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See detailHomology modeling and in vivo functional characterization of the zinc permeation pathway in a heavy metal P-type ATPase
Lekeux, Gilles ULiege; Crowet, Jean-Marc; Nouet, Cécile ULiege et al

in Journal of Experimental Botany (2019), 70

The P1B ATPase Heavy Metal ATPase 4 (HMA4) is responsible for zinc and cadmium translocation from roots to shoots in the plant Arabidopsis thaliana. It couples ATP hydrolysis to cytosolic domain movements ... [more ▼]

The P1B ATPase Heavy Metal ATPase 4 (HMA4) is responsible for zinc and cadmium translocation from roots to shoots in the plant Arabidopsis thaliana. It couples ATP hydrolysis to cytosolic domain movements enabling metal transport across the membrane. Thanks to high conservation level within the P-type ATPase family, the role of the HMA4 cytoplasmic catalytic domains can be inferred from well characterized pumps. In contrast, the function of its terminal cytosolic extensions as well as the metal permeation mechanism through the membrane remains elusive. Here, homology modeling of the HMA4 transmembrane region was conducted based on the crystal structure of a ZntA bacterial homolog. The analysis highlighted amino acids forming a metal permeation pathway, whose importance was subsequently investigated functionally through mutagenesis and complementation experiments in plants. Although the zinc pathway displayed overall conservation among the two proteins, significant differences were observed, especially in the entrance area with altered electronegativity and the presence of a salt bridge/H-bond network. The analysis also newly identified amino acids whose mutation results in total or partial loss of the protein function. In addition, comparison of zinc and cadmium accumulation in shoots of A. thaliana complemented lines revealed a number of HMA4 mutants exhibiting different abilities in zinc and cadmium translocation. These observations could be instrumental to design low cadmium accumulating crops, hence decreasing human cadmium exposure . [less ▲]

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See detailEtude à long terme de la biogéochimie des écosystèmes forestiers (2017-2018/1)
Bosman, Bernard ULiege; Klenkenberg, Sophie ULiege; Carnol, Monique ULiege

Report (2018)

Depuis les années septante, la forêt a évolué d’une fonction surtout économique et productrice de bois vers un rôle multifonctionnel qui tient également compte des aspects écologiques, récréatifs ... [more ▼]

Depuis les années septante, la forêt a évolué d’une fonction surtout économique et productrice de bois vers un rôle multifonctionnel qui tient également compte des aspects écologiques, récréatifs, éducationnels et sociétaux. L’évolution des besoins de chacun et la prise en compte des services écosystémiques que la forêt peut nous fournir sont ainsi mis en avant. Parallèlement, le caractère non renouvelable des sols a été mis en évidence et des politiques ont été développées afin de préserver leur qualité. Une gestion forestière plus ‘durable’ se développe en tenant compte des différentes fonctions de la forêt, ainsi que de l’influence des pressions anthropiques exogènes (changement climatique, polluants…). La diversification des essences est apparue comme une possibilité de gestion pour maintenir les services écosystémiques fournis par les forêts et donc pour tendre vers une forêt plus ‘durable’. Pour mesurer l’impact de cette diversification, une bonne connaissance du fonctionnement de l’écosystème est indispensable. D’où le besoin d’outils de suivi et de diagnostic. Le développement d’indicateurs biologiques de la qualité du sol constitue actuellement une priorité de recherche sur les sols à l’échelle européenne. Lors des subventions précédentes, douze placettes intensives ont été équipées de dispositifs de collecte (72 collecteurs de pluviolessivats, 96 bougies poreuses) et les récoltes ont débutées en janvier 2013 pour les pluviolessivats et en janvier 2014 pour les bougies poreuses. Des mesures de la biomasse des arbres ont été réalisées dans les placettes d’étude intensive à Waroneu en juin 2013, 2015 et 2017 et à la Robinette en juin 2014 et 2016. Des échantillonnages de sol ont été réalisés en juin 2013, en janvier, mai et novembre 2014, en mai et décembre 2015, en novembre 2016 et en novembre 2017. Les mesures de l’évolution des concentrations et flux en éléments minéraux et en carbone organique dans les pluies, pluviolessivats, la solution du sol et l’exutoire de deux bassins versants ont été poursuivis. Ce rapport résulte d’une subvention entre le SPW et l’ULiège d’une durée de 7 mois. La structure du programme 2017-2018 pour un an était : 1. Fournir des mesures à long terme sur les concentrations et flux en éléments minéraux, carbone organique dans deux bassins versants forestiers 2. Fournir des mesures à long terme sur les indicateurs biologiques de la qualité des sols forestiers 3. Gérer la base de données 4. Evaluer l’évolution à long terme des dépôts acides et azotés 5. Analyser la variabilité spatiale et temporelle des processus biogéochimiques/indicateurs biologiques de la qualité du sol 6. Etudier la dynamique du carbone et de l’azote 7. Valoriser et disséminer les résultats des recherches Dans ce rapport nous présentons les perspectives pour l'analyse statistique de l'évolution à long terme des concentrations et des flux en éléments minéraux dans les bassins versants 4 forestiers ainsi que les résultats de la diversité fonctionnelle de l’échantillonnage des sols de novembre 2017. Résultats  L’applicabilité du test de tendance de Mann-Kendall et de l’algorithme BFAST sur des séries temporelles de 23 années de mesures dans les bassins versants de Waroneu et de la Robinette a été démontrée.  Entre 1992 et 2014, une diminution significative des concentrations en sulfates dans les pluies à découvert, les pluviolessivats sous épicéas et les eaux aux exutoires des bassins versants de Waroneu et de la Robinette a été mise en évidence et quantifiée.  Durant la même période, une diminution significative des concentrations en nitrates dans les pluies à découvert, les pluviolessivats sous épicéas et les eaux aux exutoires des bassins versants de Waroneu et de la Robinette a été observée.  Toujours sur cette période de 23 années, une augmentation significative du pH dans les pluies à découvert de la Robinette, les pluviolessivats sous épicéas et les eaux aux exutoires des bassins versants de Waroneu et de la Robinette a été montrée.  La parcelle W02 de Waroneu ne peut plus être considérée comme une parcelle sans couvert forestier et sera dorénavant nommée ‘en repousse’. Cette parcelle présente une diversité potentielle métabolique, une mesure de l’AWCD et un index de Shannon plus élevé que celle d’épicéas adultes sur sol brun acide (W05) de Waroneu. [less ▲]

