RESUMEN
Basil (Ocimum basilicum L.), a medicinal plant of the Lamiaceae family, is used in traditional medicine; its essential oil is a rich source of phenylpropanoids. Methylchavicol and methyleugenol are the most important constituents of basil essential oil. Drought stress is proposed to enhance the essential oil composition and expression levels of the genes involved in its biosynthesis. In the current investigation, an experiment based on a completely randomized design (CRD) with three replications was conducted in the greenhouse to study the effect of drought stress on the expression level of four genes involved in the phenylpropanoid biosynthesis pathway in O. basilicum c.v. Keshkeni luvelou. The genes studied were chavicol O-methyl transferase (CVOMT), eugenol O-methyl transferase (EOMT), cinnamate 4-hydroxylase (C4H), 4-coumarate coA ligase (4CL), and cinnamyl alcohol dehydrogenase (CAD). The effect of drought stress on the essential oil compounds and their relationship with the expression levels of the studied genes were also investigated. Plants were subjected to levels of 100%, 75%, and 50% of field capacity (FC) at the 6-8 leaf stage. Essential oil compounds were identified by gas chromatography/mass spectrometry (GC-MS) at flowering stage and the levels of gene expression were determind by real time PCR in plant leaves at the same stage. Results showed that drought stress increased the amount of methylchavicol, methyleugenol, ß-Myrcene and α-bergamotene. The maximum amount of these compounds was observed at 50% FC. Real-time PCR analysis revealed that severe drought stress (50% FC) increased the expression level of CVOMT and EOMT by about 6.46 and 46.33 times, respectively, whereas those of CAD relatively remained unchanged. The expression level of 4CL and C4H reduced under drought stress conditions. Our results also demonstrated that changes in the expression levels of CVOMT and EOMT are significantly correlated with methylchavicol (r = 0.94, P ≤ 0.05) and methyleugenol (r = 0.98, P ≤ 0.05) content. Thus, drought stress probably increases the methylchavicol and methyleugenol content, in part, through increasing the expression levels of CVOMT and EOMT.