The genome of the Tulsi plant could help unravel the mysteries of a medicinal herb that has been cultivated for nearly 3,000 years.
Tulsi or Ocimum tenuiflorum is mentioned in the ancient Hindu scriptures of the Vedas and Puranas and has long been used by practitioners of Ayurveda, a traditional Indian alternative medicine, to treat a variety of conditions. The plant produces a wide variety of bioactive compounds, which have been found to have anti-cancer, antioxidant, antifungal and anti-inflammatory properties among others. They are believed to be part of the plant’s self-defence mechanism, but a lack of genomic data on their origin means they are poorly understood.
Now, researchers led by Sowdhamini Ramanathan from the National Centre for Biological Sciences (NCBS) in Bangalore have used DNA sequencing technology to create a draft genome of the Krishna subtype of O. tenuiflorum. Despite only assembling 61 per cent of the estimated genome size, the team was able to use the genetic map to identify several major genes responsible for the production of the herb’s medicinal compounds, known as ‘metabolites’ due to the fact they are a by-product of the plant’s metabolism.
Nearly 30 such metabolites with medicinal properties have been reported in the genus Ocimum, of which 14 have complete metabolic pathways, so the team searched for genes from these pathways in the newly produced genome. A total of 458 genes were identified, which were either homologous (have a common ancestor) or directly code for enzymes involved in the synthesis of metabolites. Expression of these genes was confirmed using RNA sequencing and reverse transcription polymerase chain reactions (q‑RT-PCR) in both the Krishna and Rama sub-types of O. tenuiflorum, as well as other Tulsi species O. gratissimum, O. saccharinum and O. kilmund.
The team, which included researchers from the Institute for Stem Cell Biology and Regenerative Medicine (inStem) and the Centre for Cellular and Molecular Platforms (C‑CAMP), also decided to investigate an important metabolic pathway that involves the production of ursolic acid — a compound with anti-inflammatory, anti-microbial and anti-tumour properties — using q‑RT-PCR as well as mass spectrometry to detect the presence of metabolites in the various parts of the plant. They discovered that the metabolites or their precursors appear to be synthesised in the young leaves of the Tulsi plant, but then remain in the mature leaves often in high abundance where they retain their medicinal properties. “The sequence reveals the interesting pathways used by Tulsi to make ursolic acid, a medically important compound. If one could now use modern synthetic biology techniques to synthesise ursolic acid — a compound with multiple chiral centers — it would be of great benefit,” said S. Ramaswamy, from inStem.
The researchers also found high expression levels of a gene that produces the pigment anthocyanin in Krishna Tulsi, which they believe helps explain the purple colouration of the sub-type’s leaves.