Roles of sugar alcohols in osmotic stress adaptation. Replacement of glycerol by mannitol and sorbitol in yeast

Enhancement of fermentation traits in industrial Baker's yeast for low or high sugar environments

Saccharomyces cerevisiae SPC-SNU 70-1 is a commercial diploid baking yeast strain valued for its excellent bread-making qualities, including superior leavening capabilities and the production of flavor-enhancing volatile organic acids. Despite its benefits, this strain faces challenges in fermenting both lean (low-sugar) and sweet (high-sugar) doughs. To address these issues, we employed the CRISPR/Cas9 genome editing system to modify genes without leaving any genetic scars. For lean doughs, we enhanced the yeast's ability to utilize maltose over glucose by deleting a gene involved in glucose repression. For sweet doughs, we increased glycerol production by overexpressing glycerol biosynthetic genes and optimizing redox balance, thereby improving the tolerence to osmotic stress during fermentation. Additionally, the glycerol-overproducing strain demonstrated enhanced freeze tolerance, and bread made from this strain exhibited improved storage properties. This study demonstrates the feasibility and benefits of using engineered yeast strains, created solely by editing their own genes without introducing foreign genes, to enhance bread making.

γ-Aminobutyric Acid Priming Alleviates Acid-Aluminum Toxicity to Creeping Bentgrass by Regulating Metabolic Homeostasis

Aluminum (Al) toxicity is a major limiting factor for plant growth and crop production in acidic soils. This study aims to investigate the effects of γ-aminobutyric acid (GABA) priming on mitigating acid-Al toxicity to creeping bentgrass (Agrostis stolonifera) associated with changes in plant growth, photosynthetic parameters, antioxidant defense, key metabolites, and genes related to organic acids metabolism. Thirty-seven-old plants were primed with or without 0.5 mM GABA for three days and then subjected to acid-Al stress (5 mmol/L AlCl3·6H2O, pH 4.35) for fifteen days. The results showed that acid-Al stress significantly increased the accumulation of Al and also restricted aboveground and underground growths, photosynthesis, photochemical efficiency, and osmotic balance, which could be effectively alleviated by GABA priming. The application of GABA significantly activated antioxidant enzymes, including superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase, to reduce oxidative damage to cells under acid-Al stress. Metabolomics analysis demonstrated that the GABA pretreatment significantly induced the accumulation of many metabolites such as quinic acid, pyruvic acid, shikimic acid, glycine, threonine, erythrose, glucose-6-phosphate, galactose, kestose, threitol, ribitol, glycerol, putrescine, galactinol, and myo-inositol associated with osmotic, antioxidant, and metabolic homeostases under acid-Al stress. In addition, the GABA priming significantly up-regulated genes related to the transportation of malic acid and citric acid in leaves in response to acid-Al stress. Current findings indicated GABA-induced tolerance to acid-Al stress in relation to scavenging of reactive oxygen species, osmotic adjustment, and accumulation and transport of organic metabolites in leaves. Exogenous GABA priming could improve the phytoremediation potential of perennial creeping bentgrass for the restoration of Al-contaminated soils.

Yeast osmoregulation - glycerol still in pole position

In response to osmotic dehydration cells sense, signal, alter gene expression, and metabolically counterbalance osmotic differences. The main compatible solute/osmolyte that accumulates in yeast cells is glycerol, which is produced from the glycolytic intermediate dihydroxyacetone phosphate. This review covers recent advancements in understanding mechanisms involved in sensing, signaling, cell-cycle delays, transcriptional responses as well as post-translational modifications on key proteins in osmoregulation. The protein kinase Hog1 is a key-player in many of these events, however, there is also a growing body of evidence for important Hog1-independent mechanisms playing vital roles. Several missing links in our understanding of osmoregulation will be discussed and future avenues for research proposed. The review highlights that this rather simple experimental system—salt/sorbitol and yeast—has developed into an enormously potent model system unravelling important fundamental aspects in biology.

Exploring the High Variability of Vegetative Desiccation Tolerance in Pteridophytes

In the context of plant evolution, pteridophytes, which is comprised of lycophytes and ferns, occupy an intermediate position between bryophytes and seed plants, sharing characteristics with both groups. Pteridophytes is a highly diverse group of plant species that occupy a wide range of habitats including ecosystems with extreme climatic conditions. There is a significant number of pteridophytes that can tolerate desiccation by temporarily arresting their metabolism in the dry state and reactivating it upon rehydration. Desiccation-tolerant pteridophytes exhibit a strategy that appears to be intermediate between the constitutive and inducible desiccation tolerance (DT) mechanisms observed in bryophytes and angiosperms, respectively. In this review, we first describe the incidence and anatomical diversity of desiccation-tolerant pteridophytes and discuss recent advances on the origin of DT in vascular plants. Then, we summarize the highly diverse adaptations and mechanisms exhibited by this group and describe how some of these plants could exhibit tolerance to multiple types of abiotic stress. Research on the evolution and regulation of DT in different lineages is crucial to understand how plants have adapted to extreme environments. Thus, in the current scenario of climate change, the knowledge of the whole landscape of DT strategies is of vital importance as a potential basis to improve plant abiotic stress tolerance.

Agro-active endo-therapy treated Xylella fastidiosa subsp. pauca-infected olive trees assessed by the first 1H-NMR-based metabolomic study

Xylella fastidiosa is a xylem-limited bacterium causing a range of economically important plant diseases in hundreds of crops. Over the last decade, a severe threat due to Olive Quick Decline Syndrome (OQDS), caused by Xylella fastidiosa subspecies pauca, affected the Salento olive groves (Apulia, South-East Italy). Very few phyto-therapeutics, including a Zn/Cu citric acid biocomplex foliar treatment, were evaluated to mitigate this disease. However, the traditional foliar applications result in the agro-actives reaching only partially their target. Therefore the development of novel endo-therapeutic systems was suggested. Metabolite fingerprinting is a powerful method for monitoring both, disease progression and treatment effects on the plant metabolism, allowing biomarkers detection. We performed, for the first time, short-term monitoring of metabolic pathways reprogramming for infected Ogliarola salentina and Cima di Melfi olive trees after precision intravascular biocomplex delivery using a novel injection system. Upon endo therapy, we observed specific variations in the leaf content of some metabolites. In particular, the ¹H NMR-based metabolomics approach showed, after the injection, a significant decrease of both the disease biomarker quinic acid and mannitol with simultaneous increase of polyphenols and oleuropein related compounds in the leaf’s extracts. This combined metabolomics/endo-therapeutic methodology provided useful information in the comprehension of plant physiology for future applications in OQDS control.

The Influence of Polyols on the Process Kinetics and Bioactive Substance Content in Osmotic Dehydrated Organic Strawberries

In recent years, an increasing interest in reducing sugar consumption has been observed and many studies are conducted on the use of polyols in the osmotic dehydration process to obtain candied or dried fruits. The studies in the literature have focused on the kinetics of the process as well as the basic physical properties. In the scientific literature, there is a lack of investigation of the influence of such polyol solutions such as sorbitol and mannitol used as osmotic substances during the osmotic dehydration process on the contents of bioactive components, including natural colourants. Thus, the aim of the study was to evaluate the impact of polyols (mannitol and sorbitol) in different concentrations on the process kinetics and on chosen physical (colour and structural changes) as well as chemical (sugars and polyol content, total anthocyanin content, total polyphenol content, vitamin C, antioxidant activity) properties of osmotic-dehydrated organic strawberries. Generally, the results showed that the best solution for osmotic dehydration is 30% or 40% sorbitol solutions, while mannitol solution is not recommended due to difficulties with preparing a high-concentration solution and its crystallization in the tissue. In the case of sorbitol, the changes of bioactive compounds, as well as colour change, were similar to the sucrose solution. However, the profile of the sugar changed significantly, in which sucrose, glucose, and fructose were reduced in organic strawberries and were partially replaced by polyols.

