Special Issue: Cell Polarity and Development   

September 2013, Volume 55 Issue 9, Pages 785每887.

Cover Caption: Cell Polarity and Development
About the cover: This special issue of JIPB highlights several aspects of plant cell polarity. Cell polarity is important for PIN FORMED (PIN) efflux carrier-driven polar transport of the plant hormone auxin. Coordinated tissue-specific PIN orientations result in auxin minimum formation during Arabidopsis fruit development (Right top; Grieneisen et al., pp. 847每863). In plant leaves, polar localization of PIN1 in files of vascular cells suggests auxin transport toward pre-existing vasculature (Left and right middle; Sawchuk and Scarpella, pp.824每834). Coordinated polar cell growth in the root epidermis gives rise to files of smaller and bigger cells (Right bottom, Löfke et al., pp. 864每875).


Cell polarity and development  
Author: Remko Offringa and Jürgen Kleine-Vehn
Journal of Integrative Plant Biology 2013 55(9): 786每788
Published Online: September 10, 2013
DOI: 10.1111/jipb.12099
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          Invited Expert Reviews
Phosphorylation-dependent Trafficking of Plasma Membrane Proteins in Animal and Plant Cells  
Author: Remko Offringa and Fang Huang
Journal of Integrative Plant Biology 2013 55(9): 789每808
Published Online: September 10, 2013
DOI: 10.1111/jipb.12096

In both unicellular and multicellular organisms, transmembrane (TM) proteins are sorted to and retained at specific membrane domains by endomembrane trafficking mechanisms that recognize sorting signals in the these proteins. The trafficking and distribution of plasma membrane (PM)-localized TM proteins (PM proteins), especially of those PM proteins that show an asymmetric distribution over the PM, has received much attention, as their proper PM localization is crucial for elementary signaling and transport processes, and defects in their localization often lead to severe disease symptoms or developmental defects. The subcellular localization of PM proteins is dynamically regulated by post-translational modifications, such as phosphorylation and ubiquitination. These modificaitons mostly occur on sorting signals that are located in the larger cytosolic domains of the cargo proteins. Here we review the effects of phosphorylation of PM proteins on their trafficking, and present the key examples from the animal field that have been subject to studies for already several decades, such as that of aquaporin 2 and the epidermal growth factor receptor. Our knowledge on cargo trafficking in plants is largely based on studies of the family of PIN FORMED (PIN) carriers that mediate the efflux of the plant hormone auxin. We will review what is known on the subcellular distribution and trafficking of PIN proteins, with a focus on how this is modulated by phosphorylation, and identify and discuss analogies and differences in trafficking with the well-studied animal examples.

Offringa R, Huang F (2013) Phosphorylation坼dependent trafficking of plasma membrane proteins in animal and plant cells. J. Integr. Plant Biol. 55(9), 789–808.

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Plasma Membrane Protein Ubiquitylation and Degradation as Determinants of Positional Growth in Plants  
Author: Barbara Korbei and Christian Luschnig
Journal of Integrative Plant Biology 2013 55(9): 809每823
Published Online: September 10, 2013
DOI: 10.1111/jipb.12059

Being sessile organisms, plants evolved an unparalleled plasticity in their post-embryonic development, allowing them to adapt and fine-tune their vital parameters to an ever-changing environment. Crosstalk between plants and their environment requires tight regulation of information exchange at the plasma membrane (PM). Plasma membrane proteins mediate such communication, by sensing variations in nutrient availability, external cues as well as by controlled solute transport across the membrane border. Localization and steady-state levels are essential for PM protein function and ongoing research identified cis- and trans-acting determinants, involved in control of plant PM protein localization and turnover. In this overview, we summarize recent progress in our understanding of plant PM protein sorting and degradation via ubiquitylation, a post-translational and reversible modification of proteins. We highlight characterized components of the machinery involved in sorting of ubiquitylated PM proteins and discuss consequences of protein ubiquitylation on fate of selected PM proteins. Specifically, we focus on the role of ubiquitylation and PM protein degradation in the regulation of polar auxin transport (PAT). We combine this regulatory circuit with further aspects of PM protein sorting control, to address the interplay of events that might control PAT and polarized growth in higher plants.

