Expression of the Populus Orthologues of AtYY1, YIN and YANG Activates the Floral Identity Genes AGAMOUS and SEPALLATA3 Accelerating Floral Transition in Arabidopsis thaliana.

YIN YANG 1 (YY1) encodes a dual-function transcription factor, evolutionary conserved between the animal and plant kingdom. In Arabidopsis thaliana, AtYY1 is a negative regulator of ABA responses and floral transition. Here, we report the cloning and functional characterization of the two AtYY1 paralogs, YIN and YANG (also named PtYY1a and PtYY1b) from Populus (Populus trichocarpa). Although the duplication of YY1 occurred early during the evolution of the Salicaceae, YIN and YANG are highly conserved in the willow tree family. In the majority of Populus tissues, YIN was more strongly expressed than YANG. Subcellular analysis showed that YIN-GFP and YANG-GFP are mainly localized in the nuclei of Arabidopsis. Stable and constitutive expression of YIN and YANG resulted in curled leaves and accelerated floral transition of Arabidopsis plants, which was accompanied by high expression of the floral identity genes AGAMOUS (AG) and SEPELLATA3 (SEP3) known to promote leaf curling and early flowering. Furthermore, the expression of YIN and YANG had similar effects as AtYY1 overexpression to seed germination and root growth in Arabidopsis. Our results suggest that YIN and YANG are functional orthologues of the dual-function transcription factor AtYY1 with similar roles in plant development conserved between Arabidopsis and Populus.

Genome-Wide Identification and Characterization of Auxin Response Factor (ARF) Gene Family Involved in Wood Formation and Response to Exogenous Hormone Treatment in Populus trichocarpa.

Auxin is a key regulator that virtually controls almost every aspect of plant growth and development throughout its life cycle. As the major components of auxin signaling, auxin response factors (ARFs) play crucial roles in various processes of plant growth and development. In this study, a total of 35 PtrARF genes were identified, and their phylogenetic relationships, chromosomal locations, synteny relationships, exon/intron structures, cis-elements, conserved motifs, and protein characteristics were systemically investigated. We also analyzed the expression patterns of these PtrARF genes and revealed that 16 of them, including PtrARF1, 3, 7, 11, 13-17, 21, 23, 26, 27, 29, 31, and 33, were preferentially expressed in primary stems, while 15 of them, including PtrARF2, 4, 6, 9, 10, 12, 18-20, 22, 24, 25, 28, 32, and 35, participated in different phases of wood formation. In addition, some PtrARF genes, with at least one cis-element related to indole-3-acetic acid (IAA) or abscisic acid (ABA) response, responded differently to exogenous IAA and ABA treatment, respectively. Three PtrARF proteins, namely PtrARF18, PtrARF23, and PtrARF29, selected from three classes, were characterized, and only PtrARF18 was a transcriptional self-activator localized in the nucleus. Moreover, Y2H and bimolecular fluorescence complementation (BiFC) assay demonstrated that PtrARF23 interacted with PtrIAA10 and PtrIAA28 in the nucleus, while PtrARF29 interacted with PtrIAA28 in the nucleus. Our results provided comprehensive information regarding the PtrARF gene family, which will lay some foundation for future research about PtrARF genes in tree development and growth, especially the wood formation, in response to cellular signaling and environmental cues.

Genome-wide analysis of R2R3-MYB transcription factors reveals their differential responses to drought stress and ABA treatment in desert poplar (Populus euphratica).

