AGROBACTERIUM-MEDIATED TRANSFORMATION OF COMMERCIAL MINTS

Commercial peppermint (P) (Mentha x piperita L. cv. Black Mitcham), na tive spearmint (NS) (M. spicata L.) and Scotch spearmint (SS) (M. x gr acillis Sole cv Baker) petioles and orange mint (OM) (M. citrata Ehrh. ) leaf disks were cocultivated with a number of Agrobacterium tumefaci ens strains. P, SS and OM initiated tumor-like callus tissue on growth regulator-free MS medium after cocultivation with strain A281, a hype rvirulent agropine strain containing Ti plasmid pTiBo542. Callus did n ot initiate from explants cocultivated with strain C58, a virulent nop aline strain; with A 136, a plasmidless strain, or from uninoculated c ontrols. A281-derived callus was maintained on growth regulator-free m edium in the absence of antibiotics for up to two years with no bacter ial outgrowth. No shoots regenerated from any of the tumors on regener ation medium. Five of seven OM callus lines assayed gave a positive si gnal for agropine. DNA extracted from OM tumor tissue hybridized to a DNA probe specific to the T-DNA region of pTi plasmid. Genomic Souther n analysis of DNA from tumors of P and SS indicated that one to a few copies of the T-DNA integrated into the mint chromosomes. PCR amplific ation of genomic DNA with primers specific for one of the T-DNA encode d genes yielded fragments that, when analyzed by restriction enzyme ma pping and on Southern blots, corresponded to the cytokinin biosynthesi s gene ipt. These results demonstrate transformation of three species of mint and the potential for using A. tumefaciens to transfer economi cally important genes into commercial mint cultivars.