Current Approaches of Preservation of Cells During (freeze-) Drying

The widespread application of therapeutic cells requires a successful stabilization of cells for the duration of transport and storage. Cryopreservation is currently considered the gold standard for the storage of active cells; however, (freeze-) drying cells could enable higher shelf life stability at ambient temperatures and facilitate easier transport and storage. During (freeze-) drying, freezing, (primary and secondary) drying and also the reconstitution step pose the risk of potential cell damage. To prevent these damaging processes, a wide range of protecting excipients has emerged, which can be classified, according to their chemical affiliation, into sugars, macromolecules, polyols, antioxidants and chelating agents. As many excipients cannot easily permeate the cell membrane, researchers have established various techniques to introduce especially trehalose intracellularly, prior to drying. This review aims to summarize the main damaging mechanisms during (freeze-) drying and to introduce the most common excipients with further details on their stabilizing properties and process approaches for the intracellular loading of excipients. Additionally, we would like to briefly explain recently discovered advantages of drying microorganisms, sperm, platelets, red blood cells, and eukaryotic cells, paying particular attention to the drying technique and residual moisture content.

Longitudinal Metatranscriptomic Analysis of a Meat Spoilage Microbiome Detects Abundant Continued Fermentation and Environmental Stress Responses during Shelf Life and Beyond

Microbial food spoilage is a complex phenomenon associated with the succession of the specific spoilage organisms (SSO) over the course of time. We performed a longitudinal metatranscriptomic study on one modified-atmosphere-packaged (MAP) beef product to increase understanding of the longitudinal behavior of a spoilage microbiome during shelf life and beyond. Based on the annotation of the mRNA reads, we recognized three stages related to the active microbiome that were descriptive of the sensory quality of the beef: acceptable product (AP), early spoilage (ES), and late spoilage (LS). Both the 16S RNA taxonomic assignments from the total RNA and functional annotations of the active genes showed that these stages were significantly different from each other. However, the functional gene annotations showed more pronounced differences than the taxonomy assignments. Psychrotrophic lactic acid bacteria (LAB) formed the core of the SSO, according to the transcribed reads. Leuconostoc species were the most abundant active LAB throughout the study period, whereas the transcription activity of Streptococcaceae (mainly Lactococcus) increased after the product had spoiled. In the beginning of the experiment, the community managed environmental stress by cold-shock responses, which were followed by expression of the genes involved in managing oxidative stress. Glycolysis, the pentose phosphate pathway, and pyruvate metabolism were active throughout the study at a relatively stable level. However, the proportional transcription activities of the enzymes in these pathways changed over time.IMPORTANCE It is generally known which organisms are the typical SSO in foods, whereas the actively transcribed genes and pathways during microbial succession are poorly understood. This knowledge is important, since better approaches to food quality evaluation and shelf life determination are needed. Therefore, we conducted this study to find longitudinal markers that are connected to quality deterioration in a MAP beef product. This kind of RNA marker could be used to develop novel types of rapid quality analysis tools in the future. New tools are needed, since even though SSO can be detected and their concentrations determined using the current microbiological methods, results from these analyses cannot predict how close in time a spoilage community is to the production of clear sensory defects. The main reason for this is that the species composition of a spoilage community does not change dramatically during late shelf life, whereas the ongoing metabolic activities lead to the development of notable sensory deterioration.

Purification and characterization of a novel medium-chain ribitol dehydrogenase from a lichen-associated bacterium Sphingomonas sp

Sugar alcohols (polyols) are abundant carbohydrates in lichen-forming algae and transported to other lichen symbionts, fungi, and bacteria. Particularly, ribitol is an abundant polyol in the lichen Cetraria sp. Polyols have important physiological roles in lichen symbiosis, but polyol utilization in lichen-associated bacteria has been largely unreported. Herein, we purified and characterized a novel ribitol dehydrogenase (RDH) from a Cetraria sp.-associated bacterium Sphingomonas sp. PAMC 26621 grown on a minimal medium containing D-ribitol (the RDH hereafter referred to as SpRDH). SpRDH is present as a trimer in its native form, and the molecular weight of SpRDH was estimated to be 39 kDa by SDS-PAGE and 117 kDa by gel filtration chromatography. SpRDH converted D-ribitol to D-ribulose using NAD+ as a cofactor. As far as we know, SpRDH is the first RDH belonging to the medium-chain dehydrogenase/reductase family. Multiple sequence alignments indicated that the catalytic amino acid residues of SpRDH consist of Cys37, His65, Glu66, and Glu157, whereas those of short-chain RDHs consist of Ser, Tyr, and Lys. Furthermore, unlike other short-chain RDHs, SpRDH did not require divalent metal ions for its catalytic activity. Despite SpRDH originating from a psychrophilic Arctic bacterium, Sphingomonas sp., it had maximum activity at 60°C and exhibited high thermal stability within the 4-50°C range. Further studies on the structure/function relationship and catalytic mechanism of SpRDH will expand our understanding of its role in lichen symbiosis.

Untargeted metabolomics as a hypothesis-generation tool in plant protection product discovery: Highlighting the potential of trehalose and glycerol metabolism of fungal conidiospores as novel targets

INTRODUCTION

The production of high quality and safe food represents a main priority for the agri-food sector in the effort to sustain the exponentially growing human population. Nonetheless, there are major challenges that require the discovery of new, alternative, and improved plant protection products (PPPs). Focusing on fungal plant pathogens, the dissection of mechanisms that are essential for their survival provides insights that could be exploited towards the achievement of the aforementioned aim. In this context, the germination of fungal spores, which are essential structures for their dispersal, survival, and pathogenesis, represents a target of high potential for PPPs. To the best of our knowledge, no PPPs that target the germination of fungal spores currently exist.

OBJECTIVES

Within this context, we have mined for changes in the metabolite profiles of the model fungus Aspergillus nidulans FGSC A4 conidiospores during germination, in an effort to discover key metabolites and reactions that could potentially become targets of PPPs.

METHODS

Untargeted GC/EI-TOF/MS metabolomics and multivariate analyses were employed to monitor time-resolved changes in the metabolomes of germinating A. nidulans conidiospores.

RESULTS

Analyses revealed that trehalose hydrolysis plays a pivotal role in conidiospore germination and highlighted the osmoregulating role of the sugar alcohols, glycerol, and mannitol.

CONCLUSION

The ineffectiveness to introduce active ingredients that exhibit new mode(s)-of-action as fungicides, dictates the urge for the discovery of PPPs, which could be exploited to combat major plant protection issues. Based on the crucial role of trehalose hydrolysis in conidiospore dormancy breakage, and the subsequent involvement of glycerol in their germination, it is plausible to suggest their biosynthesis pathways as potential novel targets for the next-generation antifungal PPPs. Our study confirmed the applicability of untargeted metabolomics as a hypothesis-generation tool in PPPs' research and discovery.

Pretreatment of the Antagonistic Yeast, Debaryomyces hansenii, With Mannitol and Sorbitol Improves Stress Tolerance and Biocontrol Efficacy

The effect of exogenous mannitol and sorbitol on the viability of the antagonist yeast, Debaryomyces hansenii, when exposed to oxidative and high-temperature stress was determined. Results indicated that both the 0.1 M mannitol (MT) and 0.1 M sorbitol (ST) treatments improved the tolerance of D. hansenii to subsequent oxidative and high-temperature stress. MT or ST cells had a significantly higher level of cell survival, elevated the gene expression of catalase 1 (CAT1) and copper-zinc superoxide dismutase (SOD1), as well as the corresponding enzyme activity. Treated cells also exhibited a lower accumulation of intracellular reactive oxygen species (ROS), and a higher content of intracellular mannitol and sorbitol relative to non-treated, control yeast cells, when exposed to a subsequent oxidative (30 mM H₂O₂) or heat (40.5°C) stress for 30 min. Additionally, MT and ST yeast exhibited a higher growth rate in kiwifruit wounds, and a greater ability to inhibit postharvest blue mold (Penicillium expansum) and gray mold (Botrytis cinerea) infections. The present study indicates that increased antioxidant response induced by mannitol and sorbitol in D. hansenii can enhance stress tolerance and biocontrol performance.