The R2R3-MYB transcription factors are widely involved in the regulation of plant growth, biotic and abiotic stress responses. Meanwhile, seed germination, which is stimulated by internal and external environments, is a critical stage in the plant life cycle. However, the identification, characterization, and expression profiling of the Populus euphratica R2R3-MYB family in drought response during seed germination have been unknown. Our study attempted to identify and characterize the R2R3-MYB genes in P. euphratica (PeR2R3-MYBs) and explore how R2R3-MYBs trigger the drought and abscisic acid (ABA) response mechanism in its seedlings. Based on the analysis of comparative genomics, 174 PeR2R3-MYBs were identified and expanded driven by whole genome duplication or segment duplication events. The analysis of Ka/Ks ratios showed that, in contrast to most PeR2R3-MYBs, the other PeR2R3-MYBs were subjected to positive selection in P. euphratica. Further, the expression data of PeR2R3-MYBs under drought stress and ABA treatment, together with available functional data for Arabidopsis thaliana MYB genes, supported the hypothesis that PeR2R3-MYBs involved in response to drought are dependent or independent on ABA signaling pathway during seed germination, especially PeR2R3-MYBs with MYB binding sites (MBS) cis-element and/or tandem duplication. This study is the first report on the genome-wide analysis of PeR2R3-MYBs, as well as the other two Salicaceae species. The duplication events and differential expressions of PeR2R3-MYBs play important roles in enhancing the adaptation to drought desert environment. Our results provide a reference for prospective functional studies of R2R3-MYBs of poplars and lay the foundation for new breeding strategies to improve the drought tolerance of P. euphratica.

Poplar leaf abscission through induced chlorophyll breakdown by Mg-dechelatase.

Chlorophyll breakdown is observed during senescence. The first step in chlorophyll breakdown is the removal of central Mg by Mg-dechelatase. This reaction is the rate-limiting step in the chlorophyll breakdown pathway. We evaluated the effect of induced chlorophyll breakdown on abscission through the removal of Mg by Mg-dechelatase. Poplar transformants carrying the dexamethasone-inducible Mg-dechelatase gene were prepared using the Arabidopsis Stay-Green1 cDNA. When leaves were treated with dexamethasone, chlorophyll was degraded, photosynthetic capacity was reduced, and an abscission zone was formed, resulting in leaf abscission. In addition, ethylene, which plays an important role during senescence, was produced in this process. Thus, chlorophyll breakdown induces the phenotype in the same way as commonly observed during leaf senescence. This study suggests a physiological role of chlorophyll breakdown in the leaf abscission of deciduous trees. Furthermore, this study shows that the dexamethasone-inducible gene expression system is an available option for deciduous tree studies.

Phosphate (Pi) stress-responsive transcription factors PdeWRKY6 and PdeWRKY65 regulate the expression of PdePHT1;9 to modulate tissue Pi concentration in poplar.

Phosphorus (P) is an important nutrient for plants. Here, we identify a WRKY transcription factor (TF) in poplar (Populus deltoides × Populus euramericana) (PdeWRKY65) that modulates tissue phosphate (Pi) concentrations in poplar. PdeWRKY65 overexpression (OE) transgenic lines showed reduced shoot Pi concentrations under both low and normal Pi availabilities, while PdeWRKY65 reduced expression (RE) lines showed the opposite phenotype. A gene encoding a Pi transporter (PHT), PdePHT1;9, was identified as the direct downstream target of PdeWRKY65 by RNA sequencing (RNA-Seq). The negative regulation of PdePHT1;9 expression by PdeWRKY65 was confirmed by DNA-protein interaction assays, including yeast one-hybrid (Y1H), electrophoretic mobility shift assay (EMSA), co-expression of the promoters of PdePHT1;9 and PdeWRKY65 in tobacco (Nicotiana benthamiana) leaves, and chromatin immunoprecipitation-quantitative PCR. A second WRKY TF, PdeWRKY6, was subsequently identified and confirmed to positively regulate the expression of PdePHT1;9 by DNA-protein interaction assays. PdePHT1;9 and PdeWRKY6 OE and RE poplar transgenic lines were used to confirm their positive regulation of shoot Pi concentrations, under both normal and low Pi availabilities. No interaction between PdeWRKY6 and PdeWRKY65 was observed at the DNA or protein levels. Collectively, these data suggest that the low Pi-responsive TFs PdeWRKY6 and PdeWRKY65 independently regulate the expression of PHT1;9 to modulate tissue Pi concentrations in poplar.

Laccaria bicolor pectin methylesterases are involved in ectomycorrhiza development with Populus tremula × Populus tremuloides.

