The association analysis of DNA methylation and transcriptomics identified BpCYCD3;2 as a participant in influencing cell division in autotetraploid birch (Betula pendula) leaves.

Polyploidization plays a crucial role in plant breeding and genetic improvement. Although the phenomenon of polyploidization affecting the area and number of plant epidermal pavement cells is well described, the underlying mechanism behind this phenomenon is still largely unknown. In this study, we found that the leaves of autotetraploid birch (Betula pendula) stopped cell division earlier and had a larger cell area. In addition, compared to diploids, tetraploids have a smaller stomatal density and fewer stomatal numbers. Genome-wide DNA methylation analysis revealed no significant difference in global DNA methylation levels between diploids and tetraploids. A total of 9154 differential methylation regions (DMRs) were identified between diploids and tetraploids, with CHH-type DMRs accounting for 91.73% of all types of DMRs. Further research has found that there are a total of 2105 differentially methylated genes (DMEGs) with CHH-type DMRs in birch. The GO functional enrichment results of DMEGs showed that differentially methylated genes were mainly involved in terms such as cellular process and metabolic process. The analysis of differentially methylated genes and differentially expressed genes suggests that hyper-methylation in the promoter region may inhibit the gene expression level of BpCYCD3;2 in tetraploids. To investigate the function of BpCYCD3;2 in birch, we obtained overexpression and repressed expression lines of BpCYCD3;2 through genetic transformation. The morphogenesis of both BpCYCD3;2-OE and BpCYCD3;2-RE lines was not affected. However, low expression of BpCYCD3;2 can lead to inhibition of cell division in leaves, and this inhibition of cell proliferation can be compensated for by an increase in cell size. Additionally, we found that the number and density of stomata in the BpCYCD3;2-RE lines were significantly reduced, consistent with the tetraploid. These data indicate that changes in cell division ability and stomatal changes in tetraploid birch can be partially attributed to low expression of the BpCYCD3;2 gene, which may be related to hyper-methylation in its promoter region. These results will provide new insights into the mechanism by which polyploidization affects plant development.

Birch WRKY transcription factor, BpWRKY32, confers salt tolerance by mediating stomatal closing, proline accumulation, and reactive oxygen species scavenging.

Plant WRKY transcription factors (TFs) play important roles in abiotic stress responses. However, how WRKY facilitate physiological changes to confer salt tolerance still needs to be studied. Here, we identified a WRKY TF from birch (Betula platyphylla Suk), BpWRKY32, which is significantly (P < 0.05) induced by salt stress. BpWRKY32 binds to W-box motif and is located in the nucleus. Under salt stress conditions, fresh weights (FW) of OE lines (BpWRKY32 overexpression lines) are increased by 66.36% than that of WT, while FW of knockout of BpWRKY32 (bpwrky32) lines are reduced by 39.49% compared with WT. BpWRKY32 regulates the expression of BpRHC1, BpNRT1, and BpMYB61 to reduce stomatal, and width-length ratio of the stomatal aperture in OE lines are reduced by 46.23% and 64.72% compared with in WT and bpwrky32 lines. BpWRKY32 induces P5CS expression, but inhibits P5CDH expression, leading to the proline content in OE lines are increased by 33.41% and 97.58% compared with WT and bpwrky32 lines. Additionally, BpWRKY32 regulates genes encoding SOD and POD family members, which correspondingly increases the activities of SOD and POD. These results suggested that BpWRKY32 regulates target genes to reduce the water loss rate, enhance the osmotic potential, and reduce the ROS accumulation, leading to improved salt tolerance.

Dissection of figured wood trait in curly birch (Betula pendula Roth var. carelica (Mercklin) Hämet-Ahti) using high-throughput genotyping.