Differential Metabolic Responses Caused by the Most Important Insect Pest of Coffee Worldwide, the Coffee Berry Borer (Hypothenemus hampei)

The world's coffee supply is threatened by the coffee berry borer, Hypothenemus hampei, the most destructive pest affecting coffee production and quality. This study hypothesized that coffee berry borer infestation induces distinct metabolic responses in the green coffee seeds of Coffea arabica and Coffea canephora (robusta). A targeted metabolomics approach was conducted using liquid chromatography tandem mass spectrometry to quantify intracellular metabolites in infested and uninfested arabica and robusta green seeds. In parallel, the seed biomass content and composition were assessed for the same conditions. Coffee berry borer attack induced increases in the levels of chlorogenic acids in arabica seeds, whereas organic acids and sugar alcohols were more abundant in infested robusta seeds. Most importantly, a set of compounds was identified as biomarkers differentiating the metabolic response of these taxa to the coffee berry borer.

Enhancement of glycerol production by UV-mutagenesis of the marine yeast Wickerhamomyces anomalus HH16: kinetics and optimization of the fermentation process

The current study aims to enhance glycerol production using UV-mutagenesis of the marine yeast Wickerhamomyces anomalus HH16 isolated from marine sediment collected from South Sinai Governorate, Egypt. Besides optimization of the culture conditions and analyzing the kinetic parameters of growth and glycerol biosynthesis by the mutant strain were studied. The marine yeast isolate HH16 was selected as the front runner glycerol-producer among all tested isolates, with glycerol yield recorded as 66.55 gl^-1. The isolate was identified based on the phenotypic and genotypic characteristics of W. anomalus. The genotypic characterization based on the internal transcribed spacer (ITS) sequence was deposited in the GenBank database with the accession number MK182824. UV-mutagenesis of W. anomalus HH16 by its exposure to UV radiation (254 nm, 200 mW cm^-2) for 5 min; increased its capability in the glycerol production rate with 16.97% (80.15 g l^-1). Based on the kinetic and Monod equations, the maximum specific growth rate (μ _max) and maximum specific glycerol production rate (v _max) by the mutant strain W. anomalus HH16MU5 were 0.21 h^-1 and 0.103 g g^-1, respectively. Optimization of the fermentation parameters such as nitrogen source, salinity and pH has been achieved. The maximum glycerol production 86.55 g l^-1 has been attained in a fermentation medium composed of 200 g l^-1 glucose, 1 g l^-1 peptone, 3 g l^-1 yeast extract, and 58.44 g l^-1 NaCl, this medium was adjusted at pH 8 and incubated for 3 days at 30° C. Moreover, results indicated the ability of this yeast to produce glycerol (73.33 g l^-1) using a seawater based medium. These findings suggest the applicability of using the yeast isolate W. anomalus HH16MU5 as a potential producer of glycerol for industrial purposes.

Characterization of gfdB, putatively encoding a glycerol 3-phosphate dehydrogenase in Aspergillus nidulans

The genome of Aspergillus nidulans accommodates two glycerol 3-phosphate dehydrogenase genes, gfdA and gfdB. Previous studies confirmed that GfdA is involved in the osmotic stress defence of the fungus. In this work, the physiological role of GfdB was characterized via the construction and functional characterization of the gene deletion mutant ΔgfdB. Unexpectedly, ΔgfdB strains showed oxidative stress sensitivity in the presence of a series of well-known oxidants including tert-butyl-hydroperoxide (tBOOH), diamide as well as hydrogen peroxide. Moderate sensitivity of the mutant towards the cell wall stress inducing agent CongoRed was also observed. Hence, both Gfd isoenzymes contributed to the environmental stress defence of the fungus but their functions were stress-type-specific. Furthermore, the specific activities of certain antioxidant enzymes, like catalase and glutathione peroxidase, were lower in ΔgfdB hyphae than those recorded in the control strain. As a consequence, mycelia from ΔgfdB cultures accumulated reactive species at higher levels than the control. On the other hand, the specific glutathione reductase activity was higher in the mutant, most likely to compensate for the elevated intracellular oxidative species concentrations. Nevertheless, the efficient control of reactive species failed in ΔgfdB cultures, which resulted in reduced viability and, concomitantly, early onset of programmed cell death in mutant hyphae. Inactivation of gfdB brought about higher mannitol accumulation in mycelia meanwhile the erythritol production was not disturbed in unstressed cultures. After oxidative stress treatment with tBOOH, only mannitol was detected in both mutant and control mycelia and the accumulation of mannitol even intensified in the ΔgfdB strain.

Soil microbial responses to drought and exotic plants shift carbon metabolism

Significant gaps in our understanding of how global change drivers interact to affect the resistance and functioning of microbial communities hinders our ability to model ecosystem responses and feedbacks to co-occurring global stressors. Here, we investigated the effects of extreme drought and exotic plants, two of the most significant threats to Mediterranean-type ecosystems, on soil microbial community composition and carbon metabolic genes within a four-year field rainfall manipulation experiment. We combined measurements of bulk microbial and soil properties with high-throughput microbial community analyses to elucidate microbial responses and microbial-mediated alterations to carbon cycling. While microbial responses to experimental droughts were weak, scant rainfall periods resulted in decreased microbial biomass and activity, and relative abundances of bacterial groups such as Proteobacteria, Verrucomicrobia, and Acidobacteria decreased concomitantly with increases in Actinobacteria, Chloroflexi, and Firmicutes abundance. Soils under exotic plants had increased temperatures, enhanced infiltration during rainfall events, and decreased water retention and labile carbon in comparison to soils under native plants. Higher peaks and more seasonally variable microbial activity were found under exotic plants and, like drought periods, the microbial community shifted towards osmotic stress life-strategies. Relationships found between microbial taxonomic groups and carbon metabolic genes support the interpretation that exotic plants change microbial carbon cycling by altering the soil microclimate and supplying easily decomposed high-quality litter. Soil microbial community responses to drought and exotic plants could potentially impact ecosystem C storage by producing a smaller, more vulnerable C pool of microbial biomass that is prone to increased pulses of heterotrophic respiration.

Alteration of Proteomes in First-Generation Cultures of Bacillus pumilus Spores Exposed to Outer Space

Bacillus pumilus SAFR-032 was originally isolated from the Jet Propulsion Lab Spacecraft Assembly Facility and thoroughly characterized for its enhanced resistance to UV irradiation and oxidative stress. This unusual resistance of SAFR-032 is of particular concern in the context of planetary protection and calls for development of novel disinfection techniques to prevent extraterrestrial contamination. Previously, spores of SAFR-032 were exposed for 18 months to a variety of space conditions on board the International Space Station to investigate their resistance to Mars-like conditions and space travel. Here, proteomic characterization of vegetative SAFR-032 cells from space-surviving spores is presented in comparison to a ground control. Vegetative cells of the first passage were processed and subjected to quantitative proteomics using tandem mass tags. Approximately 60% of all proteins encoded by SAFR-032 were identified, and 301 proteins were differentially expressed among the strains. We found that proteins predicted to be involved in carbohydrate transport/metabolism and energy production/conversion had lower abundance than those of the ground control. For three proteins, we showed that the expected metabolic activities were decreased, as expected with direct enzymatic assays. This was consistent with a decrease of ATP production in the space-surviving strains. The same space-surviving strains showed increased abundance of proteins related to survival, growth advantage, and stress response. Such alterations in the proteomes provide insights into possible molecular mechanisms of B. pumilus SAFR-032 to adapt to and resist extreme extraterrestrial environments.IMPORTANCE Spore-forming bacteria are well known for their resistance to harsh environments and are of concern for spreading contamination to extraterrestrial bodies during future life detection missions. Bacillus pumilus has been regularly isolated from spacecraft-associated surfaces and exhibited unusual resistance to ultraviolet light and other sterilization techniques. A better understanding of the mechanisms of microbial survival and enhanced resistance is essential for developing novel disinfection protocols for the purpose of planetary protection. While genomic analyses did not reveal the unique characteristics that explain elevated UV resistance of space-exposed B. pumilus, the proteomics study presented here provided intriguing insight on key metabolic changes. The observed proteomics aberrations reveal a complex biological phenomenon that plays a role in bacterial survival and adaptation under long-term exposure to outer space. This adaptive ability of microorganisms needs to be considered by those tasked with eliminating forward contamination.

