo citrange or Macrophylla rootstocks, respectively. Our work provides new insights into the mechanisms by which rootstocks modulate resistance to abiotic stress in the production variety grafted onto them.

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.

How to Cope with the Challenges of Environmental Stresses in the Era of Global Climate Change: An Update on ROS Stave off in Plants

With the advent of human civilization and anthropogenic activities in the shade of urbanization and global climate change, plants are exposed to a complex set of abiotic stresses. These stresses affect plants’ growth, development, and yield and cause enormous crop losses worldwide. In this alarming scenario of global climate conditions, plants respond to such stresses through a highly balanced and finely tuned interaction between signaling molecules. The abiotic stresses initiate the quick release of reactive oxygen species (ROS) as toxic by-products of altered aerobic metabolism during different stress conditions at the cellular level. ROS includes both free oxygen radicals {superoxide (O₂^•−) and hydroxyl (OH⁻)} as well as non-radicals [hydrogen peroxide (H₂O₂) and singlet oxygen (¹O₂)]. ROS can be generated and scavenged in different cell organelles and cytoplasm depending on the type of stimulus. At high concentrations, ROS cause lipid peroxidation, DNA damage, protein oxidation, and necrosis, but at low to moderate concentrations, they play a crucial role as secondary messengers in intracellular signaling cascades. Because of their concentration-dependent dual role, a huge number of molecules tightly control the level of ROS in cells. The plants have evolved antioxidants and scavenging machinery equipped with different enzymes to maintain the equilibrium between the production and detoxification of ROS generated during stress. In this present article, we have focused on current insights on generation and scavenging of ROS during abiotic stresses. Moreover, the article will act as a knowledge base for new and pivotal studies on ROS generation and scavenging.

Transcriptomic profiling suggests candidate molecular responses to waterlogging in cassava

Owing to climate change impacts, waterlogging is a serious abiotic stress that affects crops, resulting in stunted growth and loss of productivity. Cassava (Manihot esculenta Grantz) is usually grown in areas that experience high amounts of rainfall; however, little research has been done on the waterlogging tolerance mechanism of this species. Therefore, we investigated the physiological responses of cassava plants to waterlogging stress and analyzed global gene transcription responses in the leaves and roots of waterlogged cassava plants. The results showed that waterlogging stress significantly decreased the leaf chlorophyll content, caused premature senescence, and increased the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in the leaves and roots. In total, 2538 differentially expressed genes (DEGs) were detected in the leaves and 13364 in the roots, with 1523 genes shared between the two tissues. Comparative analysis revealed that the DEGs were related mainly to photosynthesis, amino metabolism, RNA transport and degradation. We also summarized the functions of the pathways that respond to waterlogging and are involved in photosynthesis, glycolysis and galactose metabolism. Additionally, many transcription factors (TFs), such as MYBs, AP2/ERFs, WRKYs and NACs, were identified, suggesting that they potentially function in the waterlogging response in cassava. The expression of 12 randomly selected genes evaluated via both quantitative real-time PCR (qRT-PCR) and RNA sequencing (RNA-seq) was highly correlated (R2 = 0.9077), validating the reliability of the RNA-seq results. The potential waterlogging stress-related transcripts identified in this study are representatives of candidate genes and molecular resources for further understanding the molecular mechanisms underlying the waterlogging response in cassava.

Cellular and Metabolite Changes in the Secondary Phloem of Chinese Fir (Cuninghamia lanceolata (Lamb.) Hook.) during Dormancy Release

Wood in the cold temperate zone is the product of the alternation of the growing season and the dormant period of trees, but our knowledge of the process of dormancy release in trees remains limited. Chinese fir (Cuninghamia lanceolata (Lamb.) Hook.) was used to investigate cellular and metabolite changes in the secondary phloem tissue during dormancy release. The sampling dates were 2 March, 28 March, and 13 April. The microsections of wood-forming tissue were prepared using the paraffin embedding technique to observe the formation of cambium cells; metabolites in secondary phloem cells were extracted using a methanol/chloroform organic solvent system. The results showed that the secondary phloem consists of phloem fibers, sieve cells and phloem parenchyma. The cells were regularly arranged in continuous tangential bands and were in the order of Phloem fiber-Sieve cell-Phloem parenchyma-Sieve cell-Phloem parenchyma-Sieve cell-Phloem parenchyma-Sieve cell-Sieve cell-Phloem parenchyma-. The Chinese fir cambium was in dormancy on 2 March and 28 March, while on 13 April, it was already in the active stage and two layers of xylem cells with several layers of phloem cells were newly formed. The width of the cambium zone increased from 18.7 ± 5.7 μm to 76.5 ± 3.0 μm and the average radial diameter of sieve cells expanded from 15.4 ± 7.5 μm to 21.5 ± 7.4 μm after dormancy release. The cambium zone width and the average radial diameter of sieve cells before and after dormancy release were significantly different (p < 0.01). The phloem parenchyma cells without resin were squeezed and deformed by the sieve cells, and the width of the phloem during the active period was 197.0 ± 8.5 μm, which was larger than that during the dormant period. Ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS)-based metabolomics was employed to analyze the secondary phloem of Chinese fir on 28 March and 13 April. Thirty-nine differential metabolites during dormancy release were detected. The results showed that the composition of Chinese fir metabolites was different before and after dormancy release. The relative increase in pyruvic acid and ascorbic acid contents proved that the rate of energy metabolism in Chinese fir increased substantially after dormancy release. Changes in cell development and the composition of metabolites revealed that the dormancy release of Chinese fir was at early April and the formation period of phloem tissue is earlier than xylem tissue.

