Required for the maintenance of mitochondrial structure (PubMed:21984726). Positive regulator of cell division and endoreduplication but negative regulator of cell expansion in the postembryonic root meristem, thus leading to the promotion of root growth (PubMed:21984726). Predominantly expressed in the root meristem, in the primary and lateral root tips (PubMed:21984726). Also present in leaves and pollen (PubMed:21984726). In root tips, preferentially expressed in quiescent center (QC) cells, cortex/endodermis stem cells and their daughter cells, and endodermal and stele cells of root meristems. Reduced number of dividing cells and slower rate of cell production and endoreduplication, thus affecting root meristem size and delaying postembryonic root growth (PubMed:21984726). Decreased expression of several cell cycle genes indicating a defect in cell cycle progression (PubMed:21984726). Extensive vacuolization in mitochondria (PubMed:21984726). Belongs to the RMD1/sif2 family.