Plasmodesmata as a supracellular control network in plants

The evolution of intercellular communication had an important role in the increasing complexity of both multicellular and supracellular organisms. Plasmodesmata, the intercellular organelles of the plant kingdom, establish an effective pathway for local and long-distance signalling. In higher plants, this pathway involves the trafficking of proteins and various forms of RNA that function non-cell-autonomously to affect developmental programmes.     

Phosphorylation of viral movement proteins--regulation of cell-to-cell trafficking

In plants, proteins and nucleoprotein complexes can traffic from cell to cell, via plasmodesmata. Studies based on viral movement proteins (MP) have revealed that such trafficking events are likely to be regulated at the level of protein phosphorylation. Plasmodesmal-associated protein kinases could play a central role in plant defense, in addition to regulating the translatability of endogenous MP-mRNA complexes that function at a supracellular level.     

Plasmodesmata: the battleground against intruders

Plasmodesmata are intercellular channels that establish a symplastic communication pathway between neighboring cells in plants. Owing to this role, opportunistic microbial pathogens have evolved to exploit plasmodesmata as gateways to spread infection from cell to cell within the plant. However, although these pathogens have acquired the capacity to breach the plasmodesmal trafficking pathway, plants are unlikely to relinquish control over a structure essential for their survival so easily. In this review, we examine evidence that suggests plasmodesmata play an active role in plant immunity against viral, fungal and bacterial pathogens. We discuss how these pathogens differ in their lifestyles and infection modes, and present the defense strategies that plants have adopted to prevent the intercellular spread of an infection.     

Regulation of short-distance transport of RNA and protein

The intercellular trafficking of proteins and RNAs has emerged as a novel mechanism of cell-cell communication in plant development. Plasmodesmata (PD), intercellular cytoplasmic channels, have a central role in cell-cell trafficking of regulatory proteins and RNAs. Recent studies have demonstrated that plants use either a selective or a non-selective PD trafficking pathway for regulatory proteins. Moreover, plants have developed strategies to regulate both selective and non-selective movement. Recent work has focused especially on integrating the recent understanding of the function and mechanisms of intercellular macromolecule movement through PD.     

The march of the PINs: developmental plasticity by dynamic polar targeting in plant cells

Development of plants and their adaptive capacity towards ever‐changing environmental conditions largely depend on the spatial distribution of the plant hormone auxin. At the cellular level, various internal and external signals are translated into specific changes in the polar, subcellular localization of auxin transporters from the PIN family thereby directing and redirecting the intercellular fluxes of auxin. The current model of polar targeting of PIN proteins towards different plasma membrane domains encompasses apolar secretion of newly synthesized PINs followed by endocytosis and recycling back to the plasma membrane in a polarized manner. In this review, we follow the subcellular march of the PINs and highlight the cellular and molecular mechanisms behind polar foraging and subcellular trafficking pathways. Also, the entry points for different signals and regulations including by auxin itself will be discussed within the context of morphological and developmental consequences of polar targeting and subcellular trafficking.     

Transport of macromolecules through plasmodesmata and the phloem

Cell-to-cell communication is a pivotal process in the determination of cell fate during development and physiological adaptation in response to environmental stimuli. The intercellular trafficking of proteins and RNAs has emerged as a novel mechanism of cell-to-cell signaling in plants. As a strategy for efficient intercellular communication, plants have evolved plant-specific symplasmic communication networks via plasmodesmata (PD) and the phloem. PD are symplasmic channels connecting the cytoplasm of neighboring cells and are responsible for the local exchange of metabolites and signaling molecules. The phloem is the sieve-tube system that allows rapid, long-distance translocation of molecules. Together, PD and phloem conduits have been shown to allow the transport of proteins and RNAs in non-selective or/and selective modes. This review describes the current understanding of macromolecule trafficking through PD and the phloem.     

Is There a COPII‐Mediated Membrane Traffic in Chloroplasts?

