Studies on Agrobacterium tumefaciens. V. Fate of exogenously added bacterial DNA in Nicotiana tabacum

Historical account on gaining insights on the mechanism of crown gall tumorigenesis induced by Agrobacterium tumefaciens

The plant tumor disease known as crown gall was not called by that name until more recent times. Galls on plants were described by Malpighi (1679) who believed that these extraordinary growth are spontaneously produced. Agrobacterium was first isolated from tumors in 1897 by Fridiano Cavara in Napoli, Italy. After this bacterium was recognized to be the cause of crown gall disease, questions were raised on the mechanism by which it caused tumors on a variety of plants. Numerous very detailed studies led to the identification of Agrobacterium tumefaciens as the causal bacterium that cleverly transferred a genetic principle to plant host cells and integrated it into their chromosomes. Such studies have led to a variety of sophisticated mechanisms used by this organism to aid in its survival against competing microorganisms. Knowledge gained from these fundamental discoveries has opened many avenues for researchers to examine their primary organisms of study for similar mechanisms of pathogenesis in both plants and animals. These discoveries also advanced the genetic engineering of domesticated plants for improved food and fiber.

Biophysical and genetic evidence for transformation in plants

In thiamine mutants of Arabiodopsis, genetic corrections have been obtained by treatment with DNA bearing a thiamine information. When correction is attempted under selective conditions, about 0.7% of the treated plants grow and set fruit. Their progeny and the following ones, obtained by selfing, behave as homozygotes. Segregation of characters is found only when correction is attempted under nonselective conditions or when the correcting genes were of plasmidian origin. The correction is hereditary; results of backcrosses and test crosses indicate that it is dominant, nuclear, and strongly bound to the genome. The corrective factor appears to be added to the mutated genome and not substituted for the mutation, as it can be suppressed by outcrossing with the wild type or with a plant corrected by another DNA.