Stromule

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. Protrusions from and interconnections between plastids were observed in 1888 (Gottlieb Haberlandt) and 1908 (Gustav Senn) and have been described sporadically in the literature since then.[1][2][3][4] Stromules were recently rediscovered in 1997[5] and have since been reported to exist in a number of angiosperm species including Arabidopsis thaliana, wheat, rice and tomato, but their role is not yet fully understood.[6]

This highly dynamic nature is caused by the close relationship between plastid stromules and actin microfilaments, which are anchored to the stromule extensions, either in a longitudinal fashion to pull from the stromule and guide the plastid in a given direction or in a hinge fashion allowing the plastid to rest anchored in a given place. The actin microfilaments also define the stromule shape through their interactions.[7] This dynamic random walk-like movement is probably caused by Myosin XI proteins as a recent work found.[8]

Other organelles are also associated to stromules, as mitochondria, which have been observed associated and sliding over stromule tubes. Plastids and mitochondria need to be spatially close as some metabolic pathways like photorespiration require the association of both organelles to recycle glycolate and detoxify the ammonium produced during photorespiration.

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[9] and enable the transfer of molecules as large as RuBisCO (~560 kDa) between interconnected plastids.[5] 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.[10] In general, smaller plastids produce shorter stromules, although the largest plastids, mesophyll chloroplasts produce relatively short stromules, indicating that other factors control stromule formation.[6] In general, stromules are more abundant in cells containing non-green plastids,[6] 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.

## See also

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* Membrane nanotube

## Sources

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* A Novel View of Chloroplast Structure a good introduction on stromules, with nice fluorescence pictures of chloroplasts and stromules.
* Natesan SK, Sullivan JA, Gray JC (March 2005). "Stromules: a characteristic cell-specific feature of plastid morphology". J. Exp. Bot. 56 (413): 787–97. doi:10.1093/jxb/eri088. PMID 15699062.: A good review on stromules (unfortunately access is restricted to subscribers)

## References

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1. ^ Die Gestalts- und Lageveränderung der Pflanzen-Chromatophoren, Gustav Senn, 1908, Leipzig: Wilhelm Engelmann Verlag.
2. ^ "Die Chlorophyllkörper der Selaginellen", Gottlieb Haberlandt, Flora 71 (1888), pp. 291-308.
3. ^ §2, "Stromules and the dynamic nature of plastid morphology", E. Y. Kwok and M. R. Hanson, Journal of Microscopy 214, #2 (May 2004), pp. 124-137, doi:10.1111/j.0022-2720.2004.01317.x.
4. ^ Section "The Discovery and Rediscovery of Stromules" in "Stromules: Mobile Protrusions and Interconnections Between Plastids", J. C. Gray, J. A. Sullivan, J. M. Hibberd and M. R. Hansen, Plant Biology, 3, #3 (May 2001), pp. 223-233, doi:10.1055/s-2001-15204.
5. ^ a b Köhler RH, Cao J, Zipfel WR, Webb WW, Hanson MR (1997). "Exchange of protein molecules through connections between higher plant plastids". Science. 276 (5321): 2039–42. doi:10.1126/science.276.5321.2039. PMID 9197266.
6. ^ a b c Waters, Mark; Rupert G. Fray; Kevin A. Pyke (9 July 2004). "Stromule formation is dependent upon plastid size, plastid differentiation status and the density of plastids within the cell". The Plant Journal. 39 (4): 655–67. doi:10.1111/j.1365-313X.2004.02164.x. PMID 15272881.
7. ^ Kwok EY, Hanson MR (February 2004). "In vivo analysis of interactions between GFP-labeled microfilaments and plastid stromules". BMC Plant Biol. 4: 2. doi:10.1186/1471-2229-4-2. PMC 356911. PMID 15018639.
8. ^ Amir Sattarzadeh, Johanna Krahmer, Arnaud D. Germain and Maureen R. Hanson (2009). "A Myosin XI Tail Domain Homologous to the Yeast Myosin Vacuole-Binding Domain Interacts with Plastids and Stromules in Nicotiana benthamiana". Molecular Plant. 2 (6): 1351–8. doi:10.1093/mp/ssp094. PMID 19995734.`{{cite journal}}`: CS1 maint: multiple names: authors list (link)
9. ^ Gray JC, Sullivan JA, Hibberd JM, Hansen MR (2001). "Stromules: mobile protrusions and interconnections between plastids". Plant Biology. 3 (3): 223–233. Bibcode:2001PlBio...3..223G. doi:10.1055/s-2001-15204. S2CID 84474739.
10. ^ Köhler RH, Hanson MR (2000). "Plastid tubules of higher plants are tissue-specific and developmentally regulated". Journal of Cell Science. 113: 81–9. doi:10.1242/jcs.113.1.81. PMID 10591627.