Mobile Macromolecules in Plant Development

Plant cells transmit developmental signals and distribute nutrients via dynamic intercellular channels termed plasmodesmata (PD). Multidisciplinary inquiries have provided evidence that plasmodesmatal regulation is critical to fundamental plant functions, such as development, host–pathogen interactions, and systemic RNA silencing. This review focuses on macromolecules that transport cell-to-cell through PD and describes their implications on plant development.     

Gene silencing in Arabidopsis spreads from the root to the shoot, through a gating barrier, by template-dependent, nonvascular, cell-to-cell movement

Upward long-distance mobile silencing has been shown to be phloem mediated in several different solanaceous species. We show that the Arabidopsis (Arabidopsis thaliana) seedling grafting system and a counterpart inducible system generate upwardly spreading long-distance silencing that travels not in the phloem but by template-dependent reiterated short-distance cell-to-cell spread through the cells of the central stele. Examining the movement of the silencing front revealed a largely unrecognized zone of tissue, below the apical meristem, that is resistant to the silencing signal and that may provide a gating or protective barrier against small RNA signals. Using a range of auxin and actin transport inhibitors revealed that, in this zone, alteration of vesicular transport together with cytoskeleton dynamics prevented or retarded the spread of the silencing signal. This suggests that small RNAs are transported from cell to cell via plasmodesmata rather than diffusing from their source in the phloem.     

A systemic small RNA signaling system in plants

Systemic translocation of RNA exerts non-cell-autonomous control over plant development and defense. Long-distance delivery of mRNA has been proven, but transport of small interfering RNA and microRNA remains to be demonstrated. Analyses performed on phloem sap collected from a range of plants identified populations of small RNA species. The dynamic nature of this population was reflected in its response to growth conditions and viral infection. The authenticity of these phloem small RNA molecules was confirmed by bioinformatic analysis; potential targets for a set of phloem small RNA species were identified. Heterografting studies, using spontaneously silencing coat protein (CP) plant lines, also established that transgene-derived siRNA move in the long-distance phloem and initiate CP gene silencing in the scion. Biochemical analysis of pumpkin (Cucurbita maxima) phloem sap led to the characterization of C. maxima Phloem SMALL RNA BINDING PROTEIN1 (CmPSRP1), a unique component of the protein machinery probably involved in small RNA trafficking. Equivalently sized small RNA binding proteins were detected in phloem sap from cucumber (Cucumis sativus) and lupin (Lupinus albus). PSRP1 binds selectively to 25-nucleotide single-stranded RNA species. Microinjection studies provided direct evidence that PSRP1 could mediate the cell-to-cell trafficking of 25-nucleotide single-stranded, but not double-stranded, RNA molecules. The potential role played by PSRP1 in long-distance transmission of silencing signals is discussed with respect to the pathways and mechanisms used by plants to exert systemic control over developmental and physiological processes.     

植物细胞与细胞间物质转运及其对生长发育的调控

细胞与细胞之间的物质运输和信号传递对于多细胞生物的生长发育非常重要．一些内源的大分子物质如蛋白质、核酸或核酸蛋白质复合体可以选择性地通过植物特有的亚细胞结构即胞间连丝（PD）在细胞之间运输．小分子物质主要以被动的形式在细胞间通过PD进行扩散．PD对蛋白质和核酸的运输具有选择性,这种运输受到严格调控．大分子物质在细胞间的运输对植物的生长和发育有极其重要的调控作用．KN1,STM,SHR,TRY和WER等转录因子在细胞之间的转运对于维持植物的茎尖分生组织、根尖分生组织和表皮细胞功能起重要作用．另外,某些小分子RNA也能够在植物细胞间进行选择性运输,并通过在不同细胞中降解或抑制靶mRNA的翻译来调节植物组织的生长发育．     

