tRNA-Related Sequences Trigger Systemic mRNA Transport in Plants

RBP differentiation contributes to selective transmissibility of OPT3 mRNAs

Long-distance mobile mRNAs play key roles in gene regulatory networks that control plant development and stress tolerance. However, the mechanisms underlying species-specific delivery of mRNA still need to be elucidated. Here, the use of grafts involving highly heterozygous apple (Malus) genotypes allowed us to demonstrate that apple (Malus domestica) oligopeptide transporter3 (MdOPT3) mRNA can be transported over a long distance, from the leaf to the root, to regulate iron uptake; however, the mRNA of Arabidopsis (Arabidopsis thaliana) oligopeptide transporter 3 (AtOPT3), the MdOPT3 homolog from A. thaliana, does not move from shoot to root. Reciprocal heterologous expression of the two types of mRNAs showed that the immobile AtOPT3 became mobile and moved from the shoot to the root in two woody species, Malus and Populus, while the mobile MdOPT3 became immobile in two herbaceous species, A. thaliana and tomato (Solanum lycopersicum). Furthermore, we demonstrated that the different transmissibility of OPT3 in A. thaliana and Malus might be caused by divergence in RNA-binding proteins between herbaceous and woody plants. This study provides insights into mechanisms underlying differences in mRNA mobility and validates the important physiological functions associated with this process.

Highly efficient heritable genome editing in wheat using an RNA virus and bypassing tissue culture

Genome editing provides novel strategies for improving plant traits but mostly relies on conventional plant genetic transformation and regeneration procedures, which can be inefficient. In this study, we have engineered a Barley stripe mosaic virus-based sgRNA delivery vector (BSMV-sg) that is effective in performing heritable genome editing in Cas9-transgenic wheat plants. Mutated progenies were present in the next generation at frequencies ranging from 12.9% to 100% in three different wheat varieties, and 53.8%-100% of mutants were virus free. We also achieved multiplex mutagenesis in progeny using a pool of BSMV-sg vectors harboring different sgRNAs. Furthermore, we devised a virus-induced transgene-free editing procedure to generate Cas9-free wheat mutants by crossing BSMV-infected Cas9-transgenic wheat pollen with wild-type wheat. Our study provides a robust, convenient, and tissue culture-free approach for genome editing in wheat through virus infection.

Origins and evolving functionalities of tRNA-derived small RNAs

Transfer RNA (tRNA)-derived small RNAs (tsRNAs) are among the most ancient small RNAs in all domains of life and are generated by the cleavage of tRNAs. Emerging studies have begun to reveal the versatile roles of tsRNAs in fundamental biological processes, including gene silencing, ribosome biogenesis, retrotransposition, and epigenetic inheritance, which are rooted in tsRNA sequence conservation, RNA modifications, and protein-binding abilities. We summarize the mechanisms of tsRNA biogenesis and the impact of RNA modifications, and propose how thinking of tsRNA functionality from an evolutionary perspective urges the expansion of tsRNA research into a wider spectrum, including cross-tissue/cross-species regulation and harnessing of the 'tsRNA code' for precision medicine.

Soma-to-germline RNA communication

More than a century ago, August Weissman defined a distinction between the germline (responsible for propagating heritable information from generation to generation) and the perishable soma. A central motivation for this distinction was to argue against the inheritance of acquired characters, as the germline was partly defined by its protection from external conditions. However, recent decades have seen an explosion of studies documenting the intergenerational and transgenerational effects of environmental conditions, forcing a re-evaluation of how external signals are sensed by, or communicated to, the germline epigenome. Here, motivated by the centrality of small RNAs in paradigms of epigenetic inheritance, we review across species the myriad examples of intercellular RNA trafficking from nurse cells or somatic tissues to developing gametes.

Novel insights into the pervasive role of RNA structure in post-transcriptional regulation of gene expression in plants

RNA folding is an intrinsic property of RNA that serves a key role in every step of post-transcriptional regulation of gene expression, from RNA maturation to translation in plants. Recent developments of genome-wide RNA structure profiling methods have transformed research in this area enabling focus to shift from individual molecules to the study of tens of thousands of RNAs. Here, we provide a comprehensive review of recent advances in the field. We discuss these new insights of RNA structure functionality within the context of post-transcriptional regulation including mRNA maturation, translation, and RNA degradation in plants. Notably, we also provide an overview of how plants exhibit different RNA structures in response to environmental changes.

