Arbuscular mycorrhiza enhances the production of stevioside and rebaudioside-A in Stevia rebaudiana via nutritional and non-nutritional mechanisms

The sweet herb of Paraguay, Stevia rebaudiana (Bertoni), is becoming more important worldwide in herbal care for diabetes, as it produces the zero-calorie sweeteners steviol glycosides (SGs) stevioside and rebaudioside-A. While arbuscular mycorrhizal fungi (AMF) have been shown to enhance production of secondary metabolites in many plant species, their effect on S. rebaudiana has not been studied. Moreover, relatively little is known about the mechanisms that may be involved in the increased accumulation of phytochemicals in mycorrhizal plants. Therefore, this study was performed to test the ability of Rhizophagus fasciculatus (Thaxt.) C. Walker & A. Schaler to improve the yield of SGs in S. rebaudiana and to relate this with some AMF-induced physiological changes in addition to improved phosphorus (P) uptake. The performance of plants inoculated with R. fasciculatus was compared with that of non-mycorrhizal plants with similar P concentrations. Mycorrhizal (M) and non-mycorrhizal plants with P-supplementation (NM + P) produced higher concentrations of SGs compared with control plants. However, M plants had more SGs than did NM + P plants. The higher content of SGs in M plants is due to increased concentrations of SGs and to the enhanced biomass of the shoots. The increase in biomass is directly due to the improved uptake of nutrients (N, K, Mg, Cu, Fe, Mn and Zn), and chlorophyll and carbohydrate concentrations in M plants. Higher concentrations of total carbohydrates and jasmonic acid in M plants than in NM + P plants contribute to more biosynthesis of SGs via the methyl erythritol phosphate pathway. This study suggests that AMF-mediated increases in SGs involve both nutritionally and non-nutritionally linked mechanisms. (C) 2013 Elsevier B.V. All rights reserved.