Plastosphere

Plastosphere is a term used in some discussions of plastid biology to describe a functional microenvironment that surrounds plastids within plant and algal cells. It is not an established anatomical structure in standard cell biology, and there is no formal consensus on its definition or boundaries. The term is etymologically straightforward, combining plasto- (plasts/plastids) with sphere (a surrounding region), and is used to emphasize the dynamic interface between plastids and the cytosol.

In this interpretation, the plastosphere includes the plastid envelope and its associated membranes, any extra-plastidial compartments encountered in secondary endosymbiotic plastids, and the surrounding milieu formed by stromules, lipids, ions, metabolites, and signaling molecules that mediate exchange with the cytoplasm. The concept highlights that exchange between plastids and the rest of the cell is not limited to membrane transporters alone but occurs within a locally defined physicochemical space that can influence redox state, metabolite flux, and organelle communication.

Proponents use the plastosphere to discuss coordination of plastid functions with cellular metabolism, responses to stress, and development. The term is sometimes conflated with related ideas such as the periplastidial space in complex plastids, but it remains distinct as a generalized, not strictly defined concept.

See also plastid, chloroplast, stromule, periplastidial space, endosymbiosis.