Tree architecture: A strigolactone-deficient mutant reveals a connection between branching order and auxin gradient along the tree stem.

Due to their long lifespan, trees and bushes develop higher order of branches in a perennial manner. In contrast to a tall tree, with a clearly defined main stem and branching order, a bush is shorter and has a less apparent main stem and branching pattern. To address the developmental basis of these two forms, we studied several naturally occurring architectural variants in silver birch (Betula pendula). Using a candidate gene approach, we identified a bushy kanttarelli variant with a loss-of-function mutation in the BpMAX1 gene required for strigolactone (SL) biosynthesis. While kanttarelli is shorter than the wild type (WT), it has the same number of primary branches, whereas the number of secondary branches is increased, contributing to its bush-like phenotype. To confirm that the identified mutation was responsible for the phenotype, we phenocopied kanttarelli in transgenic BpMAX1::RNAi birch lines. SL profiling confirmed that both kanttarelli and the transgenic lines produced very limited amounts of SL. Interestingly, the auxin (IAA) distribution along the main stem differed between WT and BpMAX1::RNAi. In the WT, the auxin concentration formed a gradient, being higher in the uppermost internodes and decreasing toward the basal part of the stem, whereas in the transgenic line, this gradient was not observed. Through modeling, we showed that the different IAA distribution patterns may result from the difference in the number of higher-order branches and plant height. Future studies will determine whether the IAA gradient itself regulates aspects of plant architecture.

Complex Polyploids: Origins, Genomic Composition, and Role of Introgressed Alleles.

Introgression allows polyploid species to acquire new genomic content from diploid progenitors or from other unrelated diploid or polyploid lineages, contributing to genetic diversity and facilitating adaptive allele discovery. In some cases, high levels of introgression elicit the replacement of large numbers of alleles inherited from the polyploid's ancestral species, profoundly reshaping the polyploid's genomic composition. In such complex polyploids, it is often difficult to determine which taxa were the progenitor species and which taxa provided additional introgressive blocks through subsequent hybridization. Here, we use population-level genomic data to reconstruct the phylogenetic history of Betula pubescens (downy birch), a tetraploid species often assumed to be of allopolyploid origin and which is known to hybridize with at least four other birch species. This was achieved by modeling polyploidization and introgression events under the multispecies coalescent and then using an approximate Bayesian computation rejection algorithm to evaluate and compare competing polyploidization models. We provide evidence that B. pubescens is the outcome of an autoploid genome doubling event in the common ancestor of B. pendula and its extant sister species, B. platyphylla, that took place approximately 178,000-188,000 generations ago. Extensive hybridization with B. pendula, B. nana, and B. humilis followed in the aftermath of autopolyploidization, with the relative contribution of each of these species to the B. pubescens genome varying markedly across the species' range. Functional analysis of B. pubescens loci containing alleles introgressed from B. nana identified multiple genes involved in climate adaptation, while loci containing alleles derived from B. humilis revealed several genes involved in the regulation of meiotic stability and pollen viability in plant species.

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.

Differential phenotypic plasticity of subalpine trees predicts trait integration under climate warming.

Understanding limiting factors of phenotypic plasticity is essential given its critical role in shaping biological adaptation and evolution in changing environments. It has been proposed that the pattern of phenotypic correlation could constrain trait plasticity. However, the interplay between phenotypic plasticity and integration has remained contentious. We experimentally simulated climate warming in juveniles of three subalpine tree species by exposing them to three-year in situ open-top chambers (OTCs), and then measured functional plasticity of 72 eco-physiological traits to evaluate whether phenotypic integration constituted an intrinsic constraint to plasticity. We also tested the relationship between the differences in plasticity and maintenance in trait integration. Phenotypic plasticity was positively associated with integration in deciduous tree species under warming. The difference in the plasticity of two paired traits could predict their integration in different environments, where traits displaying more similar plasticity were more likely to be correlated. Our study showed no indication that phenotypic integration constrained plasticity. More importantly, we demonstrated that differential plasticity between traits might result in a notable reorganization of the trait associations, and that warming commonly induced a tighter phenotype. Our study provides new insights into the interplay between phenotypic plasticity and integration in subalpine trees under climate warming.