Curly (Karelian) birch is a special variety of Betula pendula Roth distributed in the northwestern part of Europe. Karelian birch is well-known for its valuable figured curly wood also known as "wooden marble". The genetic basis underlying curly wood formation has been debated since last century, however, there was no data about loci responsible for the curly wood trait. In the present study, we analyzed two full-sibs populations derived from experimental crosses of curly birches and segregating for the trait. RADseq genotyping was applied to reveal how many loci are involved in 'curliness' formation and to search for genetic variants associated with this trait. One single interval on chromosome 10 was detected containing possible candidate genes. InDel marker BpCW1 was suggested for the first time for marker-assisted selection of trees with curly wood at their earliest stages of development.

Generation of CRISPR-edited birch plants without DNA integration using Agrobacterium-mediated transformation technology.

CRISPR/Cas9 system has emerged as a powerful tool in genome editing; however, generation of CRISPR-edited DNA-free plants is still challenging. In this study, Betula platyphylla (birch) was used to build a method to generate CRISPR-edited plant without foreign DNA integration using Agrobacterium-mediated transformation (CPDAT method). This technique utilizes transient genetic transformation to introduce T-DNA coding gRNA and Cas9 into birch cells, and T-DNA will express to synthesize gRNA and Cas9 protein, which will form a complex to cleave the target DNA site. The genome may be mutated due to DNA repair, and these mutations will be preserved and accumulated not dependent on whether T-DNA is integrated into the genome or not. After transient transformation, birch plants were cut into explants to induce adventitious buds without antibiotic selection pressure. Each adventitious bud can be considered as an independent potentially CRISPR-edited line for mutation detection. CRISPR-edited birch plants without foreign DNA integration are further selected by screening CRISPR-edited lines without T-DNA integration. Among 65 randomly chosen independent lines, the mutation rate was 80.00% including 40.00% of lines with both alleles mutated. In addition, 5 lines out of 65 studied lines (7.69%) were CRISPR-edited birch plants without DNA integration. In conclusion, this innovative method presents a novel strategy for generating CRISPR-edited birch plants, thereby significantly enhancing the efficiency of generating common CRISPR-edited plants. These findings offer considerable potential to develop plant genome editing techniques further.

rBet v 1a-BanLecwt induce upregulation of IL-10 and IFN-γ gene expression in Caco-2/THP-1 co-culture and secretion of IL-10 and IFN-γ/IL-4 levels in PBMCs of birch pollen allergic donors.

Novel allergen immunotherapy (AIT) approaches necessitate the use of more effective and safe therapeutics, which can be accomplished by employing novel adjuvants for improved innate immune cell activation, as well as hypoallergenic allergen forms. In this study, we investigate the immunomodulatory effects of a chimera rBet v 1a-BanLecwt (rBv1a-BLwt; Cwt) composed of the major birch pollen allergen Bet v 1a and banana lectin (BanLecwt; BLwt) and two novel chimeras, rBv1l-BLH84T (rBet v 1l-BanLecH84T; C1) and rBLH84T-Bv1l (rBanLecH84T-Bet v 1l; C2), both composed of BLH84T and hypoallergenic birch pollen allergen Bv1l in the co-culture model Caco-2/THP-1, and PBMCs from donors with birch pollen allergy. The chimeric molecules rBv1l-BLH84T (C1) and rBLH84T-Bv1l (C2) were created in silico and then produced in E. coli using recombinant DNA technology. Real-time PCR analysis of gene expression following compound treatment in the co-culture model revealed that all three chimeras have the potential to induce the anti-inflammatory cytokine IL-10 gene expression in Caco-2 cells and IFN-γ gene expression in THP-1 cells. Sandwich ELISA revealed that Cwt increased IL-10 secretion and IFN-/IL-4 levels in PBMCs from birch pollen allergic donors, whereas C1 and C2 were less effective. The findings suggest that Cwt should be analyzed further due to its potential benefit in AIT.

BpWOX11 promotes adventitious root formation in Betula pendula.

