Increased Growth Rate, Lignin, and Shikonin Levels in Onosma dichroantha Bioss. as Affected by Silicon Treatment

Onosma dichroantha Boiss. is a local medicinal herb in Iran, belonging to the Boraginaceae family that is used mainly for wound healing due to the presence of shikonin in its root cortex. Optimization of Onosma spp. in vitro cultures and shikonin production is encouraged as an alternative to harvesting the plant from its natural habitats. The present study evaluates the growth rate, several biochemical properties, and shikonin content of O. dichroantha plants treated with various concentrations of silicon (in the form of potassium silicate) in a hydroponic medium. Silicon application up to 0.5 mM increased the fresh mass, chlorophyll a/b, carotenoids, soluble proteins in shoots, and the lignin content in roots; however, phenolic compound contents were not significantly affected. In addition, silicon nutrition increased catalase and soluble ascorbate peroxidase activities, whereas polyphenol oxidase activity was not affected in roots and shoots. Interestingly, the shikonin content of O. dichroantha roots treated with increased concentrations of Si was 2-fold higher than that in the control plants, while the activity of phenylalanine ammonia lyase, a key enzyme in shikonin biosynthesis, was not affected. This suggests that the observed increase in shikonin in response to the silicon treatment could be due to increased stability or more accumulation sites of shikonin in roots. These data may be used for improvement of shikonin production in cell cultures of O. dichroantha under experimental or industrial conditions.