Reduced representation bisulfite sequencing (RRBS) analysis reveals variation in distribution and levels of DNA methylation in white birch (Betula papyrifera) exposed to nickel.

Research in understanding the role of genetics and epigenetics in plant adaptations to environmental stressors such as metals is still in its infancy. The objective of the present study is to assess the effect of nickel on DNA methylation level and distribution in white birch (Betula papyrifera Marshall) using reduced representation bisulfite sequencing (RRBS). The distribution of methylated C sites of each sample revealed that the level of methylation was much higher in CG context varying between 54% and 65%, followed by CHG (24%-31.5%), and then CHH with the methylation rate between 3.3% and 5.2%. The analysis of differentially methylated regions (DMR) revealed that nickel induced both hypermethylation and hypomethylation when compared to water. Detailed analysis showed for the first time that nickel induced a higher level of hypermethylation compared to controls, while potassium triggers a higher level of hypomethylation compared to nickel. Surprisingly, the analysis of the distribution of DMRs revealed that 38%-42% were located in gene bodies, 20%-24% in exon, 19%-20% in intron, 16%-17% in promoters, and 0.03%-0.04% in transcription start site. RRBS was successful in detecting and mapping DMR in plants exposed to nickel.

Continental-scale evaluation of downy birch pollen production: Estimating the impacts of global change.

The high prevalence of hay fever in Europe has raised concerns about the implications of climate change-induced higher temperatures on pollen production. Our study focuses on downy birch pollen production across Europe by analyzing 456 catkins during 2019-2021 in 37 International Phenological Gardens (IPG) spanning a large geographic gradient. As IPGs rely on genetically identical plants, we were able to reduce the effects of genetic variability. We studied the potential association with masting behavior and three model specifications based on mean and quantile regression to assess the impact of meteorology (e.g., temperature and precipitation) and atmospheric gases (e.g., ozone (O3) and carbon-dioxide (CO2)) on pollen and catkin production, while controlling for tree age approximated by stem circumference. The results revealed a substantial geographic variability in mean pollen production, ranging from 1.9 to 2.5 million pollen grains per catkin. Regression analyses indicated that elevated average temperatures of the previous summer corresponded to increased pollen production, while higher O3 levels led to a reduction. Additionally, catkins number was positively influenced by preceding summer's temperature and precipitation but negatively by O3 levels. The investigation of quantile effects revealed that the impacts of mean temperature and O3 levels from the previous summer varied throughout the conditional response distribution. We found that temperature predominantly affected trees characterized by a high pollen production. We therefore suggest that birches modulate their physiological processes to optimize pollen production under varying temperature regimes. In turn, O3 levels negatively affected trees with pollen production levels exceeding the conditional median. We conclude that future temperature increase might exacerbate pollen production while other factors may modify (decrease in the case of O3 and amplify for precipitation) this effect. Our comprehensive study sheds light on potential impacts of climate change on downy birch pollen production, which is crucial for birch reproduction and human health.

Herbivory and warming interact in opposing patterns of covariation between arctic shrub species at large and local scales.

A major challenge in predicting species' distributional responses to climate change involves resolving interactions between abiotic and biotic factors in structuring ecological communities. This challenge reflects the classical conceptualization of species' regional distributions as simultaneously constrained by climatic conditions, while by necessity emerging from local biotic interactions. A ubiquitous pattern in nature illustrates this dichotomy: potentially competing species covary positively at large scales but negatively at local scales. Recent theory poses a resolution to this conundrum by predicting roles of both abiotic and biotic factors in covariation of species at both scales, but empirical tests have lagged such developments. We conducted a 15-y warming and herbivore-exclusion experiment to investigate drivers of opposing patterns of covariation between two codominant arctic shrub species at large and local scales. Climatic conditions and biotic exploitation mediated both positive covariation between these species at the landscape scale and negative covariation between them locally. Furthermore, covariation between the two species conferred resilience in ecosystem carbon uptake. This study thus lends empirical support to developing theoretical solutions to a long-standing ecological puzzle, while highlighting its relevance to understanding community compositional responses to climate change.

