High quality haplotype-resolved genome assemblies of Populus tomentosa Carr., a stabilized interspecific hybrid species widespread in Asia

Study of the SPL gene family and miR156-SPL module in Populus tomentosa: Potential roles in juvenile-to-adult phase transition and reproductive phase

Populus tomentosa, a deciduous tree species distinguished by its significant economic and ecological value, enjoys a wide-ranging natural distribution. However, its long juvenile period severely restricts the advancement of breeding work. The SPL gene family, a distinctive class of transcription factors exclusive to the plant kingdom, is critical in various processes of plant growth and development. The miR156-SPL molecular module stands as an indispensable regulatory mechanism in the transition from the vegetative juvenile phase to the adult phase in plants. Consequently, this research endeavored a methodical and exhaustive exploration of the SPL gene family within the P.tomentosa species, synergistically integrating the miR156 family into the analysis. A total of 56 PtSPL genes were identified and subjected to a comprehensive analysis of their gene structure, conserved motifs, collinearity relationships, chromosomal localization, and promoter cis-acting elements. Further analysis of gene expression profiles confirmed the pivotal role of PtSPLs in the reproductive phase and tissue development of P. tomentosa. In addition, 11 members of miR156 in P. tomentosa were identified and their sequences analyzed, elucidating the miR156-SPL regulatory network. The target relationship between miR156k and PtSPLs was further validated by detecting the expression levels of PtSPLs in transgenic poplars overexpressing 35S::MIR156k. This comprehensive study lays a robust theoretical foundation for the continued exploration and application of the SPL genes in P. tomentosa, opening avenues for future research and potential advancements in plant biology and breeding strategies.

Haplotype-resolved genome assembly provides new insights into the genomic origin of purple colour in Prunus mume

Prunus mume, an important ornamental woody plant in the Rosaceae family, contains many interspecific hybridizations. Purple colour is a breeding trait of aesthetic value for P. mume, but little is known about the origin and genetic architecture of this trait. Here we address these issues by producing a haplotype-resolved genome from an interspecific hybrid cultivar of P. mume (M) and P. cerasifera 'Pissardii' (C), named P. mume 'Meiren', followed by a detailed molecular characterization. The final length of the diploid genome is 499.47 Mb, with 250.66 Mb of haplotype M (HM) and 248.79 Mb of haplotype C (HC). Approximately 95.42% (476.61 Mb) of the phased assembly is further anchored to 16 homologous chromosomes. Based on the genomic variation, we identify a 1.8 Mb large-fragmented inversion (INV) on chromosome 1b of HC, which co-segregates with purple colour traits of 'Meiren' inherited from its purple C parent 'Pissardii'. We find that a MYB transcription factor, PmmMYB10.5b, resides at the distal breakpoint of the INV, which displays consistent allele-specific expression (ASE). By directly binding to the promoter of anthocyanin synthetic alleles, PmmMYB10.5b serves as a co-activator to promote anthocyanin accumulation in 'Meiren' organs. Notably, the INV identified in 'Meiren' is generated from 'Pissardii' rather than P. cerasifera, which alters the promoter sequence of PmmMYB10.5b, activates its expression and results in the purple colour trait. Results from this study shed light on the evolutionary origin of purple colour in 'Meiren' and could potentially provide guidance on the genetic improvement of colour traits in ornamental woody plants.

Comprehensive analysis of the multi-rings mitochondrial genome of Populus tomentosa

BACKGROUND

Populus tomentosa, known as Chinese white poplar, is indigenous and distributed across large areas of China, where it plays multiple important roles in forestry, agriculture, conservation, and urban horticulture. However, limited accessibility to the mitochondrial (mt) genome of P. tomentosa impedes phylogenetic and population genetic analyses and restricts functional gene research in Salicaceae family.

