*1.1.1 Plasmids of bacteria of the genus* Agrobacterium

The *Agrobacterium* genus is a group of Gram-negative soil bacteria. Unlike the wild type, laboratory *Agrobacterium* strains are commonly used for genetic transformations of numerous plant species [3]. Different plant species show different susceptibility to infection by *Agrobacterium* bacteria [4]. In general, dicotyledonous plants are more susceptible to infection by *Agrobacterium* bacteria than monocotyledons. Bacteria of the genus *Agrobacterium* can be present in the vascular tissue of plants (xylem and phloem) but without showing disease symptoms. It indicates that the interaction of bacteria and plant cells does not always occur. Infection and pathological changes of plant tissue occuronly after injury of the plant cells when released phenolic compounds causea positive chemotoxic reaction in *Agrobacterium*. These phenolic compounds further induce genes, located on chromosomal and plasmid DNA, responsible for bacterial virulence. During genetic transformations, the plasmids in bacterial cells serve as vectors. Bacteria of the genus *Agrobacterium* have large conjugated plasmids (200–250 kb), parts of which have been successfully integrated into the plant genome during transformation processes for almost half a century [5]. In these bacteria, two basic types of plasmids are identified by the kind of disease they cause: Ti-plasmid (tumor-inducing) in *A. tumefaciens* and Ri-plasmid (root-inducing) in *A. rhizogenes*.

Both types of plasmids contain three genetic regions that are necessary for the genetic transformation process:


injured plant cell [7]. According to Simonović [8], the *chvA* and *chvB* genes are responsible for the production and secretion of cyclic 1,2-glycans while *chvE* product forms a transmembrane receptor complex together with the plasmid *virA* product.

The development of plant tumor tissue occurs after a part of the plasmid DNA (T-DNA) is transported and incorporated into the DNA of the plant. T-DNA of Ti- and Ri-plasmids carries oncogenes, the genes responsible for tumor formation and encoding enzymes that catalyze the synthesis of auxins and cytokinins. Transcription of these genes leads to an increased amount of these plant hormones, which stimulates uncontrolled cell division, tumor development and mass proliferation of transformed cells [9].