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.

Warming nondormant tree roots advances aboveground spring phenology in temperate trees.

Climate warming advances the onset of tree growth in spring, but above- and belowground phenology are not always synchronized. These differences in growth responses may result from differences in root and bud dormancy dynamics, but root dormancy is largely unexplored. We measured dormancy in roots and leaf buds of Fagus sylvatica and Populus nigra by quantifying the warming sum required to initiate above- and belowground growth in October, January and February. We furthermore carried out seven experiments, manipulating only the soil and not air temperature before or during tree leaf-out to evaluate the potential of warmer roots to influence budburst timing using seedlings and adult trees of F. sylvatica and seedlings of Betula pendula. Root dormancy was virtually absent in comparison with the much deeper winter bud dormancy. Roots were able to start growing immediately as soils were warmed during the winter. Interestingly, higher soil temperature advanced budburst across all experiments, with soil temperature possibly accounting for c. 44% of the effect of air temperature in advancing aboveground spring phenology per growing degree hour. Therefore, differences in root and bud dormancy dynamics, together with their interaction, likely explain the nonsynchronized above- and belowground plant growth responses to climate warming.

Derivative-based time-adjusted analysis of diurnal and within-tree variation in the OJIP fluorescence transient of silver birch.

The JIP test, based on fast chlorophyll fluorescence (ChlF) kinetics and derived parameters, is a dependable tool for studying photosynthetic efficiency under varying environmental conditions. We extracted additional information from the whole OJIP and the normalized variable fluorescence (Vt) transient curve using first and second-order derivatives to visualize and localize points of landmark events. To account for light-induced variations in the fluorescence transient, we present a time-adjusted JIP test approach in which the derivatives of the transient curve are used to determine the exact timing of the J and I steps instead of fixed time points. We compared the traditional JIP test method with the time-adjusted method in analyzing fast ChlF measurements of silver birch (Betula pendula) in field conditions studying diurnal and within-crown variation. The time-adjusted JIP test method showed potential for studying ChlF dynamics, as it takes into account potential time shifts in the occurrence of J and I steps. The exact occurrence times of J and I steps and other landmark events coincided with the times of significant differences in fluorescence intensity. Chlorophyll fluorescence parameters were linearly related to photosynthetic photon flux density (PPFD) at different times of day, and the values obtained by the time-adjusted JIP test showed a stronger linear regression than the traditional JIP test. For fluorescence parameters having significant differences among different times of day and crown layers, the time-adjusted JIP test resulted in more clear differences than the traditional JIP test. Diurnal ChlF intensity data indicated that differences between the southern and northern provenance were only evident under low light conditions. Taken together, our results emphasize the potential relevance of considering the time domain in the analysis of the fast ChlF induction.

Circulating mast cell progenitors increase during natural birch pollen exposure in allergic asthma patients.

BACKGROUND: Mast cells (MCs) develop from a rare population of peripheral blood circulating MC progenitors (MCps). Here, we investigated whether the frequency of circulating MCps is altered in asthma patients sensitized to birch pollen during pollen season, compared to out of season. METHODS: Asthma patients were examined during birch pollen season in late April to early June (May), and out of season in November-January. Spirometry measurements, asthma and allergy-related symptoms, asthma control questionnaire (ACQ), and asthma control test (ACT) scores were assessed at both time points. The MCp frequency was determined by flow cytometry in ficoll-separated blood samples from patients with positive birch pollen-specific IgE, and analyzed in relation to basic and disease parameters. RESULTS: The frequency of MCps per liter of blood was higher in May than in November (p = .004), particularly in women (p = .009). Patients that reported moderate to severe asthma symptoms (<.0001), nose or eye symptoms (p = .02; p = .01), or reduced asthma control (higher ACQ, p = .01) had higher MCp frequency in May than those that did not report this. These associations remained significant after adjusting for sex and BMI. The change in asthma control to a lower ACT score in May correlated with an increase in MCp frequency in May (p = .006, rho = 0.46). CONCLUSIONS: The data suggest that the frequency of MCps increases in symptomatic patients with allergic asthma. Our results unravel a link between asthma symptoms and circulating MCps, and bring new insight into the impact of natural allergen exposure on the expansion of MCs.