RESULTS

Single-molecule real-time (SMRT) sequencing technology was used to sequence, assemble, and annotate the mt genome of P. tomentosa. This genome has a complex structure composed of four circular molecules ranging from 153,004 to 330,873 base pairs (bp). Each of these four circular molecules contains unique gene sequences that constitute the mt genome of P. tomentosa. The mt genome comprises 69 functional genes, including 38 protein-coding genes (PCGs), 26 tRNA genes, and 5 rRNA genes. After removing duplications, 19 different tRNA coding genes remain, though only 10 amino acids can be recognized. The noncoding region constitutes 93.38% of the mt genome, comprising a large number of repetitive sequences, gene spacer regions, and insertion from chloroplast sequences. Specifically, 40 chloroplast-derived sequences, with a total length of 24,381 bp, were identified in P. tomentosa.

CONCLUSIONS

In the current study, the results provide mitochondrial genomic evidence for the maternal origin of P. tomentosa and enhance understanding of the gene dialog between organelle genomes, contributing to the conservation and utilization of the genetic resources of P. tomentosa.

Identifying hub genes and key functional modules in leaf tissue of Populus species based on WGCNA

As one of the most important parts of plants, the genetic mechanisms of photosynthesis or the response of leaf to a single abiotic and biotic stress have been well studied. However, few researches have involved in the integration of data analysis from system level in leaf tissue under multiple abiotic stresses by utilizing biological networks. In this study, the weighted gene co-expression network analysis (WGCNA) strategy was used to integrate multiple data in leaf tissue of Populus species under different sample treatments. The gene co-expression networks were constructed and functional modules were identified by selecting the suitable soft threshold power β in the procedure of WGCNA. The identified hub genes and gene modules were annotated by agriGO, NetAffx Analysis Center, The Plant Genome Integrative Explorer (PlantGenIE) and other annotation tools. The annotation results have displayed that the highly correlated modules and hub genes are involved in the important biological processes or pathways related to module traits. The efficiency of the WGCNA strategy can generate comprehensive understanding of gene module-traits associations in leaf tissue, which will provide novel insight into the genetic mechanism of Populus species.

Molecular Mechanisms of Heterosis and Its Applications in Tree Breeding: Progress and Perspectives

Heterosis, or hybrid vigor, refers to the phenomenon where hybrid progenies outperform their parents in traits such as yield and resistance. This phenomenon has been widely applied in plant breeding. Recent advances in high-throughput genomics have significantly advanced our understanding of heterosis. This review systematically summarizes the genetic, molecular, and epigenetic mechanisms underlying heterosis. Furthermore, we discuss recent advances in predictive methods for heterosis and their applications in improving growth rate, resistance to abiotic stresses, and wood yield in tree species. We also explore the role of tree genomics in unraveling the mechanisms underlying heterosis, emphasizing the potential of integrating high-resolution genomics, single-cell sequencing, and spatial transcriptomics to achieve a comprehensive understanding of heterosis from the molecular to spatial levels. Building on this, CRISPR-based gene-editing technologies can be employed to precisely edit heterotic loci, enabling the study of allele function. Additionally, molecular marker-assisted selection (MAS) can be utilized to identify heterotic loci in parental lines, facilitating the selection of optimal hybrid combinations and significantly reducing the labor and time costs of hybrid breeding. Finally, we review the utilization of heterosis in tree breeding and provide a forward-looking perspective on future research directions, highlighting the potential of integrating multi-omics approaches and emerging gene-editing tools to revolutionize tree hybrid breeding.

