Regulation of solute flux through plasmodesmata in the root meristem

Roles of ROS and redox in regulating cell-to-cell communication: Spotlight on viral modulation of redox for local spread

Reactive oxygen species (ROS) are important signalling molecules that influence many aspects of plant biology. One way in which ROS influence plant growth and development is by modifying intercellular trafficking through plasmodesmata (PD). Viruses have evolved to use PD for their local cell-to-cell spread between plant cells, so it is therefore not surprising that they have found ways to modulate ROS and redox signalling to optimise PD function for their benefit. This review examines how intracellular signalling via ROS and redox pathways regulate intercellular trafficking via PD during development and stress. The relationship between viruses and ROS-redox systems, and the strategies viruses employ to control PD function by interfering with ROS-redox in plants is also discussed.

A neutral invertase controls cell division besides hydrolysis of sucrose for nutrition during germination and seed setting in rice

Sucrose is the transport form of carbohydrate in plants serving as signal molecule besides nutrition, but the signaling is elusive. Here, neutral invertase 8 (OsNIN8) mutated at G461R into OsNIN8m, which increased its charge and hydrophobicity, decreased hydrolysis of sucrose to 13% and firmer binding to sucrose than the wildtype. This caused downstream metabolites and energy accumulation forming overnutrition. Paradoxically, division of subinitials in longitudinal cell lineages was only about 15 times but more than 100 times in wildtype, resulting in short radicle. Further, mutation of OsNIN8 into deficiency of hydrolysis but maintenance of sucrose binding allowed cell division until ran out of energy showing the association but not hydrolysis gave the signal. Chemically, sucrose binding to OsNIN8 was exothermic but to OsNIN8m was endothermic. Therefore, OsNIN8m lost the signal function owing to change of thermodynamic state. So, OsNIN8 sensed sucrose for cell division besides hydrolyzed sucrose.

Assaying Effector Cell-to-Cell Mobility in Plant Tissues Identifies Hypermobility and Indirect Manipulation of Plasmodesmata

In plants, plasmodesmata establish cytoplasmic continuity between cells to allow for communication and resource exchange across the cell wall. While plant pathogens use plasmodesmata as a pathway for both molecular and physical invasion, the benefits of molecular invasion (cell-to-cell movement of pathogen effectors) are poorly understood. To establish a methodology for identification and characterization of the cell-to-cell mobility of effectors, we performed a quantitative live imaging-based screen of candidate effectors of the fungal pathogen Colletotrichum higginsianum. We predicted C. higginsianum effectors by their expression profiles, the presence of a secretion signal, and their predicted and in planta localization when fused to green fluorescent protein. We assayed for cell-to-cell mobility of nucleocytosolic effectors and identified 14 that are cell-to-cell mobile. We identified that three of these effectors are "hypermobile," showing cell-to-cell mobility greater than expected for a protein of that size. To explore the mechanism of hypermobility, we chose two hypermobile effectors and measured their impact on plasmodesmata function and found that even though they show no direct association with plasmodesmata, each increases the transport capacity of plasmodesmata. Thus, our methods for quantitative analysis of cell-to-cell mobility of candidate microbe-derived effectors, or any suite of host proteins, can identify cell-to-cell hypermobility and offer greater understanding of how proteins affect plasmodesmal function and intercellular connectivity. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Parallel auxin transport via PINs and plasmodesmata during the Arabidopsis leaf hyponasty response

KEY MESSAGE

The leaf hyponasty response depends on tip-to-petiole auxin transport. This transport can happen through two parallel pathways: active trans-membrane transport mediated by PIN proteins and passive diffusion through plasmodesmata. A plant's ability to counteract potential shading by neighboring plants depends on transport of the hormone auxin. Neighbor sensing at the leaf tip triggers auxin production. Once this auxin reaches the abaxial petiole epidermis, it causes cell elongation, which leads to leaf hyponasty. Two pathways are known to contribute to this intercellular tip-to-petiole auxin movement: (i) transport facilitated by plasma membrane-localized PIN auxin transporters and (ii) diffusion enabled by plasmodesmata. We tested if these two modes of transport are arranged sequentially or in parallel. Moreover, we investigated if they are functionally linked. Mutants in which one of the two pathways is disrupted indicated that both pathways are necessary for a full hyponasty response. Visualization of PIN3-GFP and PIN7-GFP localization indicated PIN-mediated transport in parallel to plasmodesmata-mediated transport along abaxial midrib epidermis cells. We found plasmodesmata-mediated cell coupling in the pin3pin4pin7 mutant to match wild-type levels, indicating no redundancy between pathways. Similarly, PIN3, PIN4 and PIN7 mRNA levels were unaffected in a mutant with disrupted plasmodesmata pathway. Our results provide mechanistic insight on leaf hyponasty, which might facilitate the manipulation of the shade avoidance response in crops.

Multiscale Asymptotic Analysis Reveals How Cell Growth and Subcellular Compartments Affect Tissue-Scale Hormone Transport

Determining how cell-scale processes lead to tissue-scale patterns is key to understanding how hormones and morphogens are distributed within biological tissues and control developmental processes. In this article, we use multiscale asymptotic analysis to derive a continuum approximation for hormone transport in a long file of cells to determine how subcellular compartments and cell growth and division affect tissue-scale hormone transport. Focusing our study on plant tissues, we begin by presenting a discrete multicellular ODE model tracking the hormone concentration in each cell's cytoplasm, subcellular vacuole, and surrounding apoplast, represented by separate compartments in the cell-file geometry. We allow the cells to grow at a rate that can depend both on space and time, accounting for both cytoplasmic and vacuolar expansion. Multiscale asymptotic analysis enables us to systematically derive the corresponding continuum model, obtaining an effective reaction-advection-diffusion equation and revealing how the effective diffusivity, effective advective velocity, and the effective sink term depend on the parameters in the cell-scale model. The continuum approximation reveals how subcellular compartments, such as vacuoles, can act as storage vessels, that significantly alter the effective properties of hormone transport, such as the effective diffusivity and the induced effective velocity. Furthermore, we show how cell growth and spatial variance across cell lengths affect the effective diffusivity and the induced effective velocity, and how these affect the tissue-scale hormone distribution. In particular, we find that cell growth naturally induces an effective velocity in the direction of growth, whereas spatial variance across cell lengths induces effective velocity due to the presence of an extra compartment, such as the apoplast and the vacuole, and variations in the relative sizes between the compartments across the file of cells. It is revealed that hormone transport is faster across cells of decreasing lengths than cells with increasing lengths. We also investigate the effect of cell division on transport dynamics, assuming that each cell divides as soon as it doubles in size, and find that increasing the time between successive cell divisions decreases the growth rate, which enhances the effect of cell division in slowing hormone transport. Motivated by recent experimental discoveries, we discuss particular applications for transport of gibberellic acid (GA), an important growth hormone, within the Arabidopsis root. The model reveals precisely how membrane proteins that mediate facilitated GA transport affect the effective tissue-scale transport. However, the results are general enough to be relevant to other plant hormones, or other substances that are transported in a similar way in any type of cells.

