Horizontal Gene Transfer

A quarter-century ago, a sequence homologous to the Ri plasmid (pRi) T-DNA of Agrobacterium rhizogenes was detected in the genome of untransformed tree tobacco, Nicotiana glauca, and was named “cellular T-DNA” (cT-DNA). The origin of the homologous sequences in tobacco remained unknown for a long period, but at present, it has been clearly demonstrated that the cT-DNA is the pRi T-DNA inserted by ancient infection with mikimopine-type A. rhizogenes. The cT-DNA of N. glauca is composed of an imperfect inverted repeat and it contains homologues of some pRi T-DNA genes involved in adventitious root formation and opine synthesis, which are called NgrolB, NgrolC, NgORF13, NgORF14, and Ngmis. In spite of the footprint of ancient insertion of pRi T-DNA, these homologues are still expressed not only in genetic tumors of F1 hybrids of N. glauca x N. langsdorffii but also in some organs of N. glauca, although at a low level. The cT-DNA is also found in some other species of the genus Nicotiana, with mikimopine-type cT-DNA contained in at least three, N. tomentosa, N. tomentosiformis, and N. tabacum. Therefore, there is a possibility that multiple infection events occurred independently in several ancestors of Nicotiana. Furthermore, some plant species in different families also contain cT-DNA-like sequences, although the details are still unknown. Tumors are spontaneously generated on some plants in the absence of tumorigenic microorganisms. Hybrid plants of Nicotiana species also form genetic tumors, but the mechanism of this tumorigenesis is still unknown. One of the parents of the hybrid usually contains cT-DNA, implying that it is the causal factor of tumorigenesis, although the causal association between the cT-DNA and tumorigenesis remains unsolved. Since pRi-transgenic plants exhibit a peculiar phenotype, the so-called “hairy root syndrome”, which shows advantageous traits in some cases, ancient pRi-transformed plants might also have predominated in competition with parental plants or survived under a harsh climate. Therefore, the insertion events of T-DNA into the genome of plants might have influenced their evolution, resulting in the creation of new plant species.