Genome-wide investigation and analysis of NAC transcription factor family in Populus tomentosa and expression analysis under salt stress

The NAC transcription factor family is one of the largest families of TFs in plants, and members of NAC gene family play important roles in plant growth and stress response. Recent release of the haplotype-resolved genome assembly of P. tomentosa provide a platform for NAC protein genome-wide analysis. A total of 270 NAC genes were identified and a comprehensive overview of the PtoNAC gene family is presented, including gene promoter, structure and conserved motif analyses, chromosome localization and collinearity analysis, protein phylogeny, expression pattern, and interaction analysis. The results indicate that protein length, molecular weight, and theoretical isoelectric points of the NAC TF family vary, while gene structure and motif are relatively conserved. Chromosome mapping analysis showed that the P. tomentosa NAC genes are unevenly distributed on 19 chromosomes. The interchromosomal evolutionary results indicate 12 pairs of tandem and 280 segmental duplications. Segmental duplication is possibly related to amplification of P. tomentosa NAC gene family. Expression patterns of 35 PtoNAC genes from P. tomentosa subgroup were analysed under high salinity, and seven NAC genes were induced by this treatment. Promoter and protein interaction network analyses showed that PtoNAC genes are closely associated with growth, development, and abiotic and biotic stress, especially salt stress. These results provide a meaningful reference for follow-up studies of the functional characteristics of NAC genes in the mechanism of stress response and their potential roles in development of P. tomentosa.

Celine, a long interspersed nuclear element retrotransposon, colonizes in the centromeres of poplar chromosomes

Centromeres in most multicellular eukaryotes are composed of long arrays of repetitive DNA sequences. Interestingly, several transposable elements, including the well-known long terminal repeat centromeric retrotransposon of maize (CRM), were found to be enriched in functional centromeres marked by the centromeric histone H3 (CENH3). Here, we report a centromeric long interspersed nuclear element (LINE), Celine, in Populus species. Celine has colonized preferentially in the CENH3-associated chromatin of every poplar chromosome, with 84% of the Celine elements localized in the CENH3-binding domains. In contrast, only 51% of the CRM elements were bound to CENH3 domains in Populus trichocarpa. These results suggest different centromere targeting mechanisms employed by Celine and CRM elements. Nevertheless, the high target specificity seems to be detrimental to further amplification of the Celine elements, leading to a shorter life span and patchy distribution among plant species compared with the CRM elements. Using a phylogenetically guided approach, we were able to identify Celine-like LINE elements in tea plant (Camellia sinensis) and green ash tree (Fraxinus pennsylvanica). The centromeric localization of these Celine-like LINEs was confirmed in both species. We demonstrate that the centromere targeting property of Celine-like LINEs is of primitive origin and has been conserved among distantly related plant species.

Nanoplastic toxicity induces metabolic shifts in Populus × euramericana cv. '74/76' revealed by multi-omics analysis

There is increasing global concern regarding the pervasive issue of plastic pollution. We investigated the response of Populus × euramericana cv. '74/76' to nanoplastic toxicity via phenotypic, microanatomical, physiological, transcriptomic, and metabolomic approaches. Polystyrene nanoplastics (PS-NPs) were distributed throughout the test plants after the application of PS-NPs. Nanoplastics principally accumulated in the roots; minimal fractions were translocated to the leaves. In leaves, however, PS-NPs easily penetrated membranes and became concentrated in chloroplasts, causing thylakoid disintegration and chlorophyll degradation. Finally, oxidant damage from the influx of PS-NPs led to diminished photosynthesis, stunted growth, and etiolation and/or wilting. By integrating dual-omics data, we found that plants could counteract mild PS-NP-induced oxidative stress through the antioxidant enzyme system without initiating secondary metabolic defense mechanisms. In contrast, severe PS-NP treatments promoted a shift in metabolic pattern from primary metabolism to secondary metabolic defense mechanisms, an effect that was particularly pronounced during the upregulation of flavonoid biosynthesis. Our findings provide a useful framework from which to further clarify the roles of key biochemical pathways in plant responses to nanoplastic toxicity. Our work also supports the development of effective strategies to mitigate the environmental risks of nanoplastics by biologically immobilizing them in contaminated lands.

