Special Issue: Organelle Biology   

November 2012, Volume 54 Issue 11, Pages 838ĘC931.

Cover Caption: Organelle Biology
About the cover: This Special Issue focuses on plant organelle biology. Papers included in this issue provide updates on the latest research developments in organelle extensions, division, movement, trafficking, and more. The cover picture was taken from a review paper from Worden et al. (875ĘC886) that summarizes the endomembrane trafficking pathways in cell wall deposition. Endomembrane trafficking facilitates the transport of polysaccharides, structural proteins and biosynthetic enzymes from the Golgi apparatus to the plasma membrane and the apoplast (image provided by Park and Drakakaki).


Recent Advances in Plant Organelle Dynamics  
Author: Jianping Hu and Chris Hawes
Journal of Integrative Plant Biology 2012 54(11): 838-839
Published Online: October 15, 2012
DOI: 10.1111/j.1744-7909.2012.01181.x

Organelles are eukaryotic subcellular structures each possessing a specific set of functions. The morphology, abundance, and content of the organelles also undergo constant changes for optimal function and for plant adaptation to environmental variation. The six reviews and one research article published in this special issue on Organelle Biology cover a wide range of topics encompassing recent advances in the studies of the dynamic morphology, abundance, and movement of plant organelles, protein trafficking between organelles, changes in organelle composition and function in response to stress conditions, and machineries involved in the turnover of cellular constituents.

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          Invited Expert Reviews
The Endoplasmic Reticulum: A Social Network in Plant Cells  
Author: Jun Chen, Caitlin Doyle, Xingyun Qi and Huanquan Zheng
Journal of Integrative Plant Biology 2012 54(11): 840-850
Published Online: October 9, 2012
DOI: 10.1111/j.1744-7909.2012.01176.x

The endoplasmic reticulum (ER) is an interconnected network comprised of ribosome-studded sheets and smooth tubules. The ER plays crucial roles in the biosynthesis and transport of proteins and lipids, and in calcium (Ca2+) regulation in compartmentalized eukaryotic cells including plant cells. To support its well-segregated functions, the shape of the ER undergoes notable changes in response to both developmental cues and outside influences. In this review, we will discuss recent findings on molecular mechanisms underlying the unique morphology and dynamics of the ER, and the importance of the interconnected ER network in cell polarity. In animal and yeast cells, two family proteins, the reticulons and DP1/Yop1, are required for shaping high-curvature ER tubules, while members of the atlastin family of dynamin-like GTPases are involved in the fusion of ER tubules to make an interconnected ER network. In plant cells, recent data also indicate that the reticulons are involved in shaping ER tubules, while RHD3, a plant member of the atlastin GTPases, is required for the generation of an interconnected ER network. We will also summarize the current knowledge on how the ER interacts with other membrane-bound organelles, with a focus on how the ER and Golgi interplay in plant cells.

Chen J, Doyle C, Qi X, Zheng H (2012) The endoplasmic reticulum: A social network in plant cells. J. Integr. Plant Biol. 54(11), 840–850.

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Organelle Extensions in Plant Cells  
Author: Jaideep Mathur, Alena Mammone and Kiah A. Barton
Journal of Integrative Plant Biology 2012 54(11): 851-867
Published Online: October 9, 2012
DOI: 10.1111/j.1744-7909.2012.01175.x

Cell walls lock each cell in a specific position within the supra-organization of a plant. Despite its fixed location, each cell must be able to sense alterations in its immediate environment and respond rapidly to ensure the optimal functioning, continued growth and development, and eventual long-term survival of the plant. The ultra-structural detail that underlies our present understanding of the plant cell has largely been acquired from fixed and processed material that does not allow an appreciation of the dynamic nature of sub-cellular events in the cell. In recent years, fluorescent protein-aided imaging of living plant cells has added to our understanding of the dynamic nature of the plant cell. One of the major outcomes of live imaging of plant cells is the growing appreciation that organelle shapes are not fixed, and many organelles extend their surface transiently in rapid response to environmental stimuli. In many cases, the extensions appear as tubules extending from the main organelle. Specific terms such as stromules from plastids, matrixules from mitochondria, and peroxules from peroxisomes have been coined to describe the extensions. Here, we review our present understanding of organelle extensions and discuss how they may play potential roles in maintaining cellular homeostasis in plant cells.