CRISPR/Cas systems versus plant viruses: engineering plant immunity and beyond

Molecular engineering of plant immunity to confer resistance against plant viruses holds great promise for mitigating crop losses and improving plant productivity and yields, thereby enhancing food security. Several approaches have been employed to boost immunity in plants by interfering with the transmission or lifecycles of viruses. In this review, we discuss the successful application of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) (CRISPR/Cas) systems to engineer plant immunity, increase plant resistance to viruses, and develop viral diagnostic tools. Furthermore, we examine the use of plant viruses as delivery systems to engineer virus resistance in plants and provide insight into the limitations of current CRISPR/Cas approaches and the potential of newly discovered CRISPR/Cas systems to engineer better immunity and develop better diagnostics tools for plant viruses. Finally, we outline potential solutions to key challenges in the field to enable the practical use of these systems for crop protection and viral diagnostics.

RNA Viral Vectors for Accelerating Plant Synthetic Biology

The tools of synthetic biology have enormous potential to help us uncover the fundamental mechanisms controlling development and metabolism in plants. However, their effective utilization typically requires transgenesis, which is plagued by long timescales and high costs. In this review we explore how transgenesis can be minimized by delivering foreign genetic material to plants with systemically mobile and persistent vectors based on RNA viruses. We examine the progress that has been made thus far and highlight the hurdles that need to be overcome and some potential strategies to do so. We conclude with a discussion of biocontainment mechanisms to ensure these vectors can be used safely as well as how these vectors might expand the accessibility of plant synthetic biology techniques. RNA vectors stand poised to revolutionize plant synthetic biology by making genetic manipulation of plants cheaper and easier to deploy, as well as by accelerating experimental timescales from years to weeks.

tRNA-like structures and their functions

tRNA-like structures (TLSs) were first identified in the RNA genomes of turnip yellow mosaic virus. Since then, TLSs have been found in many other species including mammals, and the RNAs harboring these structures range from viral genomic RNAs to mRNAs and noncoding RNAs. Some progress has also been made on understanding their functions that include regulation of RNA replication, translation enhancement, RNA-protein interaction, and more. In this review, we summarize the current knowledge about the regulations and functions of these TLSs. Possible future directions of the field are also briefly discussed.

Tissue and regional expression patterns of dicistronic tRNA-mRNA transcripts in grapevine (Vitis vinifera) and their evolutionary co-appearance with vasculature in land plants

Transfer RNAs (tRNA) are crucial adaptor molecules between messenger RNA (mRNA) and amino acids. Recent evidence in plants suggests that dicistronic tRNA-like structures also act as mobile signals for mRNA transcripts to move between distant tissues. Co-transcription is not a common feature in the plant nuclear genome and, in the few cases where polycistronic transcripts have been found, they include non-coding RNA species, such as small nucleolar RNAs and microRNAs. It is not known, however, the extent to which dicistronic transcripts of tRNA and mRNAs are expressed in field-grown plants, or the factors contributing to their expression. We analysed tRNA-mRNA dicistronic transcripts in the major horticultural crop grapevine (Vitis vinifera) using a novel pipeline developed to identify dicistronic transcripts from high-throughput RNA-sequencing data. We identified dicistronic tRNA-mRNA in leaf and berry samples from 22 commercial vineyards. Of the 124 tRNA genes that were expressed in both tissues, 18 tRNA were expressed forming part of 19 dicistronic tRNA-mRNAs. The presence and abundance of dicistronic molecules was tissue and geographic sub-region specific. In leaves, the expression patterns of dicistronic tRNA-mRNAs significantly correlated with tRNA expression, suggesting that their transcriptional regulation might be linked. We also found evidence of syntenic genomic arrangements of tRNAs and protein-coding genes between grapevine and Arabidopsis thaliana, and widespread prevalence of dicistronic tRNA-mRNA transcripts among vascular land plants but no evidence of these transcripts in non-vascular lineages. This suggests that the appearance of plant vasculature and tRNA-mRNA occurred concurrently during the evolution of land plants.

A historical overview of long-distance signalling in plants

Be it a small herb or a large tree, intra- and intercellular communication and long-distance signalling between distant organs are crucial for every aspect of plant development. The vascular system, comprising xylem and phloem, acts as a major conduit for the transmission of long-distance signals in plants. In addition to expanding our knowledge of vascular development, numerous reports in the past two decades revealed that selective populations of RNAs, proteins, and phytohormones function as mobile signals. Many of these signals were shown to regulate diverse physiological processes, such as flowering, leaf and root development, nutrient acquisition, crop yield, and biotic/abiotic stress responses. In this review, we summarize the significant discoveries made in the past 25 years, with emphasis on key mobile signalling molecules (mRNAs, proteins including RNA-binding proteins, and small RNAs) that have revolutionized our understanding of how plants integrate various intrinsic and external cues in orchestrating growth and development. Additionally, we provide detailed insights on the emerging molecular mechanisms that might control the selective trafficking and delivery of phloem-mobile RNAs to target tissues. We also highlight the cross-kingdom movement of mobile signals during plant-parasite relationships. Considering the dynamic functions of these signals, their implications in crop improvement are also discussed.

