Sugar compartmentation as an environmental stress adaptation strategy in plants

Snowpack permanence shapes the growth and dynamic of non-structural carbohydrates in Juniperus communis in alpine tundra

Climate warming is altering snowpack permanence in alpine tundra, modifying shrub growth and distribution. Plant acclimation to snowpack changes depends on the capability to guarantee growth and carbon storage, suggesting that the content of non-structural carbohydrates (NSC) in plant organs can be a key trait to depict the plant response under different snow regimes. To test this hypothesis, we designed a 3-years long manipulative experiment aimed at evaluating the effect of snow melt timing (i.e., early, control, and late) on NSC content in needles, bark and wood of Juniperus communis L. growing at high elevation in the Alps. Starch evidenced a general decrease from late spring to summer in control and early melting, while starch was low but stable in plants subjected to a late snow melt. Leaves, bark and wood have different level of soluble NSC changing during growing season: in bark, sugars content decreased significantly in late summer, while there was no seasonal effect in needles and wood. Soluble NSC and starch were differently related with the plant growth, when considering different tissues and snow treatment. In leaf and bark we observed a starch depletion in control and early melting plants, consistently to a higher growth (i.e., twig elongation), while in late snow melt, we did not find any significant relationship between growth and NSC concentration. Our findings confirmed that snowpack duration affects the onset of the growing season promoting a change in carbon allocation in plant organs and, between bark and wood in twigs. Finally, our results suggest that plants, at this elevation, could take advantage from an early snow melt caused by climate warming, most likely due to photosynthetic activity by maintaining the level of reserves and enhancing the carbon investment for growth.

Interplay between phosphorylation and oligomerization tunes the conformational ensemble of SWEET transporters

SWEET sugar transporters are desirable biotechnological targets for improving plant growth. One engineering strategy includes modulating how SWEET transporters are regulated. Phosphorylation and oligomerization have been shown to positively regulate SWEET function, leading to increased sugar transport activity. However, constitutive phosphorylation may not be beneficial to plant health under basal conditions. Structural and mechanistic understanding of the interplay between phosphorylation and oligomerization in functional regulation of SWEETs remains limited. Using extensive molecular dynamics simulations coupled with Markov state models, we demonstrate the thermodynamic and kinetic effects of SWEET phosphorylation and oligomerization using OsSWEET2b as a model. We report that the beneficial effects of these SWEET regulatory mechanisms bias outward-facing states and improved extracellular gating, which complement published experimental findings. Our results offer molecular insights to SWEET regulation and may guide engineering strategies throughout the SWEET transport family.

Structure, evolution, and roles of SWEET proteins in growth and stress responses in plants

Carbohydrates are exported by the SWEET family of transporters, which is a novel class of carriers that can transport sugars across cell membranes and facilitate sugar's long-distance transport from source to sink organs in plants. SWEETs play crucial roles in a wide range of physiologically important processes by regulating apoplastic and symplastic sugar concentrations. These processes include host-pathogen interactions, abiotic stress responses, and plant growth and development. In the present review, we (i) describe the structure and organization of SWEETs in the cell membrane, (ii) discuss the roles of SWEETs in sugar loading and unloading processes, (iii) identify the distinct functions of SWEETs in regulating plant growth and development including flower, fruit, and seed development, (iv) shed light on the importance of SWEETs in modulating abiotic stress resistance, and (v) describe the role of SWEET genes during plant-pathogen interaction. Finally, several perspectives regarding future investigations for improving the understanding of sugar-mediated plant defenses are proposed.