The development of ectomycorrhizal (ECM) symbioses between soil fungi and tree roots requires modification of root cell walls. The pectin-mediated adhesion between adjacent root cells loosens to accommodate fungal hyphae in the Hartig net, facilitating nutrient exchange between partners. We investigated the role of fungal pectin modifying enzymes in Laccaria bicolor for ECM formation with Populus tremula × Populus tremuloides. We combine transcriptomics of cell-wall-related enzymes in both partners during ECM formation, immunolocalisation of pectin (Homogalacturonan, HG) epitopes in different methylesterification states, pectin methylesterase (PME) activity assays and functional analyses of transgenic L. bicolor to uncover pectin modification mechanisms and the requirement of fungal pectin methylesterases (LbPMEs) for ECM formation. Immunolocalisation identified remodelling of pectin towards de-esterified HG during ECM formation, which was accompanied by increased LbPME1 expression and PME activity. Overexpression or RNAi of the ECM-induced LbPME1 in transgenic L. bicolor lines led to reduced ECM formation. Hartig Nets formed with LbPME1 RNAi lines were shallower, whereas those formed with LbPME1 overexpressors were deeper. This suggests that LbPME1 plays a role in ECM formation potentially through HG de-esterification, which initiates loosening of adjacent root cells to facilitate Hartig net formation.

Visualization of phosphorus re-translocation and phosphate transporter expression profiles in a shortened annual cycle system of poplar.

Phosphorus (P) is an essential macronutrient for plant growth. In deciduous trees, P is remobilized from senescing leaves and stored in perennial tissues during winter for further growth. Annual internal recycling and accumulation of P are considered an important strategy to support the vigorous growth of trees. However, the pathways of seasonal re-translocation of P and the molecular mechanisms of this transport have not been clarified. Here we show the seasonal P re-translocation route visualized using real-time radioisotope imaging and the macro- and micro-autoradiography. We analysed the seasonal re-translocation P in poplar (Populus alba. L) cultivated under 'a shortened annual cycle system', which mimicked seasonal phenology in a laboratory. From growing to senescing season, sink tissues of 32 P and/or 33 P shifted from young leaves and the apex to the lower stem and roots. The radioisotope P re-translocated from a leaf was stored in phloem and xylem parenchyma cells and redistributed to new shoots after dormancy. Seasonal expression profile of phosphate transporters (PHT1, PHT5 and PHO1 family) was obtained in the same system. Our results reveal the seasonal P re-translocation routes at the organ and tissue levels and provide a foothold for elucidating its molecular mechanisms.

Comprehensive analysis of the BES1 gene family and its expression under abiotic stress and hormone treatment in Populus trichocarpa.

The BRI1 EMS SUPPRESSOR 1/BRASSINAZOLE RESISTANT 1 (BES1/BZR1) plays a vital role in plant growth and development and stress responses, but there are few studies on poplar BES1 genes. In this study, we identified 14 BES1 genes in the Populus trichocarpa genome and analyzed the expression under hormone treatment and abiotic stress. The PtrBES1 genes were classified into seven subgroups (I-VII) through phylogenetic analysis. All the paralogous gene pairs were shown to be subjected to expansion by segment duplication and purification selection during the PtrBES1 family evolution. Promoter cis-element analysis showed that the PtrBES1 promoter contains stress related cis-elements including ABRE-motif, MBS and TC-rich elements. Quantitative real time reverse transcription PCR (RT-qPCR) analysis showed that the PtrBES1 genes were upregulated upon NaCl, Polyethylene glycol 6000 (PEG6000) stress as well as the major stress hormone abscisic acid (ABA) treatment. Under the three treatments, PtrBES1-7 showed high expression levels in leaves and roots. Physiological experiments showed that the overexpression PtrBES1-7 line could enhance tolerance to drought stress in P. trichocarpa by improving the ability to scavenge ROS (reactive oxygen species). This is specifically reflected in the fact that the overexpression line contains less ROS (O2- and H2O2) and more antioxidant enzymes (1.42 times SOD and 1.5 times POD) than the control line. The preliminary results of this study provided a solid basis for the future functional studies of the BES1 gene family in P. trichocarpa.

Root Secondary Metabolites in Populus tremuloides: Effects of Simulated Climate Warming, Defoliation, and Genotype.