The mRNA mobileome: challenges and opportunities for deciphering signals from the noise

Organismal communication entails encoding a message that is sent over space or time to a recipient cell, where that message is decoded to activate a downstream response. Defining what qualifies as a functional signal is essential for understanding intercellular communication. In this review, we delve into what is known and unknown in the field of long-distance messenger RNA (mRNA) movement and draw inspiration from the field of information theory to provide a perspective on what defines a functional signaling molecule. Although numerous studies support the long-distance movement of hundreds to thousands of mRNAs through the plant vascular system, only a small handful of these transcripts have been associated with signaling functions. Deciphering whether mobile mRNAs generally serve a role in plant communication has been challenging, due to our current lack of understanding regarding the factors that influence mRNA mobility. Further insight into unsolved questions regarding the nature of mobile mRNAs could provide an understanding of the signaling potential of these macromolecules.

Salt Stress-Regulation of Root Water Uptake in a Whole-Plant and Diurnal Context

This review focuses on the regulation of root water uptake in plants which are exposed to salt stress. Root water uptake is not considered in isolation but is viewed in the context of other potential tolerance mechanisms of plants-tolerance mechanisms which relate to water relations and gas exchange. Plants spend between one third and half of their lives in the dark, and salt stress does not stop with sunset, nor does it start with sunrise. Surprisingly, how plants deal with salt stress during the dark has received hardly any attention, yet any growth response to salt stress over days, weeks, months and years is the integrative result of how plants perform during numerous, consecutive day/night cycles. As we will show, dealing with salt stress during the night is a prerequisite to coping with salt stress during the day. We hope to highlight with this review not so much what we know, but what we do not know; and this relates often to some rather basic questions.

Revisiting the relationship between turgor pressure and plant cell growth

Growth is central to plant morphogenesis. Plant cells are encased in rigid cell walls, and they must overcome physical confinement to grow to specific sizes and shapes. Cell wall tension and turgor pressure are the main mechanical components impacting plant cell growth. Cell wall mechanics has been the focus of most plant biomechanical studies, and turgor pressure was often considered as a constant and largely passive component. Nevertheless, it is increasingly accepted that turgor pressure plays a significant role in plant growth. Numerous theoretical and experimental studies suggest that turgor pressure can be both spatially inhomogeneous and actively modulated during morphogenesis. Here, we revisit the pressure-growth relationship by reviewing recent advances in investigating the interactions between cellular/tissular pressure and growth.

Changes in root hydraulic conductivity in wheat (Triticum aestivum L.) in response to salt stress and day/night can best be explained through altered activity of aquaporins

Salt stress reduces plant water flow during day and night. It is not known to which extent root hydraulic properties change in parallel. To test this idea, hydroponically grown wheat plants were grown at four levels of salt stress (50, 100, 150 and 200 mM NaCl) for 5-8d before harvest (d14-18) and subjected to a range of analyses to determine diurnal changes in hydraulic conductivity (Lp) at cell, root and plant level. Cell pressure probe analyses showed that the Lp of cortex cells was differentially affected by salt stress during day and night, and that the response to salt stress differed between the main axis of roots and lateral roots. The Aquaporin (AQP) inhibitor H₂ O₂ reduced Lp to a common, across treatments, level as observed in salt-stressed plants during the night. Analyses of transpiring plants and exuding root systems provided values of root Lp which were in the same range as values modeled based on cell-Lp. The results can best be explained through a change in root Lp in response to salt stress and day/night, which results from an altered activity of AQPs. qPCR gene expression analyses point to possible candidate AQP isoforms.

ROS and redox regulation of cell-to-cell and systemic signaling in plants during stress

Stress results in the enhanced accumulation of reactive oxygen species (ROS) in plants, altering the redox state of cells and triggering the activation of multiple defense and acclimation mechanisms. In addition to activating ROS and redox responses in tissues that are directly subjected to stress (termed 'local' tissues), the sensing of stress in plants triggers different systemic signals that travel to other parts of the plant (termed 'systemic' tissues) and activate acclimation and defense mechanisms in them; even before they are subjected to stress. Among the different systemic signals triggered by stress in plants are electric, calcium, ROS, and redox waves that are mobilized in a cell-to-cell fashion from local to systemic tissues over long distances, sometimes at speeds of up to several millimeters per second. Here, we discuss new studies that identified various molecular mechanisms and proteins involved in mediating systemic signals in plants. In addition, we highlight recent studies that are beginning to unravel the mode of integration and hierarchy of the different systemic signals and underline open questions that require further attention. Unraveling the role of ROS and redox in plant stress responses is highly important for the development of climate resilient crops.

Leaf vein patterning is regulated by the aperture of plasmodesmata intercellular channels

To form tissue networks, animal cells migrate and interact through proteins protruding from their plasma membranes. Plant cells can do neither, yet plants form vein networks. How plants do so is unclear, but veins are thought to form by the coordinated action of the polar transport and signal transduction of the plant hormone auxin. However, plants inhibited in both pathways still form veins. Patterning of vascular cells into veins is instead prevented in mutants lacking the function of the GNOM (GN) regulator of auxin transport and signaling, suggesting the existence of at least one more GN-dependent vein-patterning pathway. Here we show that in Arabidopsis such a pathway depends on the movement of auxin or an auxin-dependent signal through plasmodesmata (PDs) intercellular channels. PD permeability is high where veins are forming, lowers between veins and nonvascular tissues, but remains high between vein cells. Impaired ability to regulate PD aperture leads to defects in auxin transport and signaling, ultimately leading to vein patterning defects that are enhanced by inhibition of auxin transport or signaling. GN controls PD aperture regulation, and simultaneous inhibition of auxin signaling, auxin transport, and regulated PD aperture phenocopies null gn mutants. Therefore, veins are patterned by the coordinated action of three GN-dependent pathways: auxin signaling, polar auxin transport, and movement of auxin or an auxin-dependent signal through PDs. Such a mechanism of tissue network formation is unprecedented in multicellular organisms.

How do plants form vein networks, in the absence of cellular migration or direct cell-cell interaction? This study shows that a GNOM-dependent combination of polar auxin transport, auxin signal transduction, and movement of an auxin signal through plasmodesmata patterns leaf vascular cells into veins.

The receptor kinase SRF3 coordinates iron-level and flagellin dependent defense and growth responses in plants

Iron is critical for host–pathogen interactions. While pathogens seek to scavenge iron to spread, the host aims at decreasing iron availability to reduce pathogen virulence. Thus, iron sensing and homeostasis are of particular importance to prevent host infection and part of nutritional immunity. While the link between iron homeostasis and immunity pathways is well established in plants, how iron levels are sensed and integrated with immune response pathways remains unknown. Here we report a receptor kinase SRF3, with a role in coordinating root growth, iron homeostasis and immunity pathways via regulation of callose synthases. These processes are modulated by iron levels and rely on SRF3 extracellular and kinase domains which tune its accumulation and partitioning at the cell surface. Mimicking bacterial elicitation with the flagellin peptide flg22 phenocopies SRF3 regulation upon low iron levels and subsequent SRF3-dependent responses. We propose that SRF3 is part of nutritional immunity responses involved in sensing external iron levels.

Iron homeostasis is known to influence plant immune signaling. Here the authors characterize SRF3, a receptor kinase that acts as a negative regulator of callose synthesis, that is required for root responses to iron deficiency and pathogen signals.

