On the Inside Stromules: Bona Fide Chloroplast Structures

Although stroma-filled extensions and protrusions of the plastid envelope called stromules were described as long as a century ago, only recently with the advent of fluorescent protein technology have they become widely accepted as bona fide structures of chloroplasts and plastids. Stromules usually occur as filamentous tubules that can be up to 50 to 60 mm in length. The rediscovery of plastid stromules and the many unresolved questions surrounding them are the subject of a review by Hanson and Sattarzadeh (pp. 1486–1492) in this Focus Issue. The presence of stromules is not limited to vascular plants; they have been described in Euglena, Acetabularia, and a variety of lower plants. Stromules are found more frequently on nongreen plastids than on chloroplasts. There is no evidence that thylakoid membranes enter stromules, as chlorophyll autofluorescence is not visible within them; in fact, the lack of chlorophyll is one reason why the presence of tubules escaped attention for so long. Indeed, the renewed interest in plastid stromules stems largely from the ease of studying and observing GFP-labeled plastids in transgenic plants. Stromules often exhibit transient dilations that appear to move along the stromule length. Studies employing photobleaching of the GFP signal present in a plastid connected to another plastid revealed the quick reappearance of green fluorescence due to flow of GFP from the connected, unbleached plastid. Similar experiments have demonstrated that Rubisco and other stromal proteins traffic between plastids, making it likely that many other molecules, including solutes and RNA, may be transferred as well. Indeed, some stromules appear large enough to serve as conduits for the transport of multiprotein complexes as large as ribosomes. The mechanism for movement of proteins within stromules remains uncertain, but certainly diffusion plays a role. The structure and function of stromules appears to be regulated by actin and myosin. When the actin cytoskeleton is disrupted by cytochalasin D, linear stromules disappear. Double labeling of the actin cytoskeleton and stromules has revealed contact between microfilaments and stromules that may constitute anchor points for stromules. Several lines of evidence indicate that myosin motors, which operate on actin microfilaments, may also be involved in stromule motility. Perhaps the most intriguing aspect of stromules is their possible function. Do they increase the surface area of the plastid compartment for import and export of molecules? Do they serve to anchor plastids? Are they involved in the recycling of chloroplast contents? Or do they facilitate the transfer of materials between compartments? Plastids and stromules are often observed in close proximity to other organelles and the endoplasmic reticulum (ER) and may facilitate the biochemical pathways that require transfer of substrates and products between organelles. The close association of plastid bodies and stromules with the ER that has frequently been noted may assist in import of proteins that flow from the secretory pathway into plastids. This idea is reinforced by the findings of Schattat et al. (pp. 1667– 1677) who have focused their attention on the branched stromules of Arabidopsis (Arabidopsis thaliana) and Nicotiana benthamiana. Their observations suggest that stromule branching coincides with the dynamic rearrangement of cortical ER tubules. The simultaneous visualization of GFP-tagged stromules and red fluorescent proteintagged ER suggests that stromules extend and retract within ER-lined channels. These observations of coincidental stromule-ER behavior suggest that either the neighboring ER tubules shape stromules directly or the behavior of both ER and stromules is simultaneously dictated by a shared cytoskeleton-based mechanism. These findings strongly implicate the ER membrane in interactions with stromules and suggest that their interacting surfaces might serve as major conduits for the bidirectional exchange of ions, lipids, and metabolites between the two organelles. Are Plasmodesmata More Permeable Than Previously Thought?