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The endoplasmic reticulum (ER) is a non-uniform compartment in plants as regards its morphologyand function. It extends as a highly anastomosing membranous network throughout the cytoplasm, isthe major compartment of membrane biogenesis, and has been verified to function as the starting site forthe secretory pathway. Early electron microscopy studies revealed three morphological ER sub-domains: thesmooth ER, the rough ER, and the nuclear envelope. In the last two decades vital staining procedures, immunologicalmethods, and green fluorescent protein technology in connection with confocal laser scanning microscopyhave extended and augmented our knowledge regarding the morphology of the different ER domains, especiallythe three-dimensional transition between the cortical tubular network, long tubular strands, and lamellarsheets during interphase and mitosis. The cytoskeleton in connection with the respective motor proteinsand cations like Ca ^2+ and H^+play a critical role in the regulation of ER organization in dividing, differentiating, and stressedcells. Although our understanding of ER morphology in plants has improved notably, our view still remainsfounded on a rather limited number of model cells.

The targeting of proteins to specific subcellular regions is directed by a variety of signalelements. Many of these signals consist of amino acid residues (peptide sorting signals) arranged contiguouslyor in a three-dimensional array. In addition to posttranslational processes, proteins can also belocalized to specific regions of the cell by the targeting of their cognate RNA. Ongoing studies in developingrice endosperm have shown that the RNAs that code for the major storage proteins are localized to specificsubdomains of the cortical endoplasmic reticulum (ER), and that there is a tight correlation betweenthe initial site of RNA localization and the final destination of the encoded protein in the endomembranesystem. The segregation of RNA onto distinct ER subdomains may be a necessary and sufficient stepfor the localization of the coded protein in the cell.

The main resident proteins of the endoplasmic reticulum (ER) collaborate to ensure that newlysynthesized secretory proteins acquire their correct conformation. Most ER residents are therefore,directly or indirectly, folding helpers and controllers of the quality of newly synthesized secretorypolypeptides. Genetic approaches have revealed that these helpers are necessary for virtually anymajor aspect of plant life, from differentiation to reproduction to interactions with the environment.Detailed biochemical analysis on the protein–protein interactions that occur during foldingin the ER has been performed on a number of model secretory proteins, and the integration betweengenetics and biochemistry is a major future goal of this field of plant cell biology.

The endoplasmic reticulum (ER) is equipped with a quality control function that retains misfoldedand unassembled proteins and allows only structurally mature polypeptides to be transported to their finaldestination. The retained proteins are eventually retro-translocated to the cytosol and destroyed by a processcalled endoplasmic reticulum-associated degradation (ERAD). Besides being involved in the degradation ofaberrant polypeptides, the ERAD pathway is also used to regulate cellular functions and is exploited bysome plant and bacterial toxins to reach the cytosol after internalization by target cells. After summarizingthe general characteristics of the ERAD pathway, we describe the features of known plant ERAD substratesand of the plant degradative machinery, highlighting the role of protein disposal in the response to ERstress.

The endoplasmic reticulum (ER) is the starting site of the journey of newly synthesized proteinsto the apoplast, plasma membrane and to the vacuolar compartments. Transport between these membranecompartments of the secretory pathway in eukaryotic cells is mediated by vesicles, which are producedby a budding mechanism involving coat proteins that capture specific cargo molecules and helppackage them into coated vesicles. These vesicles are known as COPII-coated vesicles, and are usuallyisolated after their induction in vitro using microsomal membranes, cytosol and a non-hydrolyzableGTP-analogue. COPII-coated vesicles are formed at specific sites in the ER known as ER-exit sites(ERES). ERES are well-characterized in mammalian cells, and can be recognized in some algae, but controversysurrounds their identification in higher plant cells.

Most vacuolar proteases are transported from the endoplasmic reticulum (ER) to vacuoles via the Golgiapparatus. However, higher plants possess a unique papain-type protease, termed KDEL-tailed protease.This protease has a Lys − Asp − Glu − Leu (KDEL)sequence at its C-terminus, which is known as a retention signal of soluble proteins to the ER, althoughthe protease localizes and functions in vacuoles. Investigations on the intracellular trafficking pathwayof this unique enzyme have suggested that the protease is transported from the ER to vacuoles by bypassingthe Golgi apparatus. In this review, Golgi-dependent vacuolar trafficking of proteases is first explained,then the Golgi-independent vacuolar transport pathway of the KDEL-tailed protease is described.