Diversity of funnel plasmodesmata in angiosperms: the impact of geometry on plasmodesmal resistance

In most plant tissues, threads of cytoplasm, or plasmodesmata, connect the protoplasts via pores in the cell walls. This enables symplasmic transport, for instance in phloem loading, transport and unloading. Importantly, the geometry of the wall pore limits the size of the particles that may be transported, and also (co-)defines plasmodesmal resistance to diffusion and convective flow. However, quantitative information on transport through plasmodesmata in non-cylindrical cell wall pores is scarce. We have found conical, funnel-shaped cell wall pores in the phloem-unloading zone in growing root tips of five eudicot and two monocot species, specifically between protophloem sieve elements and phloem pole pericycle cells. 3D reconstructions by electron tomography suggested that funnel plasmodesmata possess a desmotubule but lack tethers to fix it in a central position. Model calculations showed that both diffusive and hydraulic resistance decrease drastically in conical and trumpet-shaped cell wall pores compared with cylindrical channels, even at very small opening angles. Notably, the effect on hydraulic resistance was relatively larger. We conclude that funnel plasmodesmata generally are present in specific cell-cell interfaces in angiosperm roots, where they appear to facilitate symplasmic phloem unloading. Interestingly, cytosolic sleeves of most plasmodesmata reported in the literature do not resemble annuli of constant diameter but possess variously shaped widenings. Our evaluations suggest that widenings too small for unambiguous identification on electron micrographs may drastically reduce the hydraulic and diffusional resistance of these pores. Consequently, theoretical models assuming cylindrical symmetries will underestimate plasmodesmal conductivities.

Symplastic communication in the root cap directs auxin distribution to modulate root development

Root cap not only protects root meristem, but also detects and transduces the signals of environmental changes to affect root development. The symplastic communication is an important way for plants to transduce signals to coordinate the development and physiology in response to the changing enviroments. However, it is unclear how the symplastic communication between root cap cells affects root growth. Here we exploit an inducible system to specifically block the symplastic communication in the root cap. Transient blockage of plasmodesmata (PD) in differentiated collumella cells severely impairs the root development in Arabidopsis, in particular in the stem cell niche and the proximal meristem. The neighboring stem cell niche is the region that is most sensitive to the disrupted symplastic communication and responds rapidly via the alteration of auxin distribution. In the later stage, the cell division in proximal meristem is inhibited, presumably due to the reduced auxin level in the root cap. Our results reveal the essential role of the differentiated collumella cells in the root cap mediated signaling system that directs root development.

Computational Models of Auxin-Driven Patterning in Shoots

Auxin regulates many aspects of plant development and behavior, including the initiation of new outgrowth, patterning of vascular systems, control of branching, and responses to the environment. Computational models have complemented experimental studies of these processes. We review these models from two perspectives. First, we consider cellular and tissue-level models of interaction between auxin and its transporters in shoots. These models form a coherent body of results exploring different hypotheses pertinent to the patterning of new outgrowth and vascular strands. Second, we consider models operating at the level of plant organs and entire plants. We highlight techniques used to reduce the complexity of these models, which provide a path to capturing the essence of studied phenomena while running simulations efficiently.

Plasmodesmata and their role in assimilate translocation

During multicellularization, plants evolved unique cell-cell connections, the plasmodesmata (PD). PD of angiosperms are complex cellular domains, embedded in the cell wall and consisting of multiple membranes and a large number of proteins. From the beginning, it had been assumed that PD provide passage for a wide range of molecules, from ions to metabolites and hormones, to RNAs and even proteins. In the context of assimilate allocation, it has been hypothesized that sucrose produced in mesophyll cells is transported via PD from cell to cell down a concentration gradient towards the phloem. Entry into the sieve element companion cell complex (SECCC) is then mediated on three potential routes, depending on the species and conditions, - either via diffusion across PD, after conversion to raffinose via PD using a polymer trap mechanism, or via a set of transporters which secrete sucrose from one cell and secondary active uptake into the SECCC. Multiple loading mechanisms can likely coexist. We here review the current knowledge regarding photoassimilate transport across PD between cells as a prerequisite for translocation from leaves to recipient organs, in particular roots and developing seeds. We summarize the state-of-the-art in protein composition, structure, transport mechanism and regulation of PD to apprehend their functions in carbohydrate allocation. Since many aspects of PD biology remain elusive, we highlight areas that require new approaches and technologies to advance our understanding of these enigmatic and important cell-cell connections.

More Insights from Ultrastructural and Functional Plasmodesmata Data Using PDinsight

PDinsight is a Python-based tool for computing effective wall permeability for symplasmic transport based on plasmodesma (PD) size and distribution data. PDinsight can be used for direct computation of such permeabilities if full data is available, as well as in an explorative way if some data is either not available or considered unreliable. In this chapter, we briefly describe the basic model underlying the PDinsight calculations and discuss how the different modes of PDinsight can be used in relation to typical research questions. We also offer advice on choosing appropriate values for diffusion coefficients and particle size based on the currently most used experimental probes.

The plant axis as the command centre for (re)distribution of sucrose and amino acids

Along with the increase in size required for optimal colonization of terrestrial niches, channels for bidirectional bulk transport of materials in land plants evolved during a period of about 100 million years. These transport systems are essentially still in operation - though perfected over the following 400 million years - and make use of hydrostatic differentials. Substances are accumulated or released at the loading and unloading ends, respectively, of the transport channels. The intermediate stretch between the channel termini is bifunctional and executes orchestrated release and retrieval of solutes. Analyses of anatomical and physiological data demonstrate that the release/retrieval zone extends deeper into sources and sinks than is commonly thought and covers usually much more than 99% of the translocation stretch. This review sketches the significance of events in the intermediate stretch for distribution of organic materials over the plant body. Net leakage from the channels does not only serve maintenance and growth of tissues along the pathway, but also diurnal, short-term or seasonal storage of reserve materials, and balanced distribution of organic C- and N-compounds over axial and terminal sinks. Release and retrieval are controlled by plasma-membrane transporters at the vessel/parenchyma interface in the contact pits along xylem vessels and by plasma-membrane transporters at the interface between companion cells and phloem parenchyma along sieve tubes. The xylem-to-phloem pathway vice versa is a bifacial, radially oriented system comprising a symplasmic pathway, of which entrance and exit are controlled at specific membrane checkpoints, and a parallel apoplasmic pathway. A broad range of specific sucrose and amino-acid transporters are deployed at the checkpoint plasma membranes. SUCs, SUTs, STPs, SWEETs, and AAPs, LTHs, CATs are localized to the plasma membranes in question, both in monocots and eudicots. Presence of Umamits in monocots is uncertain. There is some evidence for endo- and exocytosis at the vessel/parenchyma interface supplementary to the transporter-mediated uptake and release. Actions of transporters at the checkpoints are equally decisive for storage and distribution of amino acids and sucrose in monocots and eudicots, but storage and distribution patterns may differ between both taxa. While the majority of reserves is sequestered in vascular parenchyma cells in dicots, lack of space in monocot vasculature urges "outsourcing" of storage in ground parenchyma around the translocation path. In perennial dicots, specialized radial pathways (rays) include the sites for seasonal alternation of storage and mobilization. In dicots, apoplasmic phloem loading and a correlated low rate of release along the path would favour supply with photoassimilates of terminal sinks, while symplasmic phloem loading and a correlated higher rate of release along the path favours supply of axial sinks and transfer to the xylem. The balance between the resource acquisition by terminal and axial sinks is an important determinant of relative growth rate and, hence, for the fitness of plants in various habitats. Body enlargement as the evolutionary drive for emergence of vascular systems and mass transport propelled by hydrostatic differentials.

