Plant Organellar Homologous Recombination Depends on Accessory Factors for Recombinase Activity

Plant mitochondrial and plastid genomes rely on homologous recombination (HR) for error-free DNA repair of double strand breaks. Here, we reconstructed the homologous recombination pathway of plant organelles from Arabidopsis, aiming to understand how proteins of diverse evolutionary origins assemble into a functional repair pathway. We found that the three bacterial-derived recombinases, AtRecAs assemble D-loop intermediates, but only chloroplast AtRecA1 independently execute end-joining recombination on large DNA substrates. AtRecA1-mediated end-joining recombination is enhanced by plant-unique single-stranded DNA-binding proteins of the Whirly and Organellar Single-Stranded Binding (OSB) families, in contrast these proteins are mandatory for the recombination activities of the dual-targeted AtRecA2 and mitochondrial AtRecA3. Recombinational intermediates generated by AtRecAs are resolved by a bacterial-like RadA helicase but not bacterial-like RecG. Finally, AtRecX inhibits the recombinase activities of AtRecA2, while the activities of AtRecA1 and AtRecA3 remains largely insensitive. Collectively, our results provide an integrative reconstitution of the plant organellar homologous recombination machinery, revealing that homologous recombination arises from a complex system of bacterial, phage-derived, plant-specific, and eukaryotic components. Plant-specific ssDNA-binding proteins emerge as key regulators of this process, promoting recombination efficiency and genome stability, that supports the essential functions of homologous recombination in DNA repair and DNA replication maintenance.