Deep sequencing of the ancestral tobacco species Nicotiana tomentosiformis reveals multiple T-DNA inserts and a complex evolutionary history of natural transformation in the genus Nicotiana

Nicotiana species carry cellular T-DNA sequences (cT-DNAs), acquired by Agrobacterium-mediated transformation. We characterized the cT-DNA sequences of the ancestral Nicotiana tabacum species Nicotiana tomentosiformis by deep sequencing. N.tomentosiformis contains four cT-DNA inserts derived from different Agrobacterium strains. Each has an incomplete inverted-repeat structure. TA is similar to part of the Agrobacterium rhizogenes 1724mikimopine-type T-DNA, but has unusual orf14 and mis genes. TB carries a 1724mikimopine-type orf14-mis fragment and a mannopine-agropine synthesis region (mas2-mas1-ags). The mas2 gene codes for an active enzyme. TC is similar to the left part of the A.rhizogenes A4 T-DNA, but also carries octopine synthase-like (ocl) and c-like genes normally found in A.tumefaciens. TD shows a complex rearrangement of T-DNA fragments similar to the right end of the A4 TL-DNA, and including an orf14-like gene and a gene with unknown function, orf511. The TA, TB, TC and TD insertion sites were identified by alignment with N.tabacum and Nicotiana sylvestris sequences. The divergence values for the TA, TB, TC and TD repeats provide an estimate for their relative introduction times. A large deletion has occurred in the central part of the N.tabacum cv. Basma/Xanthi TA region, and another deletion removed the complete TC region in N.tabacum. Nicotiana otophora lacks TA, TB and TD, but contains TC and another cT-DNA, TE. This analysis, together with that of Nicotiana glauca and other Nicotiana species, indicates multiple sequential insertions of cT-DNAs during the evolution of the genus Nicotiana.