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Summary Plant MYB transcription factors control diverse biological processes, such as differentiation, development and abiotic stress responses. In this study, we characterized BplMYB46, an MYB gene from Betula platyphylla (birch) that is involved in both abiotic stress tolerance and secondary wall biosynthesis. BplMYB46 can act as a transcriptional activator in yeast and tobacco. We generated transgenic birch plants with overexpressing or silencing of BplMYB46 and subjected them to gain‐ or loss‐of‐function analysis. The results suggest that BplMYB46 improves salt and osmotic tolerance by affecting the expression of genes including SOD, POD and P5CS to increase both reactive oxygen species scavenging and proline levels. In addition, BplMYB46 appears to be involved in controlling stomatal aperture to reduce water loss. Overexpression of BplMYB46 increases lignin deposition, secondary cell wall thickness and the expression of genes in secondary cell wall formation. Further analysis indicated that BplMYB46 binds to MYBCORE and AC‐box motifs and may directly activate the expression of genes involved in abiotic stress responses and secondary cell wall biosynthesis whose promoters contain these motifs. The transgenic BplMYB46‐overexpressing birch plants, which have improved salt and osmotic stress tolerance, higher lignin and cellulose content and lower hemicellulose content than the control, have potential applications in the forestry industry.

Nitrogen (N₂)-fixing moss microbial communities play key roles in nitrogen cycling of boreal forests. Forest type and leaf litter inputs regulate moss abundance, but how they control moss microbiomes and N₂-fixation remains understudied. We examined the impacts of forest type and broadleaf litter on microbial community composition and N₂-fixation rates of Hylocomium splendens and Pleurozium schreberi. We conducted a moss transplant and leaf litter manipulation experiment at three sites with paired paper birch (Betula neoalaskana) and black spruce (Picea mariana) stands in Alaska. We characterized bacterial communities using marker gene sequencing, determined N₂-fixation rates using stable isotopes (15N₂) and measured environmental covariates. Mosses native to and transplanted into spruce stands supported generally higher N₂-fixation and distinct microbial communities compared to similar treatments in birch stands. High leaf litter inputs shifted microbial community composition for both moss species and reduced N₂-fixation rates for H. splendens, which had the highest rates. N₂-fixation was positively associated with several bacterial taxa, including cyanobacteria. The moss microbiome and environmental conditions controlled N₂-fixation at the stand and transplant scales. Predicted shifts from spruce- to deciduous-dominated stands will interact with the relative abundances of mosses supporting different microbiomes and N₂-fixation rates, which could affect stand-level N inputs.

The boreal biome represents approximately one third of the world’s forested area and plays an important role in global biogeochemical and energy cycles. Numerous studies in boreal Alaska have concluded that growth of black and white spruce is declining as a result of temperature-induced drought stress. The combined evidence of declining spruce growth and changes in the fire regime that favor establishment of deciduous tree species has led some investigators to suggest the region may be transitioning from dominance by spruce to dominance by deciduous forests and/or grasslands. Although spruce growth trends have been extensively investigated, few studies have evaluated long-term radial growth trends of the dominant deciduous species (Alaska paper birch and trembling aspen) and their sensitivity to moisture availability. We used a large and spatially extensive sample of tree cores from interior Alaska to compare long-term growth trends among contrasting tree species (white and black spruce vs. birch and aspen). All species showed a growth peak in the mid-1940s, although growth following the peak varied strongly across species. Following an initial decline from the peak, growth of white spruce showed little evidence of a trend, while black spruce and birch growth showed slight growth declines from ∼1970 to present. Aspen growth was much more variable than the other species and showed a steep decline from ∼1970 to present. Growth of birch, black and white spruce was sensitive to moisture availability throughout most of the tree-ring chronologies, as evidenced by negative correlations with air temperature and positive correlations with precipitation. However, a positive correlation between previous July precipitation and aspen growth disappeared in recent decades, corresponding with a rise in the population of the aspen leaf miner (Phyllocnistis populiella), an herbivorous moth, which may have driven growth to a level not seen since the early 20th century. Our results provide important historical context for recent growth and raise questions regarding competitive interactions among the dominant tree species and exchanges of carbon and energy in the warming climate of interior Alaska.

Summary 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.

PurposeSoil nematodes are among the most important fauna in soils and participate actively in soil ecological processes. However, whether and how soil nematodes are involved in the effects of tree species type on soil fertility remain unclear, especially during subalpine forest secondary succession.MethodsA monoculture pot experiment of two broadleaf (Betula platyphylla and Betula albosinensis) and two coniferous (Picea asperata and Abies faxoniana) trees, using sterilized soils inoculated with unsterilized soils beneath dominant plants from different successional stages, was conducted in a greenhouse. After a period of plant growth, soil nematode communities and soil fertility in each pot were investigated.ResultsSignificant differences were noted in nematode community composition under the broadleaf and coniferous trees. Coniferous trees accumulated more abundant microbivores and omnivore-predators than broadleaf trees. Moreover, the contrasting effects of tree species type on soil nematode communities were associated with successional stages, with the greatest differences noted in the early successional stages. In addition, soil nematodes might play a significant mediating role in the effects of broadleaf and coniferous trees on soil fertility. However, the indirect regulatory effects induced by soil nematodes weakened with the successional stages.ConclusionOverall, our study suggested that tree species type might affect soil fertility by regulating soil nematode communities across successional stages. Compared with broadleaf trees, more abundant microbivores under coniferous trees might contribute to the improvement of soil nitrogen mineralization but not to the increase in soil carbon storage, which might be limited by new carbon input into soils.

