Arabidopsis OBG-like GTPase (AtOBGL) is localized in chloroplasts and has an essential function in embryo development

OBG-like GTPases, a subfamily of P-loop GTPases, have divers and important functions in bacteria, including initiation of sporulation, DNA replication, and protein translation. Homologs of the Bacillus subtilis spo0B GTP-binding protein (OBG) can be found in plants and algae but their specific function in these organisms has not yet been elucidated. Here, it is shown that AT5G18570 encodes an Arabidopsis thaliana OBG-like protein (AtOBGL) that is localized in chloroplasts. In contrast to the bacterial members of this protein family, AtOBGL and other OBG-like proteins from green algae and plants possess an additional N-terminal domain, indicating functional adaptation. Disruption of the gene locus of ATOBGL by TDNA insertion resulted in an embryo-lethal phenotype and light microscopy using Normarski optics revealed that embryo maturation in the atobgl mutant is arrested at the late globular stage before development of a green embryo. Expression of 35S::ATOBGL within the atobgl mutant background could rescue the mutant phenotype, confirming that embryo-lethality is caused by the loss of AtOBGL. Together, the data show that the bacterial-derived OBG-like GTPases have retained an essential role in chloroplasts of plants and algae. They furthermore corroborate the significance of chloroplast functions for embryo development - an important stage within the Arabidopsis lifecycle.