Study of gene expression and steviol glycosides accumulation in Stevia rebaudiana Bertoni under various mannitol concentrations

Stevia rebaudiana produces sweet steviol glycosides that are 300 times sweeter than sugar and have the beneficial effects on human health including anti-hyperglycaemic. Tissue culture is the best method with high efficacy to propagate stevia. Abiotic stress has an impact on steviol glycoside contents in stevia. Therefore, we investigated the effect of mannitol on the expression of four genes involved in the biosynthesis of stevia including UGT74G1, UGT76G1, kaurene oxidase and kaurene synthase genes and steviol glycosides accumulation in stevia under in vitro conditions. The highest expression of UGT76G1 gene occurred in the plants grown under 20g/l mannitol. While for the kaurene synthase gene, the highest amount of gene expression was observed at 40g/l mannitol. The results were different about kaurene oxidase gene. As the highest and lowest gene expression were seen in 50 and 30g/l mannitol conditions respectively. There were the same results for UGT74G1 that means the most appropriate and also the most inopportune treatment for the gene expression were same as the condition for the kaurene oxidase gene. Compared with control, adding mannitol to media in all concentrations increases the expression of UGT76G1 gene. Estimation of steviol glycosides contents under different treatments of mannitol carried out by HPLC. According to the results, the highest amount of stevioside was produced under 20g/l mannitol treatment. However, rebaudioside A was accumulated in its maximum amounts under 30g/l mannitol. It can be concluded that adding mannitol to media in the certain concentration increases steviol glycoside contents in the stevia.