*1.1.2 Mechanism of genetic transformation of a plant cell using* Agrobacterium tumefaciens

Successful genetic transformations of plants using *Agrobacterium* require a bacterial strain with appropriate density, explant tissue culture, determination of the substrate and time for cocultivation, as well as temperature and selection regime [10]. In wild strains of A*grobacterium*, the Ti- and Ri- plasmids have a T-DNA region divided into two sequences (TL and TR) between which is a TC sequence that is not transmitted to the plant genome [11]. Oncogenes are located on the TL sequence while genes for opine synthesis are located on the TR sequence. Opines accumulate in transformed plant cells thus representing metabolic markers for genetic transformations. They seem useless for the plant as no appropriate enzymes for their degradation have been found in plant tissues. Pathogenic bacteria use released opines as a source of carbon and nitrogen. The genes on the plasmid itself encode the enzymes for catabolism of opines whereby the opine, whose synthesis is induced in transformed tissue, is degraded [12]. In laboratory strains of *Agrobacterium*, natural T-DNA can be replaced by a gene of interest that should be introduced into the plant genome by genetic transformation.

Genetic transformation of plants by *Agrobacterium* is a process that, in general, consists of three steps:
