DNA heterogeneity and genetic control of tumorigenesis in nicotiana tumorous and nontumorous genotypes

The possible relevance of changes in amounts of highly repetitive DNA sequences for plant differentiation and dedifferentiation processes has been suggested in several cases. Data are lacking however on (1) the genetic control of these phenomena and (2) cause-effect relationships between DNA amplification and specific ontogenetic patterns. The present study was carried out on a Nicotiana genetic system consisting of the tumorous amphidiploid N glauca X N langsdorffii, a nontumorous mutant of it, their F₁, and a backcross to the tumorous parent. Backcross segregation ratios were shown to be compatible with a “single gene” hypothesis, the F₁ plant being nontumorous but showing a low percentage of tumors induced by wounds, 6-azauracil or X-rays. In vitro studies of excised pith tissue grown on Linsmaier and Skoog medium for different periods of time showed the presence, confirmed by cytological analyses, of amplification of highly repetitive sequences only in the nontumorous stock, as judged by reassociation experiments in the first 24–96 hours of culture. CsCl analytical ultracentrifugation of those sequences showed the appearance in the same stock of a heavy DNA satellite (density = 1.721 gm/ml), whose presence was also confirmed by derivative melting curves. Amplification seemed to be essential for the initiation of cell division, which was completely inhibited in the nontumorous genotype and partially influenced in the F₁ by incorporation during the critical period (24–96 hours of the primary explant) of 5-bromo-2′-deoxy-uridine. The results are discussed in terms of an hypothesis of an integrated gene-controlled, hormone-mediated regulatory system of cell proliferation involving changes in target repetitive DNA sequences.