Identification and molecular study of medicinal Plectranthus species (Lamiaceae) from Saudi Arabia using plastid DNA regions and ITS2 of the nrDNA gene

Abstract

Plectranthus is a genus of the Lamiaceae family that includes many species of medicinal and agricultural importance. However, this genus has been the subject of taxonomic debate and contains species that are difficult to distinguish. The present study focused on six Plectranthus species commonly found in Saudi Arabia: P. arabicus, P. tenuiflorus, P. barbatus, P. pseudomarrubioides, P. asirensis, and P. hijazensis. P. hijazensis is endemic to Saudi Arabia. The capacities of five different plastid DNA barcodes (matK, rbcL, trnH-psbA, and ITS1 and ITS2 regions of the nrDNA gene) to identify and distinguish between Plectranthus species were evaluated. The following analytical methods were used to evaluate the efficiencies of the selected markers: BLAST, inter- and intraspecific distance, barcode gap, secondary structure of ITS2, and maximum likelihood (ML) phylogenetic trees. The results demonstrated that the nuclear ITS2 region can be successfully amplified and sequenced (100%), leading to a strong ability to discriminate between species and a clear barcode gap. Furthermore, there were significant differences in the ITS2 secondary structure among Plectranthus spp. Samples of Plectranthus formed monophyletic groups according to species in the ML tree, with high supported values. Our results establish that all Plectranthus species in Saudi Arabia can be classified into two groups within the Coleus clade. To our knowledge, this is the first time that local and endemic Plectranthus spp. have been identified and compared with Plectranthus samples of different geographical origins. Our results confirm the diversity of Plectranthus species growing naturally in southwestern Saudi Arabia. In addition, P. hijazensis, which is endemic to Saudi Arabia, was determined to be genetically distinct from other Plectranthus species and should, therefore, be the focus of future research, in addition to the preservation of the natural environment of these species.