Brown planthopper infestation on rice reduces plant susceptibility to Meloidogyne graminicola by reducing root sugar allocation

Plants are simultaneously attacked by different pests that rely on sugars uptake from plants. An understanding of the role of plant sugar allocation in these multipartite interactions is limited. Here, we characterized the expression patterns of sucrose transporter genes and evaluated the impact of targeted transporter gene mutants and brown planthopper (BPH) phloem-feeding and oviposition on root sugar allocation and BPH-reduced rice susceptibility to Meloidogyne graminicola. We found that the sugar transporter genes OsSUT1 and OsSUT2 are induced at BPH oviposition sites. OsSUT2 mutants showed a higher resistance to gravid BPH than to nymph BPH, and this was correlated with callose deposition, as reflected in a different effect on M. graminicola infection. BPH phloem-feeding caused inhibition of callose deposition that was counteracted by BPH oviposition. Meanwhile, this pivotal role of sugar allocation in BPH-reduced rice susceptibility to M. graminicola was validated on rice cultivar RHT harbouring BPH resistance genes Bph3 and Bph17. In conclusion, we demonstrated that rice susceptibility to M. graminicola is regulated by BPH phloem-feeding and oviposition on rice through differences in plant sugar allocation.

Transcriptomic and physiological effects of polyethylene microplastics on Zea mays seedlings and their role as a vector for organic pollutants

The widespread employment of plastics in recent decades has resulted in the accumulation of plastic residues in all ecosystems. Their presence and degradation into small particles such as microplastics (MPs) may have a negative effect on plant development and therefore on crop production. In this study, the effects of two types of polyethylene MPs on Zea mays seedlings cultured in vitro were analysed. In addition, four organic pollutants (ibuprofen, simazine, sertraline, and amoxicillin) were adsorbed by the MPs to evaluate their capacity as other contaminant vectors. The development of the plants was negatively affected by MPs alone or with the organic compounds. The strongest effect was observed in the W-MPs treatments, with a reduction in leaf and root length near 70%. Chlorophyll content was also differentially affected depending on the treatment. Transcriptome analysis showed that MPs affected gene expression in the roots of maize seedlings. As observed in the physiological parameters analysed, some gene expression changes were associated with specific treatments, such as changes in sugar transport genes in the B-MIX treatment. These results contribute to a better understanding of the molecular mechanisms of plants in regard to plastic stress responses.

Transcriptome Profiling during Sequential Stages of Cryopreservation in Banana (Musa AAA cv Borjahaji) Shoot Meristem

Cryopreservation approaches have been implemented in gene banks as a strategy to back up plant genetic resource collections that are vegetatively propagated. Different strategies have been employed to effectively cryopreserve plant tissue. There is little information on the cellular processes and molecular adjustments that confer resilience to the multiple stresses imposed during a cryoprotocol. In the present work, the cryobionomics of banana (Musa sp.), a non-model species, was investigated through the transcriptomic approach using RNA-Seq. Proliferating meristems of in vitro explants (Musa AAA cv 'Borjahaji') were cryopreserved using the droplet-vitrification technique. Transcriptome profiling analysis of eight cDNA libraries including the bio-replicates for T0 (stock cultures (control tissue), T1 (high sucrose pre-cultured), T2 (vitrification solution-treated) and T3 (liquid nitrogen-treated) meristem tissues was carried out. The raw reads obtained were mapped with a Musa acuminata reference genome sequence. A total of 70 differentially expressed genes (DEGs) comprising 34 upregulated and 36 downregulated were identified in all three phases as compared to control (T0). Among the significant DEGs (>log FC 2.0), during sequential steps, 79 in T1, 3 in T2 and the 4 in T3 were upregulated and 122 in T1, 5 in T2 and 9 in T3 were downregulated. Gene ontology (GO) enrichment analysis showed that these significant DEGs were involved in the upregulation of biological process (BP-170), cellular component (CC-10) and molecular function (MF-94) and downregulation of biological process (BP-61), cellular component (CC-3) and molecular function (MF-56). The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that DEGs were involved in the biosynthesis of secondary metabolites, glycolysis/gluconeogenesis, MAPK signaling, EIN 3-lke 1 protein, 3-ketoacy-CoA synthase 6-like, and fatty acid elongation during cryopreservation. For the first time, a comprehensive transcript profiling during four stages of cryopreservation in banana were carried out, which will pave the way for devising an effective cryopreservation protocol.