Torulaspora delbrueckii produces high levels of C5 and C6 polyols during wine fermentations

Non-Saccharomyces yeasts impact wine fermentations and can diversify the flavor profiles of wines. However, little information is available on the metabolic networks of most of these species. Here we show that unlike the main wine yeast Saccharomyces cerevisiae, Torulaspora delbrueckii and to a lesser extent Lachancea thermotolerans produce significant concentrations of C5 and C6 polyols under wine fermentation conditions. In particular, D-arabitol, D-sorbitol and D-mannitol were produced at significant levels. Their release into the extracellular matrix started when that of glycerol ceased. The data also show that polyol production is influenced by initial sugar concentration, repressed by acetic acid and induced in ethanol supplemented media. Moreover, unlike glycerol and sorbitol, mannitol was partially re-assimilated when populations started to decline. The findings suggest that polyol synthesis is a physiological adaptation to stressful conditions characteristic of alcoholic fermentation and that these polyols may serve a similar purpose as glycerol production in S. cerevisiae, including osmoadaptation and redox balancing.

Combinatorial impact of osmotic and heat shocks on the composition of membrane lipids and osmolytes in Aspergillus niger

The combinatorial action of osmotic (OS) and heat (HS) shocks on the composition of soluble cytosol carbohydrates and membrane lipids was studied. For the first time it was demonstrated that the combinatorial effect of these shocks led to the non-additive response - an increase in the trehalose level, characteristic for HS, but at the same time suppression of glycerol production, uncharacteristic of the OS response. In addition, combinatorial action resulted in a new effect - increase in the mannitol level, which was not typical for the individual HS or OS responses. On the contrary, a general pattern of change was observed in the composition of membrane lipids in response to both individual HS and OS, and their combinations, which was a twofold increase in the proportion of phosphatidic acids. At the same time, the mechanism of alteration in the degree of unsaturation of membrane phospholipids was not involved in adaptation. The response to combinatorial shocks includes the accumulation of trehalose and mannitol, and increase in the proportion of phosphatidic acids in membrane lipids.

Non-Targeted Metabolomics Reveals Sorghum Rhizosphere-Associated Exudates are Influenced by the Belowground Interaction of Substrate and Sorghum Genotype

Root exudation is an important plant process by which roots release small molecules into the rhizosphere that serve in overall plant functioning. Yet, there is a major gap in our knowledge in translating plant root exudation in artificial systems (i.e., hydroponics, sterile media) to crops, specifically for soils expected in field conditions. Sorghum (Sorghum bicolor L. Moench) root exudation was determined using both ultra-performance liquid chromatography and gas chromatography mass spectrometry-based non-targeted metabolomics to evaluate variation in exudate composition of two sorghum genotypes among three substrates (sand, clay, and soil). Above and belowground plant traits were measured to determine the interaction between sorghum genotype and belowground substrate. Plant growth and quantitative exudate composition were found to vary largely by substrate. Two types of changes to rhizosphere metabolites were observed: rhizosphere-enhanced metabolites (REMs) and rhizosphere-abated metabolites (RAMs). More REMs and RAMs were detected in sand and clay substrates compared to the soil substrate. This study demonstrates that belowground substrate influences the root exudate profile in sorghum, and that two sorghum genotypes exuded metabolites at different magnitudes. However, metabolite identification remains a major bottleneck in non-targeted metabolite profiling of the rhizosphere.

Sophorolipid exhibits antifungal activity by ROS mediated endoplasmic reticulum stress and mitochondrial dysfunction pathways in Candida albicans

In the present study, we investigated the mechanism of cell death in C. albicans due to treatment with sophorolipid (SL). SL is an extracellular glycolipid biosurfactant produced by various species of non-pathogenic yeasts and is known to inhibit the growth and biofilm formation of C. albicans. This study revealed that treatment of C. albicans cells with SL increases the ROS production and expression of oxidative stress-related genes significantly (SOD1, CAT1). Increased ROS level within the cells causes ER stress and release of Ca²⁺ in the cytoplasm and alteration of the mitochondrial membrane potential (MMP). Quantitative real time-polymerase chain reaction (qRT-PCR) data showed that SL also upregulates the Endoplasmic Reticulum (ER) stress marker HAC1. Flow cytometric analysis (AnnexinV/PI) indicated that the cell death may have occurred due to necrosis which was further confirmed by LDH release assay and transmission electron microscopy (TEM). Further experiments with the null mutant Δ hog1 strain of C. albicans SC5314 indicated the activation of the osmotic stress response pathway (HOG-MAPK) and SAP9. This study gave an insight into the mechanism of cell death initiation by glycolipids and indicated that further modification of these molecules can lead to the development of new therapeutic agent against C. albicans.

Sophorolipid induces ROS generation in C. albicans leading to mitochondrial dysfunction and ER stress followed by the release of Ca²⁺ ions (from the ER lumen) that enter mitochondria and further magnify ROS generation leading to cell death.

Disruption of Polyhydroxybutyrate Synthesis Redirects Carbon Flow towards Glycogen Synthesis in Synechocystis sp. PCC 6803 Overexpressing glgC/glgA

The photoautotrophic Synechocystis sp. PCC 6803 (hereafter Synechocystis) is known for its α-polyglucan (glycogen) synthesis to serve as a carbon storage compound. In this study, the glgC- and glgA-overexpressing Synechocystis strain with the disruption of polyhydroxybutyrate (PHB) synthesis (▴GCAX-ΔBK) showed an increased glycogen production. This engineered strain had a high glycogen content of 38.3% (g g-1 dry cell weight) as compared with 27.4% in the phaA knockout strain (ΔBK) and 34.8% in the glgC/glgA-overexpressing strain (▴GCAX) after 20 d growth. Under nitrogen-deprived growth conditions for 3 d, the ▴GCAX-ΔBK strain showed a further increase in glycogen content from 27.0% to 36.0%. Furthermore, the engineered strains grown under ionic, osmotic or oxidative stress conditions had an increase of glycogen accumulation, whereas no increase was observed in the wild type. The maximum glycogen content was 54.0% in the ▴GCAX-ΔBK strain treated with 3 mM H2O2. The overall results indicated that in the absence of PHB synthesis, Synechocystis cells redirected the carbon flow towards the synthesis of glycogen as an alternative physiological responsive compound especially under stress conditions.

