The horizontal transfer of Agrobacterium rhizogenes genes and the evolution of the genus Nicotiana.

With the aim of understanding better the distribution and evolution of Agrobacterium rhizogenes genes transferred in the genus Nicotiana, 42 species were screened for presence of rolB, rolC, ORF13, and ORF14. The transferred sequences were then compared within the genus and with current bacterial sequences. The results obtained showed the presence of at least one bacterial gene in 15 species belonging to different subgenera. Sequence analyses supported the hypothesis of coevolution of bacterial and plant sequences, thus suggesting a possible role for the transferred genes in the early events of Nicotiana species differentiation. The high level of conservation of Agrobacterium sequences and the dependence of their expression from the plant physiological context along with previous data suggesting their involvement in the determination of the plant hormonal balance were all consistent with this hypothesis. The results are finally discussed also as to their relevance for the hypothesis of mono and multi ancient infection by Agrobacterium. Copyright 2001 Academic Press.