COPII proteins facilitate membrane transport from the endoplasmic reticulum (ER) to the Golgi. They are highly conserved, although there are variations in their subcellular localization across plant, animal and yeast cells. Such variations may be needed to suit the unique organization of the ER and Golgi in the different cell systems. Earlier bioinformatics analyses have indicated that the Arabidopsis nuclear genome may encode chloroplast isoforms of the cytosolic trafficking protein machineries, including COPI and COPII, for vesicular transport within chloroplasts. These analyses suggest the intriguing possibility that plants may have evolved or adapted COP-like proteins to suit membrane trafficking events within specialized organelles. Here, we discuss recent data on the distribution and activity of the product of the At5g18570 locus, which encodes a putative chloroplast isoform of Sar1, the GTPase that regulates COPII assembly on the surface of the ER. Evidence is accumulating that the protein is targeted to the chloroplasts, that it has GTPase activity and that it may have a role in thylakoid membrane development, supporting the possibility that COPII-like trafficking machinery may be active in chloroplasts.     

Developmental regulation and significance of KNOX protein trafficking in Arabidopsis

Intercellular communication delivers critical information for position-dependent specification of cell fate. In plants, a novel mechanism for cell-to-cell communication involves the intercellular trafficking of regulatory proteins and mRNAs. The maize KNOTTED1 (KN1) gene acts non cell-autonomously in the maize leaf, and KN1 was the first plant protein shown to traffic cell-to-cell, presumably through plasmodesmata. We have compared the intercellular trafficking of green fluorescent protein (GFP) fusions of KN1 and Arabidopsis KN1-related homeobox proteins to that of the viral movement protein from turnip vein clearing tobamovirus. We show that there is specific developmental regulation of GFP approximately KN1 trafficking. GFP -- KN1 was able to traffic from the inner layers of the leaf to the epidermis, but not in the opposite direction, from epidermis to mesophyll. However, GFP or the GFP -- movement protein fusion moved readily out of the epidermis. GFP -- KN1 was however able to traffic out of the epidermal (L1) layer in the shoot apical meristem, indicating that KN1 movement out of the L1 was developmentally regulated. GFP -- KNAT1/BREVIPEDICELLUS and GFP -- SHOOTMERISTEMLESS fusions could also traffic from the L1 to the L2/L3 layers of the meristem. In a test for the functional significance of trafficking, we showed that L1-specific expression of KN1 or of KNAT1 was able to partially complement the strong shootmeristemless-11 (stm-11) mutant. However, a cell-autonomous GUS fusion to KN1 showed neither trafficking ability nor complementation of stm-11 when expressed in the L1. These results suggest that the activity of KN1 and related homeobox proteins is maintained following intercellular trafficking, and that trafficking may be required for their normal developmental function.     

胞间连丝研究的进展

胞间连丝为多细胞植物有机体提供了一个直接的细胞间物质运输和信息传递的细胞质通道,把一个个独立的“细胞王国”转变成相互连接的共质体,它是当今细胞生物学中十分活跃的研究领域。日益增多的研究结果揭示,胞间连丝协调基因表达和许多的细胞生理生化过程,对细胞的分裂与分化、形态发生、植物体的生长与发育,以及植物对环境的反应与适应等诸方面都起着十分重要的作用。本文仅就胞间连丝结构的多样性;胞间通道的调节因子;大分子蛋白质和核酸的胞间运输;胞间连丝阻断和共质体分区的形成及其与形态发生、休眠和抗逆性的关系等几个方面的新进展做一个简要的综述,借此例证胞间连丝在植物生命活动中的重要意义。     

胞间连丝研究的进展

胞间连丝为多细胞植物有机体提供了一个直接的细胞间物质运输和信息传递的细胞质通道,把一个个独立的“细胞王国”转变成相互连接的共质体,它是当今细胞生物学中十分活跃的研究领域。日益增多的研究结果揭示,胞间连丝协调基因表达和许多的细胞生理生化过程,对细胞的分裂与分化、形态发生、植物体的生长与发育,以及植物对环境的反应与适应等诸方面都起着十分重要的作用。本文仅就胞间连丝结构的多样性;胞间通道的调节因子;大分子蛋白质和核酸的胞间运输;胞间连丝阻断和共质体分区的形成及其与形态发生、休眠和抗逆性的关系等几个方面的新进展做一个简要的综述,借此例证胞间连丝在植物生命活动中的重要意义。     

Plasmodesmata form and function

Intercellular transport via plasmodesmata controls cell fate decisions in plants, and is of fundamental importance in viral movement, disease resistance, and the spread of RNAi signals. Although plasmodesmata appear to be unique to plant cells, they may have structural and functional similarities to the newly discovered tunneling nanotubes that connect animal cells. Recently, proteins that localize to plasmodesmata have been identified, and a microtubule-associated protein was found to negatively regulate the trafficking of viral movement proteins. Other advances have delivered new insights into the function and molecular nature of plasmodesmata and have shown that protein trafficking through plasmodesmata is developmentally regulated, opening up the possibility that the genetic control of plasmodesmal function will soon be understood.     