Plant Mobile Small RNAs

In plants, RNA silencing is a fundamental regulator of gene expression, heterochromatin formation, suppression of transposable elements, and defense against viruses. The sequence specificity of these processes relies on small noncoding RNA (sRNA) molecules. Although the spreading of RNA silencing across the plant has been recognized for nearly two decades, only recently have sRNAs been formally demonstrated as the mobile silencing signals. Here, we discuss the various types of mobile sRNA molecules, their short- and long-range movement, and their function in recipient cells.RNA silencing is a regulatory mechanism that controls the expression of endogenous genes and exogenous molecular parasites such as viruses, transgenes, and transposable elements. One of the most fascinating aspects of RNA silencing found in plants and invertebrates is its mobile nature—in other words, its ability to spread from the cell where it has been initiated to neighboring cells. This phenomenon relies on the movement of small noncoding RNA molecules (sRNA, 21–24 nucleotides [nt] in length) that provide the sequence specificity of the silencing effects. In plants, there are two major classes of sRNAs: short interfering RNAs (siRNAs) and micro RNAs (miRNAs). These sRNAs are generated by diverse and sometimes interacting biochemical pathways, which may influence their mobility. Movement of plant sRNAs falls into two main categories: cell-to-cell (short-range) and systemic (long-range) movement (Melnyk et al. 2011).     

Cell-to-cell transport of macromolecules during early plant development

Plant cells exchange developmental signals, distribute nutrients and ribonucleoprotein complexes through dynamic intercellular channels termed plasmodesmata (PD). Multidisciplinary investigations over the last decade have provided evidence that plasmodesmatal regulation is critical to various basic plant functions, such as development, host-pathogen interactions, and systemic RNA-silencing. This review highlights the cell-to-cell transport of micro- and macromolecules via PD during embryo and seedling growth.     

Structural insights into the arms race between host and virus along RNA silencing pathways in Arabidopsis thaliana

RNA silencing refers to a conserved sequence‐specific gene‐regulation mechanism mediated by small RNA molecules. In plants, microRNA (miRNA) and small interfering RNA (siRNA) represent two major types of small RNA molecules which play pivotal roles in plant developmental control and antiviral defences. To escape these plant defences, plant viruses have encoded a vast array of viral suppressors of RNA silencing (VSRs) to attack the host antiviral silencing pathway by interfering with small RNA processing, RNA‐induced silencing complex (RISC) assembly, viral mRNA cleavage etc. Transgenic plants expressing distinct VSRs often show developmental aberrations that resemble the phenotype of miRNA‐deficient mutants, implying a potential intrinsic link between VSRs and the miRNA pathway (at least in Arabidopsis thaliana) even though their pathogenic mechanisms remain largely unknown. In this review, we summarise our current structural understandings of the arms race between the host and virus along the RNA silencing pathway in A. thaliana by focusing on several important ribonucleoprotein (RNP) structures involved in RNA silencing and unique structural features adopted by VSRs.     

Tobacco mosaic virus movement protein enhances the spread of RNA silencing

Eukaryotic cells restrain the activity of foreign genetic elements, including viruses, through RNA silencing. Although viruses encode suppressors of silencing to support their propagation, viruses may also exploit silencing to regulate host gene expression or to control the level of their accumulation and thus to reduce damage to the host. RNA silencing in plants propagates from cell to cell and systemically via a sequence-specific signal. Since the signal spreads between cells through plasmodesmata like the viruses themselves, virus-encoded plasmodesmata-manipulating movement proteins (MP) may have a central role in compatible virus:host interactions by suppressing or enhancing the spread of the signal. Here, we have addressed the propagation of GFP silencing in the presence and absence of MP and MP mutants. We show that the protein enhances the spread of silencing. Small RNA analysis indicates that MP does not enhance the silencing pathway but rather enhances the transport of the signal through plasmodesmata. The ability to enhance the spread of silencing is maintained by certain MP mutants that can move between cells but which have defects in subcellular localization and do not support the spread of viral RNA. Using MP expressing and non-expressing virus mutants with a disabled silencing suppressing function, we provide evidence indicating that viral MP contributes to anti-viral silencing during infection. Our results suggest a role of MP in controlling virus propagation in the infected host by supporting the spread of silencing signal. This activity of MP involves only a subset of its properties implicated in the spread of viral RNA.     

