Mitochondrial ribosomal protein required for central cell maturation. May work together with PIA2 in controlling female gametophyte development, possibly by regulating the expression of some mitochondrial proteins. Interacts (via C terminus) with PIA2. Expressed in root tips, young leaves, flowers and siliques. In anthers, expressed throughout the developmental stages, especially in the mature pollen grains. In pistils, expressed in the mature female gametophyte stage and during fertilization. Aborted ovule development due to defects in fusion of polar nuclei and central cell maturation. Belongs to the universal ribosomal protein uS9 family.