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See detaildi-Cysteine motifs in the C-terminus of plant HMA4 proteins confer nanomolar affinity for zinc and are essential for HMA4 function in vivo.
Lekeux, Gilles ULiege; Laurent, Clémentine ULiege; Joris, Marine ULiege et al

in Journal of Experimental Botany (2018), 69(22), 5547-5560

The PIB ATPase heavy metal ATPase 4 (HMA4) has a central role in the zinc homeostasis network of Arabidopsis thaliana. This membrane protein loads metal from the pericycle cells into the xylem in roots ... [more ▼]

The PIB ATPase heavy metal ATPase 4 (HMA4) has a central role in the zinc homeostasis network of Arabidopsis thaliana. This membrane protein loads metal from the pericycle cells into the xylem in roots, thereby allowing root to shoot metal translocation. Moreover, HMA4 is key for zinc hyperaccumulation as well as zinc and cadmium hypertolerance in the pseudometallophyte Arabidopsis halleri. The plant-specific cytosolic C-terminal extension of HMA4 is rich in putative metal-binding residues and has substantially diverged between A. thaliana and A. halleri. To clarify the function of the domain in both species, protein variants with truncated C-terminal extension, as well as with mutated di-Cys motifs and/or a His-stretch, were functionally characterized. We show that di-Cys motifs, but not the His-stretch, contribute to high affinity zinc binding and function in planta. We suggest that the HMA4 C-terminal extension is at least partly responsible for protein targeting to the plasma membrane. Finally, we reveal that the C-terminal extensions of both A. thaliana and A. halleri HMA4 proteins share similar function, despite marginally different zinc-binding capacity. [less ▲]