Changes in Reactive Oxygen Species, Antioxidants and Carbohydrate Metabolism in Relation to Dormancy Transition and Bud Break in Apple (Malus × domestica Borkh) Cultivars

Understanding the biochemical mechanisms underlying bud dormancy and bloom time regulation in deciduous woody perennials is critical for devising effective strategies to protect these species from spring frost damage. This study investigated the accumulation profiles of carbohydrates, ROS and antioxidants during dormancy in ‘Cripps Pink’ and ‘Honeycrisp’, two apple cultivars representing the early and late bloom cultivars, respectively. Our data showed that starch levels generally declined during dormancy, whereas soluble sugars increased. However, the present study did not record significant alternations in the carbohydrate accumulation profiles between the two cultivars that could account for the differences in their bloom dates. On the other hand, H₂O₂ accumulation patterns revealed an apparent correlation with the dormancy stage and bloom dates in both cultivars; peaking early in the early-blooming cultivar, sustaining high levels for a longer time in the late-blooming cultivars, and fading by the time of bud burst in both cultivars. Also, the redox balance during dormancy appeared to be maintained mainly by catalase and, to a lesser extent, by glutathione (GSH). Overall, the present study concludes that differences in ROS and the bud redox balance could, at least partially, explain the differences in dormancy duration and bloom date among apple cultivars.

Auxin fluxes through plasmodesmata

Characterising the processes that control auxin dynamics is essential to understanding how auxin regulates plant development. Over recent years, several studies have investigated auxin diffusion through plasmodesmata, characterising this cell-to-cell diffusion and demonstrating that it affects auxin distributions. Furthermore, studies have shown that plasmodesmatal auxin diffusion affects developmental processes, including phototropism, lateral root emergence and leaf hyponasty. This short Tansley Insight review describes how these studies have contributed to our understanding of auxin dynamics and discusses potential future directions in this area.

Regulation of plasmodesmata in Arabidopsis leaves: ATP, NADPH and chlorophyll b levels matter

In mature leaves, cell-to-cell transport via plasmodesmata between mesophyll cells links the production of assimilates by photosynthesis with their export to sink organs. This study addresses the question of how signals derived from chloroplasts and photosynthesis influence plasmodesmata permeability. Cell-to-cell transport was analyzed in leaves of the Arabidopsis chlorophyll b-less ch1-3 mutant, the same mutant complemented with a cyanobacterial CAO gene (PhCAO) overaccumulating chlorophyll b, the trxm3 mutant lacking plastidial thioredoxin m3, and the ntrc mutant lacking functional NADPH:thioredoxin reductase C. The regulation of plasmodesmata permeability in these lines could not be traced back to the reduction state of the thioredoxin system or the types and levels of reactive oxygen species produced in chloroplasts; however, it could be related to chloroplast ATP and NADPH production. The results suggest that light enables plasmodesmata closure via an increase in the ATP and NADPH levels produced in photosynthesis, providing a control mechanism for assimilate export based on the rate of photosynthate production in the Calvin-Benson cycle. The level of chlorophyll b influences plasmodesmata permeability via as-yet-unidentified signals. The data also suggest a role of thioredoxin m3 in the regulation of cyclic electron flow around photosystem I.

TiO2 nanoparticles dose, application method and phosphorous levels influence genotoxicity in Rice (Oryza sativa L.), soil enzymatic activities and plant growth

The present study focused on investigating the effect of titanium dioxide nanoparticles (TiO₂NPs) on rice (Oryza sativa L.) growth and changes in soil health in two contrasting soil textures (silt-loam and clay). Moreover, response of rice to different methods of TiO₂NPs application and phosphorous fertilizer levels were also evaluated. For toxicity assessment, pot experiment was carried out. TiO₂NPs (0, 500, 750 mg kg^-1) were applied and plants were grown till vegetative stage. After harvesting, physiological parameters, stress assay, soil microbial and enzymatic activities were determined. Based on the results of toxicity study, impact of three methods of TiO₂NPs application (foliar, irrigation, soil) and four phosphorous fertilizer levels (0, 10, 20, 40 mg kg^-1) on rice growth were assessed. During the 1st phase, results showed an adverse effect of TiO₂NPs on plant growth and soil microorganisms in both soil textures at 750 mg kg^-1. The H₂O₂ production, lipid peroxidation and leaf membrane injury index were increased by 4.3-, 2.4-, and 1.9-folds in clay soil upon 750 mg kg^-1 TiO₂NPs application. Likewise, at the same level of TiO₂NPs; microbial biomass, dehydrogenase, and respiration were decreased by 0.91-, 0.79-, and 0.78- folds respectively. In 2nd phase, maximum shoot length, biomass, phosphorous uptake and rice grain protein content were observed under application of TiO₂NPs (500 mg kg^-1) through irrigation method in combination with 40 mg P kg^-1. However, 20 and 40 mg P kg^-1 performed equally well upon TiO₂NPs application and the results were not statistically significant. The results suggest that 750 mg kg^-1 of TiO₂NPs negatively affect plant growth and soil enzymatic activities. Moreover, combined application of TiO₂NPs (500 mg kg^-1) through irrigation and 20 mg P kg^-1 is recommended to be the optimum for growth of rice plant.

The fast and the furious: rapid long-range signaling in plants

Plants possess a systemic signaling system whereby local stimuli can lead to rapid, plant-wide responses. In addition to the redistribution of chemical messengers that range from RNAs and peptides to hormones and metabolites, a communication system acting through the transmission of electrical, Ca2+, reactive oxygen species and potentially even hydraulic signals has also been discovered. This latter system can propagate signals across many cells each second and researchers are now beginning to uncover the molecular machineries behind this rapid communications network. Thus, elements such as the reactive oxygen species producing NAPDH oxidases and ion channels of the two pore channel, glutamate receptor-like and cyclic nucleotide gated families are all required for the rapid propagation of these signals. Upon arrival at their distant targets, these changes trigger responses ranging from the production of hormones, to changes in the levels of primary metabolites and shifts in patterns of gene expression. These systemic responses occur within seconds to minutes of perception of the initial, local signal, allowing for the rapid deployment of plant-wide responses. For example, an insect starting to chew on just a single leaf triggers preemptive antiherbivore defenses throughout the plant well before it has a chance to move on to the next leaf on its menu.

Plasmodesmata-localized proteins and ROS orchestrate light-induced rapid systemic signaling in Arabidopsis

Systemic signaling and systemic acquired acclimation (SAA) are key to the survival of plants during episodes of abiotic stress. These processes depend on a continuous chain of cell-to-cell signaling events that extends from the initial tissue that senses the stress (the local tissue) to the entire plant (systemic tissues). Reactive oxygen species (ROS) and Ca²⁺ are key signaling molecules thought to be involved in this cell-to-cell mechanism. Here, we report that the systemic response of Arabidopsis thaliana to a local treatment of high light stress, which resulted in local ROS accumulation, required ROS generated by respiratory burst oxidase homolog D (RBOHD). ROS increased cell-to-cell transport and plasmodesmata (PD) pore size in a manner dependent on PD-localized protein 1 (PDLP1) and PDLP5, and this process was required for the propagation of the systemic ROS signals and SAA. Furthermore, aquaporins and several Ca²⁺-permeable channels in the glutamate receptor-like (GLR), mechanosensitive small conductance-like (MSL), and cyclic nucleotide-gated (CNGC) families were involved in this systemic signaling process. However, we determined that these channels were required primarily to amplify the systemic signal in each cell along the path of the systemic ROS wave, as well as to establish local and systemic acclimation. Thus, PD and RBOHD-generated ROS orchestrate light stress-induced rapid cell-to-cell spread of systemic signals in Arabidopsis.