ERF108 from Poncirus trifoliata (L.) Raf. functions in cold tolerance by modulating raffinose synthesis through transcriptional regulation of PtrRafS

Ethylene-responsive factors (ERFs) are plant-specific transcription factors involved in cold stress response, and raffinose is known to accumulate in plants exposed to cold. However, it remains elusive whether ERFs function in cold tolerance by modulating raffinose synthesis. Here, we identified a cold-responsive PtrERF108 from trifoliate orange (Poncirus trifoliata (L.) Raf.), a cold-tolerant plant closely related to citrus. PtrERF108 is localized in the nucleus and has transcriptional activation activity. Overexpression of PtrERF108 conferred enhanced cold tolerance of transgenic lemon, whereas virus-induced gene silencing (VIGS)-mediated knockdown of PtrERF108 in trifoliate orange greatly elevated cold sensitivity. Transcriptome profiling showed that PtrERF108 overexpression caused extensive reprogramming of genes associated with signaling transduction, physiological processes and metabolic pathways. Among them, a raffinose synthase (RafS)-encoding gene, PtrRafS, was confirmed as a direct target of PtrERF108. RafS activity and raffinose content were significantly increased in PtrERF108-overexpressing transgenic plants, but prominently decreased in the VIGS plants under cold conditions. Meanwhile, exogenous replenishment of raffinose could recover the cold tolerance of PtrERF108-silenced plants, whereas VIGS-mediated knockdown of PtrRafS resulted in cold-sensitive phenotype. Taken together, the current results demonstrate that PtrERF108 plays a positive role in cold tolerance by modulation of raffinose synthesis via regulating PtrRafS. Our findings reveal a new transcriptional module composed of ERF108-RafS underlying cold-induced raffinose accumulation in plants.

Phylogenetic relationships of sucrose transporters (SUTs) in plants and genome-wide characterization of SUT genes in Orchidaceae reveal roles in floral organ development

Sucrose is the primary form of photosynthetically produced carbohydrates transported long distance in many plant species and substantially affects plant growth, development and physiology. Sucrose transporters (SUTs or SUCs) are a group of membrane proteins that play vital roles in mediating sucrose allocation within cells and at the whole-plant level. In this study, we investigated the relationships among SUTs in 24 representative plant species and performed an analysis of SUT genes in three sequenced Orchidaceae species: Dendrobium officinale, Phalaenopsis equestris, and Apostasia shenzhenica. All the SUTs from the 24 plant species were classified into three groups and five subgroups, subgroups A, B1, B2.1, B2.2, and C, based on their evolutionary relationships. A total of 22 SUT genes were identified among Orchidaceae species, among which D. officinale had 8 genes (DoSUT01-08), P. equestris had eight genes (PeqSUT01-08) and A. shenzhenica had 6 genes (AsSUT01-06). For the 22 OrchidaceaeSUTs, subgroups A, B2.2 and C contained three genes, whereas the SUT genes were found to have significantly expanded in the monocot-specific subgroup B2.1, which contained 12 genes. To understand sucrose partitioning and the functions of sucrose transporters in Orchidaceae species, we analyzed the water-soluble sugar content and performed RNA sequencing of different tissues of D. officinale, including leaves, stems, flowers and roots. The results showed that although the total content of water-soluble polysaccharides was highest in the stems of D. officinale, the sucrose content was highest in the flowers. Moreover, gene expression analysis showed that most of the DoSUTs were expressed in the flowers, among which DoSUT01,DoSUT07 and DoSUT06 had significantly increased expression levels. These results indicated that stems are used as the main storage sinks for photosynthetically produced sugar in D. officinale and that DoSUTs mainly function in the cellular machinery and development of floral organs. Our findings provide valuable information on sucrose partitioning and the evolution and functions of SUT genes in Orchidaceae and other species.

Effects of antibiotics stress on growth variables, ultrastructure, and metabolite pattern of Brassica rapa ssp. chinensis

Antibiotics frequently contaminate agricultural fields and through plant uptake enter into the food chain. This study aimed to explore the effects of antibiotics; tetracycline (TC), oxytetracycline (OTC) and norfloxacin (NF) on the growth, cell ultrastructure, and metabolite pattern of Brassica rapa ssp. chinensis. Oxytetracycline accumulated more than other antibiotics followed by TC and NF. Plant growth, chlorophyll fluorescence, and antioxidant activities were negatively affected under all antibiotic treatments. Ultrastructural investigation of mesophyll of leaves performed by transmission electron microscopy indicated that antibiotic stress caused the changes in thylakoid orientation, number of plastoglobuli, and starch grains. Identification of functional groups through Fourier transform infrared analysis indicated that carboxyl group, carbonate and ammonium ions are involved in the adsorption of antibiotics. The metabolic profiling of B. rapa leaves demonstrated that all of the antibiotics treatments distorted phenylalanine, tyrosine and tryptophan biosynthesis, d-glutamine and d-glutamate metabolism, alanine, aspartate and glutamate metabolism, phenylalanine metabolism and TCA cycle. Metabolic alterations as a result of antibiotics stress provide insights of metabolites that affect the physiological changes attributed to antibiotic stress. These results will improve the understanding of antibiotic contamination effects on plants.

