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Abscisic acid (ABA), a widely known phytohormone involved in the plant response to abiotic stress, plays a vital role in mitigating Cd super(2+) toxicity in herbaceous species. However, the role of ABA in ameliorating Cd super(2+) toxicity in woody species is largely unknown. In the present study, we investigated ABA restriction on Cd super(2+) uptake and the relevance to Cd super(2+) stress alleviation in Cd super(2+)-hypersensitive Populus euphratica. ABA (5 mu M) markedly improved cell viability and growth but reduced membrane permeability in CdCl sub(2) (100 mu M)-stressed P. euphratica cells. Moreover, ABA significantly increased the activity of the antioxidant enzymes catalase (CAT), superoxide dismutase (SOD), and ascorbate peroxidase (APX), contributing to the scavenging of Cd super(2+)-elicited H sub(2)O sub(2) within P. euphratica cells during the period of CdCl sub(2) exposure (100 mu M, 24-72 h). ABA alleviation of Cd super(2+) toxicity was mainly the result of ABA restriction of Cd super(2+) uptake under Cd super(2+) stress. Steady-state and transient flux recordings showed that ABA inhibited Cd super(2+) entry into Cd super(2+)-shocked (100 mu M, 30 min) and short-term-stressed P. euphratica cells (100 mu M, 24-72 h). Non-invasive micro-test technique data showed that H sub(2)O sub(2) (3 mM) stimulated the Cd super(2+)-elicited Cd super(2+) influx but that the plasma membrane (PM) Ca super(2+) channel inhibitor LaCl sub(3) blocked it, suggesting that the Cd super(2+) influx was through PM Ca super(2+)-permeable channels. These results suggested that ABA up-regulated antioxidant enzyme activity in Cd super(2+)-stressed P. euphratica and that these enzymes scavenged the Cd super(2+)-elicited H sub(2)O sub(2) within cells. The entry of Cd super(2+) through the H sub(2)O sub(2)-mediated Ca super(2+)-permeable channels was subsequently restricted; thus, Cd super(2+) buildup and toxicity were reduced in the Cd super(2+)-hypersensitive species, P. euphratica.

The plant microbiome represents one of the key determinants of plant health and productivity by providing a plethora of functional capacities such as access to low-abundance nutrients, suppression of phytopathogens, and resistance to biotic and/or abiotic stressors. However, a robust understanding of the structural composition of the bacterial microbiome present in different plant microenvironments and especially the relationship between below-ground and above-ground communities has remained elusive. In this work, we addressed hypotheses regarding microbiome niche differentiation and structural stability of the bacterial communities within different ecological plant niches. We sampled the rhizosphere soil, root, stem, and leaf endosphere of field-grown poplar trees (Populus tremula × Populus alba) and applied 16S rRNA amplicon pyrosequencing to unravel the bacterial communities associated with the different plant habitats. We found that the structural variability of rhizosphere microbiomes in field-grown poplar trees (P. tremula × P. alba) is much lower than that of the endosphere microbiomes. Furthermore, our data not only confirm microbiome niche differentiation reports at the rhizosphere soil-root interface but also clearly show additional fine-tuning and adaptation of the endosphere microbiome in the stem and leaf compartment. Each plant compartment represents an unique ecological niche for the bacterial communities. Finally, we identified the core bacterial microbiome associated with the different ecological niches of Populus. Understanding the complex host-microbe interactions of Populus could provide the basis for the exploitation of the eukaryote-prokaryote associations in phytoremediation applications, sustainable crop production (bio-energy efficiency), and/or the production of secondary metabolites.

Although a substantial proportion of plant biomass originates from the activity of vascular cambium, the molecular basis of radial plant growth is still largely unknown. To address whether cytokinins are required for cambial activity, we studied cytokinin signaling across the cambial zones of 2 tree species, poplar (Populus trichocarpa) and birch (Betula pendula). We observed an expression peak for genes encoding cytokinin receptors in the dividing cambial cells. We reduced cytokinin levels endogenously by engineering transgenic poplar trees (P. tremula x tremuloides) to express a cytokinin catabolic gene, Arabidopsis CYTOKININ OXIDASE 2, under the promoter of a birch CYTOKININ RECEPTOR 1 gene. Transgenic trees showed reduced concentration of a biologically active cytokinin, correlating with impaired cytokinin responsiveness. In these trees, both apical and radial growth was compromised. However, radial growth was more affected, as illustrated by a thinner stem diameter than in WT at same height. To dissect radial from apical growth inhibition, we performed a reciprocal grafting experiment. WT scion outgrew the diameter of transgenic stock, implicating cytokinin activity as a direct determinant of radial growth. The reduced radial growth correlated with a reduced number of cambial cell layers. Moreover, expression of a cytokinin primary response gene was dramatically reduced in the thin-stemmed transgenic trees. Thus, a reduced level of cytokinin signaling is the primary basis for the impaired cambial growth observed. Together, our results show that cytokinins are major hormonal regulators required for cambial development.