Extremophiles as a Model of a Natural Ecosystem: Transcriptional Coordination of Genes Reveals Distinct Selective Responses of Plants Under Climate Change Scenarios

The goal of this research was to generate networks of co-expressed genes to explore the genomic responses of Rhizophora mangle L. populations to contrasting environments and to use gene network analysis to investigate their capacity for adaptation in the face of historical and future perturbations and climatic changes. RNA sequencing data were generated for R. mangle samples collected under field conditions from contrasting climate zones in the equatorial and subtropical regions of Brazil. A gene co-expression network was constructed using Pearson's correlation coefficient, showing correlations among 78,364 transcriptionally coordinated genes. Each region exhibited two distinct network profiles; genes correlated with the oxidative stress response showed higher relative expression levels in subtropical samples than in equatorial samples, whereas genes correlated with the hyperosmotic salinity response, heat response and UV response had higher expression levels in the equatorial samples than in the subtropical samples. In total, 992 clusters had enriched ontology terms, which suggests that R. mangle is under higher stress in the equatorial region than in the subtropical region. Increased heat may thus pose a substantial risk to species diversity at the center of its distribution range in the Americas. This study, which was performed using trees in natural field conditions, allowed us to associate the specific responses of genes previously described in controlled environments with their responses to the local habitat where the species occurs. The study reveals the effects of contrasting environments on gene expression in R. mangle, shedding light on the different abiotic variables that may contribute to the genetic divergence previously described for the species through the use of simple sequence repeats (SSRs). These effects may result from two fundamental processes in evolution, namely, phenotypic plasticity and natural selection.

Method for immobilization of living and synthetic cells for high-resolution imaging and single-particle tracking

Super-resolution imaging and single-particle tracking require cells to be immobile as any movement reduces the resolution of the measurements. Here, we present a method based on APTES-glutaraldehyde coating of glass surfaces to immobilize cells without compromising their growth. Our method of immobilization is compatible with Saccharomyces cerevisiae, Escherichia coli, and synthetic cells (here, giant-unilamellar vesicles). The method introduces minimal background fluorescence and is suitable for imaging of single particles at high resolution. With S. cerevisiae we benchmarked the method against the commonly used concanavalin A approach. We show by total internal reflection fluorescence microscopy that modifying surfaces with ConA introduces artifacts close to the glass surface, which are not present when immobilizing with the APTES-glutaraldehyde method. We demonstrate validity of the method by measuring the diffusion of membrane proteins in yeast with single-particle tracking and of lipids in giant-unilamellar vesicles with fluorescence recovery after photobleaching. Importantly, the physical properties and shape of the fragile GUVs are not affected upon binding to APTES-glutaraldehyde coated glass. The APTES-glutaraldehyde is a generic method of immobilization that should work with any cell or synthetic system that has primary amines on the surface.

Talaromyces marneffei simA Encodes a Fungal Cytochrome P450 Essential for Survival in Macrophages

Fungi are adept at occupying specific environmental niches and often exploit numerous secondary metabolites generated by the cytochrome P450 (CYP) monoxygenases. This report describes the characterization of a yeast-specific CYP encoded by simA ("survival in macrophages"). Deletion of simA does not affect yeast growth at 37°C in vitro but is essential for yeast cell production during macrophage infection. The ΔsimA strain exhibits reduced conidial germination and intracellular growth of yeast in macrophages, suggesting that the enzymatic product of SimA is required for normal fungal growth in vivo. Intracellular ΔsimA yeast cells exhibit cell wall defects, and metabolomic and chemical sensitivity data suggest that SimA may promote chitin synthesis or deposition in vitro. In vivo, ΔsimA yeast cells subsequently lyse and are degraded, suggesting that SimA may increase resistance to and/or suppress host cell biocidal effectors. The results suggest that simA synthesizes a secondary metabolite that allows T. marneffei to occupy the specific intracellular environmental niche within the macrophage. IMPORTANCE This study in a dimorphic fungal pathogen uncovered a role for a yeast-specific cytochrome P450 (CYP)-encoding gene in the ability of T. marneffei to grow as yeast cells within the host macrophages. This report will inspire further research into the role of CYPs and secondary metabolite synthesis during fungal pathogenic growth.

Optimizing anaerobic growth rate and fermentation kinetics in Saccharomyces cerevisiae strains expressing Calvin-cycle enzymes for improved ethanol yield

BACKGROUND

Reduction or elimination of by-product formation is of immediate economic relevance in fermentation processes for industrial bioethanol production with the yeast Saccharomyces cerevisiae. Anaerobic cultures of wild-type S. cerevisiae require formation of glycerol to maintain the intracellular NADH/NAD+ balance. Previously, functional expression of the Calvin-cycle enzymes ribulose-1,5-bisphosphate carboxylase (RuBisCO) and phosphoribulokinase (PRK) in S. cerevisiae was shown to enable reoxidation of NADH with CO2 as electron acceptor. In slow-growing cultures, this engineering strategy strongly decreased the glycerol yield, while increasing the ethanol yield on sugar. The present study explores engineering strategies to improve rates of growth and alcoholic fermentation in yeast strains that functionally express RuBisCO and PRK, while maximizing the positive impact on the ethanol yield.

RESULTS

Multi-copy integration of a bacterial-RuBisCO expression cassette was combined with expression of the Escherichia coli GroEL/GroES chaperones and expression of PRK from the anaerobically inducible DAN1 promoter. In anaerobic, glucose-grown bioreactor batch cultures, the resulting S. cerevisiae strain showed a 31% lower glycerol yield and a 31% lower specific growth rate than a non-engineered reference strain. Growth of the engineered strain in anaerobic, glucose-limited chemostat cultures revealed a negative correlation between its specific growth rate and the contribution of the Calvin-cycle enzymes to redox homeostasis. Additional deletion of GPD2, which encodes an isoenzyme of NAD+-dependent glycerol-3-phosphate dehydrogenase, combined with overexpression of the structural genes for enzymes of the non-oxidative pentose-phosphate pathway, yielded a CO2-reducing strain that grew at the same rate as a non-engineered reference strain in anaerobic bioreactor batch cultures, while exhibiting a 86% lower glycerol yield and a 15% higher ethanol yield.

CONCLUSIONS

The metabolic engineering strategy presented here enables an almost complete elimination of glycerol production in anaerobic, glucose-grown batch cultures of S. cerevisiae, with an associated increase in ethanol yield, while retaining near wild-type growth rates and a capacity for glycerol formation under osmotic stress. Using current genome-editing techniques, the required genetic modifications can be introduced in one or a few transformations. Evaluation of this concept in industrial strains and conditions is therefore a realistic next step towards its implementation for improving the efficiency of first- and second-generation bioethanol production.

Chromatin-associated regulation of sorbitol synthesis in flower buds of peach

PpeS6PDH gene is postulated to mediate sorbitol synthesis in flower buds of peach concomitantly with specific chromatin modifications. Perennial plants have evolved an adaptive mechanism involving protection of meristems within specialized structures named buds in order to survive low temperatures and water deprivation during winter. A seasonal period of dormancy further improves tolerance of buds to environmental stresses through specific mechanisms poorly known at the molecular level. We have shown that peach PpeS6PDH gene is down-regulated in flower buds after dormancy release, concomitantly with changes in the methylation level at specific lysine residues of histone H3 (H3K27 and H3K4) in the chromatin around the translation start site of the gene. PpeS6PDH encodes a NADPH-dependent sorbitol-6-phosphate dehydrogenase, the key enzyme for biosynthesis of sorbitol. Consistently, sorbitol accumulates in dormant buds showing higher PpeS6PDH expression. Moreover, PpeS6PDH gene expression is affected by cold and water deficit stress. Particularly, its expression is up-regulated by low temperature in buds and leaves, whereas desiccation treatment induces PpeS6PDH in buds and represses the gene in leaves. These data reveal the concurrent participation of chromatin modification mechanisms, transcriptional regulation of PpeS6PDH and sorbitol accumulation in flower buds of peach. In addition to its role as a major translocatable photosynthate in Rosaceae species, sorbitol is a widespread compatible solute and cryoprotectant, which suggests its participation in tolerance to environmental stresses in flower buds of peach.