Physiological and Biochemical Traits of Two Major Arabidopsis Accessions, Col-0 and Ws, Under Salinity

Salinity affects plant growth and development as shown with the glycophyte model plant, Arabidopsis thaliana (Arabidopsis). Two Arabidopsis accessions, Wassilewskija (Ws) and Columbia (Col-0), are widely used to generate mutants available from various Arabidopsis seed resources. However, these two ecotypes are known to be salt-sensitive with different degrees of tolerance. In our study, 3-week-old Col-0 and Ws plants were treated with and without 150 mM NaCl for 48, 72, or 96 h, and several physiological and biochemical traits were characterized on shoots to identify any specific traits in their tolerance to salinity. Before salt treatment was carried out, a different phenotype was observed between Col-0 and Ws, whose main inflorescence stem became elongated in contrast to Col-0, which only displayed rosette leaves. Our results showed that Col-0 and Ws were both affected by salt stress with limited growth associated with a reduction in nutrient uptake, a degradation of photosynthetic pigments, an increase in protein degradation, as well as showing changes in carbohydrate metabolism and cell wall composition. These traits were often more pronounced in Col-0 and occurred usually earlier than in Ws. Tandem Mass Tags quantitative proteomics data correlated well with the physiological and biochemical results. The Col-0 response to salt stress was specifically characterized by a greater accumulation of osmoprotectants such as anthocyanin, galactinol, and raffinose; a lower reactive oxygen detoxification capacity; and a transient reduction in galacturonic acid content. Pectin degradation was associated with an overaccumulation of the wall-associated kinase 1, WAK1, which plays a role in cell wall integrity (CWI) upon salt stress exposure. Under control conditions, Ws produced more antioxidant enzymes than Col-0. Fewer specific changes occurred in Ws in response to salt stress apart from a higher number of different fascilin-like arabinogalactan proteins and a greater abundance of expansin-like proteins, which could participate in CWI. Altogether, these data indicate that Col-0 and Ws trigger similar mechanisms to cope with salt stress, and specific changes are more likely related to the developmental stage than to their respective genetic background.

Oxidative Stress-Induced Alteration of Plant Central Metabolism

Oxidative stress is an integral component of various stress conditions in plants, and this fact largely determines the substantial overlap in physiological and molecular responses to biotic and abiotic environmental challenges. In this review, we discuss the alterations in central metabolism occurring in plants experiencing oxidative stress. To focus on the changes in metabolite profile associated with oxidative stress per se, we primarily analyzed the information generated in the studies based on the exogenous application of agents, inducing oxidative stress, and the analysis of mutants displaying altered oxidative stress response. Despite of the significant variation in oxidative stress responses among different plant species and tissues, the dynamic and transient character of stress-induced changes in metabolites, and the strong dependence of metabolic responses on the intensity of stress, specific characteristic changes in sugars, sugar derivatives, tricarboxylic acid cycle metabolites, and amino acids, associated with adaptation to oxidative stress have been detected. The presented analysis of the available data demonstrates the oxidative stress-induced redistribution of metabolic fluxes targeted at the enhancement of plant stress tolerance through the prevention of ROS accumulation, maintenance of the biosynthesis of indispensable metabolites, and production of protective compounds. This analysis provides a theoretical basis for the selection/generation of plants with improved tolerance to oxidative stress and the development of metabolic markers applicable in research and routine agricultural practice.

Transcriptomic and metabolomic analyses of non-structural carbohydrates in red maple leaves

Plant sugars serve to balance nutrition, regulate development, and respond to biotic and abiotic stresses, whereas non-structural carbohydrates (NSCs) are essential energy sources that facilitate plant growth, metabolism, and environmental adaptation. To better elucidate the mechanisms of NSCs in red maple, ultrahigh-performance liquid chromatograph Q extractive mass spectrometry (UHPLC-QE-MS) and high-throughput RNA-sequencing were performed on green, red, and yellow leaves from a selected red maple mutant. In green leaves, the fructose phosphorylation process exhibited greater flux. In yellow leaves, sucrose and starch had a stronger capacity for synthesis and degradation, whereas in red leaves, there was a greater accumulation of trehalose and manninotriose. ArTPS5 positively regulated amylose, which was negatively regulated by ArFBP2, whereas ArFRK2 and ArFBP13 played a positive role in the biosynthesis of Sucrose-6P. Sucrose-6P also regulated anthocyanins and abscisic acid in red maple by affecting transcription factors. The results of this paper can assist with the control and optimization of the biosynthesis of NSCs in red maple, which may ultimately provide the foundation for influencing sugar production in Acer.

Improved resource allocation and stabilization of yield under abiotic stress

Sugars are the main building blocks for carbohydrate storage, but also serve as signaling molecules and protective compounds during abiotic stress responses. Accordingly, sugar transport proteins fulfill multiple roles as they mediate long distance sugar allocation, but also shape the subcellular and tissue-specific carbohydrate profiles by balancing the levels of these molecules in various compartments. Accordingly, transporter activity represents a target by classical or directed breeding approaches, to either, directly increase phloem loading or to increase sink strength in crop species. The relative subcellular distribution of sugars is critical for molecular signaling affecting yield-relevant processes like photosynthesis, onset of flowering and stress responses, while controlled long-distance sugar transport directly impacts development and productivity of plants. However, long-distance transport is prone to become unbalanced upon adverse environmental conditions. Therefore, we highlight the influence of stress stimuli on sucrose transport in the phloem and include the role of stress induced cellular carbohydrate sinks, like raffinose or fructans, which possess important roles to build up tolerance against challenging environmental conditions. In addition, we report on recent breeding approaches that resulted in altered source and sink capacities, leading to increased phloem sucrose shuttling in crops. Finally, we present strategies integrating the need of cellular stress-protection into the general picture of long-distance transport under abiotic stress, and point to possible approaches improving plant performance and resource allocation under adverse environmental conditions, leading to stabilized or even increased crop yield.

