MSH 1 maintains organelle genome stability and genetically interacts with RECA and RECG in the moss Physcomitrella patens

Summary Chloroplast and mitochondrial DNA encodes genes that are essential for photosynthesis and respiration, respectively. Thus, loss of integrity of the genomic DNA of organelles leads to a decline in organelle function and alteration of organelle genetic information. RECA ( RECA 1 and RECA 2) and RECG , which are homologs of bacterial homologous recombination repair ( HRR ) factors RecA and RecG, respectively, play an important role in the maintenance of integrity of the organelle genome by suppressing aberrant recombination between short dispersed repeats ( SDR s) in the moss Physcomitrella patens . On the other hand, MutS homolog 1 ( MSH 1), a plant‐specific MSH with a C‐terminal GIY ‐ YIG endonuclease domain, is involved in the maintenance of integrity of the organelle genome in the angiosperm Arabidopsis thaliana . Here, we address the role of the duplicated MSH 1 genes, MSH 1A and MSH 1B , in P. patens , in which MSH 1A lacks the C‐terminal endonuclease domain. MSH 1A and MSH 1B localized to both chloroplast and mitochondrial nucleoids in protoplast cells. Single and double knockout ( KO ) mutants of MSH 1A and MSH 1B showed no obvious morphological defects; however, MSH 1B KO and double KO mutants, as well as MSH 1B GIY ‐ YIG deletion mutants, exhibited genomic instability due to recombination between SDR s in chloroplasts and mitochondria. Creating double KO mutations of each combination of MSH 1B , RECA 2 and RECG synergistically increased recombination between SDR s in chloroplasts and mitochondria. These results show the role of MSH 1 in the maintenance of integrity of the organelle genome and the genetic interaction between MSH 1 and homologs of HRR factors in the basal land plant P. patens .