Hedysarum L. is one of the large genera in the Fabaceae family, containing more than 160 species (Lock, 2005). Plants belonging to the genus Hedysarum are distributed in Eurasia, North Africa, and North America (Lock, 2005; Liu et al., 2017). The species occur in meadows, clayey and stony places, deserts, steppes, forests, tundra, river valleys, and mountain slopes (Choi and Ohashi, 2003). The genus Hedysarum includes perennial herbs and rarely semi-shrubs, which differ from closely related genera in pod and pollen morphology (Fedtschenko, 1948; Choi and Ohashi, 1996). Previous studies have shown that many species of the genus Hedysarum have been employed in traditional Chinese medicine to strengthen the immune system and improve the energy of the body (Dong et al., 2013).

Recent molecular studies have proposed to divide this genus into three main clades, largely corresponding to the sections Hedysarum, Stracheya, and Multicaulia (Amirahmadi et al., 2014; Nafisi et al., 2019). In these molecular phylogenetic studies, species were divided into three sections that were well supported, but intersection relationships remained unresolved, especially in section Multicaulia. This could be due to the selection of regions with low variability in the cp genome. Therefore, it is necessary to identify regions with high nucleotide diversity in the cp genome as molecular markers for future molecular phylogenetic studies of Hedysarum. Furthermore, previous studies have reported conflicting results regarding the phylogenetic relationship of section Stracheya with the other two sections within the Hedysarum genus. Duan et al. (2015) found that section Stracheya was closely related to section Multicaulia based on both nrDNA ITS and some plastid markers. However, Liu et al. (2017) reported that section Stracheya was placed together with section Hedysarum based on nrDNA ITS and plastid markers. Conversely, Nafisi et al. (2019) supported a closer relationship between section Stracheya and section Multicaulia. Therefore, determining the phylogenetic position of section Stracheya in the genus Hedysarum based on the cp genome is necessary.

Chloroplasts are important intracellular organelles, having an independent genome with several genes responsible for the process of photosynthesis in green algae and plants (Nazareno et al., 2015; Smith and Keeling, 2015; Yin et al., 2017). Most complete cp genomes harbor a typical quadripartite structure including a long single copy (LSC) region, a small single copy (SSC) region, and two copies of inverted repeat (IR) regions (Bock, 2007). Hedysarum belonging to the IRLC (Inverted Repeat Lacking Clade) clade is described by the lack of one copy of the inverted repeat (IR) region in the whole cp genome (Wojciechowski et al., 2004; She et al., 2019). Species belonging to the IRLC clade are characterized by having a cp genome size of around 121,000–133,000 bp (Moghaddam et al., 2022; Yuan et al., 2022; Tian et al., 2021). To date, the size of the cp genomes of only four Hedysarum species are known, including H. petrovii, H. semenovii, H. polybotrys, and H. taipeicum, with genome sizes of 122,571 bp, 123,407 bp, 122,232 bp, and 126,699 bp, respectively.

Comparative genomics can be used to identify important structural sequences and detect evolutionary changes across genomes since the comprehensive analysis of the cp genome of genera belonging to the IRLC clade such as Astragalus, Onobrychis, Caragana, and Glycyrrhiza have been reported (Kang et al., 2018; Moghaddam et al., 2022; Yuan et al., 2022; Tian et al., 2021). A detailed characterization of these species’ cp genome, including size, gene content, structure repeats, and GC content, as well as information about highly variable nucleotide regions, was provided. However, comprehensive studies on the genome structure of the genus Hedysarum have not been conducted so far.

In the present investigation, we detailed an overview of the complete sequence of the six Hedysarum species cp genome. We sequenced the complete cp genome of H. drobovii, H. flavescens, and H. lehmannianum to explore the relationships among Hedysarum species. We obtained the other three species (H. petrovii, H. semenovii, and H. taipeicum) from the National Center for Biotechnology Information (NCBI). The following questions were addressed: (1) what are the features of the cp genome of selected Hedysarum species? (2) How many potential microsatellite markers can the cp genome provide? (3) Which regions in the cp genome can be used as candidate molecular markers for future molecular phylogenetic studies? (4) What is the phylogenetic placement of section Stracheya within the genus Hedysarum based on the cp genome data?