RNA architecture influences plant biology

The majority of the genome is transcribed to RNA in living organisms. RNA transcripts can form astonishing arrays of secondary and tertiary structures via Watson-Crick, Hoogsteen, or wobble base pairing. In vivo, RNA folding is not a simple thermodynamic event of minimizing free energy. Instead, the process is constrained by transcription, RNA-binding proteins, steric factors, and the microenvironment. RNA secondary structure (RSS) plays myriad roles in numerous biological processes, such as RNA processing, stability, transportation, and translation in prokaryotes and eukaryotes. Emerging evidence has also implicated RSS in RNA trafficking, liquid-liquid phase separation, and plant responses to environmental variations such as temperature and salinity. At molecular level, RSS is correlated with splicing, polyadenylation, protein synthesis, and miRNA biogenesis and functions. In this review, we summarize newly reported methods for probing RSS in vivo and functions and mechanisms of RSS in plant physiology.

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.

RNA Motifs and Modification Involve in RNA Long-Distance Transport in Plants

A large number of RNA molecules have been found in the phloem of higher plants, and they can be transported to distant organelles through the phloem. RNA signals are important cues to be evolving in fortification strategies by long-distance transportation when suffering from various physiological challenges. So far, the mechanism of RNA selectively transportation through phloem cells is still in progress. Up to now, evidence have shown that several RNA motifs including Polypyrimidine (poly-CU) sequence, transfer RNA (tRNA)-related sequence, Single Nucleotide Mutation bound with specific RNA binding proteins to form Ribonucleotide protein (RNP) complexes could facilitate RNA mobility in plants. Furthermore, some RNA secondary structure such as tRNA-like structure (TLS), untranslation region (UTR) of mRNA, stem-loop structure of pre-miRNA also contributed to the mobility of RNAs. Latest researchs found that RNA methylation such as methylated 5′ cytosine (m5C) played an important role in RNA transport and function. These studies lay a theoretical foundation to uncover the mechanism of RNA transport. We aim to provide ideas and clues to inspire future research on the function of RNA motifs in RNA long-distance transport, furthermore to explore the underlying mechanism of RNA systematic signaling.

The interplay of phloem-mobile signals in plant development and stress response

Plants integrate a variety of biotic and abiotic factors for optimal growth in their given environment. While some of these responses are local, others occur distally. Hence, communication of signals perceived in one organ to a second, distal part of the plant and the coordinated developmental response require an intricate signaling system. To do so, plants developed a bipartite vascular system that mediates the uptake of water, minerals, and nutrients from the soil; transports high-energy compounds and building blocks; and traffics essential developmental and stress signals. One component of the plant vasculature is the phloem. The development of highly sensitive mass spectrometry and molecular methods in the last decades has enabled us to explore the full complexity of the phloem content. As a result, our view of the phloem has evolved from a simple transport path of photoassimilates to a major highway for pathogens, hormones and developmental signals. Understanding phloem transport is essential to comprehend the coordination of environmental inputs with plant development and, thus, ensure food security. This review discusses recent developments in its role in long-distance signaling and highlights the role of some of the signaling molecules. What emerges is an image of signaling paths that do not just involve single molecules but rather, quite frequently an interplay of several distinct molecular classes, many of which appear to be transported and acting in concert.

RNA transfer through tunneling nanotubes

It was already suggested in the early '70's that RNA molecules might transfer between mammalian cells in culture. Yet, more direct evidence for RNA transfer in animal and plant cells was only provided decades later, as this field became established. In this mini-review, we will describe evidence for the transfer of different types of RNA between cells through tunneling nanotubes (TNTs). TNTs are long, yet thin, open-ended cellular protrusions that are structurally distinct from filopodia. TNTs connect cells and can transfer many types of cargo, including small molecules, proteins, vesicles, pathogens, and organelles. Recent work has shown that TNTs can also transfer mRNAs, viral RNAs and non-coding RNAs. Here, we will review the evidence for TNT-mediated RNA transfer, discuss the technical challenges in this field, and conjecture about the possible significance of this pathway in health and disease.

Diversity of Plant Virus Movement Proteins: What Do They Have in Common?

