DNA synthesis and fragmentation in bacteroids during Astragalus sinicus root nodule development

Histo- and cytochemical techniques were used to study the DNA replication a nd fragmentation patterns in bacteroids formed by Mesorhizobium huakuii sub sp, rengei in nodules of Astragalus sinicus. DNA replication was detected b y the incorporation of 5-bromo deoxy-uridine, Signals denoting DIVA synthes is were observed in plant nuclei within the nodule meristem and in bacteroi ds near the meristem. The TUNEL (TdT-mediated dUTP nick-end labeling) assay was used to measure DNA fragmentation. In nutrient-depleted 2-mpi (month(s ) post inoculation) nodule sections, some bacteroids were in vacuoles, and DNA fragmentation signals were observed only in such bacteroids, In contras t, 1-mpi nodule sections without nutrient depletion showed neither bacteroi d localization in vacuoles nor DNA fragmentation signals. The bacteroid tra nslocation into vacuoles upon nutrient starvation might results from autoph agy of the plant. In 2-mpi nodule sections, bacteroids with DNA fragmentati on signals appeared within the cytoplasm of some nodule cells in the senesc ence zone.