Plasmodesmata and the problems with size: Interpreting the confusion

Plant tissues exhibit a symplasmic organization; the individual protoplasts are connected to their neighbors via cytoplasmic bridges that extend through pores in the cell walls. These bridges may have diameters of a micrometer or more, as in the sieve pores of the phloem, but in most cell types they are smaller. Historically, botanists referred to cytoplasmic bridges of all sizes as plasmodesmata. The meaning of the term began to shift when the transmission electron microscope (TEM) became the preferred tool for studying these structures. Today, a plasmodesma is widely understood to be a 'nano-scale' pore. Unfortunately, our understanding of these nanoscopic channels suffers from methodological limitations. This is exemplified by the fact that state-of-the-art EM techniques appear to reveal plasmodesmal pore structures that are much smaller than the tracer molecules known to diffuse through these pores. In general, transport processes in pores that have dimensions in the size range of the transported molecules are governed by different physical parameters than transport process in the macroscopic realm. This can lead to unexpected effects, as experience in nanofluidic technologies demonstrates. Our discussion of problems of size in plasmodesma research leads us to conclude that the field will benefit from technomimetic reasoning - the utilization of concepts developed in applied nanofluidics for the interpretation of biological systems.

Organelles-nucleus-plasmodesmata signaling (ONPS): an update on its roles in plant physiology, metabolism and stress responses

Plasmodesmata allow movement of metabolites and signaling molecules between plant cells and are, therefore, critical players in plant development and physiology, and in responding to environmental signals and stresses. There is emerging evidence that plasmodesmata are controlled by signaling originating from other organelles, primarily the chloroplasts and mitochondria. These signals act in the nucleus to alter expression of genetic pathways that control both trafficking via plasmodesmata and the plasmodesmatal pores themselves. This control circuit was dubbed organelle-nucleus-plasmodesmata signaling (ONPS). Here we discuss how ONPS arose during plant evolution and highlight the discovery of an ONPS-like module for regulating stomata. We also consider recent findings that illuminate details of the ONPS circuit and its roles in plant physiology, metabolism, and defense.

Uncharted routes: exploring the relevance of auxin movement via plasmodesmata

Auxin is an endogenous small molecule with an incredibly large impact on growth and development in plants. Movement of auxin between cells, due to its negative charge at most physiological pHs, strongly relies on families of active transporters. These proteins import auxin from the extracellular space or export it into the same. Mutations in these components have profound impacts on biological processes. Another transport route available to auxin, once the substance is inside the cell, are plasmodesmata connections. These small channels connect the cytoplasms of neighbouring plant cells and enable flow between them. Interestingly, the biological significance of this latter mode of transport is only recently starting to emerge with examples from roots, hypocotyls and leaves. The existence of two transport systems provides opportunities for reciprocal cross-regulation. Indeed, auxin levels influence proteins controlling plasmodesmata permeability, while cell-cell communication affects auxin biosynthesis and transport. In an evolutionary context, transporter driven cell-cell auxin movement and plasmodesmata seem to have evolved around the same time in the green lineage. This highlights a co-existence from early on and a likely functional specificity of the systems. Exploring more situations where auxin movement via plasmodesmata has relevance for plant growth and development, and clarifying the regulation of such transport, will be key aspects in coming years.This article has an associated Future Leader to Watch interview with the author of the paper.

Evaluating molecular movement through plasmodesmata

Plasmodesmata are membrane-lined cytoplasmic passageways that facilitate the movement of nutrients and various types of molecules between cells in the plant. They are highly dynamic channels, opening or closing in response to physiological and developmental stimuli or environmental challenges such as biotic and abiotic stresses. Accumulating evidence supports the idea that such dynamic controls occur through integrative cellular mechanisms. Currently, a few fluorescence-based methods are available that allow monitoring changes in molecular movement through plasmodesmata. In this chapter, following a brief introduction to those methods, we provide a detailed step-by-step protocol for the Drop-ANd-See (DANS) assay, which is advantageous when it is desirable to measure plasmodesmal permeability non-invasively, in situ and in real-time. We discuss the experimental conditions one should consider to produce reliable and reproducible DANS results along with troubleshooting ideas.

Symplasmic isolation marks cell fate changes during somatic embryogenesis

Cell-to-cell signalling is a major mechanism controlling plant morphogenesis. Transport of signalling molecules through plasmodesmata is one way in which plants promote or restrict intercellular signalling over short distances. Plasmodesmata are membrane-lined pores between cells that regulate the intercellular flow of signalling molecules through changes in their size, creating symplasmic fields of connected cells. Here we examine the role of plasmodesmata and symplasmic communication in the establishment of plant cell totipotency, using somatic embryo induction from Arabidopsis explants as a model system. Cell-to-cell communication was evaluated using fluorescent tracers, supplemented with histological and ultrastructural analysis, and correlated with expression of a WOX2 embryo reporter. We showed that embryogenic cells are isolated symplasmically from non-embryogenic cells regardless of the explant type (immature zygotic embryos or seedlings) and inducer system (2,4-dichlorophenoxyacetic acid or the BABY BOOM (BBM) transcription factor), but that the symplasmic domains in different explants differ with respect to the maximum size of molecule capable of moving through the plasmodesmata. Callose deposition in plasmodesmata preceded WOX2 expression in future sites of somatic embryo development, but later was greatly reduced in WOX2-expressing domains. Callose deposition was also associated with a decrease DR5 auxin response in embryogenic tissue. Treatment of explants with the callose biosynthesis inhibitor 2-deoxy-D-glucose supressed somatic embryo formation in all three systems studied, and also blocked the observed decrease in DR5 expression. Together these data suggest that callose deposition at plasmodesmata is required for symplasmic isolation and establishment of cell totipotency in Arabidopsis.

Chloroplast-to-nucleus retrograde signalling controls intercellular trafficking via plasmodesmata formation

The signalling pathways that regulate intercellular trafficking via plasmodesmata (PD) remain largely unknown. Analyses of mutants with defects in intercellular trafficking led to the hypothesis that chloroplasts are important for controlling PD, probably by retrograde signalling to the nucleus to regulate expression of genes that influence PD formation and function, an idea encapsulated in the organelle-nucleus-PD signalling (ONPS) hypothesis. ONPS is supported by findings that point to chloroplast redox state as also modulating PD. Here, we have attempted to further elucidate details of ONPS. Through reverse genetics, expression of select nucleus-encoded genes with known or predicted roles in chloroplast gene expression was knocked down, and the effects on intercellular trafficking were then assessed. Silencing most genes resulted in chlorosis, and the expression of several photosynthesis and tetrapyrrole biosynthesis associated nuclear genes was repressed in all silenced plants. PD-mediated intercellular trafficking was changed in the silenced plants, consistent with predictions of the ONPS hypothesis. One striking observation, best exemplified by silencing the PNPase homologues, was that the degree of chlorosis of silenced leaves was not correlated with the capacity for intercellular trafficking. Finally, we measured the distribution of PD in silenced leaves and found that intercellular trafficking was positively correlated with the numbers of PD. Together, these results not only provide further support for ONPS but also point to a genetic mechanism for PD formation, clarifying a longstanding question about PD and intercellular trafficking. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Auxin fluxes through plasmodesmata modify root-tip auxin distribution