植物转录因子的胞间运动

植物体的组织和器官由多细胞组成,细胞之间的通信对植物体的生长发育必不可少。转录因子作为一类特殊的蛋白质分子不仅在转录水平上参与植物生长发育的调控,而且新近研究发现,转录因子的胞间运动是细胞之间通信方式之一,具有重要的功能。对转录因子胞间运动的发现过程、转录因子胞间运动的机制及其通道进行了论述。转录因子的胞间运动有基于扩散作用的非目标性转运和具有目标性的主动转运两种模式。转录因子胞间运动具有明显的组织特异性和方向性。分析了影响转录因子胞间运动的因素,讨论了转录因子胞间运动的功能以及转录因子胞间运动所参与的植物生长发育及形态建成的调控。     

Redox regulation of intercellular transport

Plant cells communicate with each other via plasmodesmata (PDs) in order to orchestrate specific responses to environmental and developmental cues. At the same time, environmental signals regulate this communication by promoting changes in PD structure that modify symplastic permeability and, in extreme cases, isolate damaged cells. Reactive oxygen species (ROS) are key messengers in plant responses to a range of biotic and abiotic stresses. They are also generated during normal metabolism, and mediate signaling pathways that modulate plant growth and developmental transitions. Recent research has suggested the participation of ROS in the regulation of PD transport. The study of several developmental and stress-induced processes revealed a co-regulation of ROS and callose (a cell wall polymer that regulates molecular flux through PDs). The identification of Arabidopsis mutants simultaneously affected in cell redox homeostasis and PD transport, and the histological detection of hydrogen peroxide and peroxidases in the PDs of the tomato vascular cambium provide new information in support of this novel regulatory mechanism. Here, we describe the evidence that supports a role for ROS in the regulation of callose deposition and/or in the formation of secondary PD, and discuss the potential importance of this mechanism during plant growth or defense against environmental stresses.     

The perplexing role of autophagy in plant innate immune responses

Autophagy is a major intracellular process for the degradation of cytosolic macromolecules and organelles in the lysosomes or vacuoles for the purposes of regulating cellular homeostasis and protein and organelle quality control. In complex metazoan organisms, autophagy is highly engaged during the immune responses through interfaces either directly with intracellular pathogens or indirectly with immune signalling molecules. Studies over the last decade or so have also revealed a number of important ways in which autophagy shapes plant innate immune responses. First, autophagy promotes defence‐associated hypersensitive cell death induced by avirulent or related pathogens, but restricts unnecessary or disease‐associated spread of cell death. This elaborate regulation of plant host cell death by autophagy is critical during plant immune responses to the types of plant pathogens that induce cell death, which include avirulent biotrophic pathogens and necrotrophic pathogens. Second, autophagy modulates defence responses regulated by salicylic acid and jasmonic acid, thereby influencing plant basal resistance to both biotrophic and necrotrophic pathogens. Third, there is an emerging role of autophagy in virus‐induced RNA silencing, either as an antiviral collaborator for targeted degradation of viral RNA silencing suppressors or an accomplice of viral RNA silencing suppressors for targeted degradation of key components of plant cellular RNA silencing machinery. In this review, we summarize this important progress and discuss the potential significance of the perplexing role of autophagy in plant innate immunity.     

Quantitative imaging of directional transport through plasmodesmata in moss protonemata via single-cell photoconversion of Dendra2

Cell-to-cell transport of molecules in plants must be properly regulated for plant growth and development. One specialized mechanism that plants have evolved involves transport through plasmodesmata (PD), but when and how transport of molecules via PD is regulated among individual cells remains largely unknown, particularly at the single-cell level. Here, we developed a tool for quantitatively analyzing cell-to-cell transport via PD at a single-cell level using protonemata of Physcomitrella patens and a photoconvertible fluorescent protein, Dendra2. In the filamentous protonemal tissues, one-dimensional intercellular communication can be observed easily. Using this system, we found that Dendra2 was directionally transported toward the apex of the growing protonemata. However, this directional transport could be eliminated by incubation in the dark or treatment with a metabolic inhibitor. Thus, we propose that directional transport of macromolecules can occur via PD in moss protonemata, and may be affected by the photosynthetic and metabolic activity of cells.