Microscopical palynology: Birch woodland expansion and species hybridisation coincide with periods of climate warming during the Holocene epoch in Iceland.

Introgressive hybridisation between arctic dwarf birch (Betula nana) and European downy birch (B. pubescens) is relatively common in natural woodlands in Iceland. As dwarf birch is a diploid species and downy tree birch a tetraploid species, their hybrids are triploid. In the introgression process, triploid hybrids, which are partially fertile, can backcross with the parental species, producing progenies comprising introgressed diploid, triploid and tetraploid plants. Triploid plants produce both normal triporate pollen (with three pores) and abnormal, aborted pollen, due to dysfunctional meiosis. The type of pollen abnormality that can be detected and quantified is non-triporate pollen (with four or more pores in the pollen wall). We therefore used the presence of non-triporate pollen as a marker to trace birch hybridisation in the past. In the current study we examined fossil pollen in samples from Holocene sediments from three locations: Grímsnes (SW), Eyjafjördur (N) and Thistilsfjördur (NE Iceland). All three peat monoliths had the starting age of 10.3 cal. ka BP. Ages were calibrated based on known tephra layers and by radiocarbon dating. The size of Betula pollen grains was measured, and the species proportions calculated from size. Non-triporate grains were detected in samples from all three locations and throughout the Holocene, but with different frequencies. The peaks of intense hybridisation followed birch woodland expansion in two major periods of the Holocene, both coinciding with a warming of climate. The first period occurred in the Early Holocene, around 9.5-7 cal. ka BP, when the climate warmed rapidly after deglaciation. The second period occurred around 5-3.5 cal. ka BP, well within the mid-Holocene Northern Hemisphere warming. A new wave of birch hybridisation appears to have started in the last few decades as the climate has warmed. Birch woodlands are likely to become more widespread. Introgressed shrub birch is likely to be more competitive over dwarf birch.

Formation of xylem tissues and secondary cell walls is diminished by severe and consecutive insect defoliation.

PREMISE: Insect defoliation of trees causes unusual changes to wood anatomy and slows radial growth that decreases tree value; however, the characteristics of these anatomical changes in hardwoods remain unclear. The aim of this study was to characterize the anatomy and histochemistry of the wood in trunks of Betula maximowicziana trees after severe insect defoliation. METHODS: Secondary xylem tissues were sampled from trunks that had been defoliated by Caligula japonica at Naie and Furano in central Hokkaido during 2006-2012, then cross-dated and examined microscopically and stained histochemically to characterize anatomical and chemical changes in the cells. RESULTS: White rings with thin-walled wood fibers and greatly reduced annual ring width in the subsequent year were observed in samples from both sites. From these results, the year that the white rings formed was determined, and severe defoliation was confirmed to trigger white ring formation. The characteristics may prove useful to detect the formation year of white rings. Scanning electron microscopy and histochemical analyses of the white rings indicated that the thickness of the S2 layer in the wall of wood fiber cells decreased, but xylan and lignin were still deposited in the cell walls of wood fibers. However, the walls of the fibers rethickened after the defoliation. CONCLUSIONS: Our results suggest that B. maximowicziana responds to a temporary lack of carbon inputs due to insect defoliation by regulating the thickness of the S2 layer of the cell wall of wood fibers. For B. maximowicziana, insect defoliation late in the growing season has serious deleterious effects on wood formation and radial growth.

How does pollen production of allergenic species differ between urban and rural environments?

