Expression patterns of Kaurene oxidase (KO) and Kaurenoic acid 13-hydroxylase (KAH) genes, triggering the intermediate steps in steviol glycosides biosynthetic pathway in Stevia rebaudiana, was studied using quantitative real time polymerase chain reaction technique (Q-RTPCR). Stevia rebaudiana plants were subjected to different day lengths and irradiance conditions: long and short-day conditions in both full (4800 lux) and shady (400 lux) illuminations. Samples from both old and young leaves were collected, weekly, immersed in liquid nitrogen and kept at -80 °C for further extractions of RNA and steviol glycosides.18S rRNA and actin were used as endogenous controls for normalizing the data. The expression patterns of the target genes were depending on both the age of the leaves and the light conditions under which the plants were grown. The changes in the expression of KO showed an increase in both young and old leaves during the experiment in all conditions. The accumulation of the gene at the end of the experiment was noticeable, specifically, for full-irradiance long-day conditions. The expression of the second cytP450 gene (KAH) increased during experimental period in both young and old leaves under most conditions. On the other hand, those subjected to severe irradiance stress during long-day conditions, a very small expression was detected. The expression results for the two genes proved their roles as key regulatory factors in the flow of the metabolites to the pathway for steviol glycosides biosynthesis pathway.
Key words: Kaurenoic acid 13-hydoxylase; kaurene oxidase; quantitative real time-PCR- cytochrome P450 genes; Stevia rebaudiana
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