Exploring the neutral invertase–oxidative stress defence connection in Arabidopsis thaliana

Over the past decades, considerable advances have been made in understanding the crucial role and the regulation of sucrose metabolism in plants. Among the various sucrose-catabolizing enzymes, alkaline/neutral invertases (A/N-Invs) have long remained poorly studied. However, recent findings have demonstrated the presence of A/N-Invs in various organelles in addition to the cytosol, and their importance for plant development and stress tolerance. A cytosolic (At-A/N-InvG, At1g35580) and a mitochondrial (At-A/N-InvA, At1g56560) member of the A/N-Invs have been analysed in more detail in Arabidopsis and it was found that At-A/N-InvA knockout plants show an even more severe growth phenotype than At-A/N-InvG knockout plants. The absence of either A/N-Inv was associated with higher oxidative stress defence gene expression, while transient overexpression of At-A/N-InvA and At-A/N-InvG in leaf mesophyll protoplasts down-regulated the oxidative stress-responsive ascorbate peroxidase 2 (APX2) promoter. Moreover, up-regulation of the APX2 promoter by hydrogen peroxide or abscisic acid could be blocked by adding metabolizable sugars or ascorbate. A hypothetical model is proposed in which both mitochondrial and cytosolic A/N-Invs can generate glucose as a substrate for mitochondria-associated hexokinase, contributing to mitochondrial reactive oxygen species homeostasis

Metabolism of galactosyl-oligosaccharides in Caryophyllaceae.

The raffinose family oligosaccharides (RFOs) are the most widespread galactosyl-sucrose oligosaccharides in the plant kingdom. Raffinose (Gal-α(1→6)-Glc-α(1→2)ß-Fru), the smallest member of the family, can be further elongated by the additional transfer of galactosyl residues to the galactose of raffinose by α(1→6) linkages. In the Caryophyllaceae, however, these typical α(1→6) oligomers of raffinose appear to be replaced by other types of galactosyl-oligosaccharides (GOS). During this project, the structure and metabolism of these GOS were further elucidated using Stellaria media, a typical representative of the Caryophyllaceae. Next to raffinose the tetrasaccharide lychnose, an α-(1→1) galactosyl extension on the fructose of raffinose, is the predominant GOS in S. media. Its structure was unambiguously determined by biochemical approaches and NMR. Lychnose synthesis occurs by lychnose synthase, a raffinose:raffinose 1Fru galactosyltransferase, using only raffinose both as donor and acceptor substrate. After partial purification of the native enzyme, lychnose synthase was characterized, cloned and heterologously expressed. Although concentrations up to 375 mM were used, no saturation could be observed. Sucrose inhibits the biosynthesis reaction which is optimal at pH 5.4 - 6.0. The two other GOS accumulating in S. media are mediose, an α(1→4) galactosyl extension on the glucose of raffinose and stellariose, an α(1→4) galactosyl extension on the glucose of lychnose. Mediose and stellariose are both synthesized by a raffinose:lychnose 4Glc galactosyltransferase or stellariose synthase, which catalyzes the direct transfer of a galactose residue of raffinose to raffinose or lychnose, respectively. This enzyme however, was neither characterized nor cloned. Most likely, these GOS are subject to partial trimming by endogenous acid and alkaline α‑galactosidases. Lychnose can be broken down into the trisaccharide Glc-α(1→2)ß-Fru-α(1→1)-Gal, which structure was determined by NMR. An alkaline α-galactosidase of Silene dioica was cloned and heterologously expressed in E. coli. Finally, S. media plants were grown and acclimated (4°C) under laboratory conditions to see whether lychnose metabolism affects its frost tolerance. During low temperature acclimation a consecutive, temporal increase in the raffinose and lychnose concentration could be observed, resulting in a distinct positive correlation with increased freezing tolerance.status: publishe

