Stromule

Stromules are microscopic structures found in plant cells.

Stromules (stroma-filled tubules) are highly dynamic structures extending from the surface of all plastid types, including proplastids, chloroplasts, etioplasts, leucoplasts, amyloplasts, and chromoplasts.

Stromules are usually 0.35–0.85 µm in diameter and of variable length, from short beak-like projections to linear or branched structures up to 220 µm long. They are enclosed by the inner and outer plastid envelope membranes and enable the transfer of molecules as large as Rubisco (~560 kDa) between interconnected plastids. Stromules occur in all cell types, but stromule morphology and the proportion of plastids with stromules vary from tissue to tissue and at different stages of plant development. In general, stromules are more abundant in tissues containing non-green plastids, and in cells containing smaller plastids. The primary function of stromules is still unresolved, although the presence of stromules markedly increases the plastid surface area, potentially increasing transport to and from the cytosol. Other functions of stromules, such as transfer of macromolecules between plastids and starch granule formation in cereal endosperm, may be restricted to particular tissues and cell types.