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Crown gall, a neoplasmic transformation of plant cells, is caused by transfer and integration into nuclear plant DNA of T-DNA, a segment of the Ti plasmid of Agrobacterium tumefaciens1-8. In plant cells, this T-DNA is expressed9-12 and determines the morphogenetic growth properties of crown gall cells12-16 and the synthesis of opines17-21. The T-DNA in octopine crown galls is composed of either one or two fragments derived from a continuous Ti sequence. Thus far, all octopine tumour lines contain the TL fragment (14 kilobases (kb)), whereas TR-DNA (∼6 kb) can be present as an additional sequence22-24. Foreign DNA, inserted in the T region of a Ti plasmid, is co-transferred to the plant genome 17,18,21. However, an important requirement for the use of T-DNA as vectors for plant genetic engineering is the regeneration of normal plants which stably maintain, express and sexually transmit the transferred DNA. We report here the isolation, from tumour tissues induced by mutant Ti plasmids, of morphologically normal plants that produce octopine synthase in all tissues. This enzymatic activity is inherited as a dominant mendelian marker 25. These regenerated plants retained and expressed only the octopine synthase gene of the T-DNA. |