A nearly gapless, highly contiguous reference genome for a doubled haploid line of Populus ussuriensis, enabling advanced genomic studies

Populus species, particularly P. trichocarpa, have long served as model trees for genomics research, owing to fully sequenced genomes. However, the high heterozygosity, and the presence of repetitive regions, including centromeres and ribosomal RNA gene clusters, have left 59 unresolved gaps, accounting for approximately 3.32% of the P. trichocarpa genome. In this study, the callus induction method was improved to derive a doubled haploid (DH) callus line from P. ussuriensis anthers. Leveraging long-read sequencing, we successfully assembled a nearly gap-free, telomere-to-telomere (T2T) P. ussuriensis genome spanning 412.13 Mb. This genome assembly contains only seven gaps and has a contig N50 length of 19.50 Mb. Annotation revealed 34,953 protein-coding genes in this genome, which is 465 more than that of P. trichocarpa. Notably, centromeric regions are characterized by higher-order repeats, we identified and annotated centromere regions in all DH genome chromosomes, a first for poplars. The derived DH genome exhibits high collinearity with P. trichocarpa and significantly fills gaps present in the latter's genome. This T2T P. ussuriensis reference genome will not only enhance our understanding of genome structure, and functions within the poplar genus but also provides valuable resources for poplar genomic and evolutionary studies.

Genome-Wide Analysis of C/S1-bZIP Subfamilies in Populus tomentosa and Unraveling the Role of PtobZIP55/21 in Response to Low Energy

C/S1 basic leucine zipper (bZIP) transcription factors are essential for plant survival under energy deficiency. However, studies on the responses of C/S1-bZIPs to low energy in woody plants have not yet been reported. In this study, members of C/S1-bZIP subfamilies in Populus tomentosa were systematically analyzed using bioinformatic approaches. Four C-bZIPs and 10 S1-bZIPs were identified, and their protein properties, phylogenetic relationships, gene structures, conserved motifs, and uORFs were systematically investigated. In yeast two-hybrid assays, direct physical interactions between C-bZIP and S1-bZIP members were observed, highlighting their potential functional synergy. Moreover, expression profile analyses revealed that low energy induced transcription levels of most C/S1-bZIP members, with bZIP55 and bZIP21 (a homolog of bZIP55) exhibiting particularly significant upregulation. When the expression of bZIP55 and bZIP21 was co-suppressed using artificial microRNA mediated gene silencing in transgenic poplars, root growth was promoted. Further analyses revealed that bZIP55/21 negatively regulated the root development of P. tomentosa in response to low energy. These findings provide insights into the molecular mechanisms by which C/S1-bZIPs regulate poplar growth and development in response to energy deprivation.

Haplotype-resolved genome assembly of poplar line NL895 provides a valuable tree genomic resource

Poplar line NL895 can potentially become a model plant for poplar study as it is a widely cultivated elite line. However, the lack of genome resources hindered the use of NL895 as the major plant material in poplar. In this study, we provided a high-quality genome assembly for poplar line NL895 with PacBio single molecule real-time (SMRT) sequencing and High-throughput chromosome conformation capture (Hi-C) technology. The raw assembly of NL895 for the diploid genome included 606 contigs with a total size of ~815 Mb, and the monoploid genome included 246 contigs with a total size of ~412 Mb. The haplotype-resolved chromosomes in the diploid genomes were also generated. All the monoploid, diploid, and haplotype-resolved genomes showed more than 97% completeness and they can largely improve the mapping efficiency in RNA-Seq analysis. By comprehensively comparing the two haplotype genomes we found the heterozygosity of NL895 is much higher than other poplar lines. We also found that NL895 harbors more genomic variants and more gene diversity. The haplotype-specific genes showed higher variable gene expression patterns. These characters would be attributed to the high heterosis of poplar line NL895. The allele-specific expression (ASE) was also investigated and lots of alleles showed biased expressions in different tissues or environmental conditions. Taken together, the genome sequence for NL895 is a valuable tree genomic resource and it would greatly facilitate studies in poplar.