Auxin is a key signal regulating plant growth and development. It is well established that auxin dynamics depend on the spatial distribution of efflux and influx carriers on the cell membranes. In this study, we employ a systems approach to characterise an alternative symplastic pathway for auxin mobilisation via plasmodesmata, which function as intercellular pores linking the cytoplasm of adjacent cells. To investigate the role of plasmodesmata in auxin patterning, we developed a multicellular model of the Arabidopsis root tip. We tested the model predictions using the DII-VENUS auxin response reporter, comparing the predicted and observed DII-VENUS distributions using genetic and chemical perturbations designed to affect both carrier-mediated and plasmodesmatal auxin fluxes. The model revealed that carrier-mediated transport alone cannot explain the experimentally determined auxin distribution in the root tip. In contrast, a composite model that incorporates both carrier-mediated and plasmodesmatal auxin fluxes re-capitulates the root-tip auxin distribution. We found that auxin fluxes through plasmodesmata enable auxin reflux and increase total root-tip auxin. We conclude that auxin fluxes through plasmodesmata modify the auxin distribution created by efflux and influx carriers.

The diffusive injection micropipette (DIMP)

The microinjection of fluorescent probes into live cells is an essential component in the toolbox of modern cell biology. Microinjection techniques include the penetration of the plasma membrane and, if present, the cell wall with micropipettes, and the application of pressure or electrical currents to drive the micropipette contents into the cell. These procedures interfere with cellular functions and therefore may induce artifacts. We designed the diffusive injection micropipette (DIMP) that avoids most of the possible artifacts due to the drastically reduced volume of its fluid contents and the utilization of diffusion for cargo delivery into the target cell. DIMPs were successfully tested in plant, fungal, and animal cells. Using the continuity of cytoplasmic dynamics over ten minutes after impalement of Nicotiana trichome cells as a criterion for non-invasiveness, we found DIMPs significantly less disruptive than conventional pressure microinjection. The design of DIMPs abolishes major sources of artifacts that cannot be avoided by other microinjection techniques. Moreover, DIMPs are inexpensive, easy to produce, and can be applied without specific equipment other than a micromanipulator. With these features, DIMPs may become the tool of choice for studies that require the least invasive delivery possible of materials into live cells.

From plasmodesma geometry to effective symplasmic permeability through biophysical modelling

Regulation of molecular transport via intercellular channels called plasmodesmata (PDs) is important for both coordinating developmental and environmental responses among neighbouring cells, and isolating (groups of) cells to execute distinct programs. Cell-to-cell mobility of fluorescent molecules and PD dimensions (measured from electron micrographs) are both used as methods to predict PD transport capacity (i.e., effective symplasmic permeability), but often yield very different values. Here, we build a theoretical bridge between both experimental approaches by calculating the effective symplasmic permeability from a geometrical description of individual PDs and considering the flow towards them. We find that a dilated central region has the strongest impact in thick cell walls and that clustering of PDs into pit fields strongly reduces predicted permeabilities. Moreover, our open source multi-level model allows to predict PD dimensions matching measured permeabilities and add a functional interpretation to structural differences observed between PDs in different cell walls.

Lipids or Proteins: Who Is Leading the Dance at Membrane Contact Sites?

Understanding the mode of action of membrane contact sites (MCSs) across eukaryotic organisms at the near-atomic level to infer function at the cellular and tissue levels is a challenge scientists are currently facing. These peculiar systems dedicated to inter-organellar communication are perfect examples of cellular processes where the interplay between lipids and proteins is critical. In this mini review, we underline the link between membrane lipid environment, the recruitment of proteins at specialized membrane domains and the function of MCSs. More precisely, we want to give insights on the crucial role of lipids in defining the specificity of plant endoplasmic reticulum (ER)-plasma membrane (PM) MCSs and we further propose approaches to study them at multiple scales. Our goal is not so much to go into detailed description of MCSs, as there are numerous focused reviews on the subject, but rather try to pinpoint the critical elements defining those structures and give an original point of view by considering the subject from a near-atomic angle with a focus on lipids. We review current knowledge as to how lipids can define MCS territories, play a role in the recruitment and function of the MCS-associated proteins and in turn, how the lipid environment can be modified by proteins.

Dynamic regulation of plasmodesmatal permeability and its application to horticultural research

Effective cell-to-cell communication allows plants to fine-tune their developmental processes in accordance with the prevailing environmental stimuli. Plasmodesmata (PD) are intercellular channels that span the plant cell wall and serve as cytoplasmic bridges to facilitate efficient exchange of signaling molecules between neighboring cells. The identification of PD-associated proteins and the subsequent elucidation of the regulation of PD structure have provided vital insights into the role of PD architecture in enforcing crucial cellular processes, including callose deposition, ER-Golgi-based secretion, cytoskeleton dynamics, membrane lipid raft organization, chloroplast metabolism, and cell wall formation. In this review, we summarize the emerging discoveries from recent studies that elucidated the regulatory mechanisms involved in PD biogenesis and the dynamics of PD opening-closure. Retrospectively, PD-mediated cell-to-cell communication has been implicated in diverse cellular and physiological processes that are fundamental for the development of horticultural plants. The potential application of PD biotechnological engineering represents a powerful approach for improving agronomic traits in horticultural crops in the future.

Root Exudation of Primary Metabolites: Mechanisms and Their Roles in Plant Responses to Environmental Stimuli

Root exudation is an important process determining plant interactions with the soil environment. Many studies have linked this process to soil nutrient mobilization. Yet, it remains unresolved how exudation is controlled and how exactly and under what circumstances plants benefit from exudation. The majority of root exudates including primary metabolites (sugars, amino acids, and organic acids) are believed to be passively lost from the root and used by rhizosphere-dwelling microbes. In this review, we synthetize recent advances in ecology and plant biology to explain and propose mechanisms by which root exudation of primary metabolites is controlled, and what role their exudation plays in plant nutrient acquisition strategies. Specifically, we propose a novel conceptual framework for root exudates. This framework is built upon two main concepts: (1) root exudation of primary metabolites is driven by diffusion, with plants and microbes both modulating concentration gradients and therefore diffusion rates to soil depending on their nutritional status; (2) exuded metabolite concentrations can be sensed at the root tip and signals are translated to modify root architecture. The flux of primary metabolites through root exudation is mostly located at the root tip, where the lack of cell differentiation favors diffusion of metabolites to the soil. We show examples of how the root tip senses concentration changes of exuded metabolites and translates that into signals to modify root growth. Plants can modify the concentration of metabolites either by controlling source/sink processes or by expressing and regulating efflux carriers, therefore challenging the idea of root exudation as a purely unregulated passive process. Through root exudate flux, plants can locally enhance concentrations of many common metabolites, which can serve as sensors and integrators of the plant nutritional status and of the nutrient availability in the surrounding environment. Plant-associated micro-organisms also constitute a strong sink for plant carbon, thereby increasing concentration gradients of metabolites and affecting root exudation. Understanding the mechanisms of and the effects that environmental stimuli have on the magnitude and type of root exudation will ultimately improve our knowledge of processes determining soil CO₂ emissions, ecosystem functioning, and how to improve the sustainability of agricultural production.

