In Vitro Organ Cultures of the Cancer Herb Castilleja tenuiflora Benth. as Potential Sources of Iridoids and Antioxidant Compounds

Castilleja tenuiflora Benth. (Scrophulariaceae, “cancer herb”) is a wild plant widely recommended in Mexican folk medicine to treat tumors. Root and shoot cultures of this species were established for the production of secondary metabolites with cytotoxic and antioxidant activities. Root cultures were initiated from root tips induced in leaf explants from wild-grown plants using MS medium and 10 μM α-naphthalene acetic acid. Shoots spontaneously formed in 30-35 days and were excised. They presented continuous multiplication and elongation during subsequent subculture in free-hormone liquid medium. Antioxidant activity was measured using three in vitro models [scavenging of free radicals with 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2’-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) diammonium salt (ABTS), and the transition metal reduction in the phosphomolybdenum assay], and the strongest activity (p<0.05) was found in methanol extracts from shoot cultures. The highest contents of the phenolic compounds, flavonoid and iridoids, were also found in shoot cultures (p<0.05). Total phenolic compound levels correlated significantly with the antioxidant activity (p<0.05). Bioautography within TLC using DPPH as a detection reagent indicated that quercetin 3-β-_D-glucoside is one of the predominant contributors to the free radical scavenging of C. tenuiflora organ cultures. Aucubin, an iridoid with cytotoxic activity, was also found in the in vitro cultures. C. tenuiflora organ cultures are alternative sources for iridoids and natural antioxidants such as flavonoids and other phenolic compounds.

Keywords: Castilleja tenuiflora, Scrophulariaceae, oxidative stress, secondary metabolites, iridoid, biotechnology, micropropagation, cell suspension culture, callus, explant, organogenesis, root culture, shoot culture, chemical composition.