Optimization of a <Emphasis Type="Italic">Bacopa monnieri</Emphasis>-based genetic transformation model for testing the expression efficiency of pathway gene constructs of medicinal crops

A fast regenerating Agrobacterium tumefaciens-mediated transformation protocol for Bacopa monnieri (L.) Wettst. was developed as a model system for heterologous expression of terpenoid indole alkaloid pathway genes from Catharanthus roseus (L.) G. Don. The direct regeneration of shoots from leaf explants co-cultured with A. tumefaciens resulted in the integration of a tryptophan decarboxylase (tdc) and strictosidine synthase (str) cassette (<hpt-<Tdc2-<Str-gus>) in the regenerated progeny. The highest transformation efficiency (83.88%) was achieved when leaf explants were infected on the adaxial laminar surface by manual pricking with 48- to 72-h-old suspensions (OD₆₀₀ = 0.5–0.6) of A. tumefaciens strain LBA1119 (carrying the binary vector pMOG22). The heterologous expression of tryptophan decarboxylase and strictosidine synthase genes that are otherwise not present in B. monnieri plants was confirmed through semi-quantitative PCR and metabolite quantification assays. The entire protocol duration from co-cultivation through regeneration of transgenic plants to their establishment in the glass house took 40–45 d. The developed B. monnieri model can be used to test expression cassettes carrying genes for plant secondary metabolic pathway engineering, especially those genes that are expressed in differentiated cell, tissue, or organs.