Adventitious root formation is a key step in vegetative propagation via cuttings. It is crucial for establishing birch plantations and preserve birch varieties. Although previous studies have highlighted role of WOX11 in controlling adventitious root formation, no such study has been conducted in birch. Understanding the mechanism of adventitious root formation is essential for improvement of rooting or survival rate using stem cuttings in birch. In this study, we cloned BpWOX11 and produced BpWOX11 overexpression (OE) transgenic lines using the Agrobacterium-mediated plant transformation. OE lines exhibited early initiated adventitious root formation, leading to increase the rooting rate of stem cuttings plants. RNA sequencing analysis revealed that OE lines induced the gene expression related to expansin and cell division pathway, as well as defense and stress response genes. These may be important factors for the BpWOX11 gene to promote adventitious root formation in birch cuttings. The results of this study will help to further understand the molecular mechanisms controlling the formation of adventitious roots in birch.

BpGRP1 acts downstream of BpmiR396c/BpGRF3 to confer salt tolerance in Betula platyphylla.

Glycine-rich RNA-binding proteins (GRPs) have been implicated in the responses of plants to environmental stresses, but the function of GRP genes involved in salt stress and the underlying mechanism remain unclear. In this study, we identified BpGRP1 (glycine-rich RNA-binding protein), a Betula platyphylla gene that is induced under salt stress. The physiological and molecular responses to salt tolerance were investigated in both BpGRP1-overexpressing and suppressed conditions. BpGRF3 (growth-regulating factor 3) was identified as a regulatory factor upstream of BpGRP1. We demonstrated that overexpression of BpGRF3 significantly increased the salt tolerance of birch, whereas the grf3-1 mutant exhibited the opposite effect. Further analysis revealed that BpGRF3 and its interaction partner, BpSHMT, function upstream of BpGRP1. We demonstrated that BpmiR396c, as an upstream regulator of BpGRF3, could negatively regulate salt tolerance in birch. Furthermore, we uncovered evidence showing that the BpmiR396c/BpGRF3 regulatory module functions in mediating the salt response by regulating the associated physiological pathways. Our results indicate that BpmiR396c regulates the expression of BpGRF3, which plays a role in salt tolerance by targeting BpGRP1.

Long noncoding RNA from Betula platyphylla, BplncSIR1, confers salt tolerance by regulating BpNAC2 to mediate reactive oxygen species scavenging and stomatal movement.

Long noncoding RNAs (lncRNAs) play an important role in abiotic stress tolerance. However, their function in conferring abiotic stress tolerance is still unclear. Herein, we characterized the function of a salt-responsive nuclear lncRNA (BplncSIR1) from Betula platyphylla (birch). Birch plants overexpressing and knocking out for BplncSIR1 were generated. BplncSIR1 was found to improve salt tolerance by inducing antioxidant activity and stomatal closure, and also accelerate plant growth. Chromatin isolation by RNA purification (ChIRP) combined with RNA sequencing indicated that BplncSIR1 binds to the promoter of BpNAC2 (encoding NAC domain-containing protein 2) to activate its expression. Plants overexpressing and knocking out for BpNAC2 were generated. Consistent with that of BplncSIR1, overexpression of BpNAC2 also accelerated plant growth and conferred salt tolerance. In addition, BpNAC2 binds to different cis-acting elements, such as G-box and 'CCAAT' sequences, to regulate the genes involved in salt tolerance, resulting in reduced ROS accumulation and decreased water loss rate by stomatal closure. Taken together, BplncSIR1 serves as the regulator of BpNAC2 to induce its expression in response to salt stress, and activated BpNAC2 accelerates plant growth and improves salt tolerance. Therefore, BplncSIR1 might be a candidate gene for molecular breeding to cultivate plants with both a high growth rate and improved salt tolerance.

The miR156-SPL4/SPL9 module regulates leaf and lateral branch development in Betula platyphylla.