The essential chloroplast ribosomal protein uL15c interacts with the chloroplast RNA helicase ISE2 and affects intercellular trafficking through plasmodesmata

Chloroplasts retain part of their ancestral genomes and the machinery for expression of those genomes. The nucleus-encoded chloroplast RNA helicase INCREASED SIZE EXCLUSION LIMIT2 (ISE2) is required for chloroplast ribosomal RNA processing and chloro-ribosome assembly. To further elucidate ISE2's role in chloroplast translation, two independent approaches were used to identify its potential protein partners. Both a yeast two-hybrid screen and a pull-down assay identified plastid ribosomal protein L15, uL15c (formerly RPL15), as interacting with ISE2. The interaction was confirmed in vivo by co-immunoprecipitation. Interestingly, we found that rpl15 null mutants do not complete embryogenesis, indicating that RPL15 is an essential gene for autotrophic growth of Arabidopsis thaliana. Arabidopsis and Nicotiana benthamiana plants with reduced expression of RPL15 developed chlorotic leaves, had reduced photosynthetic capacity and exhibited defective chloroplast development. Processing of chloroplast ribosomal RNAs and assembly of ribosomal subunits were disrupted by reduced expression of RPL15. Chloroplast translation was also decreased, reducing accumulation of chloroplast-encoded proteins, in such plants compared to wild-type plants. Notably, knockdown of RPL15 expression increased intercellular trafficking, a phenotype also observed in plants with reduced ISE2 expression. This finding provides further evidence for chloroplast function in modulating intercellular trafficking via plasmodesmata.

RNA on the move: The plasmodesmata perspective

It is now widely accepted that plant RNAs can have effects at sites far away from their sites of synthesis. Cellular mRNA transcripts, endogenous small RNAs and defense-related small RNAs all move from cell to cell via plasmodesmata (PD), and may even move long distances in the phloem. Despite their small size, PD have complicated substructures, and the area of the pore available for RNA trafficking can be remarkably small. The intent of this review is to bring into focus the role of PD in cell-to-cell and long distance communication in plants. We consider how cellular RNAs could move through the cell to the PD and thence through PD. The protein composition of PD and the possible roles of PD proteins in RNA trafficking are also discussed. Recent evidence for RNA metabolism in organelles acting as a factor in controlling PD flux is also presented, highlighting new aspects of plant intra- and intercellular communication. It is clear that while the phenomenon of RNA mobility is common and essential, many questions remain, and these have been highlighted throughout this review.

Photoinducible DRONPA-s: a new tool for investigating cell-cell connectivity

The development of multicellular plants relies on the ability of their cells to exchange solutes, proteins and signalling compounds through plasmodesmata, symplasmic pores in the plant cell wall. The aperture of plasmodesmata is regulated in response to developmental cues or external factors such as pathogen attack. This regulation enables tight control of symplasmic cell-to-cell transport. Here we report on an elegant non-invasive method to quantify the passive movement of protein between selected cells even in deeper tissue layers. The system is based on the fluorescent protein DRONPA-s, which can be switched on and off repeatedly by illumination with different light qualities. Using transgenic 35S::DRONPA-s Arabidopsis thaliana and a confocal microscope it was possible to activate DRONPA-s fluorescence in selected cells of the root meristem. This enabled us to compare movement of DRONPA-s from the activated cells into the respective neighbouring cells. Our analyses showed that pericycle cells display the highest efflux capacity with a good lateral connectivity. In contrast, root cap cells showed the lowest efflux of DRONPA-s. Plasmodesmata of quiescent centre cells mediated a stronger efflux into columella cells than into stele initials. To simplify measurements of fluorescence intensity in a complex tissue we developed software that allows simultaneous analyses of fluorescence intensities of several neighbouring cells. Our DRONPA-s system generates reproducible data and is a valuable tool for studying symplasmic connectivity.

A Salutary Role of Reactive Oxygen Species in Intercellular Tunnel-Mediated Communication

The reactive oxygen species, generally labeled toxic due to high reactivity without target specificity, are gradually uncovered as signaling molecules involved in a myriad of biological processes. But one important feature of ROS roles in macromolecule movement has not caught attention until recent studies with technique advance and design elegance have shed lights on ROS signaling for intercellular and interorganelle communication. This review begins with the discussions of genetic and chemical studies on the regulation of symplastic dye movement through intercellular tunnels in plants (plasmodesmata), and focuses on the ROS regulatory mechanisms concerning macromolecule movement including small RNA-mediated gene silencing movement and protein shuttling between cells. Given the premise that intercellular tunnels (bridges) in mammalian cells are the key physical structures to sustain intercellular communication, movement of macromolecules and signals is efficiently facilitated by ROS-induced membrane protrusions formation, which is analogously applied to the interorganelle communication in plant cells. Although ROS regulatory differences between plant and mammalian cells exist, the basis for ROS-triggered conduit formation underlies a unifying conservative theme in multicellular organisms. These mechanisms may represent the evolutionary advances that have enabled multicellularity to gain the ability to generate and utilize ROS to govern material exchanges between individual cells in oxygenated environment.

You Shall Not Pass: Root Vacuoles as a Symplastic Checkpoint for Metal Translocation to Shoots and Possible Application to Grain Nutritional Quality

Plant nutrient uptake is performed mostly by roots, which have to acquire nutrients while avoiding excessive amounts of essential and toxic elements. Apoplastic barriers such as the casparian strip and suberin deposition block free diffusion from the rhizosphere into the xylem, making selective plasma membrane transporters able to control elemental influx into the root symplast, efflux into the xylem and therefore shoot translocation. Additionally, transporters localized to the tonoplast of root cells have been demonstrated to regulate the shoot ionome, and may be important for seed elemental translocation. Here we review the role of vacuolar transporters in the detoxification of elements such as zinc (Zn), manganese (Mn), cadmium (Cd), cobalt (Co) and nickel (Ni) that are co-transported with iron (Fe) during the Fe deficiency response in Arabidopsis thaliana, and the possible conservation of this mechanism in rice (Oryza sativa). We also discuss the evidence that vacuolar transporters are linked to natural variation in shoot ionome in Arabidopsis and rice, indicating that vacuolar storage might be amenable to genetic engineering without strong phenotypical changes. Finally, we discuss the possible use of root's vacuolar transporters to increase the nutritional quality of crop grains.

Symplasmic and apoplasmic transport inside feather moss stems of Pleurozium schreberi and Hylocomium splendens

Background and Aims

The ubiquitous feather mosses Pleurozium schreberi and Hylocomium splendens form a thick, continuous boundary layer between the soil and the atmosphere, and play important roles in hydrology and nutrient cycling in tundra and boreal ecosystems. The water fluxes among these mosses and environmental factors controlling them are poorly understood. The aim of this study was to investigate whether feather mosses are capable of internal transport and to provide a better understanding of species-specific morphological traits underlying this function. The impacts of environmental conditions on their internal transport rates were also investigated.

Methods

Cells involved in water and food conduction in P. schreberi and H. splendens were identified by transmission electron microscopy. Symplasmic and apoplasmic fluorescent tracers were applied to the moss stems to determine the routes of internal short- and long-distance transport and the impact of air humidity on the transport rates.

