Analysis of miRNAs and their target genes in five Melilotus albus NILs with different coumarin content

Genome-Wide Identification and Expression Profiling of the SRS Gene Family in Melilotus albus Reveals Functions in Various Stress Conditions

The plant-specific SHI-related sequence (SRS) family of transcription factors plays a vital role in growth regulation, plant development, phytohormone biosynthesis, and stress response. However, the genome-wide identification and role in the abiotic stress-related functions of the SRS gene family were not reported in white sweet clover (Melilotus albus). In this study, nine M. albus SRS genes (named MaSRS01-MaSRS09) were identified via a genome-wide search method. All nine genes were located on six out of eight chromosomes in the genome of M. albus and duplication analysis indicated eight segmentally duplicated genes in the MaSRS family. These MaSRS genes were classified into six groups based on their phylogenetic relationships. The gene structure and motif composition results indicated that MaSRS members in the same group contained analogous intron/exon and motif organizations. Further, promoter region analysis of MaSRS genes uncovered various growth, development, and stress-responsive cis-acting elements. Protein interaction networks showed that each gene has both functions of interacting with other genes and members within the family. Moreover, real-time quantitative PCR was also performed to verify the expression patterns of nine MaSRS genes in the leaves of M. albus. The results showed that nine MaSRSs were up- and down-regulated at different time points after various stress treatments, such as salinity, low-temperature, salicylic acid (SA), and methyl jasmonate (MeJA). This is the first systematic study of the M. albus SRS gene family, and it can serve as a strong foundation for further elucidation of the stress response and physiological improvement of the growth functions in M. albus.

Genome–Wide Identification of the GRAS Family Genes in Melilotus albus and Expression Analysis under Various Tissues and Abiotic Stresses

The GRAS gene family is a plant–specific family of transcription factors, which play an important role in many metabolic pathways, such as plant growth and development and stress response. However, there is no report on the comprehensive study of the GRAS gene family of Melilotus albus. Here, we identified 55 MaGRAS genes, which were classified into 8 subfamilies by phylogenetic analysis, and unevenly distributed on 8 chromosomes. The structural analysis indicated that 87% of MaGRAS genes have no intron, which is highly conservative in different species. MaGRAS proteins of the same subfamily have similar protein motifs, which are the source of functional differences of different genomes. Transcriptome and qRT–PCR data were combined to determine the expression of 12 MaGRAS genes in 6 tissues, including flower, seed, leaf, stem, root and nodule, which indicated the possible roles in plant growth and development. Five and seven MaGRAS genes were upregulated under ABA, drought, and salt stress treatments in the roots and shoots, respectively, indicating that they play vital roles in the response to ABA and abiotic stresses in M. albus. Furthermore, in yeast heterologous expression, MaGRAS12, MaGRAS34 and MaGRAS33 can enhance the drought or salt tolerance of yeast cells. Taken together, these results provide basic information for understanding the underlying molecular mechanisms of GRAS proteins and valuable information for further studies on the growth, development and stress responses of GRAS proteins in M. albus.

Functional Enrichment Analysis of Regulatory Elements

Statistical methods for enrichment analysis are important tools to extract biological information from omics experiments. Although these methods have been widely used for the analysis of gene and protein lists, the development of high-throughput technologies for regulatory elements demands dedicated statistical and bioinformatics tools. Here, we present a set of enrichment analysis methods for regulatory elements, including CpG sites, miRNAs, and transcription factors. Statistical significance is determined via a power weighting function for target genes and tested by the Wallenius noncentral hypergeometric distribution model to avoid selection bias. These new methodologies have been applied to the analysis of a set of miRNAs associated with arrhythmia, showing the potential of this tool to extract biological information from a list of regulatory elements. These new methods are available in GeneCodis 4, a web tool able to perform singular and modular enrichment analysis that allows the integration of heterogeneous information.

Genome and systems biology of Melilotus albus provides insights into coumarins biosynthesis

Melilotus species are used as green manure and rotation crops worldwide and contain abundant pharmacologically active coumarins. However, there is a paucity of information on its genome and coumarin production and function. Here, we reported a chromosome-scale assembly of Melilotus albus genome with 1.04 Gb in eight chromosomes, containing 71.42% repetitive elements. Long terminal repeat retrotransposon bursts coincided with declining of population sizes during the Quaternary glaciation. Resequencing of 94 accessions enabled insights into genetic diversity, population structure, and introgression. Melilotus officinalis had relatively larger genetic diversity than that of M. albus. The introgression existed between M. officinalis group and M. albus group, and gene flows was from M. albus to M. officinalis. Selection sweep analysis identified candidate genes associated with flower colour and coumarin biosynthesis. Combining genomics, BSA, transcriptomics, metabolomics, and biochemistry, we identified a β-glucosidase (BGLU) gene cluster contributing to coumarin biosynthesis. MaBGLU1 function was verified by overexpression in M. albus, heterologous expression in Escherichia coli, and substrate feeding, revealing its role in scopoletin (coumarin derivative) production and showing that nonsynonymous variation drives BGLU enzyme activity divergence in Melilotus. Our work will accelerate the understanding of biologically active coumarins and their biosynthetic pathways, and contribute to genomics-enabled Melilotus breeding.

