Stevia rebaudiana (Bertoni) Bertoni is a plant of economic interest in the food and pharmaceutical industries due its steviol glycosides (SG), which are rich in metabolites that are 300 times sweeter than sucrose. In addition, S. rebaudiana plants contain phenolic compounds and flavonoids with antioxidant activity. Endophytic bacteria promote the growth and development and modulate the metabolism of the host plant. However, little is known regarding the role of endophytic bacteria in the growth; synthesis of SG, flavonoids and phenolic compounds; and the relationship between trichome development and specialized metabolites in S. rebaudiana, which was the subject of this study. The 12 bacteria tested did not increase the growth of S. rebaudiana plants; however, the content of SG increased with inoculation with the bacteria Enterobacter hormaechei H2A3 and E. hormaechei H5A2. The SG content in leaves paralleled an increase in the density of glandular, short, and large trichome. The image analysis of S. rebaudiana leaves showed the presence of SG, phenolic compounds, and flavonoids principally in glandular and short trichomes. The increase in the transcript levels of the KO, KAH, UGT74G1, and UGT76G1 genes was related to the SG concentration in plants of S. rebaudiana inoculated with E. hormaechei H2A3 and E. hormaechei H5A2. In conclusion, inoculation with the stimulating endophytes E. hormaechei H2A3 and E. hormaechei H5A2 increased SG synthesis, flavonoid content and flavonoid accumulation in the trichomes of S. rebaudiana plants.

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Sphaeralcea angustifolia is a plant used for the treatment of inflammatory processes. Scopoletin, tomentin, and sphaeralcic acid were identified as the compounds with anti‐inflammatory and immunomodulatory effects. Successful establishment of the cell culture in Erlenmeyer flasks has been reported previously. The aim of this study was to evaluate the ability of cells in suspension from S. angustifolia grown in a stirred tank bioreactor and demonstrate their capacity to produce bioactive compounds. Cells in suspension grown at 200 rpm reached a maximal cell biomass in dry weight at 19.11 g/L and produced 3.47 mg/g of sphaeralcic acid. The mixture of scopoletin and tomentin was only detected at the beginning of the culture (12.13 μg/g). Considering that the profile of dissolved oxygen during the cultures was lesser than 15%, it is possible that the low growth at 100 rpm could be due to oxygen limitations or to cell sedimentation. At 400 rpm, a negative effect on cell viability could be caused by the increase in the hydrodynamic stress, including the impeller tip, average shear rate, and Reynolds number. The sphaeralcic acid content in the cell suspension of S. angustifolia obtained in the bioreactor was two orders of magnitude greater than that reported for the culture grown in Erlenmeyer flasks.

In plants, phosphorus (P) uptake occurs via arbuscular mycorrhizal (AM) symbiosis and through plant roots. The phosphate concentration is known to affect colonization by AM fungi, and the effect depends on the plant species. Stevia rebaudiana plants are valuable sources of sweetener compounds called steviol glycosides (SGs), and the principal components of SGs are stevioside and rebaudioside A. However, a detailed analysis describing the effect of the phosphate concentration on the colonization of AM fungi in the roots and the relationship of these factors to the accumulation of SGs and photochemical performance has not been performed; such an analysis was the aim of this study. The results indicated that low phosphate concentrations (20 and 200 µM KH2PO4) induced a high percentage of colonization by Rhizophagus irregularis in the roots of S. rebaudiana, while high phosphate concentrations (500 and 1,000 µM KH2PO4) reduced colonization. The morphology of the colonization structure is a typical Arum-type mycorrhiza, and a mycorrhiza-specific phosphate transporter was identified. Colonization with low phosphate concentrations improved plant growth, chlorophyll and carotenoid concentration, and photochemical performance. The transcription of the genes that encode kaurene oxidase and glucosyltransferase (UGT74G1) was upregulated in colonized plants at 200 µM KH2PO4, which was consistent with the observed patterns of stevioside accumulation. In contrast, at 200 µM KH2PO4, the transcription of UGT76G1 and the accumulation of rebaudioside A were higher in noncolonized plants than in colonized plants. These results indicate that a low phosphate concentration improves mycorrhizal colonization and modulates the stevioside and rebaudioside A concentration by regulating the transcription of the genes that encode kaurene oxidase and glucosyltransferases, which are involved in stevioside and rebaudioside A synthesis in S. rebaudiana.

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