The modern view of the mechanism of intercellular movement of viruses is based largely on data from the study of the tobacco mosaic virus (TMV) 30-kDa movement protein (MP). The discovered properties and abilities of TMV MP, namely, (a) in vitro binding of single-stranded RNA in a non-sequence-specific manner, (b) participation in the intracellular trafficking of genomic RNA to the plasmodesmata (Pd), and (c) localization in Pd and enhancement of Pd permeability, have been used as a reference in the search and analysis of candidate proteins from other plant viruses. Nevertheless, although almost four decades have passed since the introduction of the term “movement protein” into scientific circulation, the mechanism underlying its function remains unclear. It is unclear why, despite the absence of homology, different MPs are able to functionally replace each other in trans-complementation tests. Here, we consider the complexity and contradictions of the approaches for assessment of the ability of plant viral proteins to perform their movement function. We discuss different aspects of the participation of MP and MP/vRNA complexes in intra- and intercellular transport. In addition, we summarize the essential MP properties for their functioning as “conditioners”, creating a favorable environment for viral reproduction.

Phloem transport of structured RNAs: A widening repertoire of trafficking signals and protein factors

The conducting sieve tubes of the phloem consist of sieve elements (SEs), which are enucleate cells incapable of transcription and translation. Nevertheless, SEs contain a large variety of RNAs, and long-distance RNA trafficking via the phloem has been documented. The phloem transport of certain RNAs, as well as the further unloading of these RNAs at target tissues, is essential for plant individual development and responses to environmental cues. The translocation of such RNAs via the phloem is believed to be directed by RNA structural elements serving as phloem transport signals (PTSs), which are recognized by proteins that direct the PTS-containing RNAs into the phloem translocation pathway. The ability of phloem transport has been reported for several classes of structured RNAs including viroids, genuine tRNAs, mRNAs with tRNA sequences embedded into mRNA untranslated regions, tRNA-like structures in the genomic RNAs of plant viruses, and micro-RNA (miRNA) precursors (pri-miRNA). Here, three distinct types of such RNAs are discussed, along with the proteins that may specifically interact with these structures in the phloem. Three-dimensional (3D) motifs, which are characteristic of imperfect RNA duplexes, are discussed as elements of phloem-mobile structured RNAs specifically recognized by proteins involved in phloem transport, thus serving as PTSs.

To move or not to move: roles and specificity of plant RNA mobility

Intercellular communication in plants coordinates cellular functions during growth and development, and in response to environmental cues. RNAs figure prominently among the mobile signaling molecules used. Many hundreds of RNA species move over short and long distances, and can be mutually exchanged in biotic interactions. Understanding the specificity determinants of RNA mobility and the physiological relevance of this phenomenon are areas of active research. Here, we highlight the recent progress in our knowledge of small RNA and messenger RNA movement. Particular emphasis is given to novel insight into the specificity determinants of messenger RNA mobility, the role of small RNA movement in development, and the specificity of RNA exchange in plant-plant and plant-microbe interactions.

New insights into Arabidopsis transcriptome complexity revealed by direct sequencing of native RNAs

Arabidopsis thaliana transcriptomes have been extensively studied and characterized under different conditions. However, most of the current ‘RNA-sequencing’ technologies produce a relatively short read length and demand a reverse-transcription step, preventing effective characterization of transcriptome complexity. Here, we performed Direct RNA Sequencing (DRS) using the latest Oxford Nanopore Technology (ONT) with exceptional read length. We demonstrate that the complexity of the A. thaliana transcriptomes has been substantially under-estimated. The ONT direct RNA sequencing identified novel transcript isoforms at both the vegetative (14-day old seedlings, stage 1.04) and reproductive stages (stage 6.00–6.10) of development. Using in-house software called TrackCluster, we determined alternative transcription initiation (ATI), alternative polyadenylation (APA), alternative splicing (AS), and fusion transcripts. More than 38 500 novel transcript isoforms were identified, including six categories of fusion-transcripts that may result from differential RNA processing mechanisms. Aided by the Tombo algorithm, we found an enrichment of m5C modifications in the mobile mRNAs, consistent with a recent finding that m5C modification in mRNAs is crucial for their long-distance movement. In summary, ONT DRS offers an advantage in the identification and functional characterization of novel RNA isoforms and RNA base modifications, significantly improving annotation of the A. thaliana genome.

