Arbuscular mycorrhizal symbiosis in Stevia rebaudiana increases trichome development, flavonoid and phenolic compound accumulation

Stevia rebaudiana is an economically important crop species, due to its ability to produce steviol glycosides, which are compounds that are 300 times sweeter than sugar. Additionally, this species produces phenolic compounds and flavonoids, a portion of which accumulate in trichomes. Arbuscular mycorrhizal (AM) symbiosis is the most important mutualistic association between Glomeromycota soil fungi and most vascular plants. Moreover, AM symbiosis is known to alter secondary metabolite production in plants. Thus, in this study, we investigated the influence of AM symbiosis on the accumulation of phenolic compounds and flavonoids in S. rebaudiana, as well as on trichome abundance and development. Compared with noncolonized plants, colonized plants exhibited a significant increase in leaf and root dry weight, as well as total leaf area. Interestingly, trichome density was positively affected under symbiotic conditions. This effect was correlated with an increase in the expression of TTG1, which is a marker gene for trichome development. Additionally, image analysis of autofluorescence obtained at wavelengths of 435 and 485 nm demonstrated stronger fluorescence signals corresponding to flavonoid and phenolic compounds in colonized plants compared to noncolonized plants; these signals were specifically located in glandular trichomes. Furthermore, the increase in phenolic and flavonoid concentrations in colonized S. rebaudiana was associated with an increased density of glandular trichomes in those plants compared with noncolonized plants. These results indicate that the establishment of AM symbiosis increases trichome density and induces secondary metabolite accumulation, which may be of biological significance in the interactions of colonized plants with their environments.