The miR156 gene is known to play an important role in regulating growth and development in plants. This gene is involved in the transition from juvenile to adult stages, leaf morphology, and root development, among other processes. While the function of miR156 is similar in many plants, there are also differences in the function of this gene between herbaceous and native species. We obtained BpmiR156 overexpression transgenic lines in Betula platyphylla, and the transgenic lines exhibited traits such as delayed development, dwarfism, increased leaf epidermal hairs, larger leaf basal angle and altered stem curvature, which were highly consistent with the overexpression miR156 in Arabidopsis, rice and tomato. However, we also observed a lack of apical dominance, increased number of lateral branches and increased diameter of lateral branches in transgenic B. platyphylla, which is different from the effects reported in other plants. Transgenic plants showed changes in the distribution of IAA, GA3, and Zeatin in lateral branches and main stem, and the ratio of the content of the three hormones was significantly higher than in the non-transgenic plants served as control. Additionally, overexpression of BpmiR156 caused down-regulation of BpSPL4 and BpSPL9 expression, as well as differential expression of genes involved in auxin and cytokinin synthesis such as BpARR3, BpARR11 and BpmiR172.

Hormonal Status of Transgenic Birch with a Pine Glutamine Synthetase Gene during Rooting In Vitro and Budburst Outdoors.

Improving nitrogen use efficiency (NUE) is one of the main ways of increasing plant productivity through genetic engineering. The modification of nitrogen (N) metabolism can affect the hormonal content, but in transgenic plants, this aspect has not been sufficiently studied. Transgenic birch (Betula pubescens) plants with the pine glutamine synthetase gene GS1 were evaluated for hormone levels during rooting in vitro and budburst under outdoor conditions. In the shoots of the transgenic lines, the content of indoleacetic acid (IAA) was 1.5-3 times higher than in the wild type. The addition of phosphinothricin (PPT), a glutamine synthetase (GS) inhibitor, to the medium reduced the IAA content in transgenic plants, but it did not change in the control. In the roots of birch plants, PPT had the opposite effect. PPT decreased the content of free amino acids in the leaves of nontransgenic birch, but their content increased in GS-overexpressing plants. A three-year pot experiment with different N availability showed that the productivity of the transgenic birch line was significantly higher than in the control under N deficiency, but not excess, conditions. Nitrogen availability did not affect budburst in the spring of the fourth year; however, bud breaking in transgenic plants was delayed compared to the control. The IAA and abscisic acid (ABA) contents in the buds of birch plants at dormancy and budburst depended both on N availability and the transgenic status. These results enable a better understanding of the interaction between phytohormones and nutrients in woody plants.

Genome-Wide Identification and Characterization of WRKY Transcription Factors in Betula platyphylla Suk. and Their Responses to Abiotic Stresses.

The WRKY transcription factor (TF) family is one the largest plant-specific transcription factor families. It has been proven to play significant roles in multiple plant biological processes, especially stress response. Although many WRKY TFs have been identified in various plant species, WRKYs in white birch (Betula platyphylla Suk.) remain to be studied. Here, we identified a total of 68 BpWRKYs, which could be classified into four main groups. The basic physiochemical properties of these TFs were analyzed using bioinformatics tools, including molecular weight, isoelectric point, chromosome location, and predicted subcellular localization. Most BpWRKYs were predicted to be located in the nucleus. Synteny analysis found 17 syntenic gene pairs among BpWRKYs and 52 syntenic gene pairs between BpWRKYs and AtWRKYs. The cis-acting elements in the promoters of BpWRKYs could be enriched in multiple plant biological processes, including stress response, hormone response, growth and development, and binding sites. Tissue-specific expression analysis using qRT-PCR showed that most BpWRKYs exhibited highest expression levels in the root. After ABA, salt (NaCl), or cold treatment, different BpWRKYs showed different expression patterns at different treatment times. Furthermore, the results of the Y2H assay proved the interaction between BpWRKY17 and a cold-responsive TF, BpCBF7. By transient expression assay, BpWRKY17 and BpWRKY67 were localized in the nucleus, consistent with the previous prediction. Our study hopes to shed light for research on WRKY TFs and plant stress response.

