Dissection for floral micromorphology and plastid genome of valuable medicinal borages Arnebia and Lithospermum (Boraginaceae)

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.