Climate warming can influence interactions between plants and associated organisms by altering levels of plant secondary metabolites. In contrast to studies of elevated temperature on aboveground phytochemistry, the consequences of warming on root chemistry have received little attention. Herein, we investigated the effects of elevated temperature, defoliation, and genotype on root biomass and phenolic compounds in trembling aspen (Populus tremuloides). We grew saplings of three aspen genotypes under ambient or elevated temperatures (+4-6 °C), and defoliated (by 75%) half of the trees in each treatment. After 4 months, we harvested roots and determined their condensed tannin and salicinoid (phenolic glycoside) concentrations. Defoliation reduced root biomass, with a slightly larger impact under elevated, relative to ambient, temperature. Elevated temperature decreased condensed tannin concentrations by 21-43% across the various treatment combinations. Warming alone did not alter salicinoid concentrations but eliminated a small negative impact of defoliation on those compounds. Graphical vector analysis suggests that effects of warming and defoliation on condensed tannins and salicinoids were predominantly due to reduced biosynthesis of these metabolites in roots, rather than to changes in root biomass. In general, genotypes did not differ in their responses to temperature or temperature by defoliation interactions. Collectively, our results suggest that future climate warming will alter root phytochemistry, and that effects will vary among different classes of secondary metabolites and be influenced by concurrent ecological interactions such as herbivory. Temperature- and herbivory-mediated changes in root chemistry have the potential to influence belowground trophic interactions and soil nutrient dynamics.

Molecular and immunochemical characterization of Pop n 2: A new allergen of Populus nigra pollen.

BACKGROUND: Pollen is one of the most common allergens that cause respiratory allergies worldwide. Pollen grains from poplars have been reported as important sources of pollinosis in many countries. OBJECTIVE: The aim of the present study was to determine the molecular and immunochemical characterization of Pop n 2, a novel allergen of Populus nigra (P nigra) pollen extract. METHODS: In this study, the pollen extract of P nigra was analysed by SDS-PAGE, and the allergenic profile was determined by IgE immunoblotting and specific ELISA using the sera of twenty allergic patients. The coding sequence of Pop n 2 was cloned and expressed in the Escherichia coli BL21 (DE3) using plasmid the pET-21b (+). Finally, the expressed recombinant Pop n 2 was purified by affinity chromatography. RESULTS: Pop n 2 belongs to the profilin family with a molecular weight of approximately 14 kDa. Pop n 2 is the most IgE-reactive protein (about 65%) in the P nigra pollen extract. The cDNA sequencing results indicated an open reading frame 396 bp that encodes 131 amino acid residues. The results of ELISA and Immunoblotting assays showed that recombinant Pop n 2 could react with the IgE antibody in patients' sera, like its natural counterpart. CONCLUSION: Our data revealed that Pop n 2 is a significant allergen in the P nigra pollen extract. Moreover, we observed that the recombinant Pop n 2 produced by the pET-21b (+) vector in the E colisystem acts as its natural counterpart.

Phosphorus deficiency induces root proliferation and Cd absorption but inhibits Cd tolerance and Cd translocation in roots of Populus × euramericana.

To disclose how phosphorus deficiency influence phytoremediation of Cd contamination using poplars, root architecture, Cd absorption, Cd translocation and antioxidant defense in poplar roots were investigated using a clone of Populus × euramericana. Root growth was unaltered by Cd exposure regardless of P conditions, while the degree of root proliferation upon P deficiency was changed by high level of Cd exposure. The concentration and content of Cd accumulation in roots were increased by P deficiency. This can be partially explained by the increased expression of genes encoding PM H + -ATPase under the combined conditions of P deficiency and high Cd exposure, which enhanced Cd2+-H+ exchanges and led to an increment of Cd uptake under P deficiency. Despite of the increasing Cd accumulation in roots, the translocation of Cd from roots to aerial tissues sharply decreased upon P deficiency. The relative expression of genes responsible for Cd translocation (HMA4) decreased upon P deficiency and thus inhibited Cd translocation via xylem. GR activity was decreased by P deficiency, which can inhibit the form of GSH and GSH-Cd complexes and decrease Cd translocation via GSH-Cd complexes. The transportation of PC-Cd complexes into vacuole decreased under P deficiency as a result of the low expression of PCS and ABCC1, and thus suppressed Cd tolerance and Cd detoxification in roots. Moreover, P deficiency decreased the levels of antioxidase (GR and CAT) and phytohormones including JA, ABA and GA3, which synchronously reduced antioxidant capacity in roots.