An Insight of Betula platyphylla SWEET Gene Family through Genome-Wide Identification, Expression Profiling and Function Analysis of BpSWEET1c under Cold Stress.

SWEET proteins play important roles in plant growth and development, sugar loading in phloem and resistance to abiotic stress through sugar transport. In this study, 13 BpSWEET genes were identified from birch genome. Collinearity analysis showed that there were one tandem repeating gene pair (BpSWEET1b/BpSWEET1c) and two duplicative gene pairs (BpSWEET17a/BpSWEET17b) in the BpSWEET gene family. The BpSWEET gene promoter regions contained several cis-acting elements related to stress resistance, for example: hormone-responsive and low-temperature-responsive cis-elements. Analysis of transcriptome data showed that BpSWEET genes were highly expressed in several sink organs, and the most BpSWEET genes were rapidly up-regulated under cold stress. BpSWEET1c, which was highly expressed in cold stress, was selected for further analysis. It was found that BpSWEET1c was located on the cell membrane. After 6 h of 4 °C stress, sucrose content in the leaves and roots of transient overexpressed BpSWEET1c was significantly higher than that of the control. MDA content in roots was significantly lower than that of the control. These results indicate that BpSWEET1c may play a positive role in the response to cold stress by promoting the metabolism and transport of sucrose. In conclusion, 13 BpSWEET genes were identified from the whole genome level. Most of the SWEET genes of birch were expressed in the sink organs and could respond to cold stress. Transient overexpression of BpSWEET1c changed the soluble sugar content and improved the cold tolerance of birch.

Functional study of BpCOI1 reveals its role in affecting disease resistance in birch.

Plants interact with biotic and abiotic environments. Some of these interactions are detrimental including herbivory consumption and infections by microbial pathogens. The COI1 (coronatine insensitive 1) protein is the master controller of JA-regulated plant responses and plays a regulatory role in the plant defense response. However, there is little information on COI1 function in birch (Betula platyphylla × Betula pendula). Herein, we studied the F-box protein BpCOI1 which is located in the nucleus. To validate the function of this protein, we developed transgenic birch plants with overexpression or repression of BpCOI1 gene. Growth traits, such as tree height, ground diameter, number of lateral branches, did not change significantly among transgenic lines. Alternaria alternata treatment experiments indicated that low expression of BpCOI1 reduced disease resistance in birch. Furthermore, our results showed that low expression of BpCOI1 significantly reduced the sensitivity of plants to exogenous MeJA. Co-expression analysis showed gene expression patterns with similar characteristics. These genes may be closely related in function, or members involved in the same signaling pathway or physiological process with BpCOI 1. The results of transcriptome sequencing and co-expression analysis showed that BpCOI1 affects plant defense against Alternaria alternata by regulating jasmonates. This study reveals the role of BpCOI1 in disease resistance and proposes the possibility of controlling diseases through molecular breeding in birch.

Automatization of metabolite extraction for high-throughput metabolomics: case study on transgenic isoprene-emitting birch.

Metabolomics studies are becoming increasingly common for understanding how plant metabolism responds to changes in environmental conditions, genetic manipulations and treatments. Despite the recent advances in metabolomics workflow, the sample preparation process still limits the high-throughput analysis in large-scale studies. Here, we present a highly flexible robotic system that integrates liquid handling, sonication, centrifugation, solvent evaporation and sample transfer processed in 96-well plates to automatize the metabolite extraction from leaf samples. We transferred an established manual extraction protocol performed to a robotic system, and with this, we show the optimization steps required to improve reproducibility and obtain comparable results in terms of extraction efficiency and accuracy. We then tested the robotic system to analyze the metabolomes of wild-type and four transgenic silver birch (Betula pendula Roth) lines under unstressed conditions. Birch trees were engineered to overexpress the poplar (Populus × canescens) isoprene synthase and to emit various amounts of isoprene. By fitting the different isoprene emission capacities of the transgenic trees with their leaf metabolomes, we observed an isoprene-dependent upregulation of some flavonoids and other secondary metabolites as well as carbohydrates, amino acid and lipid metabolites. By contrast, the disaccharide sucrose was found to be strongly negatively correlated to isoprene emission. The presented study illustrates the power of integrating robotics to increase the sample throughput, reduce human errors and labor time, and to ensure a fully controlled, monitored and standardized sample preparation procedure. Due to its modular and flexible structure, the robotic system can be easily adapted to other extraction protocols for the analysis of various tissues or plant species to achieve high-throughput metabolomics in plant research.