Responses of sorghum to cold stress: A review focused on molecular breeding

Climate change has led to the search for strategies to acclimatize plants to various abiotic stressors to ensure the production and quality of crops of commercial interest. Sorghum is the fifth most important cereal crop, providing several uses including human food, animal feed, bioenergy, or industrial applications. The crop has an excellent adaptation potential to different types of abiotic stresses, such as drought, high salinity, and high temperatures. However, it is susceptible to low temperatures compared with other monocotyledonous species. Here, we have reviewed and discussed some of the research results and advances that focused on the physiological, metabolic, and molecular mechanisms that determine sorghum cold tolerance to improve our understanding of the nature of such trait. Questions and opportunities for a comprehensive approach to clarify sorghum cold tolerance or susceptibility are also discussed.

Effects of high air temperature, drought, and both combinations on maize: A case study

High air temperature (HAT) and natural soil drought (NSD) have seriously affected crop yield and frequently take place in a HAT-NSD combination. Maize (Zea mays) is an important crop, thermophilic but not heat tolerant. In this study, HAT, NSD, and HAT-NSD effects on maize inbred line Huangzao4 -were characterized. Main findings were as follows: H₂O₂ and O^- accumulated much more in immature young leaves than in mature old leaves under the stresses. Lateral roots were highly distributed near the upper pot mix layers under HAT and near root tips under HAT-NSD. Saccharide accumulated mainly in stressed root caps (RC) and formed a highly accumulated saccharide band at the boundary between RC and meristematic zone. Lignin deposition was in stressed roots under NSD and HAT-NSD. Chloroplasts increased in number and formed a high-density ring around leaf vascular bundles (VB) under HAT and HAT-NSD, and sparsely scattered in the peripheral area of VBs under NSD. The RC cells containing starch granules were most under NAD-HAT but least under HAT. Under NSD and HAT-NSD followed by re-watering, anther number per tassel spikelet reduced to 3. These results provide multiple clues for further distinguishing molecular mechanisms for maize to tolerate these stresses.

Metabolomic analysis reveals key metabolites alleviating green spots under exogenous sucrose spraying in air-curing cigar tobacco leaves

Cigar variety CX-010 tobacco leaves produce localized green spots during the air-curing period, and spraying exogenous sucrose effectively alleviates the occurrence of the green spots. To investigate the alleviation effect of exogenous sucrose spraying, the total water content and the number and size of green spots on tobacco leaves were investigated during the air-curing period under four treatments; CK (pure water), T1 (0.1 M sucrose), T2 (0.2 M sucrose) and T3 (0.4 M sucrose). The results showed that the total water content of tobacco leaves showed a trend of T3 < CK < T2 < T1 in the early air-curing stage, and the number and size of green spots showed a trend of T3 < T2 < T1 < CK. All sucrose treatments alleviated the green spot phenomenon, and T3 had the fewest green spots. Thus, the tobacco leaves of the T3 and CK treatments at two air-curing stages were used to perform metabolomics analysis with nontargeted liquid chromatography‒mass spectrometry to determine the physiological mechanism. A total of 259 and 178 differentially abundant metabolites (DAMs) between T3- and CK-treated tobacco leaves were identified in the early air-curing and the end of air-curing stages, respectively. These DAMs mainly included lipid and lipid-like molecules, carbohydrates, and organic acids and their derivatives. Based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, the T3 treatment significantly altered carbohydrate metabolism (pentose phosphate pathway, sucrose and starch metabolism and galactose metabolism) and amino acid metabolism (tyrosine metabolism and tryptophan metabolism) in air-curing tobacco leaves. Sucrose treatment alleviated green spots by altering DAMs that affected chlorophyll degradation, such as tyrosine and citric acid, to promote the normal degradation of chlorophyll.

Transcriptome analysis of trembling aspen (Populus tremuloides) under nickel stress

