Stromules are microscopic

Microscopy

Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...

 structures found in plant cell

Plant cell

Plant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:...

s. Stromules (stroma-filled tubules) are highly dynamic structures extending from the surface of all plastid

Plastid

Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...

 types, including proplastids, chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s, etioplast

Etioplast

Etioplasts are chloroplasts that have not been exposed to light. They are usually found in flowering plants grown in the dark. If a plant is kept out of light for several days, its normal chloroplasts will actually convert into etioplasts. Etioplasts lack active pigment and can technically be...

s, leucoplast

Leucoplast

Leucoplasts are a category of plastid and as such are organelles found in plant cells. They are non-pigmented, in contrast to other plastids such as the chloroplast....

s, amyloplast

Amyloplast

Amyloplasts are non-pigmented organelles found in some plant cells. They are responsible for the synthesis and storage of starch granules, through the polymerization of glucose. Amyloplasts also convert this starch back into sugar when the plant needs energy...

s, and chromoplast

Chromoplast

Chromoplasts are plastids responsible for pigment synthesis and storage. They, like all other plastids , are organelles found in specific photosynthetic eukaryotic species....

s. They were rediscovered in 1997 and have since been reported to exist in a number of angiosperm species including Arabidopsis thaliana

Arabidopsis thaliana

Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...

, wheat

Wheat

Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...

, rice

Rice

Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...

 and tomato

Tomato

The word "tomato" may refer to the plant or the edible, typically red, fruit which it bears. Originating in South America, the tomato was spread around the world following the Spanish colonization of the Americas, and its many varieties are now widely grown, often in greenhouses in cooler...

 but the role of stromules is not yet fully understood.

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. This dynamic random walk-like movement is probably caused by Myosin

Myosin

Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other eukaryotic motility processes. They are responsible for actin-based motility. The term was originally used to describe a group of similar...

 XI proteins as a recent work found.

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

Photorespiration

Photorespiration, or "'photo-respiration'", is a process in plant metabolism by which RuBP has oxygen added to it by the enzyme , instead of carbon dioxide during normal photosynthesis. This is the beginning step of the Calvin-Benson cycle...

 require the association of both organelles to recycle glycolate and detoxify the ammonium

Ammonium

The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...

 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 and enable the transfer of molecules as large as RuBisCO

RuBisCO

Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. RuBisCo is an abbreviation...

 (~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, smaller plastids produce shorter stromules, although the largest plastids, mesophyll

Mesophyll

Mesophyll can refer to:* Mesophyll tissue, in plant anatomy, photosynthetic parenchyma cells that lie between the upper and lower epidermis layers of a leaf...

 chloroplasts procuce relatively short stromules, indicating that other factors control stromule formation. In general, stromules are more abundant in cells 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

Starch

Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

 granule formation in cereal endosperm

Endosperm

Endosperm is the tissue produced inside the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet...

, may be restricted to particular tissues and cell types.

Sources

• A Novel View of Chloroplast Structure a good introduction on stromules, with nice fluorescence pictures of chloroplast
  Chloroplast
  Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...
  
  s and stromules.: A good review on stromules (unfortunately access is restricted to subscribers)