The cadmium tolerance development of poplar callus is influenced by silicon.

Cadmium-tolerant plants were studied for their possible usage in phytoremediation techniques. However, their response to cadmium cations at a cellular level has not been properly studied. Silicon is a beneficial element that seems to change the plant's response to the Cd2+ presence. The aim of the present study was to investigate the Cd2+ tolerance patterns of poplar callus cells exposed to Cd+2 and/or Si over short and long cultivation periods. We determined the growth parameters of the callus, the growth dynamics, cell vitality, photosynthetic pigment concentrations and the activity of antioxidant enzymes. The effects were studied over short (21 days) and long (63 days) cultivation periods. The most important result proved that the poplar callus tissue is able to build up a tolerance to Cd2+ after a longer cultivation period. On the 63rd day of the cultivation, Cd2+ stressed calli showed improvement in studied parameters and the callus cells accumulated Cd2+ more efficiently than on the 21st day. Supplementation with Si in higher concentrations (2.5 mM and 5 mM) heightened the Cd-tolerance potential of the tissue. The treatment of Cd2+, and Si in a 2.5 mM concentration was the most efficient variant for Cd2+ removal from medium. The activity of antioxidant enzymes showed that poplar callus cells effectively develop tolerance against Cd2+ after a longer cultivation period.

Cell wall O-acetyl and methyl esterification patterns of leaves reflected in atmospheric emission signatures of acetic acid and methanol.

Plants emit high rates of methanol (meOH), generally assumed to derive from pectin demethylation, and this increases during abiotic stress. In contrast, less is known about the emission and source of acetic acid (AA). In this study, Populus trichocarpa (California poplar) leaves in different developmental stages were desiccated and quantified for total meOH and AA emissions together with bulk cell wall acetylation and methylation content. While young leaves showed high emissions of meOH (140 µmol m-2) and AA (42 µmol m-2), emissions were reduced in mature (meOH: 69%, AA: 60%) and old (meOH: 83%, AA: 76%) leaves. In contrast, the ratio of AA/meOH emissions increased with leaf development (young: 35%, mature: 43%, old: 82%), mimicking the pattern of O-acetyl/methyl ester ratios of leaf bulk cell walls (young: 35%, mature: 38%, old: 51%), which is driven by an increase in O-acetyl and decrease in methyl ester content with age. The results are consistent with meOH and AA emission sources from cell wall de-esterification, with young expanding tissues producing highly methylated pectin that is progressively demethyl-esterified. We highlight the quantification of AA/meOH emission ratios as a potential tool for rapid phenotype screening of structural carbohydrate esterification patterns.

Efficient evaluation of a gene containment system for poplar through early flowering induction.

KEY MESSAGE: The early flowering system HSP::AtFT allowed a fast evaluation of a gene containment system based on the construct PsEND1::barnase-barstar for poplar. Transgenic lines showed disturbed pollen development and sterility. Vertical gene transfer through pollen flow from transgenic or non-native plant species into their crossable natural relatives is a major concern. Gene containment approaches have been proposed to reduce or even avoid gene flow among tree species. However, evaluation of genetic containment strategies for trees is very difficult due to the long-generation times. Early flowering induction would allow faster evaluation of genetic containment in this case. Although no reliable methods were available for the induction of fertile flowers in poplar, recently, a new early flowering approach was developed. In this study, early flowering poplar lines containing the gene construct PsEND1::barnase-barstar were obtained. The PsEND1 promoter was chosen due to its early expression pattern, its versality and efficiency for generation of male-sterile plants fused to the barnase gene. RT-PCRs confirmed barnase gene activity in flowers, and pollen development was disturbed, leading to sterile flowers. The system developed in this study represents a valuable tool for gene containment studies in forest tree species.