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See detailMiscanthus Biochar had Limited Effects on Soil Physical Properties, Microbial Biomass, and Grain Yield in a Four-Year Field Experiment in Norway
O'Toole, Adam; Moni, Christophe; Weldon, Simon et al

in Agriculture (2018), 8(11), 171-190

he application of biochar to soils is a promising technique for increasing soil organic C and offsetting GHG emissions. However, large-scale adoption by farmers will likely require the proof of its ... [more ▼]

he application of biochar to soils is a promising technique for increasing soil organic C and offsetting GHG emissions. However, large-scale adoption by farmers will likely require the proof of its utility to improve plant growth and soil quality. In this context, we conducted a four-year field experiment between October 2010 to October 2014 on a fertile silty clay loam Albeluvisol in Norway to assess the impact of biochar on soil physical properties, soil microbial biomass, and oat and barley yield. The following treatments were included: Control (soil), miscanthus biochar 8 t C ha−1 (BC8), miscanthus straw feedstock 8 t C ha−1 (MC8), and miscanthus biochar 25 t C ha−1 (BC25). Average volumetric water content at field capacity was significantly higher in BC25 when compared to the control due to changes in BD and total porosity. The biochar amendment had no effect on soil aggregate (2–6 mm) stability, pore size distribution, penetration resistance, soil microbial biomass C and N, and basal respiration. Biochar did not alter crop yields of oat and barley during the four growing seasons. In order to realize biochar’s climate mitigation potential, we suggest future research and development efforts should focus on improving the agronomic utility of biochar in engineered fertilizer and soil amendment products. [less ▲]

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See detailEarly stage litter decomposition across biomes
Djukic, I.; Kepfer-Rojas, S.; Schmidt, I. K. et al

in Science of the Total Environment (2018), 628-629

Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order ... [more ▼]

Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and metaanalyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging from −9 to +26 °C MAT and from 60 to 3113mm MAP) across different ecosystems. In this study we tested the effect of climate (temperature and moisture), litter type and land-use on early stage decomposition (3 months) across nine biomes. We show that litter quality was the predominant controlling factor in early stage litter decomposition, which explained about 65% of the variability in litter decomposition at a global scale. The effect of climate, on the other hand, was not litter specific and explained b0.5% of the variation for Green tea and 5% for Rooibos tea, and was of significance only under unfavorable decomposition conditions (i.e. xeric versus mesic environments).When the data were aggregated at the biome scale, climate played a significant role on decomposition of both litter types (explaining 64% of the variation for Green tea and 72% for Rooibos tea).No significant effect of land-use on early stage litter decompositionwas notedwithin the temperate biome. Our results indicate that multiple drivers are affecting early stage littermass loss with litter quality being dominant. In order to be able to quantify the relative importance of the different drivers over time, long-term studies combined with experimental trials are needed. [less ▲]

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See detailSpatial and temporal variability of biological indicators of soil quality in two forest catchments in Belgium
Ratcliffe, Sophia ULiege; Bosman, Bernard ULiege; Carnol, Monique ULiege

in Applied Soil Ecology (2018), 126

Biological indicators, measurements based on the presence and activity of soil organisms, are increasingly being considered in assessments of forest soil quality. In addition to chemical indicators, such ... [more ▼]

Biological indicators, measurements based on the presence and activity of soil organisms, are increasingly being considered in assessments of forest soil quality. In addition to chemical indicators, such as soil organic carbon and pH, biological indicators can provide an early diagnosis of changes in soil functions and processes in response to environmental change and forest management actions. We investigated the spatial and temporal variability of selected bio-indicators in the forest floor of two catchments over three years. We further evaluated the sensitivity of these indicators to changes in the dominant tree species following reforestation and natural regeneration. Indicators of microbial abundance and activity (microbial biomass, potential respiration) and carbon maintenance costs (metabolic quotient) were higher under young spruce and mixed deciduous stands than pure stands of oak and beech. Our results indicate a greater microbial activity in the autumn but a wider range of carbon substrate utilisation in the spring. Evaluation of seasonal differences in bio-indicator values is vital for the evaluation and planning of long-term studies and the development of reference values for forest soils in Belgium. Our results highlight the usefulness of these bio-indicators in identifying changes in soil functioning, particularly in response to management activities, at small spatial scales. [less ▲]

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See detailAdaptation to high zinc depends on distinct mechanisms in metallicolous populations of Arabidopsis halleri
Schvartzman Echenique, Maria Sol ULiege; Corso, Massimiliano; Fataftha, Nazeer et al

in New Phytologist (2018)

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See detailLa qualité des sols, un capital
Carnol, Monique ULiege; Krüger, Inken ULiege; Malchair, Sandrine ULiege et al

in Blerot, Philippe; Heyninck, Christophe (Eds.) Le grand livre de la Forêt (2017)

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See detailSpatial and temporal variability of microbial soil quality indicators in two forest catchments
Carnol, Monique ULiege; Bosman, Bernard ULiege; Ratcliffe, Sophia ULiege

Poster (2017, August)

