Epigenetic modification of rhizobial genome is essential for efficient nodulation

CcrM is one of the solitary bacterial DNA methyltransferases which does not have corresponding restriction enzymes. We established a stable ccrM-overexpressing mutant of Mesorhizobium loti, MlccrM-OX, and performed molecular and phenotypic characterization of this strain. In the M. loti MlccrM-OX infected plants, nodulation was apparently delayed at 7 days after inoculation (dai), however, the nodules that eventually formed on the MlccrM-OX roots showed nitrogen fixing ability by at least 21 dai. These results suggest that the initial morphogenic events were affected by ccrM-overexpression and that the correct pattern of DNA methylation of the bacterial genome is not essential for plant–microbe symbiosis, but are required for efficient nodulation.