Abstract
The genera Arnebia and Lithospermum (Lithospermeae-Boraginaceae) comprise 25–30 and 50–60 species, respectively. Some of them are economically valuable, as their roots frequently contain a purple-red dye used in the cosmetic industry. Furthermore, dried roots of Arnebia euchroma, A. guttata, and Lithospermum erythrorhizon, which have been designated Lithospermi Radix, are used as traditional Korean herbal medicine. This study is the first report on the floral micromorphology and complete chloroplast (cp) genome sequences of A. guttata (including A. tibetana), A. euchroma, and L. erythrorhizon. We reveal great diversity in floral epidermal cell patterns, gynoecium, and structure of trichomes. The cp genomes were 149,361–150,465 bp in length, with conserved quadripartite structures. In total, 112 genes were identified, including 78 protein-coding regions, 30 tRNA genes, and four rRNA genes. Gene order, content, and orientation were highly conserved and were consistent with the general structure of angiosperm cp genomes. Comparison of the four cp genomes revealed locally divergent regions, mainly within intergenic spacer regions (atpH-atpI, petN-psbM, rbcL-psaI, ycf4-cemA, ndhF-rpl32, and ndhC-trnV-UAC). To facilitate species identification, we developed molecular markers psaA- ycf3 (PSY), trnI-CAU- ycf2 (TCY), and ndhC-trnV-UAC (NCTV) based on divergence hotspots. High-resolution phylogenetic analysis revealed clear clustering and a close relationship of Arnebia to its Lithospermum sister group, which was supported by strong bootstrap values and posterior probabilities. Overall, gynoecium characteristics and genetic distance of cp genomes suggest that A. tibetana, might be recognized as an independent species rather than a synonym of A. guttata. The present morphological and cp genomic results provide useful information for future studies, such as taxonomic, phylogenetic, and evolutionary analysis of Boraginaceae.
Highlights
Arnebia Forssk., an economically and medicinally important plant group of the Boraginaceae Juss. comprises 25–30 species, distributed across Southwest and Central Asia, the Himalayas, Northeast Africa, and (Southeast) Mediterranean (Zhu et al, 1995; Mabberley, 2008; Ambrish and Srivastava, 2014; Coppi et al, 2015)
Limited variation was observed between individuals of the same species, the four species could be clearly distinguished by their floral characteristics
The corolla was yellow with or without dark spots on corolla lobes in A. guttata and A. tibetana (Figures 1A,B), dark purple to purple in A. euchroma (Figure 1C), and white in L. erythrorhizon (Figure 1D)
Summary
Arnebia Forssk., an economically and medicinally important plant group of the Boraginaceae Juss. (tribe Lithospermeae Dumort.) comprises 25–30 species, distributed across Southwest and Central Asia, the Himalayas, Northeast Africa, and (Southeast) Mediterranean (Zhu et al, 1995; Mabberley, 2008; Ambrish and Srivastava, 2014; Coppi et al, 2015). Arnebia Forssk., an economically and medicinally important plant group of the Boraginaceae Juss. The roots of certain Arnebia taxa, which frequently contain a purple-red dye, have been used as a colorant in food and cosmetic preparations. The roots of Lithospermum L., another genus in the tribe Lithospermeae, contain a red dye and have been reported to possess various phytomedical activities (Ishida and Sakaguchi, 2007). H. et al, 2019; Korea Institute of Oriental Medicine [KIOM], 2020). Because their morphological, micromorphological, and molecular characteristics have not been compared and described in detail yet, the different species of medicinal borages have been used as a single remedy. Several authors place A. tibetana as a synonym of A. guttata (Kazmi, 1970; Ambrish and Srivastava, 2014; Urgamal et al, 2014), whereas others consider that the biennial habitat and absence of blackish-purple spots on corolla lobes of A. tibetana support its classification as a separate species, independent from A. guttata (Zhu et al, 1995; Lazkov and Sultanova, 2014)
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