Microtubule motors at the intersection of trafficking and transport

Molecular motors drive the transport of vesicles and organelles within the cell. Traditionally, these transport processes have been considered separately from membrane trafficking events, such as regulated budding and fusion. However, recent progress has revealed mechanistic links that integrate these processes within the cell. Rab proteins, which function as key regulators of intracellular trafficking, have now been shown to recruit specific motors to organelle membranes. Rab-independent recruitment of motors by adaptor or scaffolding proteins is also a key mechanism. Once recruited to vesicles and organelles, these motors can then drive directed transport; this directed transport could in turn affect the efficiency of trafficking events. Here, we discuss this coordinated regulation of trafficking and transport, which provides a powerful mechanism for temporal and spatial control of cellular dynamics.     

泛素介导的植物膜蛋白转运及其研究方法

泛素(Ub)是一类小分子多肽, 可通过赖氨酸残基与靶蛋白结合, 进而决定靶蛋白的去向。泛素分子对靶蛋白进行特异性修饰的过程称为泛素化。相较于动物和酵母细胞, 植物细胞中泛素介导的蛋白动态循环, 尤其是膜蛋白胞吞动态循环研究相对滞后。随着生物化学以及显微技术的发展, 人们对泛素介导的植物细胞膜蛋白转运有了新的认识。该文阐述了泛素及类泛素在蛋白转运中的作用, 总结了泛素化(ubiquitylation)调控膜蛋白转运的分子生物学机制和常用的研究方法, 并对今后该领域的研究进行了展望。     

Small GTP-binding Proteins and their Functions in Plants

Small GTP-binding proteins exist in eukaryotes from yeast to animals to plants and constitute a superfamily whose members function as molecular switches that cycle between “active” and “inactive” states. They regulate a wide variety of cell functions such as signal transduction, cell proliferation, cytoskeletal organization, intracellular membrane trafficking, and gene expression. In yeast and animals, this superfamily is structurally classified into at least five families: the Ras, Rho, Rab, Arf/Sar1, and Ran families. However, plants contain Rab, Rho, Arf, and Ran homologs, but no Ras. Small GTP-binding proteins have become an intensively studied group of regulators not only in yeast and animals but also in plants in recent years. In this article we briefly review the class and structure of small GTP-binding proteins. Their working modes and functions in animals and yeast are listed, and the functions of individual members of these families in plants are discussed, with the emphasis on the recently revealed plant-specific roles of these proteins, including their cross-talk with plant hormones and other signals, regulation of organogenesis (leaf, root, and embryo), polar growth, cell division, and involvement in various stress and defense responses.     

Superfamily of plant monomeric GTP-binding proteins: 2. Rab proteins are the regulators of vesicles trafficking and plant responses to stresses

In plants, Rab proteins represent the largest family of monomeric GTP-binding proteins (mG-proteins). As distinct from animal cells comprising 40 subfamilies of Rab proteins, which are the key regulators of intracellular vesicular transport, numerous Rab proteins in Arabidopsis and other plant species could be grouped in only eight subfamilies on the basis of their functional properties. The available data concerning the involvement of these mG-proteins in the control of vesicle trafficking agree generally with the paradigms accepted for other eukaryotes. On the other hand, these proteins play an important role in plant responses to abiotic and biotic factors, indicating specific for plants functions of Rab proteins.     

胞间连丝与大分子物质的胞间转移

胞间连丝是细胞间细胞器,是细胞间通讯的直接途径。一般认为,胞间连丝允许通过物质的分子量上限(SEL)是800～1000 Da．近年来研究的许多证据表明,胞间连丝的SEL随组织种类及其生理状况而异。在某些情况下,它可以允许大分子物质通过,如病毒运动蛋白与胞间连丝相互作用,使病毒通过胞间连丝转移。玉米突变体 kn1基因异常表达的KN1可使包括表皮在内的各层组织结瘤,KN1是细胞间移动的信息物,P-蛋白可由伴胞通过胞间连丝转移到筛管。某些组织中胞间连丝很高的SEL和发育过程胞间连丝SEL的变化可能在植物发育调控中有重要作用。本文对大分子通过胞间连丝转移的机理进行了讨论。