Mulberry is the only reported plant species with multiple centromeres on each chromosome and is rich in miniature inverted-repeat transposable elements (MITEs) in their genomes. However, the reasons for the abundance of MITEs in the genome and their role in chromosome structure have remained unanswered. In this study, we conducted a comprehensive and detailed identification of MITEs in the genome of Morus notabilis. A total of 99.73 Mb of MITEs were identified in the 410.45 Mb genome, which can be classified into 10 superfamilies and 278 families. We found that 69.10

The silkworm Bombyx mori is a domesticated insect that serves as an animal model for research and agriculture. The silkworm super-pan-genome dataset, which we published last year, is a unique resource for the study of global genomic diversity and phenotype-genotype association. Here we present SilkMeta (http://silkmeta.org.cn), a comprehensive database covering the available silkworm pan-genome and multi-omics data. The database contains 1082 short-read genomes, 546 long-read assembled genomes, 1168 transcriptomes, 294 phenotype characterizations (phenome), tens of millions of variations (variome), 7253 long non-coding RNAs (lncRNAs), 18 717 full length transcripts and a set of population statistics. We have compiled publications on functional genomics research and genetic stock deciphering (mutant map). A range of bioinformatics tools is also provided for data visualization and retrieval. The large batch of omics data and tools were integrated in twelve functional modules that provide useful strategies and data for comparative and functional genomics research. The interactive bioinformatics platform SilkMeta will benefit not only the silkworm but also the insect biology communities.

Abiotic stress, especially drought stress, poses a significant threat to terrestrial plant growth, development, and productivity. Although mulberry has great genetic diversity and extensive stress-tolerant traits in agroforestry systems, only a few reports offer preliminary insight into the biochemical responses of mulberry leaves under drought conditions. In this study, we performed a comparative metabolomic and transcriptomic analysis on the “drooping mulberry” (Morus alba var. pendula Dippel) under PEG-6000-simulated drought stress. Our research revealed that drought stress significantly enhanced flavonoid accumulation and upregulated the expression of phenylpropanoid biosynthetic genes. Furthermore, the activities of superoxide dismutase (SOD), catalase (CAT) and malondialdehyde (MDA) content were elevated. In vitro enzyme assays and fermentation tests indicated the involvement of flavonol synthase/flavanone 3-hydroxylase (XM_010098126.2) and anthocyanidin 3-O-glucosyltransferase 5 (XM_010101521.2) in the biosynthesis of flavonol aglycones and glycosides, respectively. The recombinant MaF3GT5 protein was found to recognize kaempferol, quercetin, and UDP-glucose as substrates but not 3-/7-O-glucosylated flavonols and UDP-rhamnose. MaF3GT5 is capable of forming 3-O- and 7-O-monoglucoside, but not di-O-glucosides, from kaempferol. This implies its role as a flavonol 3, 7-O-glucosyltransferase. The findings from this study provided insights into the biosynthesis of flavonoids and could have substantial implications for the future diversified utilization of mulberry.

Mulberry is a critical economic tree with a high diversity of germplasm resources. However, the lack of primary data on genome size, chromosome number, and ploidy for this species limits the exploitation of mulberry resources. In this study, the genome size of 323 mulberry germplasm resources were examined using flow cytometry and chromosome numbers analyzed. The genome sizes ranged from 0.36 to 3.08 Gb, and seven different ploidies of mulberry germplasm resources were identified, with chromosome numbers ranging from 14 to 308. Correlation analysis indicated that genome size (1C) and chromosome number positively correlated. Here, the genome size, chromosome number, and ploidy database MGCPdb (https://mgcpdb.biodb.org) were constructed for mulberry plants, which contains 323 core mulberry germplasm resources and provides raw data of flow cytometry analysis, genome size, and chromosome count. This database is significant and valuable for mulberry genome evolution, polyploidy breeding, and genetic diversity research.

Mulberry is a fundamental component of the global sericulture industry, and its positive impact on our health and the environment cannot be overstated. However, the mulberry reference genomes reported previously remained unassembled or unplaced sequences. Here, we report the assembly and analysis of the telomere-to-telomere gap-free reference genome of the mulberry species, Morus notabilis, which has emerged as an important reference in mulberry gene function research and genetic improvement. The mulberry gap-free reference genome produced here provides an unprecedented opportunity for us to study the structure and function of centromeres. Our results revealed that all mulberry centromeric regions share conserved centromeric satellite repeats with different copies. Strikingly, we found that M. notabilis is a species with polycentric chromosomes and the only reported polycentric chromosome species up to now. We propose a compelling model that explains the formation mechanism of new centromeres and addresses the unsolved scientific question of the chromosome fusion-fission cycle in mulberry species. Our study sheds light on the functional genomics, chromosome evolution, and genetic improvement of mulberry species.