Appropriate indicators for assessing forest soil quality are needed to monitor changes in the frame of air pollution and climate change, assess impacts of forest management actions and consequently decide ... [more ▼]

Appropriate indicators for assessing forest soil quality are needed to monitor changes in the frame of air pollution and climate change, assess impacts of forest management actions and consequently decide on long-term management objectives. In addition to chemical indicators, such as for example soil organic carbon and pH, biological indicators inform on soil functions/processes and provide an early diagnosis of changes in the environment. The objectives of this work were to provide first baseline values for Belgian forest soils, to evaluate the sensitivity of selected indicators to changes in tree species on the same soil type, and to assess the temporal variability of the indicators. This work is part of a long-term study on element concentrations and fluxes in 2 small (each ca. 80 ha) forested watersheds (Waroneu and Robinette, east Belgium) in relation to forest management. Both catchments show spatial variability of tree species cover and drainage. In particular, Robinette was partially clear-cut in 1996 and replanted with deciduous tree species in 1998. Six sub-plots with a specific tree species/soil hydrology combination were defined in each catchment. Selected biological indicators (potential respiration, nitrogen mineralization, microbial biomass, metabolic diversity of bacteria) and two ecophysiological indices (microbial quotient and metabolic quotient) were measured in spring and autumn during 3 consecutive years. Our results suggest that (1) radar plots can be a useful tool to visualize the ranges of several indicators simultaneously (2) the biological indicators showed significant differences between the sub-plots, even though they are situated on the same geological substrate and in a limited geographical region, indicating high sensitivity to detect changes (3) Small scale differences detected raise the question of the appropriate spatial scale and classifying criteria to define baseline values (4) there were no marked seasonal differences, but for some indicators (N mineralization, potential respiration, microbial and metabolic quotients) the sampling moment induced a certain variability. [less ▲]

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See detailMetal binding to the N-terminal cytoplasmic domain of the PIB ATPase HMA4 is required for metal transport in Arabidopsis.
Laurent, Clémentine ULiege; Lekeux, Gilles ULiege; Ukuwela, Ashwinie A et al

in Plant Molecular Biology (2016), 90

PIB ATPases are metal cation pumps that transport metals across membranes. These proteins possess N- and C-terminal cytoplasmic extensions that contain Cys- and His-rich high affinity metal binding ... [more ▼]

PIB ATPases are metal cation pumps that transport metals across membranes. These proteins possess N- and C-terminal cytoplasmic extensions that contain Cys- and His-rich high affinity metal binding domains, which may be involved in metal sensing, metal ion selectivity and/or in regulation of the pump activity. The PIB ATPase HMA4 (Heavy Metal ATPase 4) plays a central role in metal homeostasis in Arabidopsis thaliana and has a key function in zinc and cadmium hypertolerance and hyperaccumulation in the extremophile plant species Arabidopsis halleri. <br />Here, we examined the function and structure of the N-terminal cytoplasmic metal-binding domain of HMA4. We mutagenized a conserved CCTSE metal-binding motif in the domain and assessed the impact of the mutations on protein function and localization in planta, on metal-binding properties in vitro and on protein structure by Nuclear Magnetic Resonance spectroscopy. <br />The two Cys residues of the motif are essential for the function, but not for localization, of HMA4 in planta, whereas the Glu residue is important but not essential. These residues also determine zinc coordination and affinity. Zinc binding to the N-terminal domain is thus crucial for HMA4 protein function, whereas it is not required to maintain the protein structure. <br />Altogether, combining in vivo and in vitro approaches in our study provides insights towards the molecular understanding of metal transport and specificity of metal P-type ATPases. [less ▲]

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See detailCarbon substrate utilization and microbial biomass in European forest soils are related to tree species diversity
Carnol, Monique ULiege; Baeten, Lander; Bosman, Bernard ULiege et al

Poster (2015, December)

Tree species influence biogeochemical cycling through element deposition (throughfall, litterfall), root decomposition and exudates, and through their influence on the microbial activities in the soil ... [more ▼]

