C1-FDX (Complex I-ferredoxin) has been defined as a component of CI in a ferredoxin bridge in Arabidopsis mitochondria. However, its full function remains to be addressed. We created two c1-fdx mutants in Arabidopsis using the CRISPR-Cas9 methodology. The mutants show delayed seed germination. Over-expression of C1-FDX rescues the phenotype. Molecular analyses showed that loss of the C1-FDX function decreases the abundance and activity of both CI and subcomplexes of CV. In contrast, the over-expression of C1-FDX-GFP enhances the CI* (a sub-complex of CI) and CV assembly. Immunodetection reveals that the stoichiometric ratio of the α:β subunits in the F1 module of CV is altered in the c1-fdx mutant. In the complemented mutants, C1-FDX-GFP was found to be associated with the F' and α/β sub-complexes of CV. Protein interaction assays showed that C1-FDX could interact with the β, γ, δ, and ε subunits of the F1 module, indicating that C1-FDX, a structural component of CI, also functions as an assembly factor in the assembly of F' and α/β sub-complexes of CV. These results reveal a new role of C1-FDX in the CI and CV assembly and seed germination in Arabidopsis.