Biotechnological Approach to Cultivation of Rhododendron tomentosum (Ledum palustre) as the Source of the Biologically Active Essential Oil

Rhododendron tomentosum (marsh tea, previously Ledum palustre), a fragrant shrub with characteristic evergreen leaves and white flowers, grows in Europe, Asia, and North America. It has been used for centuries in folk medicine to treat rheumatic diseases, lung problems, and infections as well as due to its repellent properties. In North America, the tonic beverage known as “Labrador tea”, derived from the indigenous tradition and made from R. tomentosum, R. groenlandicum and R. neoglandulosum leaves, is prepared until now. The modern biological research confirm anti-inflammatory, analgesic, antimicrobial and insecticidal effect of the discussed plant material, indicating an important role of the essential oil as an active ingredient. However, obtaining the volatile fraction from R. tomentosum ground material for pharmacological studies is difficult because marsh tea is the endangered species in some countries. Moreover, as many as ten chemotypes of R. tomentosum on the Eurasian continent have been distinguished, due to the chemical composition of the essential oil. Such heterogeneity of the plant material is problematic, assuming its use for medical purposes. Therefore, the shoot in vitro culture was initiated for the first time for receiving the R. tomentosum biomass, being the complex source of biologically active volatile compounds, regardless of environmental conditions. The microshoots were subsequently adapted for large laboratory scale cultivation in commercial and prototype bioreactors. The RITA® temporary immersion system, containing SH medium with 24.60 μM 2-isopentenyladenine and 592.02 μM adenine, provided the highest growth parameters of biomass (Gi = 280%) and the intensified biosynthesis of the essential oil (500 μl 100 g−1 dry weight), surpassing the productivity of the aged shoots of the mother plant (300 μl 100 g−1 dry weight). The main terpenes of the obtained volatile fraction were ledene oxide (II) (13%), shyobunone (8%), p-cymene (7%), and alloaromadendrene (6%). In order to increase the essential oil content in the R. tomentosum microshoots, elicitation strategy was applied, using methyl jasmonate and the selected abiotic and biotic elicitors. In response to stress caused by the aphid extract and Pectobacterium carotovorum lysate, the accumulation of the volatile fraction increased by 14%. In addition, the full protocol for micropropagation of marsh tea was developed, including initiation, multiplication, elongation, rooting, hardening, and adaptation of the seedlings to in vivo conditions, for ex situ protection of the discussed endangered species. This article reviews the importance of R. tomentosum from a medical point of view as well as the biotechnological approach obtaining an alternative source of this valuable biomass.