Plants have evolved heavy metal tolerance mechanisms to adapt and cope with nickel (Ni) toxicity. Decrypting whole gene expression of Trembling Aspen (Pinus tremuloides) under nickel stress could elucidate the nickel resistance/tolerance mechanisms. The main objectives of the present research were to 1) characterize the P. tremuloides transcriptome, and 2) compare gene expression dynamics between nickel-resistant and nickel-susceptible P. tremuloides genotypes with Whole Transcriptome (WT) sequencing. Illumina Sequencing generated 27–45 million 2X150 paired-end reads of raw data per sample. The alignment performed with StringTie Software added two groups of transcripts to the draft genome annotation. One group contained 32,677 new isoforms that match to 17,254 genes. The second group contained 17,349 novel transcripts that represent 16,157 novel genes. Overall, 52,987 genes were identified from which 36,770 genes were selected as differently expressed. With the high stringency (two-fold change, FDR value ≤ 0.05 and logFC value ≥1 (upregulated) or ≤ -1 (downregulated), after GSEA analysis and filtering for gene set size, 575 gene sets were upregulated and 146 were downregulated in nickel resistant phenotypes compared to susceptible genotypes. For biological process, genes associated with translation were significantly upregulated while signal transduction and cellular protein process genes were downregulated in resistant compared to susceptible genotypes. For molecular function, there was a significant downregulation of genes associated with DNA binding in resistant compared to susceptible lines. Significant upregulation was observed in genes located in ribosome while downregulation of genes in chloroplast and mitochondrion were preponderant in resistant genotypes compared to susceptible. Hence, from a whole transcriptome level, an upregulation in ribosomal and translation activities was identified as the main response to Ni toxicity in the resistant plants. More importantly, this study revealed that a metal transport protein (Potrs038704g29436 –ATOX1-related copper transport) was among the top upregulated genes in resistant genotypes when compared to susceptible plants. Other identified upregulated genes associated with abiotic stress include genes coding for Dirigent Protein 10, GATA transcription factor, Zinc finger protein, Auxin response factor, Bidirectional sugar transporter, and thiamine thiazole synthase.

Genome-Wide Investigation and Characterization of SWEET Gene Family with Focus on Their Evolution and Expression during Hormone and Abiotic Stress Response in Maize

The sugar will eventually be exported transporters (SWEET) family is an important group of transport carriers for carbon partitioning in plants and has important functions in growth, development, and abiotic stress tolerance. Although the SWEET family is an important sugar transporter, little is known of the functions of the SWEET family in maize (Zea mays), especially in response to abiotic stresses. To further explore the response pattern of maize SWEET to abiotic stress, a bioinformatics-based approach was used to predict and identify the maize SWEET gene (ZmSWEET) family. Twenty-four ZmSWEET genes were identified using the MaizeGDB database. Phylogenetic analysis resolved these twenty-four genes into four clades. One tandem and five segmental duplication events were identified, which played a major role in ZmSWEET family expansion. Synteny analysis provided insight into the evolutionary characteristics of the ZmSWEET genes with those of three graminaceous crop species. A heatmap showed that most ZmSWEET genes responded to at least one type of abiotic stress. By an abscisic acid signaling pathway, among which five genes were significantly induced under NaCl treatment, eight were obviously up-regulated under PEG treatment and five were up-regulated under Cd stress, revealing their potential functions in response to abiotic stress. These findings will help to explain the evolutionary links of the ZmSWEET family and contribute to future studies on the functional characteristics of ZmSWEET genes, and then improve abiotic stress tolerance in maize through molecular breeding.

Genome-wide identification and expression profiling of sugar transporter genes in tobacco

Sugars are both nutrients and important signal molecules in higher plants. Sugar transporters (STs) are involved in sugar loading and unloading and facilitate sugar transport across membranes. Tobacco (Nicotiana tabacum) is a model plant and one of the most significant plants economically. In our research, 92 N. tabacum ST (NtST) genes were identified and classified into eight distinct subfamilies in the tobacco genome based on phylogenetic analysis. Exon-intron analysis revealed that each subfamily manifested closely associated gene architectural features based on a comparable number or length of exons. Tandem repetition and purifying selection were the main factors of NtST gene evolution. A search for cis-regulatory elements in the promoter sequences of the NtST gene families suggested that they are probably regulated by light, plant hormones, and abiotic stress factors. We performed a comprehensive expression study in different tissues, viarious abiotic and phytohormone stresses. The results revealed different expression patterns and the functional diversification of NtST genes. The resulting data showed that NtSFP1 was highly expressed all measured five tobacco tissues, and also regulated by the MeJA, and temperature stress. In addition, the virus-induced NibenSFP1 silencing in tobacco and detected dramatically enhanced glucose content, indicating the NtSFP1 might regulate the glucose content and involved in MeJA signaling way to response the temperature stress. In general, our findings provide useful information on understanding the roles of STs in phytohormone signaling way and abiotic stresses in N. tabacum.