Cold-Triggered Induction of ROS- and Raffinose Metabolism in Freezing-Sensitive Taproot Tissue of Sugar Beet

Sugar beet (Beta vulgaris subsp. vulgaris) is the exclusive source of sugar in the form of sucrose in temperate climate zones. Sugar beet is grown there as an annual crop from spring to autumn because of the damaging effect of freezing temperatures to taproot tissue. A collection of hybrid and non-hybrid sugar beet cultivars was tested for winter survival rates and freezing tolerance. Three genotypes with either low or high winter survival rates were selected for detailed study of their response to frost. These genotypes differed in the severity of frost injury in a defined inner region in the upper part of the taproot, the so-called pith. We aimed to elucidate genotype- and tissue-dependent molecular processes during freezing and combined analyses of sugar beet anatomy and physiology with transcriptomic and metabolite profiles of leaf and taproot tissues at low temperatures. Freezing temperatures induced strong downregulation of photosynthesis in leaves, generation of reactive oxygen species (ROS), and ROS-related gene expression in taproots. Simultaneously, expression of genes involved in raffinose metabolism, as well as concentrations of raffinose and its intermediates, increased markedly in both leaf and taproot tissue at low temperatures. The accumulation of raffinose in the pith tissue correlated with freezing tolerance of the three genotypes. We discuss a protective role for raffinose and its precursors against freezing damage of sugar beet taproot tissue.

Biochemical characterization of recombinant UDP-sugar pyrophosphorylase and galactinol synthase from Brachypodium distachyon

Raffinose (Raf) protects plant cells during seed desiccation and under different abiotic stress conditions. The biosynthesis of Raf starts with the production of UDP-galactose by UDP-sugar pyrophosphorylase (USPPase) and continues with the synthesis of galactinol by galactinol synthase (GolSase). Galactinol is then used by Raf synthase to produce Raf. In this work, we report the biochemical characterization of USPPase (BdiUSPPase) and GolSase 1 (BdiGolSase1) from Brachypodium distachyon. The catalytic efficiency of BdiUSPPase was similar with galactose 1-phosphate and glucose 1-phosphate, but 5- to 17-fold lower with other sugar 1-phosphates. The catalytic efficiency of BdiGolSase1 with UDP-galactose was three orders of magnitude higher than with UDP-glucose. A structural model of BdiGolSase1 allowed us to determine the residues putatively involved in the binding of substrates. Among these, we found that Cys²⁶¹ lies within the putative catalytic pocket. BdiGolSase1 was inactivated by oxidation with diamide and H₂O₂. The activity of the diamide-oxidized enzyme was recovered by reduction with dithiothreitol or E. coli thioredoxin, suggesting that BdiGolSase1 is redox-regulated.

Subcellular Compartmentation of Metabolites Involved in Cold Acclimation

Plant cells are heavily compartmentalized, and metabolite concentrations in the various compartments differ significantly. Thus, determination of metabolite abundance in whole-cell extracts may be misleading, when the role of a compound in plant freezing tolerance shall be evaluated. Here, we describe a method for the separation of the largest compartments of plant cells, the vacuole, plastid, and cytosol. With more elaborate analysis, this method can be expanded to also resolve mitochondria and other compartments.

Raffinose synthase enhances drought tolerance through raffinose synthesis or galactinol hydrolysis in maize and Arabidopsis plants

Raffinose and its precursor galactinol accumulate in plant leaves during abiotic stress. RAFFINOSE SYNTHASE (RAFS) catalyzes raffinose formation by transferring a galactosyl group of galactinol to sucrose. However, whether RAFS contributes to plant drought tolerance and, if so, by what mechanism remains unclear. In this study, we report that expression of RAFS from maize (or corn, Zea mays) (ZmRAFS) is induced by drought, heat, cold, and salinity stresses. We found that zmrafs mutant maize plants completely lack raffinose and hyper-accumulate galactinol and are more sensitive to drought stress than the corresponding null-segregant (NS) plants. This indicated that ZmRAFS and its product raffinose contribute to plant drought tolerance. ZmRAFS overexpression in Arabidopsis enhanced drought stress tolerance by increasing myo-inositol levels via ZmRAFS-mediated galactinol hydrolysis in the leaves due to sucrose insufficiency in leaf cells and also enhanced raffinose synthesis in the seeds. Supplementation of sucrose to detached leaves converted ZmRAFS from hydrolyzing galactinol to synthesizing raffinose. Taken together, we demonstrate that ZmRAFS enhances plant drought tolerance through either raffinose synthesis or galactinol hydrolysis, depending on sucrose availability in plant cells. These results provide new avenues to improve plant drought stress tolerance through manipulation of the raffinose anabolic pathway.

The Consequences of a Disruption in Cyto-Nuclear Coadaptation on the Molecular Response to a Nitrate Starvation in Arabidopsis

Mitochondria and chloroplasts are important actors in the plant nutritional efficiency. So, it could be expected that a disruption of the coadaptation between nuclear and organellar genomes impact plant response to nutrient stresses. We addressed this issue using two Arabidopsis accessions, namely Ct1 and Jea, and their reciprocal cytolines possessing the nuclear genome from one parent and the organellar genomes of the other one. We measured gene expression, and quantified proteins and metabolites under N starvation and non-limiting conditions. We observed a typical response to N starvation at the phenotype and molecular levels. The phenotypical response to N starvation was similar in the cytolines compared to the parents. However, we observed an effect of the disruption of genomic coadaptation at the molecular levels, distinct from the previously described responses to organellar stresses. Strikingly, genes differentially expressed in cytolines compared to parents were mainly repressed in the cytolines. These genes encoded more mitochondrial and nuclear proteins than randomly expected, while N starvation responsive ones were enriched in genes for chloroplast and nuclear proteins. In cytolines, the non-coadapted cytonuclear genomic combination tends to modulate the response to N starvation observed in the parental lines on various biological processes.

