COM1, a Factor of Alternative Non-homologous End Joining (NHEJ), Lagging Behind the Classic NHEJ Pathway in Rice Somatic Cells.

The role of rice (Oryza sativa) COM1 in meiotic homologous recombination (HR) is well-understood, but its part in somatic double-strand break (DSB) repair remains unclear. Here, we show that COM1 confers the tolerance against DNA damages for rice plants caused by the chemicals, bleomycin and mitomycin C, while COM1 mutation did not compromise HR efficiencies and HR factors (RAD51 and RAD51 paralogs) localization to irradiation-induced DSBs. The similar retarded growth at post-germination stage were observed in com1-2 mre11 double mutant and the mre11 single mutant, while combined mutations of the COM1 with HR pathway gene (RAD51C) or classic NHEJ pathway genes (KU70, KU80, and LIG4) caused more phenotypic defects. In response to γ-irradiation, COM1 was loaded normally onto DSBs in ku70 mutant, but could not be properly loaded in MRE11^RNAi plant and wortmannin-treated wild-type plant. Under non-irradiated condition, more DSB sites were occupied by the factors (MRE11, COM1, and LIG4) than RAD51 paralogs (RAD51B, RAD51C, and XRCC3) in the nucleus of wild-type, and the protein loading of COM1 and XRCC3 was increased in the ku70 mutant. Thus, quite different with its role for HR in meiocytes, rice COM1 specifically acts in an alternative NHEJ pathway in somatic cells, based on the Mre11-Rad50-Nbs1 (MRN) complex and facilitated by PI3K-like kinases. And the NHEJ factors, not the HR factors, preferentially load on the endogenous DSBs, with the KU70 restricting DSB-localization of COM1 and XRCC3 in plant somatic cells.