Quantitative analysis of uridine diphosphate glycosyltransferase UGT85C2, UGT74G1 and UGT76G1 genes expression in Stevia rebaudiana under different irrigations

Stevia rebaudiana Bertoni is known to the scientific world for its sweetness and steviol glycosides. Steviol glycosides are the secondary metabolites responsible for the sweetness of stevia and have many medical applications. They are synthesized by a biosynthesis pathway operating in the leaves. Stevioside (Stev) and rebaudioside A (Reb A) are two main glycosides compounds of Stevia. Translation products of three Uridine diphosphate glycosyltransferase candidate genes, UGT85C2, UGT74G1 and UGT76G1, were found to have an activity towards the synthesis of Stev and Reb A. In this study, a experiment was designed to measure the expression level of three candidate genes contributing to the biosynthesis of steviol glycosides under three different irrigation intervals using quantitative real-time PCR analysis technique. Irrigation was done at 3, 6 and 9 days irrigation intervals, based on soil moisture content 90, 75 and 60% of field capacity (FC) respectively. The results showed that soil water depletion up to 75% FC lead to significant increase in expression of UGT85C2 and UGT74G1 genes, whereas a significant growth in expression of UGT76G1 gene occurred at 60% FC. Measurement of Reb A contents in treated leaves showed that a significant correlation between UGT76G1 transcription and Reb A accumulation under 60% FC. Also, the comparison of expression patterns of the three target UGT transcripts at 75 and 60% FC showed a highly significant difference in UGT74G1 transcripts in comparison of UGT85C2 and UGT76G1 transcripts. These results indicated that the highest value of Reb A/ Stev ratio as sweetness quality index likely belongs to 60% FC and suggested 9 days irrigation intervals to improve the quantity and quality of glycoside profile by up-regulation and down-regulation of mentioned genes.