Component of the plastid division machinery consisting in a binary fission accomplished by the simultaneous constriction of the FtsZ ring on the stromal side of the inner envelope membrane, and the ARC5 ring on the cytosolic side of the outer envelope membrane (PubMed:28248291, PubMed:29466386). Involved in the initiation of proplastid and plastid division (including chloroplasts, statoliths and leukoplasts) (PubMed:29466386). Promotes the assembly and/or stabilization of the plastid-dividing FtsZ ring, functioning as an antagonistic regulator of FtsZ dynamics against CDP1 and facilitating MCD1 positioning to membrane tethered FtsZ filaments to form the chloroplast Z-Ring; inhibits GDP-induced disassembly of FTSZ2 but enables ARC3 binding to FTSZ2-1 (PubMed:29466386, PubMed:29967285, PubMed:29769312, PubMed:29138260). Relays plastid division site position between stroma and outer surface via interactions with the stromal FtsZ ring and the outer membrane PDV2 that recruits cytoplasmic ARC5 ring (PubMed:28248291). Required for plastid equatorial positioning of PDV2 and ARC5. May contribute to gravitropism in stems and hypocotyls. Seems to influence stromule (stroma-filled tubular extensions of the plastid envelope membrane) length and frequency. Self-interacts (PubMed:28248291, PubMed:29769312). Part of a complex made of ARC3, ARC6, FTSZ1 and FTSZ2 (PubMed:22823492). Interacts with FTSZ2-1 and FTSZ2-2 (via C-terminus), but not with FTSZ1; this interaction enables ARC3 binding to FTSZ2 (PubMed:29967285, PubMed:29769312, PubMed:29138260, PubMed:26527658). Binds to CDT1A. Interacts (via C-terminus) with PDV2 (via C-terminus) in the chloroplast intermembrane space; this interaction induces homodimerization and leads to the formation of an heterotetramer containing two ARC6 and two PDV2 subunits (PubMed:28248291). Interacts with MCD1 in the chloroplast stroma and facilitates its subsequent binding to FtsZ2-1 (PubMed:29967285). Interacts (via J domain) with CJD1 (via J-like domain) (PubMed:22028775). Localized to a ring at the center of the chloroplasts (equatorial positioning) prior to and during constriction. Mostly expressed in young leaves. Levels decrease slightly from young developing leaves to mature ones (at protein level). Slightly induced by gibberellic acid (GA). Defective in proplastid and plastid division, with only one or two grossly enlarged plastids per cell, sometimes exhibiting alteration in stromule length and frequency in non-green tissues (e.g. increase in the frequency of stromules in nearly all cells) and in stomatal guard cells (GCs) (PubMed:29466386). Heterogeneous chloroplasts sizes and shapes such as giant and mini-plastids in leaf epidermal pavement cells (PCs) (PubMed:29466386). Abnormal subplastidial localization of the key plastid division proteins FTSZ1 and FTSZ2 (numerous short and disorganized FtsZ filament fragments) (PubMed:29967285). Root cells statoliths, chloroplasts, and other plastids are also abnormally large. Impaired gravitropism of inflorescence stems and hypocotyls, but not of roots. Several mesophyll and stomatal guard cells contain chlorophyll-free plastids, probably missing chloroplastic DNA. Misexpression and mislocalization of ADT2 (PubMed:30252596). The double mutant mcd1 arc6 exhibits similar chloroplast defect than the single mutant arc6, including the abnormal localization of FTSZ1 to short filaments and dots within chloroplasts (PubMed:29967285). The double mutant arc6 cjd1 exhibits both phenotypes of single mutants cjd1 and arc6 including altered fatty acid profiles and heterogeneous chloroplasts sizes and shapes, respectively (PubMed:22028775).