Key Results

Symplasmic transport over short distances occurs via food-conducting cells in both mosses. Pleurozium schreberi is also capable of apoplasmic internal long-distance transport via a central strand of hydroids. These are absent in H. splendens. Reduced air humidity significantly increased the internal transport of both species, and the increase was significantly faster for P. schreberi than for H. splendens.

Conclusions

Pleurozium schreberi and Hylocomium splendens are capable of internal transport but the pathway and conductivity differ due to differences in stem anatomy. These results help explain their varying desiccation tolerance and possibly their differing physiology and autecology and, ultimately, their impact on ecosystem functioning.

Architecture and permeability of post-cytokinesis plasmodesmata lacking cytoplasmic sleeves

Plasmodesmata are remarkable cellular machines responsible for the controlled exchange of proteins, small RNAs and signalling molecules between cells. They are lined by the plasma membrane (PM), contain a strand of tubular endoplasmic reticulum (ER), and the space between these two membranes is thought to control plasmodesmata permeability. Here, we have reconstructed plasmodesmata three-dimensional (3D) ultrastructure with an unprecedented level of 3D information using electron tomography. We show that within plasmodesmata, ER-PM contact sites undergo substantial remodelling events during cell differentiation. Instead of being open pores, post-cytokinesis plasmodesmata present such intimate ER-PM contact along the entire length of the pores that no intermembrane gap is visible. Later on, during cell expansion, the plasmodesmata pore widens and the two membranes separate, leaving a cytosolic sleeve spanned by tethers whose presence correlates with the appearance of the intermembrane gap. Surprisingly, the post-cytokinesis plasmodesmata allow diffusion of macromolecules despite the apparent lack of an open cytoplasmic sleeve, forcing the reassessment of the mechanisms that control plant cell-cell communication.

Water transport and energy

Water transport in plants occurs along various paths and is driven by gradients in its free energy. It is generally considered that the mode of transport, being either diffusion or bulk flow, is a passive process, although energy may be required to sustain the forces driving water flow. This review aims at putting water flow at the various organisational levels (cell, organ, plant) in the context of the energy that is required to maintain these flows. In addition, the question is addressed (1) whether water can be transported against a difference in its chemical free energy, 'water potential' (Ψ), through, directly or indirectly, active processes; and (2) whether the energy released when water is flowing down a gradient in its energy, for example during day-time transpiration and cell expansive growth, is significant compared to the energy budget of plant and cell. The overall aim of review is not so much to provide a definite 'Yes' and 'No' to these questions, but rather to stimulate discussion and raise awareness that water transport in plants has its real, associated, energy costs and potential energy gains.

Sucrose transporters and plasmodesmal regulation in passive phloem loading

An essential step for the distribution of carbon throughout the whole plant is the loading of sugars into the phloem in source organs. In many plants, accumulation of sugars in the sieve element-companion cell (SE-CC) complex is mediated and regulated by active processes. However, for poplar and many other tree species, a passive symplasmic mechanism of phloem loading has been proposed, characterized by symplasmic continuity along the pre-phloem pathway and the absence of active sugar accumulation in the SE-CC complex. A high overall leaf sugar concentration is thought to enable diffusion of sucrose into the phloem. In this review, we critically evaluate current evidence regarding the mechanism of passive symplasmic phloem loading, with a focus on the potential influence of active sugar transport and plasmodesmal regulation. The limited experimental data, combined with theoretical considerations, suggest that a concomitant operation of passive symplasmic and active phloem loading in the same minor vein is unlikely. However, active sugar transport could well play an important role in how passively loading plants might modulate the rate of sugar export from leaves. Insights into the operation of this mechanism has direct implications for our understanding of how these plants utilize assimilated carbon.

Experimental and Theoretical Methods to Approach the Study of Vascular Patterning in the Plant Shoot

The plant vascular system provides transport and mechanical support functions that are essential for suitable plant growth and development. In Arabidopsis thaliana (Arabidopsis), the vascular tissues at the shoot inflorescence stems are disposed in organized vascular bundles. The vascular patterning emergence and development within the shoot inflorescence stems is under the control of plant growth regulators (De Rybel et al., Nat Rev Mol Cell Biol 17:30-40, 2016; Caño-Delgado et al., Annu Rev Cell Dev Biol 26:605-637, 2010). By using a combined approach of experimental methods for vascular tissues visualization and quantification together with theoretical methods through mathematical and computational modeling, we have reported that auxin transport and brassinosteroid signaling play complementary roles in the formation of the periodic vascular patterning in the shoot (Ibañes et al., Proc Natl Acad Sci U S A 106:13630-13635, 2009; Fàbregas et al., Plant Signal Behav 5:903-906, 2010; Fàbregas et al., PLoS Genet 11:e1005183, 2015). Here, we report the methodology for the interdisciplinary analysis of the shoot vascular patterning in the plant model Arabidopsis into a handle procedure for visualization, quantification, data analysis, and modeling implementation.

The missing link: does tunnelling nanotube-based supercellularity provide a new understanding of chronic and lifestyle diseases?

Tunnelling nanotubes (TNTs) are increasingly recognized as central players in a multitude of cellular mechanisms and diseases. Although their existence and functions in animal organisms are still elusive, emerging evidence suggests that they are involved in developmental processes, tissue regeneration, viral infections or pathogen transfer, stem cell differentiation, immune responses as well as initiation and progression of neurodegenerative disorders and cancer (see Sisakhtnezhad & Khosravi 2015 Eur. J. Cell Biol. 94, 429-443. (doi:10.1016/j.ejcb.2015.06.010)). A broader field of vision, including their striking functional and structural resemblance with nanotube-mediated phenomena found throughout the phylogenetic tree, from plants down to bacteria, points to a universal, conserved and tightly regulated mechanism of cellular assemblies. Based on our initial definition of TNTs as open-ended channels mediating membrane continuity between connected cells (Rustom et al. 2004 Science 303, 1007-1010. (doi:10.1126/science.1093133)), it is suggested that animal tissues represent supercellular assemblies that-besides opening discrete communication pathways-balance diverse stress factors caused by pathological changes or fluctuating physiological and environmental conditions, such as oxidative stress or nutrient shortage. By combining current knowledge about nanotube formation, intercellular transfer and communication phenomena as well as associated molecular pathways, a model evolves, predicting that the linkage between reactive oxygen species, TNT-based supercellularity and the intercellular shuttling of materials will have significant impact on diverse body functions, such as cell survival, redox/metabolic homeostasis and mitochondrial heteroplasmy. It implies that TNTs are intimately linked to the physiological and pathological state of animal cells and represent a central joint element of diverse diseases, such as neurodegenerative disorders, diabetes or cancer.

Receptor Complex Mediated Regulation of Symplastic Traffic

Plant receptor kinases (RKs) and receptor proteins (RPs) are involved in a plethora of cellular processes, including developmental decisions and immune responses. There is increasing evidence that plasmodesmata (PD)-localized RKs and RPs act as nexuses that perceive extracellular signals and convey them into intra- and intercellular responses by regulating the exchange of molecules through PD. How RK/RP complexes regulate the specific and nonspecific traffic of molecules through PD, and how these receptors are specifically targeted to PD, have been elusive but underpin comprehensive understanding of the function and regulation of the symplast. In this review we gather the current knowledge of RK/RP complex function at PD and how they might regulate intercellular traffic.