Robbsia betulipollinis sp. nov., Isolated from Pollen of Birch (Betula pendula).

One gram-negative strain designated Bb-Pol-6 T was isolated from birch (Betula pendula) pollen at Giessen area, Germany. The analysis of 16S rRNA gene-based phylogenies indicated the next-relative genera were Robbsia, Chitinasiproducens, Pararobbsia and Paraburkholderia (96-95.6%). Further comparative genome analysis and phylogenetic tree-based methods revealed its phylogenetic position under the genus Robbsia. The genome of strain Bb-Pol-6 T was 5.04 Mbp with 4401 predicted coding sequences and a G + C content of 65.31 mol%. Average amino acid identity, average nucleotide identity, digital DNA-DNA hybridization and percentage of conserved proteins values to Robbsia andropogonis DSM 9511 T were 68.0, 72.5, 22.7 and 65.85%, respectively. Strain Bb-Pol-6 T was rod-shaped, non-motile, facultative anaerobic and grew optimally at 28 °C and pH 6-7. Ubiquinone 8 was the major respiratory quinone and the major cellular fatty acids were C16:0, C19:0 cyclo ω7c, C17:0 cyclo ω7c and C17:1 ω6c. The dominant polar lipids were diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol and an unidentified aminophospholipid. Based on the genomic physiological and phenotypic characteristics, strain Bb-Pol-6 T was considered a novel species under the genus Robbsia, for which the name Robbsia betulipollinis sp. nov. was proposed. The type strain is Bb-Pol-6 T (= LMG 32774 T = DSM 114812 T).

Genome-wide identification and expression profiling of the bZIP gene family in Betula platyphylla and the functional characterization of BpChr04G00610 under low-temperature stress.

The basic leucine zipper (bZIP) gene, which plays a significant role in the regulation of tolerance to biotic/abiotic stresses, has been characterized in many plant species. Betula platyphylla is a significant afforestation species. To elucidate the stress resistance mechanism of birch, previous studies identified some stress resistance genes. However, the genome-wide identification and characterization of bZIP gene family in the birch have not been reported. Here, the 56 BpbZIP genes were identified and classified into 13 groups in birch. Cis-element analysis showed that the promoters of 56 family genes contained 108 elements, of which 16 were shared by 13 groups. There were 8 pairs of fragment repeats and 1 pair of tandem repeats, indicating that duplication may be the major reason for the amplification of the BpbZIP gene family. Tissue-specific of BpbZIP genes showed 18 genes with the highest expression in roots, 15 in flowers, 11 in xylem and 9 in leaves. In addition, five differentially expressed bZIP genes were identified from the RNA-seq data of birch under low-temperature stress, and the co-expressed differentially expressed genes were further screened. The analysis of gene ontology (GO) enrichment of each co-expression regulatory network showed that they were related to membrane lipids and cell walls. Furthermore, the transient overexpression of BpChr04G00610 decreased the ROS scavenging ability of birch under low-temperature stress, suggesting that it may be more sensitive to low-temperature. In conclusion, this study provides a basis for the study of the function of BpbZIP genes.

Genome-wide identification of the TIFY family reveals JAZ subfamily function in response to hormone treatment in Betula platyphylla.