High quality genomes produced from single MinION flow cells clarify polyploid and demographic histories of critically endangered Fraxinus (ash) species

With populations of threatened and endangered species declining worldwide, efforts are being made to generate high quality genomic records of these species before they are lost forever. Here, we demonstrate that data from single Oxford Nanopore Technologies (ONT) MinION flow cells can, even in the absence of highly accurate short DNA-read polishing, produce high quality de novo plant genome assemblies adequate for downstream analyses, such as synteny and ploidy evaluations, paleodemographic analyses, and phylogenomics. This study focuses on three North American ash tree species in the genus Fraxinus (Oleaceae) that were recently added to the International Union for Conservation of Nature (IUCN) Red List as critically endangered. Our results support a hexaploidy event at the base of the Oleaceae as well as a subsequent whole genome duplication shared by Syringa, Osmanthus, Olea, and Fraxinus. Finally, we demonstrate the use of ONT long-read sequencing data to reveal patterns in demographic history.

Haplotype-resolved genome of heterozygous African cassava cultivar TMEB117 (Manihot esculenta)

Cassava (Manihot esculenta Crantz) is a vital tropical root crop providing essential dietary energy to over 800 million people in tropical and subtropical regions. As a climate-resilient crop, its significance grows as the human population expands. However, yield improvement faces challenges from biotic and abiotic stress and limited breeding. Advanced sequencing and assembly techniques enabled the generation of a highly accurate, nearly complete, haplotype-resolved genome of the African cassava cultivar TMEB117. It is the most accurate cassava genome sequence to date with a base-level accuracy of QV > 64, N50 > 35 Mbp, and 98.9% BUSCO completeness. Over 60% of the genome comprises repetitive elements. We predicted over 45,000 gene models for both haplotypes. This achievement offers valuable insights into the heterozygosity genome organization of the cassava genome, with improved accuracy, completeness, and phased genomes. Due to its high susceptibility to African Cassava Mosaic Virus (ACMV) infections compared to other cassava varieties, TMEB117 provides an ideal reference for studying virus resistance mechanisms, including epigenetic variations and smallRNA expressions.

Subgenome phasing for complex allopolyploidy: case-based benchmarking and recommendations

Accurate subgenome phasing is crucial for understanding the origin, evolution and adaptive potential of polyploid genomes. SubPhaser and WGDI software are two common methodologies for subgenome phasing in allopolyploids, particularly in scenarios lacking known diploid progenitors. Triggered by a recent debate over the subgenomic origins of the cultivated octoploid strawberry, we examined four well-documented complex allopolyploidy cases as benchmarks, to evaluate and compare the accuracy of the two software. Our analysis demonstrates that the subgenomic structure phased by both software is in line with prior research, effectively tracing complex allopolyploid evolutionary trajectories despite the limitations of each software. Furthermore, using these validated methodologies, we revisited the controversial issue regarding the progenitors of the octoploid strawberry. The results of both methodologies reaffirm Fragaria vesca and Fragaria iinumae as progenitors of the octoploid strawberry. Finally, we propose recommendations for enhancing the accuracy of subgenome phasing in future studies, recognizing the potential of integrated tools for advanced complex allopolyploidy research and offering a new roadmap for robust subgenome-based phylogenetic analysis.