Mathur J, Mammone A, Barton KA (2012) Organelle extensions in plant cells. J. Integr. Plant Biol. 54(11), 851–867.

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Putting the Breaks On: Regulating Organelle Movements in Plant Cells  
Author: Julianna K. Vick and Andreas Nebenführ
Journal of Integrative Plant Biology 2012 54(11): 868-874
Published Online: October 22, 2012
DOI: 10.1111/j.1744-7909.2012.01180.x

A striking characteristic of plant cells is that their organelles can move rapidly through the cell. This movement, commonly referred to as cytoplasmic streaming, has been observed for over 200 years, but we are only now beginning to decipher the mechanisms responsible for it. The identification of the myosin motor proteins responsible for these movements allows us to probe the regulatory events that coordinate organelle displacement with normal cell physiology. This review will highlight several recent developments that have provided new insight into the regulation of organelle movement, both at the cellular level and at the molecular level.

Vick JK, Nebenführ A (2012) Putting the breaks on: Regulating organelle movements in plant cells. J. Integr. Plant Biol. 54(11), 868–874.

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Trans-Golgi Networkí¬An Intersection of Trafficking Cell Wall Components  
Author: Natasha Worden, Eunsook Park and Georgia Drakakaki
Journal of Integrative Plant Biology 2012 54(11): 875-886
Published Online: October 22, 2012
DOI: 10.1111/j.1744-7909.2012.01179.x

The cell wall, a crucial cell compartment, is composed of a network of polysaccharides and proteins, providing structural support and protection from external stimuli. While the cell wall structure and biosynthesis have been extensively studied, very little is known about the transport of polysaccharides and other components into the developing cell wall. This review focuses on endomembrane trafficking pathways involved in cell wall deposition. Cellulose synthase complexes are assembled in the Golgi, and are transported in vesicles to the plasma membrane. Non-cellulosic polysaccharides are synthesized in the Golgi apparatus, whereas cellulose is produced by enzyme complexes at the plasma membrane. Polysaccharides and enzymes that are involved in cell wall modification and assembly are transported by distinct vesicle types to their destinations; however, the precise mechanisms involved in selection, sorting and delivery remain to be identified. The endomembrane system orchestrates the delivery of Golgi-derived and possibly endocytic vesicles carrying cell wall and cell membrane components to the newly-formed cell plate. However, the nature of these vesicles, their membrane compositions, and the timing of their delivery are largely unknown. Emerging technologies such as chemical genomics and proteomics are promising avenues to gain insight into the trafficking of cell wall components.

Worden N, Park E, Drakakaki G (2012) Trans-golgi network–an intersection of trafficking cell wall components. J. Integr. Plant Biol. 54(11), 875–886.

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Mitochondrial Composition, Function and Stress Response in Plants  
Author: Richard P. Jacoby, Lei Li, Shaobai Huang, Chun Pong Lee, A. Harvey Millar and Nicolas L. Taylor
Journal of Integrative Plant Biology 2012 54(11): 887-906
Published Online: October 9, 2012
DOI: 10.1111/j.1744-7909.2012.01177.x

The primary function of mitochondria is respiration, where catabolism of substrates is coupled to ATP synthesis via oxidative phosphorylation. In plants, mitochondrial composition is relatively complex and flexible and has specific pathways to support photosynthetic processes in illuminated leaves. This review begins with outlining current models of mitochondrial composition in plant cells, with an emphasis upon the assembly of the complexes of the classical electron transport chain (ETC). Next, we focus upon the comparative analysis of mitochondrial function from different tissue types. A prominent theme in the plant mitochondrial literature involves linking mitochondrial composition to environmental stress responses, and this review then gives a detailed outline of how oxidative stress impacts upon the plant mitochondrial proteome with particular attention to the role of transition metals. This is followed by an analysis of the signaling capacity of mitochondrial reactive oxygen species, which studies the transcriptional changes of stress responsive genes as a framework to define specific signals emanating from the mitochondrion. Finally, specific mitochondrial roles during exposure to harsh environments are outlined, with attention paid to mitochondrial delivery of energy and intermediates, mitochondrial support for photosynthesis, and mitochondrial processes operating within root cells that mediate tolerance to anoxia and unfavorable soil chemistries.