Korbei B, Luschnig C (2013) Plasma membrane protein ubiquitylation and degradation as determinants of positional growth in plants. J. Integr. Plant Biol. 55(9), 809–823.

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Polarity, Continuity, and Alignment in Plant Vascular Strands  
Author: Megan G. Sawchuk and Enrico Scarpella
Journal of Integrative Plant Biology 2013 55(9): 824每834
Published Online: September 10, 2013
DOI: 10.1111/jipb.12086

Plant vascular cells are joined end to end along uninterrupted lines to connect shoot organs with roots; vascular strands are thus polar, continuous, and internally aligned. What controls the formation of vascular strands with these properties? The “auxin canalization hypothesis”—based on positive feedback between auxin flow through a cell and the cell's capacity for auxin transport—predicts the selection of continuous files of cells that transport auxin polarly, thus accounting for the polarity and continuity of vascular strands. By contrast, polar, continuous auxin transport—though required—is insufficient to promote internal alignment of vascular strands, implicating additional factors. The auxin canalization hypothesis was derived from the response of mature tissue to auxin application but is consistent with molecular and cellular events in embryo axis formation and shoot organ development. Objections to the hypothesis have been raised based on vascular organizations in callus tissue and shoot organs but seem unsupported by available evidence. Other objections call instead for further research; yet the inductive and orienting influence of auxin on continuous vascular differentiation remains unique.

Sawchuk MG, Scarpella E (2013) Polarity, continuity, and alignment in plant vascular strands. J. Integr. Plant Biol. 55(9), 824–834.

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Targeting and Regulation of Cell Wall Synthesis During Tip Growth in Plants  
Author: Fangwei Gu and Erik Nielsen
Journal of Integrative Plant Biology 2013 55(9): 835每846
Published Online: September 10, 2013
DOI: 10.1111/jipb.12077

Root hairs and pollen tubes are formed through tip growth, a process requiring synthesis of new cell wall material and the precise targeting and integration of these components to a selected apical plasma membrane domain in the growing tips of these cells. Presence of a tip-focused calcium gradient, control of actin cytoskeleton dynamics, and formation and targeting of secretory vesicles are essential to tip growth. Similar to cells undergoing diffuse growth, cellulose, hemicelluloses, and pectins are also deposited in the growing apices of tip-growing cells. However, differences in the manner in which these cell wall components are targeted and inserted in the expanding portion of tip-growing cells is reflected by the identification of elements of the plant cell wall synthesis machinery which have been shown to play unique roles in tip-growing cells. In this review, we summarize our current understanding of the tip growth process, with a particular focus on the subcellular targeting of newly synthesized cell wall components, and their roles in this form of plant cell expansion.

Gu F, Nielsen E (2013) Targeting and regulation of cell wall synthesis during tip growth in plants. J. Integr. Plant Biol. 55(9), 835–846.

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Juicy Stories on Female Reproductive Tissue Development: Coordinating the Hormone Flows  
Author: Verônica A. Grieneisen, Athanasius F.M. Marée and Lars Østergaard
Journal of Integrative Plant Biology 2013 55(9): 847每863
Published Online: September 10, 2013
DOI: 10.1111/jipb.12092

In the past 2–3 decades, developmental biologists have made tremendous progress in identifying genes required for the specification of individual cell types of an organ and in describing how they interact in genetic networks. In comparison, very little is known about the mechanisms that regulate tissue polarity and overall organ patterning. Gynoecia and fruits from members of the Brassicaceae family of flowering plants provide excellent model systems to study organ patterning and tissue specification because they become partitioned into distinct domains whose formation is determined by polarity establishment both at a cellular and whole tissue level. Interactions among key regulators of Arabidopsis gynoecium and fruit development have revealed a network of upstream transcription factor activities required for such tissue differentiation. Regulation of the plant hormone auxin is emerging as both an immediate downstream output and input of these activities, and here we aim to provide an overview of the current knowledge regarding the link between auxin and female reproductive development in plants. In this review we will also demonstrate how available data can be exploited in a mathematical modelling approach to reveal and understand the feedback regulatory circuits that underpin the polarity establishment, necessary to guide auxin flows.