Identification of microRNAs involved in ectomycorrhizal formation in Populus tomentosa

The majority of woody plants are able to form ectomycorrhizal (ECM) symbioses with fungi. During symbiotic development, plants undergo a complex re-programming process involving a series of physiological and morphological changes. MicroRNAs (miRNAs) are important components of the regulatory network underlying symbiotic development. To elucidate the mechanisms of miRNAs and miRNA-mediated mRNA cleavage during symbiotic development, we conducted high-throughput sequencing of small RNAs and degradome tags from roots of Populus tomentosa inoculated with Cenococcum geophilum. This process led to the annotation of 51 differentially expressed miRNAs between non-mycorrhizal and mycorrhizal roots of P. tomentosa, including 13 novel miRNAs. Increased or decreased accumulation of several novel and conserved miRNAs in ECM roots, including miR162, miR164, miR319, miR396, miR397, miR398, novel-miR44 and novel-miR47, suggests essential roles for these miRNAs in ECM formation. The degradome analysis identified root transcripts as miRNA-mediated mRNA cleavage targets, which was confirmed using real-time quantitative PCR. Several of the identified miRNAs and corresponding targets are involved in arbuscular mycorrhizal symbioses. In summary, increased or decreased accumulation of specific miRNAs and miRNA-mediated cleavage of symbiosis-related genes indicate that miRNAs play important roles in the regulatory network underlying symbiotic development.

Haplotype-resolved genome assembly of Populus tremula × P. alba reveals aspen-specific megabase satellite DNA

Populus species play a foundational role in diverse ecosystems and are important renewable feedstocks for bioenergy and bioproducts. Hybrid aspen Populus tremula × P. alba INRA 717-1B4 is a widely used transformation model in tree functional genomics and biotechnology research. As an outcrossing interspecific hybrid, its genome is riddled with sequence polymorphisms which present a challenge for sequence-sensitive analyses. Here we report a telomere-to-telomere genome for this hybrid aspen with two chromosome-scale, haplotype-resolved assemblies. We performed a comprehensive analysis of the repetitive landscape and identified both tandem repeat array-based and array-less centromeres. Unexpectedly, the most abundant satellite repeats in both haplotypes lie outside of the centromeres, consist of a 147 bp monomer PtaM147, frequently span >1 megabases, and form heterochromatic knobs. PtaM147 repeats are detected exclusively in aspens (section Populus) but PtaM147-like sequences occur in LTR-retrotransposons of closely related species, suggesting their origin from the retrotransposons. The genomic resource generated for this transformation model genotype has greatly improved the design and analysis of genome editing experiments that are highly sensitive to sequence polymorphisms. The work should motivate future hypothesis-driven research to probe into the function of the abundant and aspen-specific PtaM147 satellite DNA.

Morphology, sucrose metabolism and gene network reveal the molecular mechanism of seed fiber development in poplar

Poplar is an important tree species for ecological protection, wood production, bioenergy and urban greening; it has been widely planted worldwide. However, the catkin fibers produced by female poplars can cause environmental pollution and safety hazards during spring. This study focused on Populus tomentosa, and revealed the sucrose metabolism regulatory mechanism of catkin fibers development from morphological, physiological and molecular aspects. Paraffin section suggested that poplar catkin fibers were not seed hairs and produced from the epidermal cells of funicle and placenta. Sucrose degradation via invertase and sucrose synthase played the dominant role during poplar catkin fibers development. The expression patterns revealed that sucrose metabolism-related genes played important roles during catkin fibers development. Y1H analysis indicated that there was a potential interaction between sucrose synthase 2 (PtoSUS2)/vacuolar invertase 3 (PtoVIN3) and trichome-regulating MYB transcription factors in poplar. Finally, the two key genes, PtoSUS2 and PtoVIN3, had roles in Arabidopsis trichome density, indicating that sucrose metabolism is important in poplar catkin fibers development. This study is not only helpful for clarifying the mechanism of sucrose regulation during trichome development in perennial woody plants, but also establishes a foundation to solve poplar catkin fibers pollution through genetic engineering methods.