This study is the first to comprehensively examine the features of cp genomes in Hedysarum species. We compared the cp genomes of six species belonging to three sections that were distributed in different regions. We sequenced the cp genome of H. drobovii, H. flavescens, and H. lehmannianum for this study. The sizes of the six cp genomes ranged from 121,176 bp (H. drobovii) to 126,738 bp (H. taipeicum). It is worth noting that many related genera with similar cp genome sizes to Hedysarum have been reported in recent years (Tian et al., 2021; Bei et al., 2022; Moghaddam et al., 2022).

The cp genomes of Hedysarum species have 110 genes, including 76 protein-coding genes, 30 transfer RNA genes, and 4 ribosomal RNA genes. The structural composition of Hedysarum cp genomes revealed similarity with other IRLC clade species (Su et al., 2019; Bei et al., 2022; Moghaddam et al., 2022). The cp genomes of six Hedysarum species showed high similarity with regard to genome size (except for H. taipeicum which was 126,738 bp), gene sequences, gene classes, and the lacking IR region. All selected Hedysarum species were found to have lost one copy of the IR region, which was first identified in H. taipeicum by She et al. (2019). This loss of the IR region is common in most species belonging to the subfamily Papilionoideae in the family Fabaceae, forming a clade named the IR-lacking clade (IRLC) (Wojciechowski et al., 2004). The GC content of the six Hedysarum species in this study was highly similar, which is an important indicator of species affinity according to Tamura et al. (2011).

Introns are recognized as being central to the regulation of gene expression in plants and animals (Callis et al., 1987; Emami et al., 2013: Choi et al., 1991). In the present study, 15 genes with one intron and one gene (ycf3) with two introns were identified in each of the cp genomes of the six studied Hedysarum species. Most of the 16 identified genes have a high similarity in the structure of introns. However, a structural change was detected in the intron of the petB and clpP genes of H. drobovii and H. lehmannianum, respectively. The intron of the petB gene in the cp genome of H. drobovii is very short (9 bp); whereas, in the other five species, it ranges from 806 bp (H. flavescens) to 864 bp (H. taipeicum). Similarly, the intron of the clpP gene in the cp genome of H. lehmannianum is 6 bp long, while in other species it is from 159 bp (H flavescens) to 613 bp (H. taipeicum). However, the implications or link between gene expression and short or long introns for Hedysarum have not been studied. Further experimental work on the roles of introns in Hedysarum is therefore essential and should prove interesting. In consonance with previous studies, the trnK-UUU gene in the Hedysarum cp genome was observed to be harboring the largest intron (2407-2503 bp) which includes the matK gene.

The chloroplast SSRs were used in evolutionary studies, phylogenetic relationships, and plant population genetics and species identification as molecular markers (Olmstead and Palmer, 1994; Saski et al., 2005). A total of 172 SSRs (H. taipeicum) to 190 SSRs (H. semenovii) were found in the cp genome of six Hedysarum species. Several studies found that the mononucleotide repeats were dominant among SSRs in the cp genome, where A/T bases account for the majority (Ellegren, 2004; George et al., 2015; Ren et al., 2021). Likewise, A/T mononucleotide repeats were dominant among SSRs in the six Hedysarum cp genomes, ranging from 78.7% to 84.3%. Furthermore, the identified common and unique SSRs might play an important role in the analysis of the genetic diversity of the genus Hedysarum. In particular, the unique pentanucleotide SSRs identified in H. drobovii (AAAGG, AAAAT, AAGAC, ATTTT, and TTGTC) and H. taipeicum (AACCG), as well as the unique hexanucleotide SSRs identified in H. semenovii (ATAGCT and TTTTTC) and H. taipeicum (ATTCTT) may be effectively utilized in the future for species identification and assessment of genetic diversity in their populations. In addition, repeat sequences are known to play an important role in cp genome rearrangement, recombination, gene duplication, deletion, and gene expression (Gemayel et al., 2010; Do et al., 2014; Vieira et al., 2014; Li and Zheng, 2018),. They also have been reported to be responsible for substitutions and indels in the cp genome (Yi et al., 2013). We identified 51 (H. petrovii) to 214 (H. flavescens) repeat sequences among the six Hedysarum cp genomes analyzed, with forward repeats being the most common in H. petrovii, H. flavescens, H. semenovii, H. lehmannianum, and H. taipeicum, whereas palindromic and forward repeats were the most abundant in H. drobovii. Notably, H. drobovii and H. petrovii, both belonging to section Multicaulia, had significantly fewer repeat regions (53 and 51, respectively) compared to the other species (104–214). Further investigation into repeat sequences in section Multicaulia is necessary.

