Describe role of Agrobacterium tumefaciens in transforming a plant cell. 

Agrobacterium tumefaciens infects walnuts, grapevines, sugar beets, horse radish. etc.
$?$ It infects plants usually through an open wound. 
$?$ Once It enters a plant host, it Injects a section of its DNA called the T-DNA which Is derived from its Ti (tumor inducing) plasmid Into its host. 
$?$ The T-DNA first directs the plant cells to make auxins and cytokinins, which causes the cells to undergo rapid divisions and forms a visible tumor called a gall. 
$?$ The T-DNA then directs the plant cell to start making opines (usually nopaline or agropine) which A. tumefaciens use as an energy source. Thus, it  creates a special niche for itself inside the gall. 
This property of Ti-plasmid has been exploited for cloning of gene of interest and stably integrating them in the plant genomes. Therefore, by using Ti-plasmid or its derivations, recombinant plant cells with desired genes of Interest stably integrated in the plant genome has been successfully produced.