Such studies also implicated the actin cytoskeleton as an organizer of the ER network Quader et al.
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However, it was the advent of fluorescent protein technology that permitted a true appreciation of the dynamic nature of the ER Boevink et al. Based on EM, these bodies reside in the lumen of the ER, although they cause massive distortion of the tubules Hawes et al. The movement of these ER bodies is actin mediated and the implication is that, as there is no evidence of cytoskeletal elements within the lumen of the ER, their movement must reflect movement of the whole ER network.
In the absence of actin, activated membrane diffused evenly through the ER from the activation spot. It is now clear that in leaf epidermal cells, there are at least three if not four distinct forms of movement of the ER: i movement of the membrane surface; ii growth and shrinkage of tubules; iii conversion of tubules to cisternae and vice versa; and iv movement of the tubules forming the polygons of the cortical network which along with tubule growth permits polygon ring formation and closure Sparkes et al.
All of these are distinct from the extremely rapid ER translocation caused by classic cytoplasmic streaming. The significance of this static ER is not known, although the mapping also revealed sets of persistent puncta, which could represent anchor points to the plasma membrane. Apart from the obvious dynamics of the system, the most remarkable feature of the cortical ER network is the polygonal arrangement of the tubules and their tendency to morph in to small cisternae, mainly at three way junctions, and for these to convert back to tubules.
To date, there is no real functional significance attributed to these membrane forms, although previously it has been suggested that this could be related to the secretory capacity of the cell Stephenson and Hawes Recently, it has been suggested that loss of the GTPase RHD3 root hair defective 3, see below , which is involved in shaping the ER network, disrupts the unfolded protein response, and this may indicate a direct relationship between ER form and function Lai et al.
There are a number of proposed mechanisms by which proteins can induce membrane curvature in organelles or in vesicle formation including classic coats such as clathrin, external scaffolding such as BAR domain proteins, and direct insertion into membranes Shen et al. In plants, it has been shown that the reticulon family of proteins is most likely primarily responsible for tubulation of the ER Tolley et al. The proteins are associated with curved ER membranes, tubules or edges of cisternae, and are not regularly found on cisternal sheets Sparkes et al.
Reticulons are integral ER membrane proteins with an unusual topology in that they feature two major hydrophobic helical domains which are far too long to simply span the ER unit membrane. The protein can then dimerize or oligomerize with itself creating differential tension in the ER membrane resulting in induction of curvature Figure 2 ; Sparkes et al.
Sparkes et al. A Diagram showing the proposed insertion of reticulons into the endoplasmic reticulum ER membrane. Another family of proteins involved in the organization of the ER network is the atlastins in mammals, with Sey1p being the yeast functional ortholog Hu et al. Again, it has been suggested that this may be due to a problem involving the fusion of ER tubules preventing polygon formation Zhang et al.
Interestingly, Chen et al. However, it was reported by another group that RHD3 could complement yeast sey1 mutants and was used in an assay to show that RHD3 can promote ER fusion in yeast Zhang et al.
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Although the majority of the ER network appears to be in a constantly dynamic state as stated above, a low percentage of tubules appear to be static. Also, Sparkes et al.
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An analysis of cortical ER networks from time lapse series of leaf epidermal cells identified the presence of persistent puncta, which did not move with the ER network Sparkes et al. An Arabidopsis leaf in which Golgi bodies are marked with a red fluorescent protein and endoplasmic reticulum ER with green fluorescent protein. Two Golgi bodies were trapped white arrows and an attached ER tubule follows them as they are pulled through the cortical cytoplasm. At 9 s, the Golgi bodies pick up another ER tubule which fuses with the trailing tubule and forms a new three way junction.
At 31 s, the trailing ER tubule attaches to a putative plasma membrane contact site yellow arrow and remains anchored to it. Figure adapted from Sparkes et al. In yeast and mammalian cells, it has been proposed that amongst a number of different proteins discovered at sites where the ER is anchored to the plasma membrane, members of the VAP family are be particularly important. In yeast, the VAP homolog, Scs2, anchors the ER to the plasma membrane, especially at the bud tips and interacts with sterol transfer proteins Loewen and Levine ; Loewen et al.