Complete NMR characterization of lychnose from Stellaria media (L.) Vill

Lychnose (alpha-D-Gal-(1-->6)-alpha-D-Glc-(1-->2)-beta-D-Fru-(1-->1)-alpha-D-Gal) was isolated from Stellaria media, a representative member of the Caryophyllaceae plant family. Weak acid hydrolysis, enzymatic hydrolysis and complete NMR characterization were performed to confirm the identity of the tetrasaccharide. All (1)H and (13)C resonances were unambiguously assigned and the conformation of the sugars was determined using one and two dimensional NMR techniques. Anomeric characterizations in lychnose were confirmed from HMBC and NOESY spectra.status: publishe

Genome-wide characterization of ISR induced in Arabidopsis thaliana by Trichoderma hamatum T382 against Botrytis cinerea infection

In this study, the molecular basis of the induced systemic resistance (ISR) in Arabidopsis thaliana by the biocontrol fungus Trichoderma hamatum T382 against the phytopathogen Botrytis cinerea B05-10 was unraveled by microarray analysis both before (ISR-prime) and after (ISR-boost) additional pathogen inoculation. The observed high numbers of differentially expressed genes allowed us to classify them according to the biological pathways in which they are involved. By focusing on pathways instead of genes, a holistic picture of the mechanisms underlying ISR emerged. In general, a close resemblance is observed between ISR-prime and systemic acquired resistance (SAR), the systemic defense response that is triggered in plants upon pathogen infection leading to increased resistance towards secondary infections. Treatment with Trichoderma hamatum T382 primes the plant (ISR-prime), resulting in an accelerated activation of the defense response against Botrytis cinerea during ISR-boost and a subsequent moderation of the Botrytis cinerea induced defense response (BIDR). Microarray results were validated for representative genes by qRT-PCR. The involvement of various defense-related pathways was confirmed by phenotypic analysis of mutants affected in these pathways, thereby proving the validity of our approach. Combined with additional anthocyanin analysis data these results all point to the involvement of the phenylpropanoid pathway in Trichoderma hamatum T382-induced ISR.status: publishe

Global cytosine methylation in Daphnia magna depends on genotype, environment and their interaction

The authors characterized global cytosine methylation levels in two different genotypes of the ecotoxicological model organism Daphnia magna after exposure to a wide array of biotic and abiotic environmental stressors. The study aimed to improve the authors' understanding of the role of cytosine methylation in the organism‘s response to environmental conditions. The authors observed a significant genotype effect, an environment effect and a genotype x environment effect. In particular, global cytosine methylation levels were significantly altered after exposure to Triops predation cues, Microcystis and sodium chloride compared to control conditions. Significant differences between the two genotypes were observed when animals were exposed to Triops predation cues, Microcystis, Cryptomonas and sodium chloride. Despite the low global methylation rate under control conditions (0.49-0.52%), global cytosine methylation levels upon exposure to Triops demonstrated a five-fold difference between the genotypes (0.21% versus 1.02%). No effects were found in response to arsenic, cadmium, fish, lead, pH of 5.5, pH of 8, temperature, hypoxia and white fat cell disease. The authors’ results point to the potential role of epigenetic effects under changing environmental conditions such as predation (i.e. Triops), diet (i.e. Cryptomonas and Microcystis) and salinity. The current results indicate that, despite global cytosine methylation levels are low, epigenetic effects may be important in environmental studies on Daphnia. This article is protected by copyright. All rights reservedstatus: publishe

The importance of the terrigenous fraction within a cold-water coral mound: A case study

