An in vitro molecular study of the Nicotiana tabacum L. genome in the presence or absence of the herbicide atrazine

  Nicotiana tabacum L. somaclones both selected and not selected for tolerance to the triazine herbicide atrazine were used to compare tissue culture-induced variability in the presence or absence of stress. Two types of repeated sequences (rDNA and a randomly cloned, anonymous sequence) were analysed both qualitatively and quantitatively, and overall genome variation was assessed by RAPDs. Multiplicity differences were found for the two sequences both between the tolerant and susceptible group and within each group with respect to leaf DNA, but no qualitative differences were detected with either RFLPs or RAPDs. Moreover, we investigated whether stressinduced variation in the atrazine target gene, the chloroplast psbA gene, was responsible for herbicide tolerance by analysing two possible resistance mechanisms: the presence of a specific point mutation in the gene and its amplification and/or increased expression. Some somaclones were shown to be a mosaic for psbA gene mutation, but the number of cells or plastid genomes involved seemed too low to account for tolerance in the whole tissue. Atrazine tolerance could then be due to an increase in the number of plastids/plastid genomes or/and to a permanent response to respiration inhibition whose basis is, up to now, unknown.