Biodiversity in targeted metabolomics analysis of filamentous fungal pathogens by 1H NMR-based studies

The taxonomical classification among fungi kingdom in the last decades was evolved. In this work the targeted metabolomics study based on 1H NMR spectroscopy combined with chemometrics tools was reported to be useful for differentiation of three model of fungal strains, which represent various genus of Ascomycota (Aspergillus pallidofulvus, Fusarium oxysporum, Geotrichum candidum) were selected in order to perform metabolomics studies. Each tested species, revealed specific metabolic profile of primary endo-metabolites. The species of A. pallidofulvus is represented by the highest concentration of glycerol, glucitol and Unk5. While, F. oxysporum species is characterised by increased level of propylene glycol, ethanol, 4-aminobutyrate, succinate, xylose, Unk1 and Unk4. In G. candidum, 3-methyl-2-oxovalerate, glutamate, pyruvate, glutamine and citrate were elevated. Additionally, a detailed analysis of metabolic changes among A. pallidofulvus, F. oxysporum and G. candidum showed that A. pallidofulvus seems to be the most pathogenic fungi. The obtained results demonstrated that targeted metabolomics analysis could be utilized in the future as a supporting taxonomical tool for currently methods.

Dose-dependent effects of higher methionine levels on the transcriptome and metabolome of transgenic Arabidopsis seeds

Higher methionine levels in transgenic Arabidopsis seeds trigger the accumulation of stress-related transcripts and primary metabolites. These responses depend on the levels of methionine within seeds. Methionine, a sulfur-containing amino acid, is a key metabolite in plant cells. To reveal the regulatory role of the Arabidopsis thaliana CYSTATHIONINE γ-SYNTHASE (AtCGS), methionine main regulatory enzyme, in the synthesis of methionine in seeds, we generated transgenic RNAi seeds with targeted repression of AtCGS during late developmental stages of seeds. Unexpectedly, these seeds accumulated 2.5-fold more methionine than wild-type seeds. To study the nature of these seeds, transcriptomic and primary metabolite profiling were employed using Affymetrix ATH1 microarray and gas chromatography-mass spectrometry analyses, respectively. The results were compared to transgenic Arabidopsis seeds expressing a feedback-insensitive form of AtCGS (named SSE-AtD-CGS) that were previously showed to accumulate up to sixfold more soluble methionine than wild-type seeds. Statistical assessments showed that the nature of transcriptomic and metabolic changes that occurred in RNAi::AtCGS seeds were relatively similar, but to lesser extents, to those previously reported for SSE-AtD-CGS seeds, and linked to the induction of global transcriptomic and metabolic responses associated with stronger desiccation stress. As transgenic seeds obtained by both manipulations exhibited higher, but different methionine levels, the data strongly suggest that these changes depend on the absolute amounts of methionine within seeds and much less to the expression level of AtCGS.

Metabolic engineering strategies for optimizing acetate reduction, ethanol yield and osmotolerance in Saccharomyces cerevisiae

BACKGROUND

Glycerol, whose formation contributes to cellular redox balancing and osmoregulation in Saccharomyces cerevisiae, is an important by-product of yeast-based bioethanol production. Replacing the glycerol pathway by an engineered pathway for NAD⁺-dependent acetate reduction has been shown to improve ethanol yields and contribute to detoxification of acetate-containing media. However, the osmosensitivity of glycerol non-producing strains limits their applicability in high-osmolarity industrial processes. This study explores engineering strategies for minimizing glycerol production by acetate-reducing strains, while retaining osmotolerance.

RESULTS

GPD2 encodes one of two S. cerevisiae isoenzymes of NAD⁺-dependent glycerol-3-phosphate dehydrogenase (G3PDH). Its deletion in an acetate-reducing strain yielded a fourfold lower glycerol production in anaerobic, low-osmolarity cultures but hardly affected glycerol production at high osmolarity. Replacement of both native G3PDHs by an archaeal NADP⁺-preferring enzyme, combined with deletion of ALD6, yielded an acetate-reducing strain the phenotype of which resembled that of a glycerol-negative gpd1Δ gpd2Δ strain in low-osmolarity cultures. This strain grew anaerobically at high osmolarity (1 mol L^-1 glucose), while consuming acetate and producing virtually no extracellular glycerol. Its ethanol yield in high-osmolarity cultures was 13% higher than that of an acetate-reducing strain expressing the native glycerol pathway.

CONCLUSIONS

Deletion of GPD2 provides an attractive strategy for improving product yields of acetate-reducing S. cerevisiae strains in low, but not in high-osmolarity media. Replacement of the native yeast G3PDHs by a heterologous NADP⁺-preferring enzyme, combined with deletion of ALD6, virtually eliminated glycerol production in high-osmolarity cultures while enabling efficient reduction of acetate to ethanol. After further optimization of growth kinetics, this strategy for uncoupling the roles of glycerol formation in redox homeostasis and osmotolerance can be applicable for improving performance of industrial strains in high-gravity acetate-containing processes.

Ecological genomics of local adaptation in Cornus florida L. by genotyping by sequencing

Discovering local adaptation, its genetic underpinnings, and environmental drivers is important for conserving forest species. Ecological genomic approaches coupled with next-generation sequencing are useful means to detect local adaptation and uncover its underlying genetic basis in nonmodel species. We report results from a study on flowering dogwood trees (Cornus florida L.) using genotyping by sequencing (GBS). This species is ecologically important to eastern US forests but is severely threatened by fungal diseases. We analyzed subpopulations in divergent ecological habitats within North Carolina to uncover loci under local selection and associated with environmental-functional traits or disease infection. At this scale, we tested the effect of incorporating additional sequencing before scaling for a broader examination of the entire range. To test for biases of GBS, we sequenced two similarly sampled libraries independently from six populations of three ecological habitats. We obtained environmental-functional traits for each subpopulation to identify associations with genotypes via latent factor mixed modeling (LFMM) and gradient forests analysis. To test whether heterogeneity of abiotic pressures resulted in genetic differentiation indicative of local adaptation, we evaluated Fst per locus while accounting for genetic differentiation between coastal subpopulations and Piedmont-Mountain subpopulations. Of the 54 candidate loci with sufficient evidence of being under selection among both libraries, 28-39 were Arlequin-BayeScan Fst outliers. For LFMM, 45 candidates were associated with climate (of 54), 30 were associated with soil properties, and four were associated with plant health. Reanalysis of combined libraries showed that 42 candidate loci still showed evidence of being under selection. We conclude environment-driven selection on specific loci has resulted in local adaptation in response to potassium deficiencies, temperature, precipitation, and (to a marginal extent) disease. High allele turnover along ecological gradients further supports the adaptive significance of loci speculated to be under selection.

[Biochemical basis of tolerance to osmotic stress in phytopathogenic fungus: The case of Macrophomina phaseolina (Tassi) Goid.]

Fungus Macrophomina phaseolina (Tassi) Goid. is the causative agent of charcoal rot disease which causes significant yield losses in major crops such as maize, sorghum, soybean and common beans in Mexico. This fungus is a facultative parasite which shows broad ability to adapt itself to stressed environments where water deficits and/or high temperature stresses commonly occur. These environmental conditions are common for most cultivable lands throughout Mexico. Here we describe some basic facts related to the etiology and epidemiology of the fungus as well as to the importance of responses to stressed environments, particularly to water deficits, based on morphology and growth traits, as well as on physiology, biochemistry and pathogenicity of fungus M. phaseolina. To conclude, we show some perspectives related to future research into the genus, which emphasize the increasing need to improve the knowledge based on the application of both traditional and biotechnological tools in order to elucidate the mechanisms of resistance to environmental stress which can be extrapolated to other useful organisms to man.