Sugar transporters and their molecular tradeoffs during abiotic stress responses in plants

Sugars as photosynthates are well known as energy providers and as building blocks of various structural components of plant cells, tissues and organs. Additionally, as a part of various sugar signaling pathways, they interact with other cellular machinery and influence many important cellular decisions in plants. Sugar signaling is further reliant on the differential distribution of sugars throughout the plant system. The distribution of sugars from source to sink tissues or within organelles of plant cells is a highly regulated process facilitated by various sugar transporters located in plasma membranes and organelle membranes, respectively. Sugar distribution, as well as signaling, is impacted during unfavorable environments such as extreme temperatures, salt, nutrient scarcity, or drought. Here, we have discussed the mechanism of sugar transport via various types of sugar transporters as well as their differential response during environmental stress exposure. The functional involvement of sugar transporters in plant's abiotic stress tolerance is also discussed. Besides, we have also highlighted the challenges in engineering sugar transporter proteins as well as the undeciphered modules associated with sugar transporters in plants. Thus, this review provides a comprehensive discussion on the role and regulation of sugar transporters during abiotic stresses and enables us to target the candidate sugar transporter(s) for crop improvement to develop climate-resilient crops.

Xylella fastidiosa and Drought Stress in Olive Trees: A Complex Relationship Mediated by Soluble Sugars

Simple Summary

Carbohydrates play important roles in tolerance to both biotic and abiotic stressors. Xylella fastidiosa, the causal agent of “Olive Quick Decline Syndrome”, is a quarantine pathogen that induces drought stress in the host, aggravated by eventual water shortage, which is a frequent environmental condition in Mediterranean olive groves. At present, the resistance mechanisms shown by few resistant olive cultivars (e.g., cv Leccino) are not completely known; therefore, the aim of this research is to understand whether sugar metabolism is involved in the cross-talk mechanisms of biotic and abiotic responses. The results show that drought stress response induces effects beneficial to resistance of Xylella fastidiosa in cv Leccino. In the current context of global climate change, this study supports the importance of investigating the complex drought–disease interaction to detect resistance traits and thus find ways to counter the threat of this pathogen in the future.

Abstract

Xylella fastidiosa (Xf) subsp. pauca “De Donno” is the etiological agent of “Olive Quick Decline Syndrome” (OQDS) on olive trees (Olea europaea L.); the presence of the bacterium causes xylem vessel occlusions inducing a drought stress and the development of leaf scorch symptoms, which may be worsened by water shortage in summer. In order to evaluate how the two stress factors overlap each other, the carbohydrate content and the expression patterns of genes related to carbohydrate metabolism have been evaluated in two olive cvs trees (Cellina di Nardò, susceptible to Xf, and Leccino, resistant to Xf) reporting transcriptional dynamics elicited by Xf infection, drought, or combined stress (drought/Xf). In the Xf-susceptible Cellina di Nardò plants, Xf and its combination with drought significantly decrease total sugars compared to control (−27.0% and −25.7%, respectively). In contrast, the Xf-resistant Leccino plants show a more limited reduction in sugar content in Xf-positive conditions (−20.1%) and combined stresses (−11.1%). Furthermore, while the amount of glucose decreases significantly in stressed Cellina di Nardò plants (≈18%), an increase was observed in Leccino plants under drought/Xf combined stresses (+11.2%). An opposite behavior among cvs was also observed for sucrose, as an accumulation of the disaccharide was recorded in stressed Leccino plants (≈37%). The different response to combined stress by Xf-resistant plants was confirmed considering genes coding for the sucrose or monosaccharide transporter (OeSUT1, OeMST2), the cell wall or vacuolar invertase (OeINV-CW, OeINV-V), the granule-bound starch synthase I (OeGBSSI) and sucrose synthase (OeSUSY), with a higher expression than at least one single stress (e.g., ≈1-fold higher or more than Xf for OeMST2, OeINV-CW, OeINV-V, OeGBSSI). It is probable that the pathways involved in drought stress response induce positive effects useful for pathogen resistance in cv Leccino, confirming the importance of investigating the mechanisms of cross-talk of biotic and abiotic responses.