It is worth noting that these contact sites are not found at three way junctions in the ER network, which are formed by tubule fusion as suggested by Hamada et al. Various functions have been attributed to ER—plasma membrane contact sites in mammalian cells besides anchoring of the ER network. These include cell signaling, direct lipid transfer, and calcium release Manford et al. Thus, the membrane surface of the ER may function as a kind of highway transporting organelles and other structures.
It has long been known that in many plant cell types Golgi bodies move with the surface of the ER see below. Certainly, in tobacco leaf epidermal cells, it has been demonstrated that viral movement proteins and RNA can be located to the ER and even be transported along its surface to plasmodesmata Heinlein et al. More recently, it has been confirmed that the ER supports the movement of viruses with the triple gene block TGB organization to plasmodesmata. Viral nucleocapsid proteins such as those of tomato spotted wilt Tospovirus have also been shown to move on the surface of the ER in tobacco leaf epidermal cells as motile inclusions.
A concept is also emerging of the ER offering an extensive surface for the location of blocks of metabolic enzymes, thus not only regulating the spatial organization of metabolic pathways but also enabling the mixing of metabolic products throughout the cytoplasm. Organellar compartmentation provides the possibility for cells to bring enzymes and their substrates in closer proximity and increase local concentration for process optimization.
The concept of these so called metabolons was introduced by Srere in the context of glycolytic and Krebs cycle enzymes. Metabolons provide a variety of crucial advantages: these structures can improve catalytic efficiency in terms of turnover rate by bringing the active sites of enzymes converting adjacent steps in closer proximity.
Such complex coordination has been shown for protein translocases found in ER, chloroplasts, and mitochondria Jarvis et al. Most metabolons involved in secondary metabolism comprise less stable interactions than reported for metabolons in primary metabolism, requiring refined microscopy methods and computational analysis to verify their existence Bassard et al.
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It is further suggested that cytochrome P localizes even in specific ER subdomains, and that this is facilitated by interactions between metabolon and the cytoskeletal elements Bassard et al. In sorghum, the production machinery of the cyanogenic glucoside, dhurrin, forms metabolons in specific ER domains Winkel In tapetal cells, four enzymes responsible for the biosynthesis of sporopollenin, the main building blocks of the outer layer of pollen walls, mainly localize to the ER. The ER is a hotspot for active lipid synthesis and tapetal cells have been reported to show signs of a strong lipid metabolism, such as dilated ER and accumulation of lipid bodies Murgia et al.
The high demand for sporopollenin would benefit from a quick and efficiently coordinated production of pollen cell wall components as afforded by the metabolon, allowing for rapid pollen development. This could potentially be due to such a complex being membrane anchored, which would hinder detection by conventional approaches.
Additionally, a number of other YUCCA proteins are predicted, in silico , to have hydrophobic domains and signal peptides, indicating a wider significance for enzyme localization in auxin biosynthesis Kriechbaumer et al. As total IAA was normalized against leaf dry weight, this difference in productivity could indicate a certain level of protein stabilization or enhanced accumulation on ER membranes. Insert shows labeling of nucleus but not the nuclear envelope which is typical for cytosolic fluorescent proteins. Panels A—C are adapted from Kriechbaumer et al. D The two alternatively localized enzymes were overexpressed in tobacco epidermal leaves followed by total indole acetic acid IAA extraction and analysis by thin layer chromatography.
13 ERA-CAPS Plant Endoplasmic Reticulum Architecture and Seed Productivity
Additionally, metabolons would not be limited to one organellar membrane system. For instance, partially fused membranes at interaction sites between the ER and plastids Whatley et al.