ZmDREB1A Regulates RAFFINOSE SYNTHASE Controlling Raffinose Accumulation and Plant Chilling Stress Tolerance in Maize

Raffinose accumulation is positively correlated with plant chilling stress tolerance; however, the understanding of the function and regulation of raffinose metabolism under chilling stress remains in its infancy. RAFFINOSE SYNTHASE (RAFS) is the key enzyme for raffinose biosynthesis. In this study, we report that two independent maize (Zea mays) zmrafs mutant lines, in which raffinose was completely abolished, were more sensitive to chilling stress and their net photosynthetic product (total soluble sugars and starch) accumulation was significantly decreased compared with controls after chilling stress. A similar characterization of the maize dehydration responsive element (DRE)-binding protein 1A mutant (zmdreb1a) showed that ZmRAFS expression and raffinose content were significantly decreased compared with its control under chilling stress. Overexpression of maize ZmDREB1A in maize leaf protoplasts increased ZmDREB1A amounts, which consequently upregulated the expression of maize ZmRAFS and the Renilla LUCIFERASE (Rluc), which was controlled by the ZmRAFS promoter. Deletion of the single dehydration-responsive element (DRE) in the ZmRAFS promoter abolished ZmDREB1A's influence on Rluc expression, while addition of three copies of the DRE in the ZmRAFS promoter dramatically increased Rluc expression when ZmDREB1A was simultaneously overexpressed. Electrophoretic mobility shift assays and chromatin immunoprecipitation-quantitative PCR demonstrated that ZmDREB1A directly binds to the DRE motif in the promoter of ZmRAFS both in vitro and in vivo. These data demonstrate that ZmRAFS, which was directly regulated by ZmDREB1A, enhances both raffinose biosynthesis and plant chilling stress tolerance.

Fructan and antioxidant metabolisms in plants of Lolium perenne under drought are modulated by exogenous nitric oxide

Drought is a major environmental factor that can trigger oxidative stress and affect plant growth and productivity. Previous studies have shown that exogenous nitric oxide (NO) can minimize oxidative stress-related damage through the modulation of antioxidant enzyme activity. Fructan accumulation also has an important role in drought tolerance, since these carbohydrates participate in osmoregulation, membrane protection and oxidant scavenging. Currently, there are few studies investigating NO-regulated fructan metabolism in response to abiotic stresses. In the present study, we sought to determine if treating plants of Lolium perenne with S-nitrosoglutathione (GSNO), a NO donor, improved drought tolerance. Two-month-old plants received water (control), GSNO and reduced glutathione (GSH) as foliar spray treatments and were then maintained under drought or well-watered conditions for 23 days. At the end of drought period, we evaluated growth, pigment content and antioxidant and fructan metabolisms. None of these conditions influenced dry mass accumulation, but the leaves of plants treated with GSNO exhibited a slight increase in pigment content under drought. GSNO treatment also induced 1-SST activity, which was associated with a 3-fold increase in fructan content. GSNO-treated plants presented higher GR activity and, consequently, increased GSH levels. L. perenne cv. AberAvon was relatively tolerant to the water stress condition employed herein, maintaining ROS homeostasis and mitigating oxidative stress, possibly due to fructan, ascorbate and glutathione pools.

Subcellular dynamics of proteins and metabolites under abiotic stress reveal deferred response of the Arabidopsis thaliana hexokinase-1 mutant gin2-1 to high light

Stress responses in plants imply spatio-temporal changes in enzymes and metabolites, including subcellular compartment-specific re-allocation processes triggered by sudden changes in environmental parameters. To investigate interactions of primary metabolism with abiotic stress, the gin2-1 mutant, defective in the sugar sensor hexokinase 1 (HXK1) was compared with its wildtype Landsberg erecta (Ler) based on time resolved, compartment-specific metabolome and proteome data obtained over a full diurnal cycle. The high light sensitive gin2-1 mutant was substantially delayed in subcellular re-distribution of metabolites upon stress, and this correlated with a massive reduction in proteins belonging to the ATP producing electron transport chain under high light, while fewer changes occurred in the cold. In the wildtype, compounds specifically protecting individual compartments could be identified, e.g., maltose and raffinose in plastids, myo-inositol in mitochondria, but gin2-1 failed to recruit these substances to the respective compartments, or responded only slowly to high irradiance. No such delay was obtained in the cold. At the whole cell level, concentrations of the amino acids, glycine and serine, provided strong evidence for an important role of the photorespiratory pathway during stress exposure, and different subcellular allocation of serine may contribute to the slow growth of the gin2-1 mutant under high irradiance.

Dynamics of Plant Metabolism during Cold Acclimation

Plants have evolved strategies to tightly regulate metabolism during acclimation to a changing environment. Low temperature significantly constrains distribution, growth and yield of many temperate plant species. Exposing plants to low but non-freezing temperature induces a multigenic processes termed cold acclimation, which eventually results in an increased freezing tolerance. Cold acclimation comprises reprogramming of the transcriptome, proteome and metabolome and affects communication and signaling between subcellular organelles. Carbohydrates play a central role in this metabolic reprogramming. This review summarizes current knowledge about the role of carbohydrate metabolism in plant cold acclimation with a focus on subcellular metabolic reprogramming, its thermodynamic constraints under low temperature and mathematical modelling of metabolism.