Salt tolerance genes constitute an important class of loci in plant genomes. Little is known about the extent to which natural selection in saline environments has acted upon these loci, and what types of nucleotide diversity such selection has given rise to. Here, we surveyed genetic diversity in three types of Na super(+)/H super(+) antiporter gene (SOS, NhaD, and NHX, belonging to the cation/proton antiporter 1 family), which have well-characterized essential roles in plant salt tolerance. Ten Na super(+)/H super(+) antiporter genes and 16 neutral loci randomly selected as controls were sequenced from 17 accessions of two closely related members of the genus Populus, Populus euphratica and Populus pruinosa, section Turanga, which are native to northwest China. The results show that salt tolerance genes are common targets of natural selection in P. euphratica and P. pruinosa. Moreover, the patterns of nucleotide variation across the three types of Na super(+)/H super(+) antiporter gene are distinctly different in these two closely related Populus species, and gene flow from P. pruinosa to P. euphratica is highly restricted. Our results suggest that natural selection played an important role in shaping the current distinct patterns of Na super(+)/H super(+) antiporter genes, resulting in adaptive evolution in P. euphratica and P. pruinosa. Salt tolerance genes are common targets of natural selection in Populus euphratica and Populus pruinosa. The patterns of nucleotide variation across three types of Na+/H+ antiporter gene are distinctly different in two closely related Populus species.

Aims: In this study we quantified the annual soil CO sub(2) efflux (annual SCE) of a short rotation coppice plantation in its establishment phase. We aimed to examine the effect of former (agricultural) land use type, inter-row spacing and genotype. Methods: Annual SCE was quantified during the second growth year of the establishment rotation in a large scale poplar plantation in Flanders. Automated chambers were distributed over the two former land use types, the two different inter-row spacings and under two poplar genotypes. Additional measurements of C, N, P, K, Mg, Ca and Na concentrations of the soil, pH, bulk density, fine root biomass, microbial biomass C, soil mineralization rate, distance to trees and tree diameters were performed at the end of the second growth year. Results: Total carbon loss from soil CO sub(2) efflux was valued at 589 g m super(-2) yr super(-1). Annual SCE was higher in former pasture as compared to cropland, higher in the narrow than in the wider inter-row spacings, but no effect of genotype was found. Conclusions: Spatial differences in site characteristics are of great importance for understanding the effect of ecosystem management and land use change on soil respiration processes and need to be taken into account in modeling efforts of the carbon balance.

Fast-growing poplar plantations are considered of great benefit to both timber production and carbon (C) sequestration, and are increasingly planted for multiple purposes worldwide. Irrigation and fertilization are common management practices in plantations in semiarid regions. However, quantitative investigation of the integrative effect of surface drip irrigation and fertigation (SDIF) on biomass and C storage in poplar plantations remains limited. In this study, we conducted a field experiment on a fast-growing poplar cultivar (Populus euramerica na cv. Guariento) plantation to compare the combination of surface drip irrigation and fertigation in growing seasons with conventional management (control; CK). Experiments repeated over 2 years showed that SDIF significantly increased biomass and C storage in both trees and soil in the plantation compared with the CK. Tree biomass C in SDIF-treated and CK stands after the first year of the experiment (age 5) was 6.20 and 4.05 t C ha super(-1), respectively, and the difference further increased, i.e., 15.18 and 8.63 t C ha super(-1), respectively, after the second year of the experiment (age 6). There was 53 and 76 % higher C storage in SDIF-treated trees than in the CK trees after the first and second years of the experiment, respectively. The SDIF increased the soil C concentration, especially in the surface soil at 0- to 40-cm depth. Soil organic C at a depth of 0-60 cm under the SDIF treatment was 45.42, 50.87 and 61.32 t C ha super(-1) in the 1st, 2nd and 3rd years, respectively, with annual increases of 12 and 21 % between the first and second, and second and third year, respectively. The corresponding soil organic C in the CK was 43.08, 43.57 and 47.92 t C ha super(-1) in the 1st, 2nd and 3rd years; the annual increases were only 1 and 10 %, respectively. The results confirmed the significant effect of the combined management on C storage in poplar plantations, thus we suggest it can be applied in forestry management, even though it generally did not change C concentrations of tree components.