As the prevalence of diabetes continues to increase, the number of individuals living with diabetes complications will reach an unprecedented magnitude. Continuous use of some synthetic agents to reduce blood glucose levels causes severe side effects, and thus, the demand for nontoxic, affordable drugs persists. Naturally occurring compounds, such as iminosugars derived from the mulberry (Morus spp.), have been shown to reduce blood glucose levels. In mulberry, 1-deoxynojirimycin (DNJ) is the predominant iminosugar. However, the mechanism underlying DNJ biosynthesis is not completely understood. Here, we showed that DNJ in mulberry is derived from sugar and catalyzed through 2-amino-2-deoxy-D-mannitol (ADM) dehydrogenase MnGutB1. Combining both targeted and nontargeted metabolite profiling methods, DNJ and its precursors ADM and nojirimycin (NJ) were quantified in mulberry samples from different tissues. Purified His-tagged MnGutB1 oxidized the hexose derivative ADM to form the 6-oxo compound DNJ. The mutant MnGutB1 D283N lost this remarkable capability. Furthermore, in contrast to virus-induced gene silencing of MnGutB1 in mulberry leaves that disrupted the biosynthesis of DNJ, overexpression of MnGutB1 in hairy roots and light-induced upregulation of MnGutB1 enhanced DNJ accumulation. Our results demonstrated that hexose derivative ADM, rather than lysine derivatives, is the precursor in DNJ biosynthesis, and it is catalyzed by MnGutB1 to form the 6-oxo compound. These results represent a breakthrough in producing DNJ and its analogs for medical use by metabolic engineering or synthetic biology.

SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes, as unique plant transcription factors, play important roles in plant developmental regulation and stress response adaptation. Although mulberry is a commercially valuable tree species, there have been few systematic studies on SPL genes. In this work, we identified 15 full-length SPL genes in the mulberry genome, which were distributed on 4 Morus notabilis chromosomes. Phylogenetic analysis clustered the SPL genes from five plants (Malus × domestica Borkh, Populus trichocarpa, M. notabilis, Arabidopsis thaliana, and Oryza sativa) into five groups. Two zinc fingers (Zn1 and Zn2) were found in the conserved SBP domain in all of the MnSPLs. Comparative analyses of gene structures and conserved motifs revealed the conservation of MnSPLs within a group, whereas there were significant structure differences among groups. Gene quantitative analysis showed that the expression of MnSPLs had tissue specificity, and MnSPLs had much higher expression levels in older mulberry leaves. Furthermore, transcriptome data showed that the expression levels of MnSPL7 and MnSPL14 were significantly increased under silkworm herbivory. Molecular experiments revealed that MnSPL7 responded to herbivory treatment through promoting the transcription of MnTT2L2 and further upregulating the expression levels of catechin synthesis genes (F3′H, DFR, and LAR).

Mulberry (Morus spp.) is an economically important plant as the main food plant used for rearing domesticated silkworm and it has multiple uses in traditional Chinese medicine. Two basic chromosome numbers (Morus notabilis, n = 7, and Morus alba, n = 14) have been reported in the genus Morus, but the evolutionary history and relationship between them remain unclear. In the present study, a 335-Mb high-quality chromosome-scale genome was assembled for the wild mulberry species M. notabilis. Comparative genomic analyses indicated high chromosomal synteny between the 14 chromosomes of cultivated M. alba and the six chromosomes of wild M. notabilis. These results were successfully verified by fluorescence in situ hybridization. Chromosomal fission/fusion events played crucial roles in the chromosome restructuring process between M. notabilis and M. alba. The activity of the centromere was another key factor that ensured the stable inheritance of chromosomes. Our results also revealed that long terminal repeat retrotransposons were a major driver of the genome divergence and evolution of the mulberry genomes after they diverged from each other. This study provides important insights and a solid foundation for studying the evolution of mulberry, allowing the accelerated genetic improvement of cultivated mulberry species.