Tree species influence biogeochemical cycling through element deposition (throughfall, litterfall), root decomposition and exudates, and through their influence on the microbial activities in the soil. Yet, the effect of mixing tree species on soil functioning is unclear, in particular concerning the microbial diversity and activity in soils. Here we synthesize results from the Exploratory Platform of the FunDivEUROPE project (http://www.fundiveurope.eu/). This network of 209 comparative plots covering tree diversity levels of 1 to 5 species was established in existing mature forests in 6 European regions. These six focal regions represent a gradient of major European forest types from boreal to Mediterranean forests. The aims of this study were to determine the soil microbial biomass and metabolic diversity of soil bacteria for these 6 European forest regions, presenting each a tree species richness gradient and to analyse the impact of tree species richness and the role of other controlling factors. We analysed the relation between tree species diversity, the proportion of coniferous tree species and soil factors (pH, soil organic carbon, water soluble carbon and nitrogen) and the carbon substrate utilisation pattern of soil bacteria (BIOLOG Ecoplate), soil microbial biomass (fumigation-extraction), hot water carbon and nitrogen in the forest floor and the upper mineral soil horizon (linear mixed models, GLM for multivariate abundance data, discriminant correspondence analysis). Mean values of microbial biomass carbon ranged from 3264 (Italy) to 8717 (Finland) mg kg-1 in the forest floor and from 465 (Italy) to 3748 (Finland) mg kg-1 in the mineral soil. Statistical models predicted microbial biomass to increase in both soil layers by 7-8% with each step increase in tree diversity. Increased proportion of conifers was linked to a decrease in the number of carbon substrates used by soil bacteria. The types of carbon sources used were dependent on region, proportion of conifers, soil pH and water-soluble carbon and nitrogen. [less ▲]

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See detailZinc triggers a complex transcriptional and post-transcriptional regulation of the metal homeostasis gene FRD3 in Arabidopsis relatives
Charlier, Jean_Benoit; Polese, Catherine; Nouet, Cécile ULiege et al

in Journal of Experimental Botany (2015), 66

In Arabidopsis thaliana, FRD3 (Ferric Chelate Reductase Defective 3) plays a central role in metal homeostasis. FRD3 is among a set of metal homeostasis genes that are constitutively highly expressed in ... [more ▼]

In Arabidopsis thaliana, FRD3 (Ferric Chelate Reductase Defective 3) plays a central role in metal homeostasis. FRD3 is among a set of metal homeostasis genes that are constitutively highly expressed in roots and shoots of Arabidopsis halleri, a zinc hyperaccumulating and hypertolerant species. Here, we examined the regulation of FRD3 by zinc in both species to shed light on the evolutionary processes underlying the evolution of hyperaccumulation in A. halleri. We combined gene expression studies with the use of GUS and GFP reporter constructs to compare the expression profile, transcriptional and post-transcriptional regulation of FRD3 in both species. The AtFRD3 and AhFRD3 genes display a conserved expression profile. In A. thaliana, alternative transcription initiation sites from two promoters determine transcript variants which are differentially regulated by zinc supply in roots and shoots to favour the most highly translated variant under zinc excess conditions. In A. halleri, a single transcript variant with higher transcript stability and enhanced translation has been maintained. The FRD3 gene thus undergoes complex transcriptional and post-transcriptional regulation in Arabidopsis relatives. Our study reveals that a diverse set of mechanisms underlie increased gene dosage in the A. halleri lineage and illustrates how an environmental challenge can alter gene regulation. [less ▲]

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See detailMicrobial biomass increases with tree species diversity in European forest soils
Carnol, Monique ULiege; Baeten, Lander; Bosman, Bernard ULiege et al

Conference (2015)

Increasing tree species diversity in forests might contribute to ecosystem-service maintenance, as well as to the reconciliation of regulating, provisioning and supporting services within the frame of ... [more ▼]

Increasing tree species diversity in forests might contribute to ecosystem-service maintenance, as well as to the reconciliation of regulating, provisioning and supporting services within the frame of multifunctional and sustainable forestry. Individual tree species influence biogeochemical cycling through element deposition (throughfall, litterfall), and through microbial activities in the soil. Yet, the influence of mixing tree species on these ecosystem processes is unclear, in particular concerning the microbial diversity and activity in soils. Here we synthesize results from the Exploratory Platform of the FunDivEUROPE project (http://www.fundiveurope.eu/). This network of 209 comparative plots covering a tree diversity gradient of 1 to 5 tree species was established in existing mature forests in 6 European regions. These six focal regions represent a gradient of major European forest types from boreal to Mediterranean forests. We analysed the impact of tree species diversity and the role of other controlling factors on the metabolic diversity of soil bacteria (BIOLOG Ecoplate), soil microbial biomass (fumigation-extraction) and potential nitrification (shaken soil slurry) in the forest floor and the upper organo-mineral soil horizon. Mean values of microbial biomass carbon ranged from 3264 (Italy) to 8717 (Finland) mg kg-1 in the forest floor. Statistical models predict microbial biomass to increase in both horizons by 7-8% with each step increase in tree diversity. Increased proportion of conifers was linked to a decrease in the metabolic diversity of soil bacteria. These tree diversity effects could be linked to soil drivers, such as pH, total and labile carbon and nitrogen. [less ▲]