Examples for pathways with these criteria are gibberellic acid biosynthesis Zybailov et al. It has been shown across the kingdoms that the ER can make close contacts with a number of organelles besides the plasma membrane see above , such as mitochondria Rowland and Voeltz and the Golgi apparatus, directly via structures such as ER exit sites and tubular vesicular carriers Ladinsky et al.
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Less is known about contacts between the ER and other plant organelles, although some potentially interesting stories are beginning to emerge Mathur et al. The Golgi apparatus lies at the heart of the secretory pathway and it modifies proteins and moves lipids around as well as being responsible for the biosynthesis of the major matrix polysaccharides of the cell wall. The interaction of the Golgi with the ER Juniper et al. It is now accepted that in vacuolated cells, Golgi bodies together with ER exit sites move over, or with, the surface of the ER Brandizzi et al.
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So much so that when Golgi bodies are trapped and manipulated by optical tweezers, the attached ER is remodeled behind the moving Golgi body Figure 3 ; Sparkes et al. The number of stacks in a cell would then reflect the secretory capacity of the cell, with ER exit sites multiplying possibly by fission and generating a new Golgi stack when required Hawes et al. It is now known that all the components of COPII carrier coats are present in plants and they are most likely required for the efficient transfer of cargo between the ER and Golgi De Craene et al.
However, whether COPII vesicle carriers are used in many cell types, other than a few specific tissues, is still an open and controversial question, and tubular continuity for cargo transfer is a distinct possibility Hawes However, the mobile secretory unit concept is gaining in acceptance Langhans et al. However, an equivalent retrograde pathway most likely exists, as evidenced by work employing secretory pathway inhibiting drugs such as brefeldin A BFA , and genetic inhibition of exit from the ER, both of which result in redistribution of Golgi membrane to the ER system Osterrieder et al.
Unfortunately, due to the lack of marker proteins, little is known about these putative ER import sites. At the same time, Lerich et al. Interestingly, Schoberer et al. Interestingly, from numerous observations it is apparent that Golgi bodies and their associated ER exit sites are only attached to curved ER membranes: the surface of tubules or the curved edges of cisternae C Hawes, unpubl.
As mentioned above, the existence of functional continuity between ER and plastid envelopes has been extrapolated from the ability of enzymes in the tocopherol and carotenoid synthetic pathways that are normally resident in the plastid to complement their loss of function mutant when targeted to the lumen of the ER Mehrshahi et al. Contacts between ER and chloroplasts have also been demonstrated in cytoplasm from burst protoplasts by using optical tweezers, and the authors suggested that this indicated a functional relationship between the structures in vivo Andersson et al.
It has also been shown using fluorescent protein expression and live cell imaging that the tubular extensions from plastids, known as stomules, can align along the ER but the functional significance of this alignment has yet to be elucidated Schattat et al. Such contacts between the ER and chloroplasts most likely mediate lipid transfer between the organelles Hurlock et al. Peroxules, tubular extensions from peroxisomes, are particularly apparent in cells under conditions of oxidative stress. Sinclair et al. Such data suggest direct connection or interaction between the two organelles.
Although in yeast and mammalian cells it has been shown that the ER is directly involved in peroxisome biogenesis Smith and Aitchison , there is little evidence that this is the case in plants Sparkes et al. As described earlier, the ER displays a diverse range of movements which are driven by the cell's actomyosin system with members of the myosin XI family being the key motor proteins driving the movements Sparkes et al.
Using such an approach, Griffing et al. Or filter your current search. Current Opinion in Plant Biology [28 Sep , 9 6 ]. Type: Research Support, Non-U. Gov't, Review, Research Support, U. Gov't, Non-P. Abstract Significant advances have been made in recent years that have increased our understanding of the trafficking to and from membranes that are functionally linked to the Golgi apparatus in plants. New routes from the Golgi to organelles outside the secretory pathway are now being identified, revealing the importance of the Golgi apparatus as a major sorting station in the plant cell.
This review discusses our current perception of Golgi structure and organization as well as the molecular mechanisms that direct traffic in and out of the Golgi. Read Article at publisher's site. How does Europe PMC derive its citations network?