Scleromitrula shiraiana is a necrotrophic fungus with a narrow host range, and is one of the main causal pathogens of mulberry sclerotial disease. However, its molecular mechanisms and pathogenesis are unclear. Here, we report a 39.0 Mb high-quality genome sequence for S. shiraiana strain SX-001. The S. shiraiana genome contains 11,327 protein-coding genes. The number of genes and genome size of S. shiraiana are similar to most other Ascomycetes. The cross-similarities and differences of S. shiraiana with the closely related Sclerotinia sclerotiorum and Botrytis cinerea indicated that S. shiraiana differentiated earlier from their common ancestor. A comparative genomic analysis showed that S. shiraiana has fewer genes encoding cell wall-degrading enzymes (CWDEs) and effector proteins than that of S. sclerotiorum and B. cinerea, as well as many other Ascomycetes. This is probably a key factor in the weaker aggressiveness of S. shiraiana to other plants. S. shiraiana has many species-specific genes encoding secondary metabolism core enzymes. The diversity of secondary metabolites may be related to the adaptation of these pathogens to specific ecological niches. However, melanin and oxalic acid are conserved metabolites among many Sclerotiniaceae fungi, and may be essential for survival and infection. Our results provide insights into the narrow host range of S. shiraiana and its adaptation to mulberries.

Ciboria carunculoides is the dominant causal agent of mulberry sclerotial disease, and it is a necrotrophic fungal pathogen with a narrow host range that causes devastating diseases in mulberry fruit. However, little is known about the interaction between C. carunculoides and mulberry. Here, our transcriptome sequencing results showed that the transcription of genes in the secondary metabolism and defense-related hormone pathways were significantly altered in infected mulberry fruit. Due to the antimicrobial properties of proanthocyanidins (PAs), the activation of PA biosynthetic pathways contributes to defense against pathogens. Salicylic acid (SA) and jasmonic acid (JA) are major plant defense hormones. However, SA signaling and JA signaling are antagonistic to each other. Our results showed that SA signaling was activated, while JA signaling was inhibited, in mulberry fruit infected with C. carunculoides. Yet SA mediated responses are double-edged sword against necrotrophic pathogens, as SA not only activates systemic acquired resistance (SAR) but also suppresses JA signaling. We also show here that the small secreted protein CcSSP1 of C. carunculoides activates SA signaling by targeting pathogenesis-related protein 1 (PR1). These findings reveal that the infection strategy of C. carunculoides functions by regulating SA signaling to inhibit host defense responses.

Expansin gene (EXP) family plays important roles in plant growth and crop improvement. However, it has not been well studied in the Brassica genus that includes several important agricultural and horticultural crops. To get insight to the evolution and expansion of EXP family in Brassica, Brassica EXPs which are homologues of 35 known AtEXPs of Arabidopsis were comprehensively and systematically analyzed in the present study. In total, 340 Brassica EXPs were clustered into four groups that corresponded multiple alignment to four subfamilies of AtEXPs, with divergent conserved motifs and cis-acting elements among groups. To understand the expansion of EXP family, an integrated genomic block system was constructed among Arabidopsis and Brassica species based on 24 known ancestral karyotype blocks. Obvious gene loss, segmental duplication, tandem duplication and DNA sequence repeat events were found during the expansion of Brassica EXPs, of which the segmental duplication was possibly the major driving force. The divergence time was estimated in 1109 orthologs pairs of EXPs, revealing the divergence of Brassica EXPs from AtEXPs during ~30 MYA, and the divergence of EXPs among Brassica species during 13.50-17.94 MYA. Selective mode analysis revealed that the purifying selection was the major contributor to expansion of Brassica EXPs. This study provides new insights into the evolution and expansion of the EXP family in Brassica genus.

DNA methylation has been proposed to regulate plant stress resistance. However, the dynamic changes in DNA methylation in woody plants and their correlations with pathogenic responses are not fully understood. Here, we present single-base maps of the DNA methylomes of mulberry ( Morus notabilis ) leaves that were subjected to a mock treatment or inoculation with Botrytis cinerea . Compared with the former, the latter showed decreased mCG and mCHG levels and increased mCHH levels. DNA methylation inhibitors reduced resistance gene methylation levels and enhanced mulberry resistance, suggesting that the hypomethylation of resistance genes affects mulberry resistance to B. cinerea . Virus-induced gene silencing of MnMET1 enhanced the expression of mulberry-resistance genes, thereby increasing the plant’s resistance to B. cinerea . We also found that MITE s play a dominant role in controlling DNA methylation levels. MITE s appear to be the main sources of 24-nt siRNAs that regulate gene expression through the RNA-directed DNA methylation pathway.