DNA barcodes with high variability are crucial for species identification, resource conservation, and phylogenetic analyses (Gregory, 2005; Bringloe and Saunders, 2019; Chen et al., 2019; Liu et al., 2019). Our comparison of Hedysarum species’ cp genomes revealed high similarity in gene content and gene order, with genome lengths ranging from 121,176 to 126,738 bp. However, mVISTA analyses indicated that sequence variation was higher in non-coding regions than in other regions. Nucleotide diversity analysis identified six highly variable regions in the whole cp genome of Hedysarum species, mainly located in non-coding regions. Three protein-coding genes, clpP, accD, and atpF, exhibited higher Pi values and were found to be highly variable regions. The variability of the clpP gene can be attributed to the large variation in its Exon I length between species, ranging from 3 bp (H. drobovii) to 219 bp (H. taipeicum and H. lehmannianum) ( Supplementary Table 2 ). The clpP and accD genes have been reported to play an important role in counteracting biotic and abiotic stress (Singh et al., 2015; Sinha et al., 2018; Ali and Baek, 2020), while the atpF gene is involved in the synthesis of ATF during photosynthesis (Ghulam et al., 2012), which is greatly influenced by altitude conditions (Wang et al., 2017). The high Pi values observed in these genes may reflect adaptation to different environmental conditions. Moreover, these highly variable regions can serve as candidate molecular markers and a reference for identifying future Hedysarum species. The clpP, accD, and atpF gene exon regions have similarly been identified as some of the most highly variable hotspot regions in cp genomes of some species (Mo et al., 2020; Mascarello et al., 2021; Moghaddam et al., 2022; Long et al., 2023).

Our phylogenetic analysis based on complete cp genomes and protein-coding genes confirmed previous studies on cp genome data of IRLC clade species, determining the phylogenetic position of Hedysarum as a sister to Onobrychis (She et al., 2019; Jin et al., 2021; Moghaddam et al., 2022; Tian et al., 2021). Our study also confirms the monophyly of Hedysarum based on plastid DNA genes, which is consistent with previous studies by Duan et al. (2015); Liu et al. (2017), and Nafisi et al. (2019). Although a limited number of species were used in our study, the phylogenetic relationships among the three sections of Hedysarum were analyzed using complete cp genomes and 76 protein-coding genes for the first time. Our results suggested that sections of Hedysarum could be monophyletic based on both cp genome data. However, further studies with more species, particularly from section Stracheya, are necessary to confirm this outcome. Furthermore, both our phylogenetic reconstructions revealed a close relationship between section Stracheya and Hedysarum, which is consistent with previous findings by Liu et al. (2017), but incongruous with the outcomes of Duan et al. (2015) and Nafisi et al. (2019). Additionally, this relationship is supported by the shared morphological characteristics between species of section Stracheya and Hedysarum, including leaves with numerous leaflets (4-15 paired), wings longer than half of the keel, and pods lacking ribs, bristles, or spines.

Our study is the first research work to investigate the genome characteristics of the genus Hedysarum. We sequenced, assembled, and annotated the cp genome of H. drobovii, H. flavescens va H. lehmannianum using high-throughput technology. Our study is based on cp genome data from a total of six Hedysarum species, including three previously published species. The cp genomes of all six Hedysarum species analyzed contained 110 genes, including 76 protein-coding genes, 4 rRNA genes, and 30 tRNA genes. We identified between 172 and 190 microsatellites and 51 to 214 pairs of repeat sequences among the six Hedysarum species cp genomes. In addition, we identified seven common SSRs and 14 unique SSRs in the studied Hedysarum species. Furthermore, we detected highly variable regions in the clpP, accD, and atpF protein-coding genes. These repeat motifs and highly variable genes could be used for evolutionary studies, phylogenetic relationships, plant population genetics, and species identification. Our phylogenetic reconstructions using the complete cp genome and protein-coding genes confirmed the monophyly of Hedysarum. Additionally, we supported the close relationship between section Stracheya and section Hedysarum using all three BI, ML, and MP methods. However, future studies using more species will provide a better understanding of the relationships among Hedysarum sections.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/ Supplementary Material .

IJ: Conceptualization, methodology, data analysis, identification, visualization, writing, original draft preparation, reviewing, editing, and discussing. DM: methodology and data analysis. ZY: methodology and collection. KT: Supervision, investigation, identification, reviewing, editing, and discussing. All authors contributed to the article and approved the submitted version.