Genome-Wide Analysis of the UDP-Glycosyltransferase Family Reveals Its Roles in Coumarin Biosynthesis and Abiotic Stress in Melilotus albus

Coumarins, natural products abundant in Melilotus albus, confer features in response to abiotic stresses, and are mainly present as glycoconjugates. UGTs (UDP-glycosyltransferases) are responsible for glycosylation modification of coumarins. However, information regarding the relationship between coumarin biosynthesis and stress-responsive UGTs remains limited. Here, a total of 189 MaUGT genes were identified from the M. albus genome, which were distributed differentially among its eight chromosomes. According to the phylogenetic relationship, MaUGTs can be classified into 13 major groups. Sixteen MaUGT genes were differentially expressed between genotypes of Ma46 (low coumarin content) and Ma49 (high coumarin content), suggesting that these genes are likely involved in coumarin biosynthesis. About 73.55% and 66.67% of the MaUGT genes were differentially expressed under ABA or abiotic stress in the shoots and roots, respectively. Furthermore, the functions of MaUGT68 and MaUGT186, which were upregulated under stress and potentially involved in coumarin glycosylation, were characterized by heterologous expression in yeast and Escherichia coli. These results extend our knowledge of the UGT gene family along with MaUGT gene functions, and provide valuable findings for future studies on developmental regulation and comprehensive data on UGT genes in M. albus.

Genome-wide development and application of miRNA-SSR markers in Melilotus genus

Genetic diversity of plants is the brace of biodiversity and diversity within species, between species, and of ecosystems. SSR markers are the most preferable molecular marker tool that has been successfully used to study the genetic diversity of plant species. Development of miRNA-SSR markers has been deed in animals but is still limited in plants. In this study, 365 precursors miRNA were extracted from Melilotus albus (Ma) genome and used to design Ma miRNA-SSR primers. 137 Ma primer pairs (79 from known and 58 from novel pre-miRNAs) were obtained. 66 pairs of Ma miRNA-SSR primers were selected with polymorphisms and expected fragment size. The polymorphisms of primers were evaluated in 60 individuals of 15 Ma accessions. A total of 66 primer pairs showed high polymorphism, with average polymorphic information content of 0.49 among 15 Ma accessions and 0.63 among 18 Melilotus species, indicating that these primers have high polymorphisms. The number of alleles produced per primer ranged from 2 to 6 with an average of 3.6 alleles per locus in Ma accessions, and 2 to 10 numbers of alleles with a mean of 5.24 alleles per locus in Melilotus spp. For further studies, the genetic relationship was examined and the cluster analysis showed that 15 Ma accessions were grouped in three groups, on the other hand, 18 Melilotus species clustered into two groups. The analysis of molecular variance (AMOVA) revealed that 64.82% of the variation was found within the species and 35.18% between the species. The population structure analysis showed similar results with PCA analysis in that 18 species were grouped in two groups. In addition, 16,450 miRNA target genes were identified and used for GO and KEGG analysis. This is the first study to develop miRNA-SSR molecular markers in Melilotus spp., which has a great potential for marker-assisted, genetic improvement, genotyping applications, QTL analysis, and molecular-assisted selection studies for plant breeders and other researchers.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01086-z.

Genome-Wide Identification and Development of LTR Retrotransposon-Based Molecular Markers for the Melilotus Genus

Melilotus is an important genus of legumes with industrial and medicinal value, partly due to the production of coumarin. To explore the genetic diversity and population structure of Melilotus, 40 accessions were analyzed using long terminal repeat (LTR) retrotransposon-based markers. A total of 585,894,349 bp of LTR retrotransposon sequences, accounting for 55.28% of the Melilotus genome, were identified using bioinformatics tools. A total of 181,040 LTR retrotransposons were identified and classified as Gypsy, Copia, or another type. A total of 350 pairs of primers were designed for assessing polymorphisms in 15 Melilotus albus accessions. Overall, 47 polymorphic primer pairs were screened for their availability and transferability in 18 Melilotus species. All the primer pairs were transferable, and 292 alleles were detected at 47 LTR retrotransposon loci. The average polymorphism information content (PIC) value was 0.66, which indicated that these markers were highly informative. Based on unweighted pair group method with arithmetic mean (UPGMA) dendrogram cluster analysis, the 18 Melilotus species were classified into three clusters. This study provides important data for future breeding programs and for implementing genetic improvements in the Melilotus genus.

NAC and MYB Families and Lignin Biosynthesis-Related Members Identification and Expression Analysis in Melilotus albus

Melilotus albus is an annual or biennial legume species that adapts to extreme environments via its high stress tolerance. NAC and MYB transcription factors (TFs) are involved in the regulation of lignin biosynthesis, which has not been studied in M. albus. A total of 101 MaNAC and 299 MaMYB members were identified based on M. albus genome. Chromosome distribution and synteny analysis indicated that some genes underwent tandem duplication. Ka/Ks analysis suggested that MaNACs and MaMYBs underwent strong purifying selection. Stress-, hormone- and development-related cis-elements and MYB-binding sites were identified in the promoter regions of MaNACs and MaMYBs. Five MaNACs, two MaMYBs and ten lignin biosynthesis genes were identified as presenting coexpression relationships according to weighted gene coexpression network analysis (WGCNA). Eleven and thirteen candidate MaNAC and MaMYB genes related to lignin biosynthesis were identified, respectively, and a network comprising these genes was constructed which further confirmed the MaNAC and MaMYB relationship. These candidate genes had conserved gene structures and motifs and were highly expressed in the stems and roots, and qRT-PCR further verified the expression patterns. Overall, our results provide a reference for determining the precise role of NAC and MYB genes in M. albus and may facilitate efforts to breed low-lignin-content forage cultivars in the future.