Determination of the T-DNA transfer and the T-DNA integration frequencies upon cocultivation of Arabidopsis thaliana root explants

Using the Cre/lox recombination system, we analyzed the extent to which T-D NA transfer to the plant cell and T-DNA integration into the plant genome d etermine the transformation and cotransformation frequencies of Arabidopsis root cells. Without selection for transformation competence, the stable tr ansformation frequency of shoots obtained after cocultivation and regenerat ion on nonselective medium is below 0,5%, T-DNA transfer and expression occ ur in 5% of the shoots, indicating that the T-DNA integrates in less than 1 0% of the transiently expressing plant cells. A limited fraction of root ce lls, predominantly located at the wounded sites and in the pericycle, are c ompetent for interaction with agrobacteria and the uptake of a T-DNA, as de monstrated by histochemical GUS staining. When selection for transformation competence is applied, the picture is completely different, Then, approxim ately 50% of the transformants show transient expression of a second, nonse lected T-DNA and almost 50% of these cotransferred T-DNAs are integrated in to the plant genome, Our results indicate that both T-DNA transfer and T-DN A integration limit the transformation and cotransformation frequencies and that plant cell competence for transformation is based on these two factor s.