Early stage metabolic events associated with the establishment of Vitis vinifera - Plasmopara viticola compatible interaction

Grapevine (Vitis vinifera L.) is the most widely cultivated and economically important fruit crop in the world, with 7.5 million of production area in 2017. The domesticated varieties of grapevine are highly susceptible to many fungal infections, of which downy mildew, caused by the biotrophic oomycete Plasmopara viticola (Berk. et Curt.) Berl. et de Toni is one of the most threatening. In V. vinifera, several studies have shown that a weak and transient activation of a defense mechanism occurs, but it is easily overcome by the pathogen leading to the establishment of a compatible interaction. Major transcript, protein and physiologic changes were shown to occur at later infection time-points, but comprehensive data on the first hours of interaction is scarce. In the present work, we investigated the major physiologic and metabolic changes that occur in the first 24 h of interaction between V. vinifera cultivar Trincadeira and P. viticola. Our results show that there was a negative modulation of several metabolic classes associated to pathogen defense such as flavonoids or phenylpropanoids as well as an alteration of carbohydrate content after inoculation with the pathogen. We also found an accumulation of hydrogen peroxide and increase of lipid peroxidation but to a low extent, that seems to be insufficient to restrain pathogen growth during the initial biotrophic phase of the interaction.

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.

A molecular perspective on starch metabolism in woody tissues

MAIN CONCLUSION

The elucidation of the molecular mechanisms of starch synthesis and mobilization in perennial woody tissues is of the utmost scientific and agricultural importance. Starch is the main carbohydrate reserve in plants and is fundamental in human nutrition and several industrial processes. In leaves, starch accumulated during the day is degraded throughout the night and the resulting sugars, glucose and maltose, are exported to the cytosol by the specialized transmembrane translocators pGT and MEX, respectively. Nevertheless, the degradation of the starch granule is a complex process not completely elucidated. While the mechanisms of starch mobilization during germination in the dead endosperm of cereal seeds are well described, the molecular and biochemical mechanisms involved in starch storage in the heterotrophic tissues of woody plants and its utilization in spring and winter are still puzzling. It is known that some biochemical steps of starch synthesis are conserved in heterotrophic tissues and in the leaves, but some aspects are particular to sink organs. From an agronomic standpoint, the knowledge on starch storage and mobilization in woody tissues is pivotal to understand (and to optimize) some common practices in the field that modify source-sink relationships, such as pruning and defoliation. Soluble sugars resulting from starch are also pivotal to cold adaptation, and in several fruits, such as banana and kiwifruit, starch may provide soluble sugars during ripening. In this review, we explore the recent advances on the molecular mechanisms and regulations involved in starch synthesis and mobilization, with a focus on perennial woody tissues.

Metabolic process of raffinose family oligosaccharides during cold stress and recovery in cucumber leaves

Raffinose family oligosaccharides (RFOs) accumulate under stress conditions in many plants and have been suggested to act as stress protectants. To elucidate the metabolic process of RFOs under cold stress, levels of RFOs, and related carbohydrates, the expression and activities of main metabolic enzymes and their subcellular compartments were investigated during low-temperature treatment and during the recovery period in cucumber leaves. Cold stress induced the accumulation of stachyose in vacuoles, galactinol in vacuoles and cytosol, and sucrose and raffinose in vacuoles, cytosol, and chloroplasts. After cold stress removal, levels of these sugars decreased gradually in the respective compartments. Among four galactinol synthase genes (CsGS), CsGS1 was not affected by cold stress, while the other three CsGSs were up-regulated by low temperature. RNA levels of acid-α-galactosidase (GAL) 3 and alkaline-α-galactosidase (AGA) 2 and 3, and the activities of GAL and AGA, were up-regulated after cold stress removal. GAL3 protein and GAL activity were exclusively located in vacuoles, whereas AGA2 and AGA 3 proteins were found in cytosol and chloroplasts, respectively. The results indicate that RFOs, which accumulated during cold stress in different subcellular compartments in cucumber leaves, could be catabolized in situ by different galactosidases after stress removal.

Sugar compartmentation as an environmental stress adaptation strategy in plants

The sessile nature of plants has driven their evolution to cope flexibly with ever-changing surrounding environments. The development of stress tolerance traits is complex, and a broad range of cellular processes are involved. Recent studies have revealed that sugar transporters contribute to environmental stress tolerance in plants, suggesting that sugar flow is dynamically fluctuated towards optimization of cellular conditions in adverse environments. Here, we highlight sugar compartmentation mediated by sugar transporters as an adaptation strategy against biotic and abiotic stresses. Competition for sugars between host plants and pathogens shapes their evolutionary arms race. Pathogens, which rely on host-derived carbon, manipulate plant sugar transporters to access sugars easily, while plants sequester sugars from pathogens by enhancing sugar uptake activity. Furthermore, we discuss pathogen tactics to circumvent sugar competition with host plants. Sugar transporters also play a role in abiotic stress tolerance. Exposure to abiotic stresses such as cold or drought stress induces sugar accumulation in various plants. We also discuss how plants allocate sugars under such conditions. Collectively, these findings are relevant to basic plant biology as well as potential applications in agriculture, and provide opportunities to improve crop yield for a growing population.

Subcellular reprogramming of metabolism during cold acclimation in Arabidopsis thaliana

Metabolite changes in plant leaves during exposure to low temperatures involve re-allocation of a large number of metabolites between sub-cellular compartments. Therefore, metabolite determination at the whole cell level may be insufficient for interpretation of the functional significance of cellular compounds. To investigate the cold-induced metabolite dynamics at the level of individual sub-cellular compartments, an integrative platform was developed that combines quantitative metabolite profiling by gas chromatography coupled to mass spectrometry (GC-MS) with the non-aqueous fractionation technique allowing separation of cytosol, vacuole and the plastidial compartment. Two mutants of Arabidopsis thaliana representing antipodes in the diversion of carbohydrate metabolism between sucrose and starch were compared to Col-0 wildtype before and after cold acclimation to investigate interactions of cold acclimation with subcellular re-programming of metabolism. A multivariate analysis of the data set revealed dominant effects of compartmentation on metabolite concentrations that were modulated by environmental condition and genetic determinants. While for both, the starchless mutant of plastidial phospho-gluco mutase (pgm) and a mutant defective in sucrose-phosphate synthase A1, metabolic constraints, especially at low temperature, could be uncovered based on subcellularly resolved metabolite profiles, only pgm had lowered freezing tolerance. Metabolic profiles of pgm point to redox imbalance as a possible reason for reduced cold acclimation capacity.