The vegetative phase transition is a prerequisite for flowering in angiosperm plants. Mulberry miR156 has been confirmed to be a crucial factor in the vegetative phase transition in Arabidopsis thaliana. The over-expression of miR156 in transgenic Populus × canadensis dramatically prolongs the juvenile phase. Here, we find that the expression of mno-miR156 decreases with age in all tissues in mulberry, which led us to study the hierarchical action of miR156 in mulberry. Utilizing degradome sequencing and dual-luciferase reporter assays, nine MnSPLs were shown to be directly regulated by miR156. The results of yeast one-hybrid and dual-luciferase reporter assays also revealed that six MnSPLs could recognize the promoter sequences of mno-miR172 and activate its expression. Our results demonstrate that mno-miR156 performs its role by repressing MnSPL/mno-miR172 pathway expression in mulberry. This work uncovered a miR156/SPLs/miR172 regulation pathway in the development of mulberry and fills a gap in our knowledge about the molecular mechanism of vegetative phase transition in perennial woody plants.

The acquisition of new metabolic activities is a major force driving evolution. We explored, from the perspectives of gene family expansion and the evolutionary adaptability of proteins, how new functions have arisen in which terpene synthases diverged. Monoterpenoids are diverse natural compounds that can be divided into cyclic and acyclic skeleton forms according to their chemical structure. We demonstrate, through phylogenetic reconstructions and genome synteny analyses, that the (E)-β-ocimene synthases, which are acyclic monoterpene synthases (mTPSs), appear to have arisen several times in independent lineages during plant evolution. Bioinformatics analyses and classical mutation experiments identified four sites (I388, F420, S446, and F485) playing important roles in the neofunctionalization of mTPSs. Incubation of neryl diphosphate with Salvia officinalis 1,8-cineole synthase (SCS) and mutated proteins show that these four sites obstruct the isomerization of geranyl diphosphate. Quantum mechanical/molecular mechanical molecular dynamics simulations of models of SCS, SCSY420F/I446S, and SCSN338I/Y420F/I446S/L485F with (3R)-linalyl diphosphate suggest that mutations changed the configuration of the intermediate to obtain new activities. These results provide new perspectives on the evolution of mTPSs, explain the convergent evolution of (E)-β-ocimene synthases at the molecular level, and identify key residues to control the specificity of engineered mTPSs.

Proanthocyanidins (PAs) are major defense-related phenolics in mulberry, but the mechanism underlying their biosynthesis remains uncharacterized. In this study, the relationship between the expression of genes encoding anthocyanidin reductase (ANR) or leucoanthocyanidin reductase (LAR) and PA biosynthesis was investigated in white and red mulberry fruits. In ripening fruits, the MnANR and MnLAR transcription levels tended to decrease, whereas the catechin and epicatechin contents initially increased and then decreased. In contrast, the PA content exhibited a clearly different trend. The ectopic expression of MnANR and MnLAR in tobacco increased the resistance to Botrytis cinerea, as evidenced by the less extensive disease symptoms of the transgenic plants compared with the wild-type plants. In vitro experiments revealed that the transgenic tobacco crude leaf extract had an obvious inhibitory effect on B. cinerea. Moreover, the ectopic expression of MnANR and MnLAR in tobacco inhibited the expression of anthocyanin biosynthesis genes, resulting in decreased anthocyanin contents in flowers. The results of this study may be useful for elucidating the mechanism underlying PA biosynthesis. Furthermore, ANR and LAR represent potential targets for improving the resistance of mulberry and related plant species to B. cinerea.

Mulberry is an important economic crop plant and traditional medicine. It contains a huge array of bioactive metabolites such as flavonoids, amino acids, alkaloids and vitamins. Consequently, mulberry has received increasing attention in recent years. MMHub (version 1.0) is the first open public repository ofmass spectra of small chemical compounds (<1000 Da) in mulberry leaves. The database contains 936 electrospray ionization tandemmass spectrometry (ESI-MS2) data and lists the specific distribution of compounds in 91 mulberry resources with two biological duplicates. ESI-MS2 data were obtained under non-standardized and independent experimental conditions. In total, 124 metabolites were identified or tentatively annotated and details of 90 metabolites with associated chemical structures have been deposited in the database. Supporting information such as PubChem compound information, molecular formula and metabolite classification are also provided in the MS2 spectral tag library. The MMHub provides important and comprehensive metabolome data for scientists working with mulberry. This information will be useful for the screening of quality resources and specific metabolites of mulberry.

