COM1, a factor of alternative non-homologous end joining, lagging behind the classic non-homologous end joining pathway in rice somatic cells

The role of rice (Oryza sativa)COM1in meiotic homologous recombination (HR) is well understood, but its part in somatic double-stranded break (DSB) repair remains unclear. Here, we show that for rice plantsCOM1conferred tolerance against DNA damage caused by the chemicals bleomycin and mitomycin C, while theCOM1mutation did not compromise HR efficiencies and HR factor (RAD51 and RAD51 paralogues) localization to irradiation-induced DSBs. Similar retarded growth at the post-germination stage was observed in thecom1-2 mre11double mutant and themre11single mutant, while combined mutations inCOM1with the HR pathway gene (RAD51C) or classic non-homologous end joining (NHEJ) pathway genes (KU70,KU80, andLIG4) caused more phenotypic defects. In response to gamma-irradiation, COM1 was loaded normally onto DSBs in theku70mutant, but could not be properly loaded in theMRE11(RNAi)plant and in the wortmannin-treated wild-type plant. Under non-irradiated conditions, more DSB sites were occupied by factors (MRE11, COM1, and LIG4) than RAD51 paralogues (RAD51B, RAD51C, and XRCC3) in the nucleus of wild-type; protein loading of COM1 and XRCC3 was increased in theku70mutant. Therefore, quite differently to its role for HR in meiocytes, riceCOM1specifically acts in an alternative NHEJ pathway in somatic cells, based on the Mre11-Rad50-Nbs1 (MRN) complex and facilitated by PI3K-like kinases. NHEJ factors, not HR factors, preferentially load onto endogenous DSBs, with KU70 restricting DSB localization of COM1 and XRCC3 in plant somatic cells.