Plant Tumours Caused by Bacterial Plasmids: Crown Gall
Crown galls develop on dicotyledonous plants after infection by Agrobacterium tumefaciens. In virulent agrobacteria a large plasmid which is called Ti-plasmid, carries the genetic information for oncogenicity. A fragment of this plasmid is present and is expressed in crown gall cells. The fragment is called T-DNA and it probably harbours the genes responsible for the neoplastic condition of the cells. It is assumed that the autonomous growth of crown gall cells is due to the activity of the hormones auxin and cytokinin in the cell. Ti-plasmid mutants, which induce tumours with different phenotype, have led to the supposition that T-DNA genes are responsible for both hormone activities.
The neoplastic condition can be reversibly suppressed with regard to the capacity of the tumour cells to develop teratomas or shoots. However, no root formation from crown gall callus or tumour shoots has been observed so far. Using a receptor model for auxin action at the gene level it is postulated that suppression of the neoplastic condition is caused by a regulatory mechanism of the cell which eliminates or inactivates a cytoplasmic auxin receptor and which normally might act at the start of cell differentiation and shooting.
The tumour-phenotype is reestablished by wounding. This is explained by assuming that as a wound response the auxin receptor is synthesized or again released.
KeywordsAdventitious Root Plant Tissue Culture Callus Tissue Crown Gall Neoplastic Condition
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