The design of DNA barcode-specific PCR primers for medicinal plant authentication

The DNA Barcode of Life initiative aims to obtain designated „barcode“ sequences for every known species on Earth. Proponents of plant DNA barcoding have cited medicinal plant authentication as one of the potential applications of barcode information. However, DNA sequencing of barcode regions may not be suitable for routine quality assurance testing of plant materials, which could contain mixtures of plant species and/or degraded DNA. The value of DNA barcoding in this arena may in fact lie in its role as a platform for the design of standardised DNA-based tests.

We have studied three groups of plant species (Hypericum spp. Actaea spp. and Rhodiola spp.), each comprising a target commercial medicinal plant and known or potential adulterant species. The suitability of four „barcode“ regions for the design of species-specific PCR primers was determined; the designated plastid rbcL and matK barcode regions, the candidate plastid trnH-psbA spacer barcode region and the nuclear ribosomal ITS region. Problems were encountered with the plastid barcodes (low inter-specific variation of rbcL, lack of reliable generic primers for matK, repetitive sequences in trnH-psbA) and for all three groups of plants the nrITS region proved to be most appropriate for primer design. Targeting the nuclear genome also allows discrimination of hybrids that may not be detected using plastid barcodes. Whilst DNA barcoding may prove to have a role in plant species identification, the current choice of universal plant barcodes may not be ideal for the development of routine authentication tests.

Keywords: authentication, DNA barcode, PCR primer, ribosomal ITS