A trichome-specific promoter for advanced bio-engineering in lavender

The 1, 8-Cineole synthase promoter directs the specific expression of Green Fluorescent Protein (GFP) in Lavandula latifolia glandular trichomes, and can help improve monoterpene metabolism in lavender through metabolic engineering. Lavender produces large amounts of a monoterpene-rich essential oil (EO) in glandular trichomes (GTs) present on its aerial parts. There is significant interest in improving EO quality and yield, and in using lavender GTs as bio-factories for mass-producing specialized metabolites, in particular terpenoids. Metabolic engineering is often carried out using constitutive promoters, like CaMV 35S, to drive terpene synthase gene expression. However, this non-specific expression can cause metabolite accumulation in non-specialized cells, potentially leading to cytotoxicity. GT-specific promoters offer a superior approach, confining terpene biosynthesis exclusively to the GTs' secretory cells. This study evaluated the GT specificity of four fragments of the L. x intermedia 1,8-cineole synthase (LiCINS) promoter region in stably transformed L. latifolia plants. Initial transformation using the β-glucuronidase (GUS) reporter gene resulted in inconsistent staining, even in CaMV 35S positive controls. To overcome this limitation, we generated new L. latifolia transformants in which the promoter fragments were used to drive the expression of the Green Fluorescent Protein (GFP). GFP fluorescence provided clear visualization of expression in leaf cells, including the GT secretory cells. The full-length promoter fragment (LiCINS-F1) exhibited exclusive GT-specific activity. In stark contrast, the shorter fragments (LiCINS-F2, LiCINS-F3, and LiCINS-F4) drove strong but non-specific GUS/GFP expression. These results confirm that the GT-specific promoter LiCINS-F1 is a valuable tool for metabolic engineering. By restricting the production of rare and valuable terpenes or other compounds exclusively to GTs, this approach allows the trichomes to function as specialized plant bio-factories without compromising the plant's overall wellbeing.