Molecular Bases of Desiccation Tolerance in Plant Cells and Potential Applications in Food Dehydration

Desiccation has many detrimental effects on the structure and function of biological membranes and proteins and this molecular damage decreases the freshness appearance of dehydrated foods. Phospholipid membranes are destabilised upon water stress by insertion of cellular amphiphiles, phase transition into the gel phase and membrane fusion. Proteins are denatured and electron transport chains are perturbed leading to increased formation of reactive oxygen species which cause irreversible damage of cellular structures. Cells respond to water stress by generating defense proteins and metabolites and eventually develop outstanding desiccation tolerance such as in the case of plant seeds and pollen, fungal spores, crustacean cysts, etc. The molecular bases for this remarkable phenomenon are not completely understood but several important principles have been identified. Three biological systems seem to act in concert to achieve desiccation tolerance: enzymes involved in osmolyte synthesis; proteins specialised in desiccation protection of membranes and proteins (LEA proteins), and antioxidant enzymes and molecules. Both osmolytes and LEA proteins contribute to stabilisation of membrane and protein structures by conferring preferential hydration at moderate desiccation and replacing water at extreme desiccation. Osmolytes also contribute to osmotic adjustment and act as hydroxyl radical scavengers. Genetically modified plants with increased production of these defenses could be useful to improve the quality of dried food.

Potential application of glycerol in the production of plant beneficial microorganisms

This review highlights the importance of research for development of biofertilizer and biocontrol products based on the use of glycerol for further process scale-up to industrial microbiology. Glycerol can be used successfully in all stages of production of plant beneficial microorganisms. It serves as an excellent substrate in both submerged and solid-state fermentation processes with free and immobilized microbial cells. Glycerol is also one of the most attractive formulation agents that ensures high cell density and viability including in harsh environmental conditions. Future research is discussed to make this inexpensive material a base for industrial production of plant beneficial microorganisms.

Improving ethanol yield in acetate-reducing Saccharomyces cerevisiae by cofactor engineering of 6-phosphogluconate dehydrogenase and deletion of ALD6

BACKGROUND

Acetic acid, an inhibitor of sugar fermentation by yeast, is invariably present in lignocellulosic hydrolysates which are used or considered as feedstocks for yeast-based bioethanol production. Saccharomyces cerevisiae strains have been constructed, in which anaerobic reduction of acetic acid to ethanol replaces glycerol formation as a mechanism for reoxidizing NADH formed in biosynthesis. An increase in the amount of acetate that can be reduced to ethanol should further decrease acetic acid concentrations and enable higher ethanol yields in industrial processes based on lignocellulosic feedstocks. The stoichiometric requirement of acetate reduction for NADH implies that increased generation of NADH in cytosolic biosynthetic reactions should enhance acetate consumption.

RESULTS

Replacement of the native NADP(+)-dependent 6-phosphogluconate dehydrogenase in S. cerevisiae by a prokaryotic NAD(+)-dependent enzyme resulted in increased cytosolic NADH formation, as demonstrated by a ca. 15% increase in the glycerol yield on glucose in anaerobic cultures. Additional deletion of ALD6, which encodes an NADP(+)-dependent acetaldehyde dehydrogenase, led to a 39% increase in the glycerol yield compared to a non-engineered strain. Subsequent replacement of glycerol formation by an acetate reduction pathway resulted in a 44% increase of acetate consumption per amount of biomass formed, as compared to an engineered, acetate-reducing strain that expressed the native 6-phosphogluconate dehydrogenase and ALD6. Compared to a non-acetate reducing reference strain under the same conditions, this resulted in a ca. 13% increase in the ethanol yield on glucose.

CONCLUSIONS

The combination of NAD(+)-dependent 6-phosphogluconate dehydrogenase expression and deletion of ALD6 resulted in a marked increase in the amount of acetate that was consumed in these proof-of-principle experiments, and this concept is ready for further testing in industrial strains as well as in hydrolysates. Altering the cofactor specificity of the oxidative branch of the pentose-phosphate pathway in S. cerevisiae can also be used to increase glycerol production in wine fermentation and to improve NADH generation and/or generation of precursors derived from the pentose-phosphate pathway in other industrial applications of this yeast.

Contrasting strategies used by lichen microalgae to cope with desiccation-rehydration stress revealed by metabolite profiling and cell wall analysis

Most lichens in general, and their phycobionts in particular, are desiccation tolerant, but their mechanisms of desiccation tolerance (DT) remain obscure. The physiological responses and cell wall features of two putatively contrasting lichen-forming microalgae, Trebouxia sp. TR9 (TR9), isolated from Ramalina farinacea (adapted to frequent desiccation-rehydration cycles), and Coccomyxa solorina-saccatae (Csol), obtained from Solorina saccata (growing in usually humid limestone crevices, subjected to seasonal dry periods) was characterized. Microalgal cultures were desiccated under 25%-30% RH and then rehydrated. Under these conditions, RWC and ψw decreased faster and simultaneously during dehydration in Csol, whereas TR9 maintained its ψw until 70% RWC. The metabolic profile indicated that polyols played a key role in DT of both microalgae. However, TR9 constitutively accumulated higher amounts of polyols, whereas Csol induced the polyol synthesis under desiccation-rehydration. Csol also accumulated ascorbic acid, while TR9 synthesized protective raffinose-family oligosaccharides (RFOs) and increased its content of phenolics. Additionally, TR9 exhibited thicker and qualitatively different cell wall and extracellular polymeric layer compared with Csol, indicating higher water retention capability. The findings were consistent with the notion that lichen microalgae would have evolved distinct strategies to cope with desiccation-rehydration stress in correspondence with the water regime of their respective habitats.

Inactivation of glutamate racemase (MurI) eliminates virulence in Streptococcus mutans

Inhibition of enzymes required for bacterial cell wall synthesis is often lethal or leads to virulence defects. Glutamate racemase (MurI), an essential enzyme in peptidoglycan biosynthesis, has been an attractive target for therapeutic interventions. Streptococcus mutans, one of the many etiological factors of dental caries, possesses a series of virulence factors associated with cariogenicity. However, little is known regarding the mechanism by which MurI influences pathogenesis of S. mutans. In this work, a stable mutant of S. mutans deficient in glutamate racemase (S. mutans FW1718) was constructed to investigate the impact of murI inactivation on cariogenic virulence in S. mutans UA159. Microscopy revealed that the murI mutant exhibited an enlarged cell size, longer cell chains, diminished cell⬜cell aggregation, and altered cell surface ultrastructure compared with the wild-type. Characterization of this mutant revealed that murI deficiency weakened acidogenicity, aciduricity, and biofilm formation ability of S. mutans (P<0.05). Real-time quantitative polymerase chain reaction (qRT-PCR) analysis demonstrated that the deletion of murI reduced the expression of the acidogenesis-related gene ldh by 44-fold (P<0.0001). The expression levels of the gene coding for surface protein antigen P (spaP) and the acid-tolerance related gene (atpD) were down-regulated by 99% (P<0.0001). Expression of comE, comD, gtfB and gtfC, genes related to biofilm formation, were down-regulated 8-, 43-, 85- and 298-fold in the murI mutant compared with the wild-type (P<0.0001), respectively. Taken together, the current study provides the first evidence that MurI deficiency adversely affects S. mutans virulence properties, making MurI a potential target for controlling dental caries.

Hydrophilins in the filamentous fungus Neosartorya fischeri (Aspergillus fischeri) have protective activity against several types of microbial water stress

Hydrophilins are proteins that occur in all domains of life and protect cells and organisms against drought and other stresses. They include most of the late embryogenesis abundant (LEA) proteins and the heat shock protein (HSP) Hsp12. Here, the role of a predicted LEA-like protein (LeamA) and two Hsp12 proteins (Hsp12A and Hsp12B) of Neosartorya fischeri was studied. This filamentous fungus forms ascospores that belong to the most stress-resistant eukaryotic cells described to date. Heterologous expression of LeamA, Hsp12A and Hsp12B resulted in increased tolerance against salt and osmotic stress in Escherichia coli. These proteins were also shown to protect lactate dehydrogenase against dry heat and freeze-thaw cycles in vitro. Deletion of leamA caused diminished viability of sexual ascospores after drought and heat. This is the first report on functionality of Hsp12 and putative LeamA proteins derived from filamentous fungi, and their possible role in N. fischeri ascospore resistance against desiccation, high temperature and osmotic stress is discussed.