TMT based proteomic profiling of Sophora alopecuroides leaves reveal flavonoid biosynthesis processes in response to salt stress

Salt stress is the major abiotic stress worldwide, adversely affecting crop yield and quality. Utilizing salt tolerance genes for the genetic breeding of crops is one of the most effective measures to withstand salinization. Sophora alopecuroides is a well-known saline-alkaline and drought-tolerant medicinal plant. Understanding the underlying molecular mechanism for Sophora alopecuroides salt tolerance is crucial to identifying the salt-tolerant genes. In this study, we performed tandem mass tag (TMT) based proteomic profiling of S. alopecuroides leaves under 150 mM NaCl induced salt stress condition for 3 d and 7 d. Data are available on ProteomeXchange (PXD027627). Furthermore, the proteomic findings were validated through parallel reaction monitoring (PRM). We observed that the expression levels of several transporter proteins related to the secondary messenger signaling pathway were altered under salt stress conditions induced for 3 d. However, the expression of the certain transferase, oxidoreductase, dehydrogenase, which are involved in the biosynthesis of flavonoids, alkaloids, phenylpropanoids, and amino acid metabolism, were mainly alerted after 7 d post-salt-stress induction. Several potential genes that might be involved in salt stress conditions were identified; however, it demands further investigation. Although salt stress affects the level of secondary metabolites, their correlation needs to be investigated further. SIGNIFICANCE: Salinization is the most severe abiotic adversity, which has had a significant negative effect on world food security over the time. Excavating salt-tolerant genes from halophytes or medicinal plants is one of the important measures to cope with salt stress. S. alopecuroides is a well-known medicinal plant with anti-tumor, anti-inflammatory, and antibacterial effects, anti-saline properties, and resistance to drought stress. Currently, only a few studies have explored the S. alopecuroides' gene function, and regulation and these studies are mostly related to the unpublished genome sequence information of S. alopecuroides. Recently, transcriptomics and metabolomics studies have been carried on the abiotic stress in S. alopecuroides roots. Multiple studies have shown that altered gene expression at the transcript level and altered metabolite levels do not correspond to the altered protein levels. In this study, TMT and PRM based proteomic analyses of S. alopecuroides leaves under salt stress condition induced using 150 mM NaCl for 3 d and 7 d was performed. These analyses elucidated the activation of different mechanisms in response to salt stress. A total of 434 differentially abundant proteins (DAPs) in salt stress conditions were identified and analyzed. For the first time, this study utilized proteomics technology to dig out plentiful underlying salt-tolerant genes from the medicinal plant, S. alopecuroides. We believe that this study will be of great significance to crop genetics and breeding.

Analysis of Soluble Sugar Content in Minute Quantities of Rice Tissues by GC-MS

Soluble sugars play key roles in plant growth, development, and adaption to the environment. Characterizing sugar content profiling of plant tissues promotes our understanding of the mechanisms underlying these plant processes. Several technologies have been developed to quantitate soluble sugar content in plant tissues; however, it is difficult with only minute quantities of plant tissues available. Here, we provide a detailed protocol for gas chromatography mass spectrometry (GC-MS)-based soluble sugar profiling of rice tissues that offers a good balance of sensitivity and reliability, and is considerably more sensitive and accurate than other reported methods. We summarize all the steps from sample collection and soluble sugar extraction to derivatization procedures of the soluble extracted sugars, instrumentation settings, and data analysis.