Biotechnological mechanism for improving plant remobilization of phosphorus during leaf senescence.

Phosphorus enrichment of aquatic ecosystems through diffuse source pollution is an ongoing issue worldwide. A potential solution lies in the use of fast-growing, multipurpose feedstocks, such as trees, to limit the flow of phosphorus into riparian areas through luxury consumption. However, the perennial nature of trees and their use of leaves as storage organs for excess phosphorus may reduce the effectiveness of contaminant removal during periods of leaf abscission. In an attempt to improve phosphorus remobilization during autumnal senescence, transgenic hybrid poplar P39 (Populus alba × Populus grandidentata) and Arabidopsis thaliana harbouring a constitutively expressed low-affinity potato phosphate transporter (35S::StPht1-1) were generated using Agrobacterium-mediated transformation. For both species, the highest expressing 35S::StPht1-1 lines were grown alongside wild-type plants and subjected to increasing phosphate applications. StPht1-1 expression in A. thaliana led to a reduction in biomass when grown under high-phosphate conditions and had no effect on phosphate remobilization during senescence. In contrast, StPht1-1 constitutive expression in P39 resulted in increased leaf phosphate content in the highest expressing transgenic line and minimal to no effect on P resorption efficiency. Surprisingly, sulphate resorption showed the greatest improvement in all three transgenic poplar lines, displaying a 31%-37% increase in resorption efficiency. These results highlight the complexity of nutrient resorption mechanisms in plants.

Sex-specific strategies of phosphorus (P) acquisition in Populus cathayana as affected by soil P availability and distribution.

Soil phosphorus (P) availability and its distribution influence plant growth and productivity, but how they affect the growth dynamics and sex-specific P acquisition strategies of dioecious plant species is poorly understood. In this study, the impact of soil P availability and its distribution on dioecious Populus cathayana was characterized. P. cathayana males and females were grown under three levels of P supply, and with homogeneous or heterogeneous P distribution. Females had a greater total root length, specific root length (SRL), biomass and foliar P concentration under high P supply. Under P deficiency, males had a smaller root system than females but a greater exudation of soil acid phosphatase, and a higher colonization rate and arbuscular mycorrhizal hyphal biomass, suggesting a better capacity to mine P and a stronger association with arbuscular mycorrhizal fungi to forage P. Heterogeneous P distribution enhanced growth and root length density (RLD) in females. Female root proliferation in P-rich patches was related to increased foliar P assimilation. Localized P application for increasing P availability did not enhance the biomass accumulation and the morphological plasticity of roots in males, but it raised hyphal biomass. The findings herein indicate that sex-specific strategies in P acquisition relate to root morphology, root exudation and mycorrhizal symbioses, and they may contribute to sex-specific resource utilization patterns and niche segregation.

Phosphorus influence Cd phytoextraction in Populus stems via modulating xylem development, cell wall Cd storage and antioxidant defense.

The soils in mining lands with cadmium (Cd) contamination usually are deficient in nutrients. Disclosing how P nutrition and N:P stoichiometric ratio influences Cd accumulation and stress tolerance in stems of Populus spp. will facilitate the phytoremediation of mining sites polluted by Cd. In this study, investigations at the anatomical and physiological levels were conducted using a clone of Populus × euramericana. Both phosphorus deficiency and cadmium exposure inhibited xylem development via reducing cell layers in the xylem. Under P-sufficient condition, appropriate P status and balanced N:P ratio in stem promoted xylem development under Cd exposure via stimulating cell division, which enhanced Cd accumulation in stems. Cd accumulation in cell walls of collenchyma tissues of the stem was enhanced by P application due to increased polysaccharide production and cell wall affinity for Cd. The low P concentrations (0.3-0.4 mg g-1) and imbalanced N:P ratio under P deficiency inhibited the production of APX and ascorbate-GSH cycle, which increased oxidative stress and lipid peroxidation as indicated by high MDA concentration in stem. Under P-sufficient condition, the interactions between phytohormones and antioxidants play crucial roles in the process of antioxidant defense under Cd exposure. In conclusions, appropriate P addition and balanced N:P ratio enhanced secondary xylem development and promoted cadmium accumulation and stress tolerance in Populus stems, which can benefit the phytoextraction of Cd from Cd-contaminated soil.