The endoplasmic reticulum (ER) differentiates to generate various types of compartments, each of whichhas its own function. The ER-derived compartments are classified into two types according to their contents.The first type is found in maturing seeds and accumulates seed storage proteins. The second type of compartmentaccumulates a hydrolytic enzyme. Here we focus on two typical ER-derived compartments: precursor-accumulating(PAC) vesicles as a storage protein type, and ER bodies as a hydrolytic enzyme type. PAC vesiclesmediate the mass transport of storage protein precursors directly from the ER to protein storage vacuolesin maturing seeds. A vacuolar sorting receptor of storage protein is localized in the membrane ofthe PAC vesicles. The vesicles provide a clue to the molecular mechanism of vacuolar sorting of storageproteins. In contrast, ER bodies accumulate a large amount of β-glucosidase with an ER retentionsignal. They are distributed in the epidermal cells of seedlings and roots. Wounding and chewing by insectsinduce many ER bodies in rosette leaves, which have no ER bodies under normal conditions. The ER bodiesmight therefore play a role in a defense strategy of plants. Most of the ER-derived compartmentsare induced in specific tissues in response to internal and external signals. Hence, the induction of ER-derivedcompartments is controlled in a sophisticated way by the conditions under which plants grow.

The endoplasmic reticulum (ER) in plants plays a key role in the synthesis of a wide rangeof lipids which are essential structural components of all cellular membranes. Lipids also represent themajor form of storage carbon in the seeds, pollen and fruit of many plant species and in some cases over75% of the dry mass of these tissues has been metabolised by the ER. The world vegetable-oils market isworth over $30 billion per year and is of great importance to the agricultural economy. There is thereforeparticular interest in aspects of ER function relating to triacylglycerol synthesis. In the epidermis,lipids made by the ER are exported to form the cuticular barrier protecting the plant against water loss,biotic and abiotic stresses. In addition, ER-derived glycerolipids, sphingolipids and sterols have essentialroles as components of signal transduction pathways. This chapter describes the biochemical pathways ofmembrane and storage lipid synthesis in the plant ER and charts progress in the identification and characterisationof the genes involved.

Three types of related subcellular oil-rich particles are present in plants: storage oil bodies inseeds for gluconeogenesis during germination, storage oil bodies in pollen providing acyl moieties for membranesynthesis in the pollen tube, and tapetosomes in the anther tapetum for delivering lipids and proteins tothe maturing pollen surface. Each of these oil-rich particles has a basic structure of an oil body,which consists of a triacylglycerol matrix enclosed by a layer of phospholipids and the structuralprotein oleosins. All components of an oil body are synthesized and assembled in endoplasmic reticulum(ER), from which a budding oil body is released. An oleosin molecule has a highly conserved centraldomain of 72 uninterrupted hydrophobic residues flanked by variable amphipathic N -and C -terminal segments. Its unique central domain is presumed to haveevolved from a transmembrane segment of an ER protein. An oleosin molecule does not have an N -terminal ER-targeting signal and is targeted to the signal recognition particleand then ER via its central hydrophobic domain. Targeting studies of oleosin molecules that have been modifiedby adding a  N -terminal ER-targeting signal, shortening the centralhydrophobic stretch and eliminating the N - or C -terminalamphipathic stretch, have provided a model delineating the mechanism of oleosin targeting to ER andoil bodies. A tapetosome possesses numerous oleosin-coated oil bodies associated ionically with abundantmembranous vesicles, both of which are assembled in and then detached from ER.

Diverse and compelling evidence is presented in support of the participation of the ER in the biogenesisof different kinds of plant peroxisomes. New and previous data coupled with interpretations and opinionsare embedded within four multistep peroxisome assembly models derived from studies with diverse organisms.The main objective of this Chapter is to compare and contrast the varied involvement of the ER in the biogenesisof peroxisomes within the context of the four general models for peroxisome origination, assembly, maturation,and replication. Two of the models depict a unique participation of the ER in the origin and subsequentmaturation of nascent pre-peroxisomes . In the third autonomous model, the ER is not involved, whereas in the fourth semi-autonomous model, ER-derived vesicles contribute to the maturation/differentiation and replication of Pre-existingperoxisome . The semi-autonomous model pertains to the biogenesis of plant peroxisomes. Withinthis scheme, a subset of peroxisomal membrane proteins (PMPs), collectively called group I proteins,e.g. peroxin 16 and ascorbate peroxidase, are synthesized in the cytosol and trafficked indirectly to peroxisomes via ER-derived vesicles. Interestingly, current evidence does not predict the origin ofnew plant peroxisomes directly from domains of the ER. Instead, mature pre-existing peroxisomes apparentlyreplicate via constitutive duplication (fission) in response to the action of one or more isoforms of a peroxinhomolog designated as peroxin 11. Nascent daughter organelles acquire membrane phospholipids and PMPs fromER-derived vesicles.