BACKGROUND: The TIFY family is a plant-specific gene family and plays an important role in plant growth and development. But few reports have been reported on the phylogenetic analysis and gene expression profiling of TIFY family genes in birch (Betula platyphylla). RESULTS: In this study, we characterized TIFY family and identified 12 TIFY genes and using phylogeny and chromosome mapping analysis in birch. TIFY family members were divided into JAZ, ZML, PPD and TIFY subfamilies. Phylogenetic analysis revealed that 12 TIFY genes were clustered into six evolutionary branches. The chromosome distribution showed that 12 TIFY genes were unevenly distributed on 5 chromosomes. Some TIFY family members were derived from gene duplication in birch. We found that six JAZ genes from JAZ subfamily played essential roles in response to Methyl jasmonate (MeJA), the JAZ genes were correlated with COI1 under MeJA. Co-expression and GO enrichment analysis further revealed that JAZ genes were related to hormone. JAZ proteins involved in the ABA and SA pathways. Subcellular localization experiments confirmed that the JAZ proteins were localized in the nucleus. Yeast two-hybrid assay showed that the JAZ proteins may form homologous or heterodimers to regulate hormones. CONCLUSION: Our results provided novel insights into biological function of TIFY family and JAZ subfamily in birch. It provides the theoretical reference for in-depth analysis of plant hormone and molecular breeding design for resistance.

Population structure and genetic variation of fragmented mountain birch forests in Iceland.

Betula pubescens Ehrh. (mountain birch) is the only forest-forming tree in Iceland. Since human settlement (874 AD), the continuous 25,000 to 30,000 km2 forest has shrunk to 1.200 km2 of fragmented patches, making it a good object to study population genetic consequences of habitat fragmentation and disturbance. Further, genetic studies have also shown that hybridization between the tetraploid (2n = 56) B. pubescens and the diploid (2n = 28) Betula nana L. (dwarf birch) occurs among Iceland's natural populations. This study assessed the genetic variation within and among 11 birch forests remaining across Iceland. Genotype-by-sequencing methodology provided a total of 24,585 single nucleotide polymorphisms (SNP´s), with a minor allele frequency >5% for genetic analyses. The analysis showed similar diversity within forests, suggesting that fragmentation and hybridization have had a limited effect on the genetic variation within sites. A clear genetic divergence is found among forests from the different regions of Iceland that may reflect historical isolation; the differentiation between forests increased with geographic distances reflecting isolation by distance. Information on the distribution of genetic variation of birch in Iceland is essential for its conservation and to establish genotype-phenotype associations to predict responses to new environmental conditions imposed by climate change and novel biotic/abiotic stressors.

Genomic signals of local adaptation and hybridization in Asian white birch.

Disentangling the numerous processes that affect patterns of genome-wide diversity in widespread tree species has important implications for taxonomy, conservation, and forestry. Here, we investigate the population genomic structure of Asian white birch (Betula platyphylla) in China and seek to explain it in terms of hybridization, demography and adaptation. We generate whole genome sequence data from 83 individuals across the species range in China. Combining this with an existing data set for 79 European and Russian white birches, we show a clear distinction between B. pendula and B. platyphylla, which have sometimes been lumped taxonomically. Genomic diversity of B. platyphylla in north-western China and Central Russia is affected greatly by hybridization with B. pendula. Excluding these hybridized populations, B. platyphylla in China has a linear distribution from north-eastern to south-western China, along the edge of the inland mountainous region. Within this distribution, three genetic clusters are found, which we model as long diverged with subsequent episodes of gene flow. Patterns of covariation between allele frequencies and environmental variables in B. platyphylla suggest the role of natural selection in the distribution of diversity at 7609 SNPs of which 3767 were significantly differentiated among the genetic clusters. The putative adaptive SNPs are distributed throughout the genome and span 1633 genic regions. Of these genic regions, 87 were previously identified as candidates for selective sweeps in Eurasian B. pendula. We use the 7609 environmentally associated SNPs to estimate the risk of nonadaptedness for each sequenced B. platyphylla individual under a scenario of future climate change, highlighting areas where populations may be under future threat from rising temperatures.