Jacoby RP, Li L, Huang S, Lee CP, Millar AH, Taylor NL (2012) Mitochondrial composition, function and stress response in plants. J. Integr. Plant Biol. 54(11), 887–906.

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What to Eat: Evidence for Selective Autophagy in Plants  
Author: Brice E. Floyd, Stephanie C. Morriss, Gustavo C. MacIntosh and Diane C. Bassham
Journal of Integrative Plant Biology 2012 54(11): 907-920
Published Online: October 9, 2012
DOI: 10.1111/j.1744-7909.2012.01178.x

Autophagy is a macromolecular degradation pathway by which cells recycle their contents as a developmental process, housekeeping mechanism, and response to environmental stress. In plants, autophagy involves the sequestration of cargo to be degraded, transport to the cell vacuole in a double-membrane bound autophagosome, and subsequent degradation by lytic enzymes. Autophagy has generally been considered to be a non-selective mechanism of degradation. However, studies in yeast and animals have found numerous examples of selective autophagy, with cargo including proteins, protein aggregates, and organelles. Recent work has also provided evidence for several types of selective autophagy in plants. The degradation of protein aggregates was the first selective autophagy described in plants, and, more recently, a hybrid protein of the mammalian selective autophagy adaptors p62 and NBR1, which interacts with the autophagy machinery and may function in autophagy of protein aggregates, was described in plants. Other intracellular components have been suggested to be selectively targeted by autophagy in plants, but the current evidence is limited. Here, we discuss recent findings regarding the selective targeting of cell components by autophagy in plants.

Floyd BE, Morriss SC, MacIntosh GC, Bassham DC (2012) What to eat: Evidence for selective autophagy in plants. J. Integr. Plant Biol. 54(11), 907–920.

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          Research Articles
Differential Roles of Arabidopsis Dynamin-Related Proteins DRP3A, DRP3B, and DRP5B in Organelle Division  
Author: Kyaw Aung and Jianping Hu
Journal of Integrative Plant Biology 2012 54(11): 921-931
Published Online: October 9, 2012
DOI: 10.1111/j.1744-7909.2012.01174.x

Dynamin-related proteins (DRPs) are key components of the organelle division machineries, functioning as molecular scissors during the fission process. In Arabidopsis, DRP3A and DRP3B are shared by peroxisomal and mitochondrial division, whereas the structurally-distinct DRP5B (ARC5) protein is involved in the division of chloroplasts and peroxisomes. Here, we further investigated the roles of DRP3A, DRP3B, and DRP5B in organelle division and plant development. Despite DRP5B's lack of stable association with mitochondria, drp5B mutants show defects in mitochondrial division. The drp3A-2 drp3B-2 drp5B-2 triple mutant exhibits enhanced mitochondrial division phenotypes over drp3A-2 drp3B-2, but its peroxisomal morphology and plant growth phenotypes resemble those of the double mutant. We further demonstrated that DRP3A and DRP3B form a supercomplex in vivo, in which DRP3A is the major component, yet DRP5B is not a constituent of this complex. We thus conclude that DRP5B participates in the division of three types of organelles in Arabidopsis, acting independently of the DRP3 complex. Our findings will help elucidate the precise composition of the DRP3 complex at organelle division sites, and will be instrumental to studies aimed at understanding how the same protein mediates the morphogenesis of distinct organelles that are linked by metabolism.

Aung K, Hu J (2012) Differential roles of Arabidopsis dynamin-related proteins DRP3A, DRP3B, and DRP5B in organelle division. J. Integr. Plant Biol. 54(11), 921–931.

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