Study on the interaction preference between CYCD subclass and CDK family members at the poplar genome level

Cyclin-dependent kinases (CDKs) control the progression of the cell cycle. D-type cyclin (CYCD) is generally believed to form a complex with CDK and control the G1/S transition. In plants, CYCD and CDK gene families can be divided into 6 (D1-D7) and 7 (CDKA-CDKG) subclasses, respectively. Different subclasses in the CYCD and CDK families have different numbers, structures and functions. In some heterologous woody plants, the functions of these subclass family members remain unclear. In this study, 43 CYCD and 27 CDK gene family members were identified in the allodiploid Populus tomentosa Carr. Phylogenetic analysis suggested that these CYCDs and CDKs were divided into 6 and 7 subclasses, respectively, which were the same as other species. The analysis of protein properties, gene structure, motifs, domains, cis-acting elements and tissue-specific expression of all members of these CYCDs and CDKs showed that the differences between members of different subclasses varied widely, but members of the same subclass especially in the CDK gene family were very similar. These findings also demonstrated a strong correlation between CYCD and CDK gene family members in response to hormones and specific expression. The collinear analysis of P. tomentosa, Populus trichocarpa and Arabidopsis thaliana showed that the expansion patterns of CYCD and CDK gene families were predominantly whole genome duplications (WGD). The protein interaction prediction results of different subclasses of CYCD and CDKs showed that the interaction between different subclasses of CYCD and CDKs was significantly different. Our previous study found that transgenic PtoCYCD2;1 and PtoCYCD3;3 poplars exhibited opposite phenotypes. Y2H and BIFC results showed that the interaction between PtoCYCD2;1 and PtoCYCD3;3 was significantly different with CDKs. This finding might suggest that the functional differences of different CYCD subclasses in plant growth and development were closely related to the different interactions between CYCD and CDK. Our results provide a good idea and direction for the functional study of CYCD and CDK proteins in woody plants.

Genome-wide identification of the AlkB homologs gene family, PagALKBH9B and PagALKBH10B regulated salt stress response in Populus

The AlkB homologs (ALKBH) gene family regulates N⁶-methyladenosine (m⁶A) RNA methylation and is involved in plant growth and the abiotic stress response. Poplar is an important model plant for studying perennial woody plants. Poplars typically have a long juvenile period of 7-10 years, requiring long periods of time for studies of flowering or mature wood properties. Consequently, functional studies of the ALKBH genes in Populus species have been limited. Based on AtALKBHs sequence similarity with Arabidopsis thaliana, 23 PagALKBHs were identified in the genome of the poplar 84K hybrid genotype (P. alba  ×  P. tremula var. glandulosa), and gene structures and conserved domains were confirmed between homologs. The PagALKBH proteins were classified into six groups based on conserved sequence compared with human, Arabidopsis, maize, rice, wheat, tomato, barley, and grape. All homologs of PagALKBHs were tissue-specific; most were highly expressed in leaves. ALKBH9B and ALKBH10B are m⁶A demethylases and overexpression of their homologs PagALKBH9B and PagALKBH10B reduced m⁶A RNA methylation in transgenic lines. The number of adventitious roots and the biomass accumulation of transgenic lines decreased compared with WT. Therefore, PagALKBH9B and PagALKBH10B mediate m⁶A RNA demethylation and play a regulatory role in poplar growth and development. Overexpression of PagALKBH9B and PagALKBH10B can reduce the accumulation of H₂O₂ and oxidative damage by increasing the activities of SOD, POD, and CAT, and enhancing protection for Chl a/b, thereby increasing the salt tolerance of transgenic lines. However, overexpression lines were more sensitive to drought stress due to reduced proline content. This research revealed comprehensive information about the PagALKBH gene family and their roles in growth and development and responsing to salt stress of poplar.

PtoNF-YC9-SRMT-PtoRD26 module regulates the high saline tolerance of a triploid poplar

BACKGROUND

Sensing and responding to stresses determine the tolerance of plants to adverse environments. The triploid Chinese white poplar is widely cultivated in North China because of its adaptation to a wide range of habitats including highly saline ones. However, its triploid genome complicates any detailed investigation of the molecular mechanisms underlying its adaptations.

