“A stromule is a microscopic structure 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.”
They are ubiquitous, being found extending from the surface of plastids in all multicellular plants - however :
“[…] their function is still largely a matter of conjecture. They may increase the plastid surface area to facilitate movement of materials into or out of plastids, be involved in sensing the cellular environment, and/or have signaling functions due to close apposition of stromules and nuclei, plasma membrane and other cell organelles. Stromule formation appears unrelated to chloroplast division or to light-intensity-dependent chloroplast movement.”
Source: Plastid Development in Leaves during Growth and Senescence pp 169-186
Further reading American Society of Plant Biologists
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