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.

Metabolite transport and associated sugar signalling systems underpinning source/sink interactions

Metabolite transport between organelles, cells and source and sink tissues not only enables pathway co-ordination but it also facilitates whole plant communication, particularly in the transmission of information concerning resource availability. Carbon assimilation is co-ordinated with nitrogen assimilation to ensure that the building blocks of biomass production, amino acids and carbon skeletons, are available at the required amounts and stoichiometry, with associated transport processes making certain that these essential resources are transported from their sites of synthesis to those of utilisation. Of the many possible posttranslational mechanisms that might participate in efficient co-ordination of metabolism and transport only reversible thiol-disulphide exchange mechanisms have been described in detail. Sucrose and trehalose metabolism are intertwined in the signalling hub that ensures appropriate resource allocation to drive growth and development under optimal and stress conditions, with trehalose-6-phosphate acting as an important signal for sucrose availability. The formidable suite of plant metabolite transporters provides enormous flexibility and adaptability in inter-pathway coordination and source-sink interactions. Focussing on the carbon metabolism network, we highlight the functions of different transporter families, and the important of thioredoxins in the metabolic dialogue between source and sink tissues. In addition, we address how these systems can be tailored for crop improvement.

OEP40, a Regulated Glucose-permeable β-Barrel Solute Channel in the Chloroplast Outer Envelope Membrane

Chloroplasts and mitochondria are unique endosymbiotic cellular organelles surrounded by two membranes. Essential metabolic networking between these compartments and their hosting cells requires the exchange of a large number of biochemical pathway intermediates in a directed and coordinated fashion across their inner and outer envelope membranes. Here, we describe the identification and functional characterization of a highly specific, regulated solute channel in the outer envelope of chloroplasts, named OEP40. Loss of OEP40 function in Arabidopsis thaliana results in early flowering under cold temperature. The reconstituted recombinant OEP40 protein forms a high conductance β-barrel ion channel with subconductant states in planar lipid bilayers. The OEP40 channel is slightly cation-selective PK+/PCl- ≈ 4:1 and rectifying (i⃗/i⃖ ≅ 2) with a slope conductance of Ḡmax ≅ 690 picosiemens. The OEP40 channel has a restriction zone diameter of ≅1.4 nm and is permeable for glucose, glucose 1-phosphate and glucose 6-phosphate, but not for maltose. Moreover, channel properties are regulated by trehalose 6-phosphate, which cannot permeate. Altogether, our results indicate that OEP40 is a "glucose-gate" in the outer envelope membrane of chloroplasts, facilitating selective metabolite exchange between chloroplasts and the surrounding cell.

PtrA/NINV, an alkaline/neutral invertase gene of Poncirus trifoliata, confers enhanced tolerance to multiple abiotic stresses by modulating ROS levels and maintaining photosynthetic efficiency

BACKGROUND

Alkaline/neutral invertase (A/N-INV), an enzyme that hydrolyzes sucrose irreversibly into glucose and fructose, is essential for normal plant growth,development, and stress tolerance. However, the physiological and/or molecular mechanism underpinning the role of A/N-INV in abiotic stress tolerance is poorly understood.

RESULTS

In this report, an A/N-INV gene (PtrA/NINV) was isolated from Poncirus trifoliata, a cold-hardy relative of citrus, and functionally characterized. PtrA/NINV expression levels were induced by cold, salt, dehydration, sucrose, and ABA, but decreased by glucose. PtrA/NINV was found to localize in both chloroplasts and mitochondria. Overexpression of PtrA/NINV conferred enhanced tolerance to multiple stresses, including cold, high salinity, and drought, as supported by lower levels of reactive oxygen species (ROS), reduced oxidative damages, decreased water loss rate, and increased photosynthesis efficiency, relative to wild-type (WT). The transgenic plants exhibited higher A/N-INV activity and greater reducing sugar content under normal and stress conditions.

CONCLUSIONS

PtrA/NINV is an important gene implicated in sucrose decomposition, and plays a positive role in abiotic stress tolerance by promoting osmotic adjustment, ROS detoxification and photosynthesis efficiency. Thus, PtrA/NINV has great potential to be used in transgenic breeding for improvement of stress tolerance.

Presence of Inulin-Type Fructo-Oligosaccharides and Shift from Raffinose Family Oligosaccharide to Fructan Metabolism in Leaves of Boxtree (Buxus sempervirens)

Fructans are known to occur in 15% of flowering plants and their accumulation is often associated with stress responses. Typically, particular fructan types occur within particular plant families. The family of the Buxaceae, harboring Pachysandra terminalis, an accumulator of graminan- and levan-type fructans, also harbors boxtree (Buxus sempervirens), a cold and drought tolerant species. Surprisingly, boxtree leaves do not accumulate the expected graminan- and levan-type fructans, but small inulin fructo-oligosaccharides (FOS: 1-kestotriose and nystose) and raffinose family oligosaccharides (RFOs: raffinose and stachyose) instead. The seasonal variation in concentrations of glucose, fructose, sucrose, FOS and RFOs were followed. Raffinose and stachyose peaked during the winter months, while FOS peaked at a very narrow time-interval in spring, immediately preceded by a prominent sucrose accumulation. Sucrose may function as a reserve carbohydrate in winter and early spring leaves. The switch from RFO to fructan metabolism in spring strongly suggests that fructans and RFOs fulfill distinct roles in boxtree leaves. RFOs may play a key role in the cold acclimation of winter leaves while temporal fructan biosynthesis in spring might increase sink strength to sustain the formation of new shoots.

