Transformation of plant-associated bacteria by plant DNA has never been
demonstrated in agricultural fields. In total 552 bacterial isolates from
stems of Ralstonia solanacearum-infected and healthy tomato plants and from stems and leaves of healthy potato plants were tested for natural genetic competence using
plasmid pSKTG DNA and homologous DNA extracts. Control strain
Acinetobacterbaylyi ADP1 was transformable with both DNA extracts. No transformable isolates
were observed after treatment with plasmid pSKTG DNA. Two isolates, P34,
identified as Pseudomonas trivialis and A19, identified as Pseudomonas fragi, were selected on the basis of the
consistently higher Rp-resistant CFU numbers after treatment with DNA from
Rp-resistant cells than with that from wild-type cells. P34 showed 2.1-fold
and A19 1.5-fold higher Rp-resistant CFU numbers after treatment with DNA
from homologous Rp-resistant cells versus that from wild-type cells. It is
concluded that bacteria capable of in vitro capture and integration of exogenous DNA
into their genomes are relatively rare in culturable bacterial communities
associated with tomato and potato plants, or that conditions conducive to
transformation were not met in transformation assays.