Many disturbances are shifting in severity, frequency, and extent due to changing climate and human activities. Altered disturbance regimes can trigger shifts in ecosystem state where recovery to the pre-disturbance ecosystem is uncertain. In the western North American boreal forest, the intensification of wildfire can cause canopy dominance to switch from black spruce (Picea mariana) to deciduous trees such as Alaska paper birch (Betula neoalaskana) and trembling aspen (Populus tremuloides). Understanding the key mechanisms that determine the resilience and stability of these alternative community types is required for the prediction of future forest dynamics. Here, we assess patterns of post-fire tree recovery across a pre-fire gradient of spruce- to deciduous-dominated forests in Interior Alaska and quantify compositional and environmental thresholds that support the resilience of alternative canopy types. We found post-fire organic soil depth of stands on a recovery trajectory to deciduous dominance (7.3 ± 5.5 cm) were similar regardless of pre-fire composition and significantly shallower than spruce (14.9 ± 9.0 cm) or mixed trajectories (10.4 ± 5.9 cm). Deciduous-dominated stands were highly resilient to fire, as 100% remained deciduous-dominated post-fire. Even when deciduous trees only accounted for a small proportion (12%) of the pre-fire stand, deciduous trees often became dominant after wildfire. We conclude that the establishment of deciduous bud banks and seed sources creates a strong hysteresis in stand recovery that reinforces the resilience of deciduous-dominated boreal forests to wildfire. Accounting for the resilience of this alternative stable state to wildfire suggests that shifts from spruce to deciduous dominance caused by shifting wildfire will have long-term effects on future structure and function of boreal forests and vegetation feedbacks to climate change.

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.

Background Salt stress is one of the major environmental factors affecting plant growth and productivity. BRI1-EMS suppressor 1/brassinazole-resistant 1 ((BES1/BZR1) plays an important role in responding to abiotic stress in plants. Although the impacts of BES1/BZR1 on plant growth and resistance have been documented, the potential mechanisms are not fully elucidated in Betula platyphylla . This work contributes to the understanding of how BES1/BZR1 promotes stress tolerance in woody plants. Results Six BES1/BZR1 family members were identified from Betula platyphylla . Cis -element analysis showed that the promoters of six genes were rich in ABA-responsive element (ABRE), MYB and MBS cis -acting elements, which are reported to be involved in abiotic stress responses. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis indicated that BpBZR1-6 (BPChr10G06000) could be induced by salt stress, ABA and BRs. BpBZR1-6 was localized in the nucleus and had transactivation activity. Ectopic expression of BpBZR1-6 enhanced Arabidopsis tolerance and decreased abscisic acid (ABA) sensitivity under salt treatment. Specifically, the seed germination rate, root length, fresh weight and chlorophyll content were significantly higher in BpBZR1-6 -overexpressing (OE) transgenic plants than in wild-type (WT) plants after salt stress ( P  < 0.05). Additionally, BpBZR1-6 overexpression showed enhanced the reactive oxygen species (ROS) scavenging capability under salt stress, including increasing the activities of antioxidant enzyme, resulting in a decrease in O 2 − and H 2 O 2 accumulation, and reducing malondialdehyde (MDA) content. Meanwhile, the expression levels of six antioxidant enzyme genes were higher in OE plants than in WT plants after stress. Conclusion BpBZR1-6 overexpression enhanced the salt tolerance of transgenic plants by modulating antioxidant enzyme gene expression and ROS scavenging, which may provide underlying strategy for breeding of salt-tolerant plants.

Summary 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.

AimsIn order to better understand the changes of soil carbon sequestration capacity in forest after forest mixing, the effects of broadleaf tree invasion on soil aggregate stability and carbon sequestration were studied.MethodsIn northern China, the pure Larix principis-rupprechtii plantations and the Larix principis-rupprechtii plantations invaded by Betula platyphylla at various degrees with the same site conditions were selected (Betula platyphylla had mixed degrees of 0.2 and 0.4). The distribution and stability of soil aggregates were analyzed, and soil organic carbon and active carbon components were determined.ResultsThe distribution of soil macroaggregates (> 0.25 mm) increased with the increase in the mixed degree of Betula platyphylla. The mixture of Betula platyphylla could effectively increased SOC, EOC, DOC and MBC of the original soil and soil aggregates of different diameter classes. The invasion of Betula platyphylla had a positive indirect impact on soil carbon sequestration by affecting the soil physical and chemical properties and the aggregate stability.ConclusionThe invasion of Betula platyphylla had significant positive effects on soil aggregate stability, erosion resistance and soil nutrient status in Larix principis-rupprechtii plantation. Maybe the selection of suitable broadleaf mixed species can improve the soil quality and soil organic carbon sequestration of the Larix principis-rupprechtii plantation in this area.