Differential Metabolic Rearrangements after Cold Storage Are Correlated with Chilling Injury Resistance of Peach Fruits

Reconfiguration of the metabolome is a key component involved in the acclimation to cold in plants; however, few studies have been devoted to the analysis of the overall metabolite changes after cold storage of fruits prior to consumption. Here, metabolite profiling of six peach varieties with differential susceptibility to develop mealiness, a chilling-injury (CI) symptom, was performed. According to metabolic content at harvest; after cold treatment; and after ripening, either following cold treatment or not; peach fruits clustered in distinct groups, depending on harvest-time, cold treatment, and ripening state. Both common and distinct metabolic responses among the six varieties were found; common changes including dramatic galactinol and raffinose rise; GABA, Asp, and Phe increase; and 2-oxo-glutarate and succinate decrease. Raffinose content after long cold treatment quantitatively correlated to the degree of mealiness resistance of the different peach varieties; and thus, raffinose emerges as a candidate biomarker of this CI disorder. Xylose increase after cold treatment was found only in the susceptible genotypes, indicating a particular cell wall reconfiguration of these varieties while being cold-stored. Overall, results indicate that peach fruit differential metabolic rearrangements due to cold treatment, rather than differential metabolic priming before cold, are better related with CI resistance. The plasticity of peach fruit metabolism renders it possible to induce a diverse metabolite array after cold, which is successful, in some genotypes, to avoid CI.

Accumulation of extra-chloroplastic triacylglycerols in Arabidopsis seedlings during heat acclimation

Heat acclimation enables plants to tolerate and survive short-term heat stress on hot days. In Arabidopsis thaliana, a genetically programmed heat shock response can be rapidly triggered in the temperature range of 32-38°C through activation of heat shock transcription factors (HSF). The heat shock response leads to heat acclimation and confers short-term protection against temperatures above 40°C. However, little is known about metabolic adjustments during heat acclimation.Untargeted metabolite analyses of A. thaliana seedlings revealed that levels of polyunsaturated triacylglycerols (TG) rapidly and dramatically increase during heat acclimation. TG accumulation was found to be temperature dependent in a temperature range of 32-50°C (optimum at 42°C) and reversible after a return from 37°C to normal growth temperatures. Heat-induced TGs accumulated in extra-chloroplastic compartments and increased in both roots and shoots to a similar extent. Analysis of mutants deficient in all four HSFA1 master regulator genes or the HSFA2 gene revealed that TG accumulation was not dependent on HSFs. Moreover, the TG response was not limited to heat stress because drought and salt stress also triggered an accumulation of TGs, but not short-term osmotic, cold, and high light stress. Lipid analysis revealed that heat-induced accumulation of TGs was not due to massive de novo fatty acid synthesis. It is hypothesized that TGs serve as transient stores for fatty acids that may be required for membrane remodelling during heat acclimation.

Sugars as hydroxyl radical scavengers: proof-of-concept by studying the fate of sucralose in Arabidopsis

Substantial formation of reactive oxygen species (ROS) is inevitable in aerobic life forms. Due to their extremely high reactivity and short lifetime, hydroxyl radicals are a special case, because cells have not developed enzymes to detoxify these most dangerous ROS. Thus, scavenging of hydroxyl radicals may only occur by accumulation of higher levels of simple organic compounds. Previous studies have demonstrated that plant-derived sugars show hydroxyl radical scavenging capabilities during Fenton reactions with Fe(2+) and hydrogen peroxide in vitro, leading to formation of less detrimental sugar radicals that may be subject of regeneration to non-radical carbohydrates in vivo. Here, we provide further evidence for the occurrence of such radical reactions with sugars in planta, by following the fate of sucralose, an artificial analog of sucrose, in Arabidopsis tissues. The expected sucralose recombination and degradation products were detected in both normal and stressed plant tissues. Oxidation products of endogenous sugars were also assessed in planta for Arabidopsis and barley, and were shown to increase in abundance relative to the non-oxidized precursor during oxidative stress conditions. We concluded that such non-enzymatic reactions with hydroxyl radicals form an integral part of plant antioxidant mechanisms contributing to cellular ROS homeostasis, and may be more important than generally assumed. This is discussed in relation to the recently proposed roles for Fe(2+) and hydrogen peroxide in processes leading to the origin of metabolism and the origin of life.

Galactinol synthase transcriptional profile in two genotypes of Coffea canephora with contrasting tolerance to drought

Increased synthesis of galactinol and raffinose family oligosaccharides (RFOs) has been reported in vegetative tissues in response to a range of abiotic stresses. In this work, we evaluated the transcriptional profile of a Coffea canephora galactinol synthase gene (CcGolS1) in two clones that differed in tolerance to water deficit in order to assess the contribution of this gene to drought tolerance. The expression of CcGolS1 in leaves was differentially regulated by water deficit, depending on the intensity of stress and the genotype. In clone 109A (drought-susceptible), the abundance of CcGolS1 transcripts decreased upon exposure to drought, reaching minimum values during recovery from severe water deficit and stress. In contrast, CcGolS1 gene expression in clone 14 (drought-tolerant) was stimulated by water deficit. Changes in galactinol and RFO content did not correlate with variation in the steady-state transcript level. However, the magnitude of increase in RFO accumulation was higher in the tolerant cultivar, mainly under severe water deficit. The finding that the drought-tolerant coffee clone showed enhanced accumulation of CcGolS1 transcripts and RFOs under water deficit suggests the possibility of using this gene to improve drought tolerance in this important crop.