RESULTS

We report a haplotype-resolved genome of this triploid poplar and characterize, using reverse genetics and biochemical approaches, a MYB gene, SALT RESPONSIVE MYB TRANSCRIPTION FACTOR (SRMT), which combines NUCLEAR FACTOR Y SUBUNIT C 9 (PtoNF-YC9) and RESPONSIVE TO DESICCATION 26 (PtoRD26), to regulate an ABA-dependent salt-stress response signaling. We reveal that the salt-inducible PtoRD26 is dependent on ABA signaling. We demonstrate that ABA or salt drives PtoNF-YC9 shuttling into the nucleus where it interacts with SRMT, resulting in the rapid expression of PtoRD26 which in turn directly regulates SRMT. This positive feedback loop of SRMT-PtoRD26 can rapidly amplify salt-stress signaling. Interference with either component of this regulatory module reduces the salt tolerance of this triploid poplar.

CONCLUSION

Our findings reveal a novel ABA-dependent salt-responsive mechanism, which is mediated by the PtoNF-YC9-SRMT-PtoRD26 module that confers salt tolerance to this triploid poplar. These genes may therefore also serve as potential and important modification targets in breeding programs.

SubPhaser: a robust allopolyploid subgenome phasing method based on subgenome-specific k-mers

With advanced sequencing technology, dozens of complex polyploid plant genomes have been characterized. However, for many polyploid species, their diploid ancestors are unknown or extinct, making it impossible to unravel the subgenomes and genome evolution directly. We developed a novel subgenome-phasing algorithm, SubPhaser, specifically designed for a neoallopolyploid or a homoploid hybrid. SubPhaser first searches for the subgenome-specific sequence (k-mer), then assigns homoeologous chromosomes into subgenomes, and further provides tools to annotate and investigate specific sequences. SubPhaser works well on neoallopolyploids and homoploid hybrids containing subgenome-specific sequences like wheat, but fails on autopolyploids lacking subgenome-specific sequences like alfalfa, indicating that SubPhaser can phase neoallopolyploid/homoploid hybrids with high accuracy, sensitivity and performance. This highly accurate, highly sensitive, ancestral data free chromosome phasing algorithm, SubPhaser, offers significant application value for subgenome phasing in neoallopolyploids and homoploid hybrids, and for the subsequent exploration of genome evolution and related genetic/epigenetic mechanisms.

Conversion between duplicated genes generated by polyploidization contributes to the divergence of poplar and willow

BACKGROUND

Gene conversion has an important effect on duplicate genes produced by polyploidization. Poplar (Populus trichocarpa) and willow (Salix brachista) are leading models and excellent green plants in the Salicaceae. Although much attention has been paid to the evolution of duplicated genes in poplar and willow, the role of conversion between duplicates generated from polyploidization remains poorly understood.

RESULTS

Here, through genomic synteny analyses, we identified duplicate genes generated by the Salicaceae common tetraploidization (SCT) in the poplar and willow genomes. We estimated that at least 0.58% and 0.25% of poplar and willow duplicates were affected by whole-gene conversion after the poplar-willow divergence, with more (5.73% and 2.66%) affected by partial-gene conversion. Moreover, we found that the converted duplicated genes were unevenly distributed on each chromosome in the two genomes, and the well-preserved homoeologous chromosome regions may facilitate the conversion of duplicates. Notably, we found that conversion maintained the similarity of duplicates, likely contributing to the conservation of certain sequences, but is essentially accelerated the rate of evolution and increased species divergence. In addition, we found that converted duplicates tended to have more similar expression patterns than nonconverted duplicates. We found that genes associated with multigene families were preferentially converted. We also found that the genes encoding conserved structural domains associated with specific traits exhibited a high frequency of conversion.

CONCLUSIONS

Extensive conversion between duplicate genes generated from the SCT contributes to the diversification of the family Salicaceae and has had long-lasting effects on those genes with important biological functions.