In the nineties, cold-water coral mounds were discovered in the Porcupine Seabight (NE Atlantic, west of Ireland). A decade later, this discovery led to the drilling of the entire Challenger cold-water coral mound (Eastern slope, Porcupine Seabight) during IODP Expedition 307. As more than 50% of the sediment within Challenger Mound consists of terrigenous material, the terrigenous component is equally important for the build-up of the mound as the framework-building corals. Moreover, the terrigenous fraction contains important information on the dynamics and the conditions of the depositional environment during mound development. In this study, the first in-depth investigation of the terrigenous sediment fraction of a cold-water coral mound is performed, combining clay mineralogy, sedimentology, petrography and Sr–Nd-isotopic analysis on a gravity core (MD01-2451G) collected at the top of Challenger Mound.Sr- and Nd-isotopic fingerprinting identifies Ireland as the main contributor of terrigenous material in Challenger Mound. Besides this, a variable input of volcanic material from the northern volcanic provinces (Iceland and/or the NW British Isles) is recognized in most of the samples. This volcanic material was most likely transported to Challenger Mound during cold climatic stages. In three samples, the isotopic ratios indicate a minor contribution of sediment deriving from the old cratons on Greenland, Scandinavia or Canada. The grain-size distributions of glacial sediments demonstrate that ice-rafted debris was deposited with little or no sorting, indicating a slow bottom-current regime. In contrast, interglacial intervals contain strongly current-sorted sediments, including reworked glacio-marine grains. The micro textures of the quartz-sand grains confirm the presence of grains transported by icebergs in interglacial intervals. These observations highlight the role of ice-rafting as an important transport mechanism of terrigenous material towards the mound during the Late Quaternary.Furthermore, elevated smectite content in the siliciclastic, glaciomarine sediment intervals is linked to the deglaciation history of the British-Irish Ice Sheet (BIIS). The increase of smectite is attributed to the initial stage of chemical weathering processes, which became activated following glacial retreat and the onset of warmer climatic conditions. During these deglaciations a significant change in the signature of the detrital fraction and a lack of coral growth is observed. Therefore, we postulate that the deglaciation of the BIIS has an important effect on mound growth. It can seriously alter the hydrography, nutrient supply and sedimentation processes, thereby affecting both sediment input and coral growth and hence, coral mound development

(Table 1) Neodymium and Strontium measurements from sediment core MD01-2451

In the nineties, cold-water coral mounds were discovered in the Porcupine Seabight (NE Atlantic, west of Ireland). A decade later, this discovery led to the drilling of the entire Challenger cold-water coral mound (Eastern slope, Porcupine Seabight) during IODP Expedition 307. As more than 50% of the sediment within Challenger Mound consists of terrigenous material, the terrigenous component is equally important for the build-up of the mound as the framework-building corals. Moreover, the terrigenous fraction contains important information on the dynamics and the conditions of the depositional environment during mound development. In this study, the first in-depth investigation of the terrigenous sediment fraction of a cold-water coral mound is performed, combining clay mineralogy, sedimentology, petrography and Sr-Nd-isotopic analysis on a gravity core (MD01-2451G) collected at the top of Challenger Mound. Sr- and Nd-isotopic fingerprinting identifies Ireland as the main contributor of terrigenous material in Challenger Mound. Besides this, a variable input of volcanic material from the northern volcanic provinces (Iceland and/or the NW British Isles) is recognized in most of the samples. This volcanic material was most likely transported to Challenger Mound during cold climatic stages. In three samples, the isotopic ratios indicate a minor contribution of sediment deriving from the old cratons on Greenland, Scandinavia or Canada. The grain-size distributions of glacial sediments demonstrate that ice-rafted debris was deposited with little or no sorting, indicating a slow bottom-current regime. In contrast, interglacial intervals contain strongly current-sorted sediments, including reworked glacio-marine grains. The micro textures of the quartz-sand grains confirm the presence of grains transported by icebergs in interglacial intervals. These observations highlight the role of ice-rafting as an important transport mechanism of terrigenous material towards the mound during the Late Quaternary. Furthermore, elevated smectite content in the siliciclastic, glaciomarine sediment intervals is linked to the deglaciation history of the British-Irish Ice Sheet (BIIS). The increase of smectite is attributed to the initial stage of chemical weathering processes, which became activated following glacial retreat and the onset of warmer climatic conditions. During these deglaciations a significant change in the signature of the detrital fraction and a lack of coral growth is observed. Therefore, we postulate that the deglaciation of the BIIS has an important effect on mound growth. It can seriously alter the hydrography, nutrient supply and sedimentation processes, thereby affecting both sediment input and coral growth and hence, coral mound development