Plasmodesmata play pivotal role in sucrose supply to Meloidogyne graminicola-caused giant cells in rice

On infection, plant-parasitic nematodes establish feeding sites in roots from which they take up carbohydrates among other nutrients. Knowledge on how carbohydrates are supplied to the nematodes' feeding sites is limited. Here, gene expression analyses showed that RNA levels of OsSWEET11 to OsSWEET15 were extremely low in both Meloidogyne graminicola (Mg)-caused galls and noninoculated roots. All the rice sucrose transporter genes, OsSUT1 to OsSUT5, were either down-regulated in Mg-caused galls compared with noninoculated rice roots or had very low transcript abundance. OsSUT1 was the only gene up-regulated in galls, at 14 days postinoculation (dpi), after being highly down-regulated at 3 and 7 dpi. OsSUT4 was down-regulated at 3 dpi. No noticeable OsSUTs promoter activities were detected in Mg-caused galls of pOsSUT1 to -5::GUS rice lines. Loading experiments with carboxyfluorescein diacetate (CFDA) demonstrated that symplastic connections exist between phloem and Mg-caused giant cells (GCs). According to data from OsGNS5- and OsGSL2-overexpressing rice plants that had decreased and increased callose deposition, respectively, callose negatively affected Mg parasitism and sucrose supply to Mg-caused GCs. Our results suggest that plasmodesmata-mediated sucrose transport plays a pivotal role in sucrose supply from rice root phloem to Mg-caused GCs, and OsSWEET11 to -15 and OsSUTs are not major players in it, although further functional analysis is needed for OsSUT1 and OsSUT4.

Imperative role of sugar signaling and transport during drought stress responses in plants

Cellular sugar status is essentially maintained during normal growth conditions but is impacted negatively during various environmental perturbations. Drought presents one such unfavorable environmental cue that hampers the photosynthetic fixation of carbon into sugars and affects their transport by lowering the cellular osmotic potential. The transport of cellular sugar is facilitated by a specific set of proteins known as sugar transporters. These transporter proteins are the key determinant of influx/ efflux of various sugars and their metabolite intermediates that support the plant growth and developmental process. Abiotic stress and especially drought stress-mediated injury results in reprogramming of sugar distribution across the cellular and subcellular compartments. Here, we have reviewed the imperative role of sugar accumulation, signaling, and transport under typical and atypical stressful environments. We have discussed the physiological effects of drought on sugar accumulation and transport through different transporter proteins involved in monosaccharide and disaccharide sugar transport. Further, we have illustrated sugar-mediated signaling and regulation of sugar transporter proteins along with the overall crosstalk of this signaling with the phytohormone module of abiotic stress response under osmotic stress. Overall, the present review highlights the critical role of sugar transport, distribution and signaling in plants under drought stress conditions.

Relationships between population traits, nonstructural carbohydrates, and elevation in alpine stands of Vaccinium myrtillus

PREMISE

Despite great attention given to the relationship between plant growth and carbon balance in alpine tree species, little is known about shrubs at the treeline. We hypothesized that the pattern of main nonstructural carbohydrates (NSCs) across elevations depends on the interplay between phenotypic trait plasticity, plant-plant interaction, and elevation.

METHODS

We studied the pattern of NSCs (i.e., glucose, fructose, sucrose, and starch) in alpine stands of Vaccinium myrtillus (above treeline) across an elevational gradient. In the same plots, we measured key growth traits (i.e., anatomical stem features) and shrub cover, evaluating putative relationships with NSCs.

RESULTS

Glucose content was positively related with altitude, but negatively related with shrub cover. Sucrose decreased at high altitude and in older populations and increased with higher percentage of vascular tissue. Starch content increased at middle and high elevations and in stands with high shrub cover. Moreover, starch content was negatively related with the number of xylem rings and the percentage of phloem tissue, but positively correlated with the percentage of xylem tissue.

CONCLUSIONS

We found that the increase in carbon reserves across elevations was uncoupled from plant growth, supporting the growth limitation hypothesis, which postulates NSCs accumulate at high elevation as a consequence of low temperature. Moreover, the response of NSC content to the environmental stress caused by elevation was buffered by phenotypic plasticity of plant traits, suggesting that, under climate warming conditions, shrub expansion due to enhanced plant growth would be pronounced in old but sparse stands.