The need for renewable energy sources will lead to a considerable expansion in the planting of dedicated fast-growing biomass crops across Europe. These are commonly cultivated as short-rotation coppice (SRC), and currently poplar (Populus spp.) is the most widely planted. In this study, we report the greenhouse gas (GHG) fluxes of carbon dioxide (CO sub(2)), methane (CH sub(4)) and nitrous oxide (N sub(2)O) measured using eddy covariance technique in an SRC plantation for bioenergy production. Measurements were made during the period 2010-2013, that is, during the first two rotations of the SRC. The overall GHG balance of the 4 years of the study was an emission of 1.90 ( plus or minus 1.37) Mg CO sub(2)eq ha super(-1); this indicated that soil trace gas emissions offset the CO sub(2) uptake by the plantation. CH sub(4) and N sub(2)O contributed almost equally to offset the CO sub(2) uptake of -5.28 ( plus or minus 0.67) Mg CO sub(2)eq ha super(-1) with an overall emission of 3.56 ( plus or minus 0.35) Mg CO sub(2)eq ha super(-1) of N sub(2)O and of 3.53 ( plus or minus 0.85) Mg CO sub(2)eq ha super(-1) of CH sub(4). N sub(2)O emissions mostly occurred during one single peak a few months after the site was converted to SRC; this peak comprised 44% of the total N sub(2)O loss during the two rotations. Accurately capturing emission events proved to be critical for deriving correct estimates of the GHG balance. The nitrogen (N) content of the soil and the water table depth were the two drivers that best explained the variability in N sub(2)O and CH sub(4,) respectively. This study underlines the importance of the 'non-CO sub(2) GHGs' on the overall balance. Further long-term investigations of soil trace gas emissions should monitor the N content and the mineralization rate of the soil, as well as the microbial community, as drivers of the trace gas emissions.

Forest trees display a perennial growth behavior characterized by a multiple-year delay in flowering and, in temperate regions, an annual cycling between growth and dormancy. We show here that the CO/FT regulatory module, which controls flowering time in response to variations in daylength in annual plants, controls flowering in aspen trees. Unexpectedly, however, it also controls the short-day-induced growth cessation and bud set occurring in the fall. This regulatory mechanism can explain the ecogenetic variation in a highly adaptive trait: the critical daylength for growth cessation displayed by aspen trees sampled across a latitudinal gradient spanning northern Europe.

A total of 2 542 lincRNAs were identified from Populus trichocarpa and some of them play key roles in drought stress tolerance or regulate microRNA through target mimicry patterns.

The primary obstacle to producing renewable fuels from lignocellulosic biomass is a plant's recalcitrance to releasing sugars bound in the cell wall. From a sample set of wood cores representing 1,100 individual undomesticated Populus trichocarpa trees, 47 extreme phenotypes were selected across measured lignin content and ratio of syringyl and guaiacyl units (S/G ratio). This subset was tested for total sugar release through enzymatic hydrolysis alone as well as through combined hot-water pretreatment and enzymatic hydrolysis using a high-throughput screening method. The total amount of glucan and xylan released varied widely among samples, with total sugar yields of up to 92% of the theoretical maximum. A strong negative correlation between sugar release and lignin content was only found for pretreated samples with an S/G ratio < 2.0. For higher S/G ratios, sugar release was generally higher, and the negative influence of lignin was less pronounced. When examined separately, only glucose release was correlated with lignin content and S/G ratio in this manner, whereas xylose release depended on the S/G ratio alone. For enzymatic hydrolysis without pretreatment, sugar release increased significantly with decreasing lignin content below 20%, irrespective of the S/G ratio. Furthermore, certain samples featuring average lignin content and S/G ratios exhibited exceptional sugar release. These facts suggest that factors beyond lignin and S/G ratio influence recalcitrance to sugar release and point to a critical need for deeper understanding of cell-wall structure before plants can be rationally engineered for reduced recalcitrance and efficient biofuels production.