Monitoring of a long term phytoremediation process of a soil contaminated by heavy metals and hydrocarbons in Tuscany.

The purpose of this study was to monitor and model indicators of soil contamination, organic matter evolution and biochemical processes involved in a long-term phytoremediation process. Populus nigra L., Paulownia tomentosa Steud., Cytisus scoparius L. and natural vegetation were used in differently contaminated areas (high, medium and low levels of contamination). Parameters indicating contamination (total petroleum hydrocarbons (TPH) and heavy metals) and agronomic (C, N and P) and functional (enzyme activities) soil recovery were monitored for 3.5 years. Three subareas with different levels of contamination (high, medium and low) were identified according to the Nemerow Index. A considerable decrease in TPH (52% on average) over time in the whole site was measured, while the metal reduction was only of about 22% at surface level. A stimulation in metabolic soil processes and improvement in the chemical quality of the soil was also observed throughout the experimental site. Statistical analysis modelling showed that the contaminant content decreased following a one-phase decay model, while the dramatic increase in enzyme activities could be represented by an exponential growth equation. On the basis of our data, it is possible to conclude that the initial contamination level affected neither the decontamination process nor the improvement in soil quality, which occurred similarly in the three different contaminated areas.

Element content and expression of genes of interest in guard cells are connected to spatiotemporal variations in stomatal conductance.

Element content and expression of genes of interest on single cell types, such as stomata, provide valuable insights into their specific physiology, improving our understanding of leaf gas exchange regulation. We investigated how far differences in stomatal conductance (gs ) can be ascribed to changes in guard cells functioning in amphistomateous leaves. gs was measured during the day on both leaf sides, on well-watered and drought-stressed trees (two Populus euramericana Moench and two Populus nigra L. genotypes). In parallel, guard cells were dissected for element content and gene expressions analyses. Both were strongly arranged according to genotype, and drought had the lowest impact overall. Normalizing the data by genotype highlighted a structure on the basis of leaf sides and time of day both for element content and gene expression. Guard cells magnesium, phosphorus, and chlorine were the most abundant on the abaxial side in the morning, where gs was at the highest. In contrast, genes encoding H+ -ATPase and aquaporins were usually more abundant in the afternoon, whereas genes encoding Ca2+ -vacuolar antiporters, K+ channels, and ABA-related genes were in general more abundant on the adaxial side. Our work highlights the unique physiology of each leaf side and their analogous rhythmicity through the day.

Impact of poplar-based phytomanagement on metal bioavailability in low-phosphorus calcareous soil with multi-metal contamination.

Bioavailability of trace metals (TMs) is the key component in the management of TM-contaminated soils; however, its impact mechanism is unclear in low-phosphorus (P) calcareous soils afforested by fast-growing tree species for a long duration (>10 years). We selected a site contaminated with multiple TMs and phytomanaged by poplar (Populus hopeiensis Hu & Chow) to study the impact mechanism of plant-soil interactions on TM bioavailability along a long-term chronosequence (i.e., 10, 15, 20, and 25 years). We found that phytomanagement significantly decreased soil organic carbon (SOC) content, soil total nitrogen (N) content, and soil C/P and N/P ratios with stand age, but did not significantly change soil total P content. In contrast, soil available P content significantly changed in rhizospheric soils compared with the bulk soil, suggesting the tight coupling between the amplification of P turnover and N availability. Soil pH in rhizospheric soils significantly decreased by 0.22 to 0.32 units, while calcium carbonate (CaCO3) content decreased by 14% to 39%, as compared with the bulk soil. Bioavailable concentrations of cadmium, lead, and zinc were positively correlated with soil available P, whereas bioavailable nickel concentration was negatively correlated with soil pH. Furthermore, TM bioavailability in rhizospheric soils significantly increased with stand age, regardless of the metal type. Our results suggest that P mobilization associated with SOC depletion induced soil acidification followed by CaCO3 dissolution, collectively leading to metal mobilization with stand age.