Grieneisen VA, Marée AFM, Østergaard L (2013) Juicy stories on female reproductive tissue development: Coordinating the hormone flows. J. Integr. Plant Biol. 55(9), 847–863.

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          Research Articles
Epidermal Patterning Genes Impose Non-cell Autonomous Cell Size Determination and have Additional Roles in Root Meristem Size Control  
Author: Christian Löfke, Kai Dünser and Jürgen Kleine-Vehn
Journal of Integrative Plant Biology 2013 55(9): 864每875
Published Online: September 10, 2013
DOI: 10.1111/jipb.12097

The regulation of cellular growth is of vital importance for embryonic and postembryonic patterning. Growth regulation in the epidermis has importance for organ growth rates in roots and shoots, proposing epidermal cells as an interesting model for cellular growth regulation. Here we assessed whether the root epidermis is a suitable model system to address cell size determination. In Arabidopsis thaliana L., root epidermal cells are regularly spaced in neighbouring tricho- (root hair) and atrichoblast (non-hair) cells, showing already distinct cell size regulation in the root meristem. We determined cell sizes in the root meristem and at the onset of cellular elongation, revealing that not only division rates but also cellular shape is distinct in tricho- and atrichoblasts. Intriguingly, epidermal-patterning mutants, failing to define differential vacuolization in neighbouring epidermal cell files, also display non-differential growth. Using these epidermal-patterning mutants, we show that polarized growth behaviour of epidermal tricho- and atrichoblast is interdependent, suggesting non-cell autonomous signals to integrate tissue expansion. Besides the interweaved cell-type-dependent growth mechanism, we reveal an additional role for epidermal patterning genes in root meristem size and organ growth regulation. We conclude that epidermal cells represent a suitable model system to study cell size determination and interdependent tissue growth.

Löfke C, Dünser K, Kleine坼Vehn J (2013) Epidermal patterning genes impose non坼cell autonomous cell size determination and have additional roles in root meristem size control. J. Integr. Plant Biol. 55(9), 864–875.

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OsRRMh, a Spen-like Gene, Plays an Important Role during the Vegetative to Reproductive Transition in Rice  
Author: Derui Liu and Xiuling Cai
Journal of Integrative Plant Biology 2013 55(9): 876每887
Published Online: August 26, 2013
DOI: 10.1111/jipb.12056

OsRRMh, a homologue of OsRRM, encodes a Spen-like protein, and is composed of two N-terminal RNA recognition motifs (RRM) and one C-terminal Spen paralogue and an orthologue C-terminal domain (SPOC). The gene has been found to be constitutively expressed in the root, stem, leaf, spikelet, and immature seed, and alternative splicing patterns were confirmed in different tissues, which may indicate diverse functions for OsRRMh. The OsRRMh dsRNAi lines exhibited late-flowering and a larger panicle phenotype. When full-length OsRRMh and/or its SPOC domain were overexpressed, the fertility rate and number of spikelets per panicle were both markedly reduced. Also, overexpression of OsRRMh in the Arabidopsis fpa mutant did not restore the normal flowering time, and it delayed flowering in Col plants. Therefore, we propose that OsRRMh may confer one of its functions in the vegetative-to-reproductive transition in rice (Oryza sativa L. subsp. japonica cv. Zhonghua No. 11 (ZH11)).

Liu D, Cai X (2013) OsRRMh, a Spen坼like gene, plays an important role during the vegetative to reproductive transition in rice. J. Integr. Plant Biol. 55(9), 876–887.

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