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See detailFunctional analysis of the three HMA4 copies of the metal hyperaccumulator Arabidopsis halleri
Nouet, Cécile ULiege; Charlier, Jean-Benoit; Carnol, Monique ULiege et al

in Journal of Experimental Botany (2015), 66

In Arabidopsis halleri, the AhHMA4 gene has an essential function in Zn/Cd hypertolerance and hyperaccumulation by mediating root to shoot translocation of metals. Constitutive high expression of AhHMA4 ... [more ▼]

In Arabidopsis halleri, the AhHMA4 gene has an essential function in Zn/Cd hypertolerance and hyperaccumulation by mediating root to shoot translocation of metals. Constitutive high expression of AhHMA4 results from a tandem triplication and cis-activation of the promoter of all three copies. The three AhHMA4 copies possess divergent promoter sequences, but highly conserved coding sequences, and display identical expression profiles in the root and shoot vascular system. Here, we expressed an AhHMA4::GFP fusion under the control of each three A. halleri HMA4 promoters in a hma2hma4 double mutant of Arabidopsis thaliana to individually examine the function of each A. halleri AhHMA4 copy. The protein localized non-polarly at the plasma membrane of the root pericycle cells of both A. thaliana and A. halleri. The expression of each AhHMA4::GFP copy complemented the severe Zn deficiency phenotype of the hma2hma4 mutant by restoring root-to-shoot translocation of zinc. However, each copy had different impact on metal homeostasis in the A. thaliana genetic background: AhHMA4 copies 2 and 3 were more highly expressed and provided higher Zn tolerance in roots and accumulation in shoots than copy 1, whereas AhHMA4 copy 3 also increased Cd tolerance in roots. Our data suggest a certain extent of functional differentiation among the three A. halleri HMA4 copies, stemming from differences in expression levels rather than in expression profile. HMA4 is a key node of the Zn homeostasis network and small changes in expression level can have major impact on Zn allocation to root or shoot tissues. [less ▲]

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See detailTree species diversity effects on soil microbial biomass, diversity and activity across European forest types
Carnol, Monique ULiege; Baeten, Lander; Bosman, Bernard ULiege et al

Conference (2014, December)

Increasing tree species diversity in forests might contribute to ecosystem-service maintenance, as well as to the reconciliation of regulating, provisioning and supporting services within the frame of ... [more ▼]

Increasing tree species diversity in forests might contribute to ecosystem-service maintenance, as well as to the reconciliation of regulating, provisioning and supporting services within the frame of multifunctional and sustainable forestry. Individual tree species influence biogeochemical cycling through element deposition (throughfall, litterfall), and through microbial activities in the soil. Yet, the influence of mixing tree species on these ecosystem processes is unclear, in particular concerning the microbial diversity and activity in soils. Here we synthesize results from the Exploratory Platform of the FunDivEUROPE project (http://www.fundiveurope.eu/). This network of 209 comparative plots covering a tree diversity gradient of 1 to 5 tree species was established in existing mature forests in 6 European regions. These six focal regions represent a gradient of major European forest types from boreal to Mediterranean forests. We analysed the impact of tree species diversity and the role of other controlling factors on the metabolic diversity of soil bacteria (BIOLOG Ecoplate), soil microbial biomass (fumigation-extraction) and potential nitrification (shaken soil slurry) in the forest floor and the upper organo-mineral soil horizon. Mean values of microbial biomass carbon ranged from 240 (Poland) to 1762 (Germany) mg kg-1 in the forest floor and from 4197 (Italy) to 11207 (Finland) mg kg-1 in the upper organo-mineral horizon. Tree diversity and soil water content were important controlling factors. Statistical models predict microbial biomass to increase in both horizons by 7-8% with each step increase in tree diversity. Metabolic diversity of soil bacteria (% of substrates used) showed high variability both within and between sites. Further results analysed with mixed linear models will be presented and discussed. [less ▲]

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