Question

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Answer 1

Plant transformation mediated by the soil plant pathogen Agrobacterium tumefaciens has become the most used method for plant transformation. A. tumefaciens naturally infects the wound sites in dicotyledonous plant causing the formation of the crown gall tumours. Since the discovery of the bacterial origin of this neoplastic diseases a large number of researches have focused on the study of this process, firstly with the hope to understand the mechanisms of oncogenesis in general and applied it to study of cancer disease in animals and humans. Later this hypothesis was discarded and the interest on crown gall disease largely decreased until it was evident that A. tumefaciens is capable to transfer a particular DNA segment (T-DNA) of the tumour-inducing (Ti) plasmid into the nucleus of infected cells where it is subsequently stable integrated into the host genome and transcribed, causing the crown gall disease. The T-DNA contains two types of genes: the oncogenic genes, encoding for enzymes involved in the synthesis of auxins and cytokinins and responsible for tumour formation; and the genes encoding for the synthesis of opines, a product resulted from condensation between amino acids and sugars, which are produced and excreted by the crown gall cells and consume by A. tumefaciens as carbon and nitrogen sources. Outside the T-DNA, are located the genes for the opine catabolism, the genes involved in the process of T-DNA transfer from the bacterium to the plant cell and for the bacterium-bacterium plasmid conjugative transfer genes