Plant immunity in signal integration between biotic and abiotic stress responses

Plants constantly monitor and cope with the fluctuating environment while hosting a diversity of plant-inhabiting microbes. The mode and outcome of plant-microbe interactions, including plant disease epidemics, are dynamically and profoundly influenced by abiotic factors, such as light, temperature, water and nutrients. Plants also utilize associations with beneficial microbes during adaptation to adverse conditions. Elucidation of the molecular bases for the plant-microbe-environment interactions is therefore of fundamental importance in the plant sciences. Following advances into individual stress signaling pathways, recent studies are beginning to reveal molecular intersections between biotic and abiotic stress responses and regulatory principles in combined stress responses. We outline mechanisms underlying environmental modulation of plant immunity and emerging roles for immune regulators in abiotic stress tolerance. Furthermore, we discuss how plants coordinate conflicting demands when exposed to combinations of different stresses, with attention to a possible determinant that links initial stress response to broad-spectrum stress tolerance or prioritization of specific stress tolerance.

Metabolic response of soybean plants to Sclerotinia sclerotiorum infection

White mold is a disease caused by the fungus Sclerotinia sclerotiorum, a highly destructive necrotrophic pathogen that infects soybean crops, among others. Usually, the infection triggers the plant defense system to minimize the damages. However, the effects of the infection on soybean plant metabolism are still unclear. In this work, the metabolic profiles of soybean stems and leaves were accessed using 1H HR-MAS NMR spectroscopy to identify metabolic changes as a response to S. sclerotiorum infection. This fungus widely affects the central metabolism of soybean plants, and most of the altered metabolites are involved in carbon metabolism, as suggested by the results. Furthermore, the metabolites of central metabolism can be associated with the production of several polyphenolic metabolites. Changes in metabolic profile of leaves indicate systemic effects.

Elucidating Drought Stress Tolerance in European Oaks Through Cross-Species Transcriptomics

The impact of climate change that comes with a dramatic increase of long periods of extreme summer drought associated with heat is a fundamental challenge for European forests. As a result, forests are expected to shift their distribution patterns toward north-east, which may lead to a dramatic loss in value of European forest land. Consequently, unraveling key processes that underlie drought stress tolerance is not only of great scientific but also of utmost economic importance for forests to withstand future heat and drought wave scenarios. To reveal drought stress-related molecular patterns we applied cross-species comparative transcriptomics of three major European oak species: the less tolerant deciduous pedunculate oak (Quercus robur), the deciduous but quite tolerant pubescent oak (Q. pubescens), and the very tolerant evergreen holm oak (Q. ilex). We found 415, 79, and 222 differentially expressed genes during drought stress in Q. robur, Q. pubescens, and Q. ilex, respectively, indicating species-specific response mechanisms. Further, by comparative orthologous gene family analysis, 517 orthologous genes could be characterized that may play an important role in drought stress adaptation on the genus level. New regulatory candidate pathways and genes in the context of drought stress response were identified, highlighting the importance of the antioxidant capacity, the mitochondrial respiration machinery, the lignification of the water transport system, and the suppression of drought-induced senescence - providing a valuable knowledge base that could be integrated in breeding programs in the face of climate change.

Evolution and Stress Responses of Gossypium hirsutum SWEET Genes

The SWEET (sugars will eventually be exported transporters) proteins are sugar efflux transporters containing the MtN3_saliva domain, which affects plant development as well as responses to biotic and abiotic stresses. These proteins have not been functionally characterized in the tetraploid cotton, Gossypium hirsutum, which is a widely cultivated cotton species. In this study, we comprehensively analyzed the cotton SWEET gene family. A total of 55 putative G. hirsutumSWEET genes were identified. The GhSWEET genes were classified into four clades based on a phylogenetic analysis and on the examination of gene structural features. Moreover, chromosomal localization and an analysis of homologous genes in Gossypium arboreum, Gossypium raimondii, and G. hirsutum suggested that a whole-genome duplication, several tandem duplications, and a polyploidy event contributed to the expansion of the cotton SWEET gene family, especially in Clade III and IV. Analyses of cis-acting regulatory elements in the promoter regions, expression profiles, and artificial selection revealed that the GhSWEET genes were likely involved in cotton developmental processes and responses to diverse stresses. These findings may clarify the evolution of G. hirsutum SWEET gene family and may provide a foundation for future functional studies of SWEET proteins regarding cotton development and responses to abiotic stresses.