Pollen production is one plant characteristic that is considered to be altered by changes in environmental conditions. In this study, we investigated pollen production of the three anemophilous species Betula pendula, Plantago lanceolata, and Dactylis glomerata along an urbanization gradient in Ingolstadt, Germany. We compared pollen production with the potential influencing factors urbanization, air temperature, and the air pollutants nitrogen dioxide (NO2) and ozone (O3). While we measured air temperature in the field, we computed concentration levels of NO2 and O3 from a land use regression model. The results showed that average pollen production (in million pollen grains) was 1.2 ± 1.0 per catkin of Betula pendula, 5.0 ± 2.4 per inflorescence of Plantago lanceolata, and 0.7 ± 0.5 per spikelet of Dactylis glomerata. Pollen production was higher in rural compared to urban locations on average for B. pendula (+ 73%) and P. lanceolata (+ 31%), while the opposite was the case for D. glomerata (- 14%). We found that there was substantial heterogeneity across the three species with respect to the association of pollen production and environmental influences. Pollen production decreased for all species with increasing temperature and urbanization, while for increasing pollutant concentrations, decreases were observed for B. pendula, P. lanceolata, and increases for D. glomerata. Additionally, pollen production was found to be highly variable across species and within species-even at small spatial distances. Experiments should be conducted to further explore plant responses to altering environmental conditions.

Islands in the shade: scattered ectomycorrhizal trees influence soil inoculum and heterospecific seedling response in a northeastern secondary forest.

The eastern deciduous forest is a mix of arbuscular (AM) and ectomycorrhizal (EM) trees, but land use legacies have increased the abundance of AM trees like Acer spp. (maple). Although these legacies have not changed the abundance of some EM trees like Betula spp. (birch), EM conifers like Tsuga canadensis (hemlock), and Pinus strobus (pine) have declined. We used a soil bioassay to investigate if the microbial community near EM birch (birch soil) contains a greater abundance and diversity of EM fungal propagules compatible with T. canadensis and P. strobus compared to the community associated with the surrounding AM-dominated secondary forest matrix (maple soil). We also tested the effectiveness of inoculation with soil from a nearby EM-dominated old-growth forest as a restoration tool to reintroduce EM fungi into secondary forest soils. Finally, we examined how seedling growth responded to EM fungi associated with each treatment. Seedlings grown with birch soil were colonized by EM fungi mostly absent from the surrounding maple forest. Hemlock seedlings grown with birch soil grew larger than hemlock seedlings grown with maple soil, but pine seedling growth did not differ with soil treatment. The addition of old-growth soil inoculum increased hemlock and pine growth in both soils. Our results found that EM trees are associated with beneficial EM fungi that are mostly absent from the surrounding AM-dominated secondary forest, but inoculation with old-growth soil is effective in promoting the growth of seedlings by reintroducing native EM fungi to the AM-dominated forests.

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.

Genome-Wide Identification and Expression Profiles of C-Repeat Binding Factor Transcription Factors in Betula platyphylla under Abiotic Stress.

CBF (C-repeat binding factor) transcription factor subfamily belongs to AP2/ERF (Apetala 2/ethylene-responsive factor) transcription factor family, known for playing a vital role in plant abiotic stress response. Although some CBF transcription factors have been identified in several species, such as Arabidopsis, tobacco, tomato and poplar, research of CBF focus mainly on model plant Arabidopsis and have not been reported in Betula platyphylla yet. In this study, a total of 20 BpCBF subfamily members were identified. The conserved domains, physicochemical properties, exon-intron gene structure and the structure of conserved protein motifs of BpCBFs were analyzed via bioinformatic tools. The collinearity analysis of CBF genes was performed between Betula platyphylla and Arabidopsis thaliana, Betula platyphylla, and Populus trichocarpa. The cis-acting elements in the promoter region of BpCBFs were identified, which were mainly environmental stress-related and hormone-related element components. In this case, the expression patterns of the 20 BpCBFs upon ABA or salt treatment were investigated. Most of these transcription factors were responsive to ABA or salt stress in different plant tissues. The up-regulation trend upon cold treatment of the six cold-responsive genes validated by qRT-PCR was consistent with the result of RNA-seq. BpCBF7 showed transcription activating activity. This study sheds light on the responses of BpCBFs to abiotic stress and provides a reference for further study of CBF transcription factors 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.