Functional expression of Saccharomyces cerevisiae CYP51A1 encoding lanosterol-14-demethylase in tobacco results in bypass of endogenous sterol biosynthetic pathway and resistance to an obtusifoliol-14-demethylase herbicide inhibitor

Nicotiana tabacum protoplasts have been transformed by Agrobacterium tumefaciens containing a T-DNA in which the gene CYP51A1 encoding lanosterol-14-demethylase (LAN14DM) from Saccharomyces cerevisiae is under the control of a cauliflower mosaic virus (CaMV) 35S promoter. Two transformants strongly expressed the LAN14DM as shown by Northern and Western experiments. These transgenic calli were killed by LAB 170250F (LAB) (a phytotoxic fungicide inhibiting both plant obtusifoliol-14-demethylase (OBT14DM) and LAN14DM) but were resistant to γ-ketotriazole (γ-kt), a herbicide which has been shown to inhibit OBT14DM but not LAN14DM at a concentration that was lethal to control calli. However, these transgenic calli were killed by mixtures of γ-kt plus fungicide inhibitors of LAN14DM such as ketoconazole, itraconazole or flusilazole which alone were not effective. Further analysis of the transgenic calli grown in the presence of γ-kt showed that their Δ⁵-sterol content was close to that of untreated control calli obtained from protoplasts transformed with control plasmid; this is in agreement with evidence that the LAN14DM expressed from the transgene could bypass the blocked OBT14DM by using the plant substrate obtusifoliol. In contrast, control calli when treated with γ-kt, displayed a sterol content strongly enriched in 14α-methyl sterols and depressed in physiological Δ⁵-sterols. When the transgenic calli were cultured in mixtures of γ-kt and LAN14DM inhibitors sterol compositions enriched in 14α-methyl sterols were obtained, reflecting a strong inhibition of both ‘endogenous’ OBT14DM and ‘exogenous’ LAN14DM. Taken together these results show that in tobacco calli transformed with CYP51A1, resistance to a triazole herbicide arises from expression of a functional LAN14DM enzyme; its activity in transgenic tissues creates a bypass of the sterol biosynthetic pathway at the 14-demethylase level when this latter is blocked by an OBT14DM herbicide inhibitor.