Flavonoids are widely distributed in mulberry leaves and have been recognized for their beneficial physiological effects on the human health. Here, we analyzed variations in 44 flavonoid compounds among 91 mulberry resources. Metabolic profiling revealed that O-rhamnosylated flavonols and malonylated flavonol glycosides, including rutin and quercetin 3-O-(6″-O-malonylglucoside) (Q3MG), were absent from Morus notabilis and multiple mulberry (Morus alba L.) resources. Transcriptome and phylogenetic analyses of flavonoid-related UDP-glycosyltransferases (UGTs) suggested that the flavonol 3-O-glucoside-O-rhamnosyltransferase (FGRT) KT324624 is a key enzyme involved in rutin synthesis. A recombinant FGRT protein was able to convert kaempferol/quercetin 3-O-glucoside to kaempferol 3-O-rutinoside (K3G6″Rha) and rutin. The recombinant FGRT was able to use 3-O-glucosylated flavonols but not flavonoid aglycones or 7-O-glycosylated flavonoids as substrates. The enzyme preferentially used UDP-rhamnose as the sugar donor, indicating that it was a flavonol 3-O-glucoside: 6″-O-rhamnosyltransferase. This study provided insights into the biosynthesis of rutin in mulberry.

BACKGROUND:Miniature inverted-repeat transposable elements (MITEs) are common in eukaryotic genomes, and are important for genomic evolution.RESULTS:In the present study, the identification of MITEs in the mulberry genome revealed 286,122 MITE-related sequences, including 90,789 full-length elements. The amplification of mulberry MITEs and the influence of MITEs on the evolution of the mulberry genome were analyzed. The timing of MITE amplifications varied considerably among the various MITE families. Fifty-one MITE families have undergone a single round of amplification, while the other families developed from multiple amplifications. Most mulberry MITEs were inserted near genes and some could regulate gene expression through small RNAs. An analysis of transgenic plants indicated that MITE insertions can upregulate the expression of a target gene. Moreover, MITEs are frequently associated with alternative splicing events (exonizations).CONCLUSION:The data presented herein provide insights into the generation of MITEs as well as their impact on gene regulation and evolution in mulberry.

The evolutionary dynamics of long terminal repeat (LTR) retrotransposons in tree genomes has remained largely unknown. The availability of the complete genome sequences of the mulberry tree (Morus notabilis) has offered an unprecedented opportunity for us to characterize these retrotransposon elements. We investigated 202 and 114 families of Copia and Gypsy superfamilies, respectively, comprising 2916 intact elements in the mulberry genome. The tRNAMet was the most frequently used type of tRNA in both superfamilies. Phylogenetic analysis suggested that Copia and Gypsy from mulberry can be grouped into eight and six lineages, respectively. All previously characterized families of such elements could also be found in the mulberry genome. About 95% of the identified Copia and Gypsy full elements were estimated to have been inserted into the mulberry genome within the past 2–3 million years. Meanwhile, the estimated insertion times of members of the three most abundant families of the Copia superfamily (908 members from the three most abundant families) and Gypsy superfamily (783 members from the three most abundant families) revealed divergent life histories. Compared with the situation in Gypsy elements, three families of Copia elements are under positive selection pressure, which suggested that Copia elements may have a dominant influence in the evolution of mulberry genes. Analysis of insertion and deletion dynamics suggested that Copia and Gypsy elements exhibited a very long half-life in the mulberry genome. The present work provides new insights into the insertion and deletion dynamics of LTR retrotransposons, and it will greatly improve our understanding of the important roles transposable elements play in the architecture of the mulberry genome.

Mulberry ( Morus notabilis C. K. Schneid) leaves have been used as the food for the domesticated silkworm, Bombyx mori , for more than 5000 yr, and the mulberry–silkworm relationship is one of the best‐known and oldest models of plant defense–insect adaptation. The availability of a genome assembly of mulberry provides us with an opportunity to mine the characteristics and distribution of transposable elements (TEs) in this species and to examine their relationship to genes and gene expression. In this study, a significantly correlated inverse relationship between the percentage coverage of genes and TEs was observed. The TE‐rich regions appeared to have a lower percentage of putatively expressed genes. Distribution patterns between different TE superfamilies were detected in the mulberry genome. The Copia elements (the TE making up the greatest proportion of the mulberry genome) were significantly overrepresented within genes in the mulberry genome, and they may have a dominant influence on evolution of the mulberry genome. Approximately 96.93% (330/344) of the TE‐containing genes assigned to pathways were assigned to metabolism‐related pathways. The TE‐related alternative splicing events accounted for 7.58% (402/5,302) of all alternative splicing types in the mulberry genome, suggesting that TEs are one of the driving forces in the formation of the alternatively spliced genes. The results will be valuable in improving our understanding of the important roles of TEs in mulberry genome evolution.