Crystal Structure and Comparative Sequence Analysis of GmhA from Colwellia psychrerythraea Strain 34H Provides Insight into Functional Similarity with DiaA

The psychrophilic organism Colwellia psychrerythraea strain 34H produces extracellular polysaccharide substances to tolerate cold environments. Sedoheptulose 7-phosphate isomerase (GmhA) is essential for producing d-glycero-d-mannoheptose 7-phosphate, a key mediator in the lipopolysaccharide biosynthetic pathway. We determined the crystal structure of GmhA from C. psychrerythraea strain 34H (CpsGmhA, UniProtKB code: Q47VU0) at a resolution of 2.8 Å. The tetrameric structure is similar to that of homologous GmhA structures. Interestingly, one of the catalytic residues, glutamate, which has been reported to be critical for the activity of other homologous GmhA enzymes, is replaced by a glutamine residue in the CpsGmhA protein. We also found differences in the conformations of several other catalytic residues. Extensive structural and sequence analyses reveal that CpsGmhA shows high similarity to Escherichia coli DnaA initiator-associating protein A (DiaA). Therefore, the CpsGmhA structure reported here may provide insight into the structural and functional correlations between GmhA and DiaA among specific microorganisms.

Symbiosis dependent accumulation of primary metabolites in arbuscule-containing cells

BACKGROUND

The arbuscular mycorrhizal symbiosis is characterized by the presence of different symbiotic structures and stages within a root system. Therefore tools allowing the analysis of molecular changes at a cellular level are required to reveal insight into arbuscular mycorrhizal (AM) symbiosis development and functioning.

RESULTS

Here we describe the analysis of metabolite pools in arbuscule-containing cells, which are the site of nutrient transfer between AM fungus and host plant. Laser capture microdissection (LCM) combined with gas chromatography mass spectrometry (GC-EI/TOF-MS) enabled the analysis of primary metabolite levels,which might be of plant or fungal origin, within these cells.

CONCLUSIONS

High levels of the amino acids, aspartate, asparagine, glutamate, and glutamine, were observed in arbuscule-containing cells. Elevated amounts of sucrose and the steady-state of hexose levels indicated a direct assimilation of monosaccharides by the fungal partner.

Mannitol metabolism during pathogenic fungal-host interactions under stressed conditions

Numerous plants and fungi produce mannitol, which may serve as an osmolyte or metabolic store; furthermore, mannitol also acts as a powerful quencher of reactive oxygen species (ROS). Some phytopathogenic fungi use mannitol to stifle ROS-mediated plant resistance. Mannitol is essential in pathogenesis to balance cell reinforcements produced by both plants and animals. Mannitol likewise serves as a source of reducing power, managing coenzymes, and controlling cytoplasmic pH by going about as a sink or hotspot for protons. The metabolic pathways for mannitol biosynthesis and catabolism have been characterized in filamentous fungi by direct diminishment of fructose-6-phosphate into mannitol-1-phosphate including a mannitol-1-phosphate phosphatase catalyst. In plants mannitol is integrated from mannose-6-phosphate to mannitol-1-phosphate, which then dephosphorylates to mannitol. The enzyme mannitol dehydrogenase plays a key role in host–pathogen interactions and must be co-localized with pathogen-secreted mannitol to resist the infection.

TdERF1, an ethylene response factor associated with dehydration responses in durum wheat (Triticum turgidum L. subsp. durum)

Water deficit and increasing salinization reduce productivity of wheat, the leading crop for human diet. While the complete genome sequence of this crop has not been deciphered, a BAC library screening allowed the isolation of TdERF1, the first ethylene response factor gene from durum wheat. This gene is putatively involved in mediating salt stress tolerance and its characterization provides clues toward understanding the mechanisms underlying the adaptation/tolerance of durum wheat to suboptimal growth conditions. TdERF1 expression is differentially induced by high salt treatment in 2 durum wheat varieties, the salt-tolerant Grecale (GR) and the salt-sensitive Om Rabiaa (OR). To further extend these findings, we show here that the expression of this ERF is correlated with physiological parameters, such as the accumulation of osmo-regulators and membrane integrity, that discriminate between the 2 contrasted wheat genotypes. The data confirm that GR and OR are 2 contrasted wheat genotypes with regard to salt-stress and show that TdERF1 is also induced by water stress with an expression pattern clearly discriminating between the 2 genotypes. These findings suggest that TdERF1 might be involved in responses to salt and water stress providing a potential genetic marker discriminating between tolerant and sensitive wheat varieties.

Potassium Retention under Salt Stress Is Associated with Natural Variation in Salinity Tolerance among Arabidopsis Accessions

Plants are exposed to various environmental stresses during their life cycle such as salt, drought and cold. Natural variation mediated plant growth adaptation has been employed as an effective approach in response to the diverse environmental cues such as salt stress. However, the molecular mechanism underlying this process is not well understood. In the present study, a collection of 82 Arabidopsis thaliana accessions (ecotypes) was screened with a view to identify variation for salinity tolerance. Seven accessions showed a higher level of tolerance than Col-0. The young seedlings of the tolerant accessions demonstrated a higher K(+) content and a lower Na(+)/K(+) ratio when exposed to salinity stress, but its Na(+) content was the same as that of Col-0. The K(+) transporter genes AtHAK5, AtCHX17 and AtKUP1 were up-regulated significantly in almost all the tolerant accessions, even in the absence of salinity stress. There was little genetic variation or positive transcriptional variation between the selections and Col-0 with respect to Na+-related transporter genes, as AtSOS genes, AtNHX1 and AtHKT1;1. In addition, under salinity stress, these selections accumulated higher compatible solutes and lower reactive oxygen species than did Col-0. Taken together, our results showed that natural variation in salinity tolerance of Arabidopsis seems to have been achieved by the strong capacity of K(+) retention.

Metabolite responses to exogenous application of nitrogen, cytokinin, and ethylene inhibitors in relation to heat-induced senescence in creeping bentgrass

The exogenous application of ethylene inhibitors, cyotkinins, or nitrogen has previously been shown to suppress heat-induced senescence and improve heat tolerance in cool -season grasses. The objectives of this study were to examine metabolic profiles altered by exogenous treatment of creeping bentgrass with an ethylene inhibitor, cytokinin or nitrogen under heat stress and to determine metabolic pathways regulated by those compounds in association with their effectiveness for improving heat tolerance. Creeping bentgrass (Agostis stolonifera) plants (cv. Penncross) were foliar sprayed with 18 mM carbonyldiamide (N source), 25μM aminoethoxyvinylglycine (AVG, ethylene inhibitor), 25μM zeatin riboside (ZR, cytokinin), or a water control, and then exposed to 20/15°C (day/night) or 35/30°C (heat stress) in growth chambers. All three exogenous treatments suppressed leaf senescence, as manifested by increased turf quality and chlorophyll content, and reduced electrolyte leakage under heat stress. Polar metabolite profiling identified increases in the content of certain organic acids (i.e. citric and malic acid), sugar alcohols, disaccharides (sucrose), and decreased accumulations of monosaccharides (i.e. glucose and fructose) with exogenous treatment of N, AVG, or ZR at the previously mentioned concentrations when compared to the untreated control under heat stress. Nitrogen stimulated amino acid accumulation whereas AVG and ZR reduced amino acid accumulation compared to the untreated control under heat stress. These results revealed that the alleviation of heat-induced leaf senescence by N, AVG, and ZR could be due to changes in the accumulation of metabolites involved in osmoregulation, antioxidant metabolism, carbon and nitrogen metabolism, as well as stress signaling molecules.