Cuscuta australis (dodder) parasite eavesdrops on the host plants' FT signals to flower

Many plants use environmental cues, including seasonal changes of day length (photoperiod), to control their flowering time. Under inductive conditions, FLOWERING LOCUS T (FT) protein is synthesized in leaves, and FT protein is a mobile signal, which is able to travel to the shoot apex to induce flowering. Dodders (Cuscuta, Convolvulaceae) are root- and leafless plants that parasitize a large number of autotrophic plant species with varying flowering time. Remarkably, some dodder species, e.g., Cuscuta australis, are able to synchronize their flowering with the flowering of their hosts. Detailed sequence inspection and expression analysis indicated that the FT gene in dodder C. australis very likely does not function in activating flowering. Using soybean host plants cultivated under inductive and noninductive photoperiod conditions and soybean and tobacco host plants, in which FT was overexpressed and knocked out, respectively, we show that FT-induced flowering of the host is likely required for both host and parasite flowering. Biochemical analysis revealed that host-synthesized FT signals are able to move into dodder stems, where they physically interact with a dodder FD transcription factor to activate dodder flowering. This study demonstrates that FTs can function as an important interplant flowering signal in host-dodder interactions. The unique means of flowering regulation of dodder illustrates how regressive evolution, commonly found in parasites, may facilitate the physiological synchronization of parasite and host, here allowing the C. australis parasite to time reproduction exactly with that of their hosts, likely optimizing parasite fitness.

Rhizobial infection triggers systemic transport of endogenous RNAs between shoots and roots in soybean

Legumes have evolved a symbiotic relationship with rhizobial bacteria and their roots form unique nitrogen-fixing organs called nodules. Studies have shown that abiotic and biotic stresses alter the profile of gene expression and transcript mobility in plants. However, little is known about the systemic transport of RNA between roots and shoots in response to rhizobial infection on a genome-wide scale during the formation of legume-rhizobia symbiosis. In our study, we found that two soybean (Glycine max) cultivars, Peking and Williams, show a high frequency of single nucleotide polymorphisms; this allowed us to characterize the origin and mobility of transcripts in hetero-grafts of these two cultivars. We identified 4,552 genes that produce mobile RNAs in soybean, and found that rhizobial infection triggers mass transport of mRNAs between shoots and roots at the early stage of nodulation. The majority of these mRNAs are of relatively low abundance and their transport occurs in a selective manner in soybean plants. Notably, the mRNAs that moved from shoots to roots at the early stage of nodulation were enriched in many nodule-related responsive processes. Moreover, the transcripts of many known symbiosis-related genes that are induced by rhizobial infection can move between shoots and roots. Our findings provide a deeper understanding of endogenous RNA transport in legume-rhizobia symbiotic processes.

Extracellular tRNAs and tRNA-derived fragments

Fragmentation of tRNAs generates a family of small RNAs collectively known as tRNA-derived fragments. These fragments vary in sequence and size but have been shown to regulate many processes involved in cell homoeostasis and adaptations to stress. Additionally, the field of extracellular RNAs (exRNAs) is rapidly growing because exRNAs are a promising source of biomarkers in liquid biopsies, and because exRNAs seem to play key roles in intercellular and interspecies communication. Herein, we review recent descriptions of tRNA-derived fragments in the extracellular space in all domains of life, both in biofluids and in cell culture. The purpose of this review is to find consensus on which tRNA-derived fragments are more prominent in each extracellular fraction (including extracellular vesicles, lipoproteins and ribonucleoprotein complexes). We highlight what is becoming clear and what is still controversial in this field, in order to stimulate future hypothesis-driven studies which could clarify the role of full-length tRNAs and tRNA-derived fragments in the extracellular space.

Long-Distance Movement of mRNAs in Plants

Long-distance transport of information molecules in the vascular tissues could play an important role in regulating plant growth and enabling plants to cope with adverse environments. Various molecules, including hormones, proteins, small peptides and small RNAs have been detected in the vascular system and proved to have systemic signaling functions. Sporadic studies have shown that a number of mRNAs produced in the mature leaves leave their origin cells and move to distal tissues to exert important physiological functions. In the last 3-5 years, multiple heterograft systems have been developed to demonstrate that a large quantity of mRNAs are mobile in plants. Further comparison of the mobile mRNAs identified from these systems showed that the identities of these mRNAs are very diverse. Although species-specific mRNAs may regulate the unique physiological characteristic of the plant, mRNAs with conserved functions across multiple species are worth more effort in identifying universal physiological mechanisms existing in the plant kingdom.

What determines the composition of the phloem sap? Is there any selectivity filter for macromolecules entering the phloem sieve elements?

In view of recent findings, it is still a matter of debate whether the composition of the phloem sap of higher plants is specific and based on a plasmodesmal selectivity filter for macromolecular transport, or whether simply related to size, abundance and half-life of the macromolecules within the phloem sap. A range of reports indicates specific function of phloem-mobile signaling molecules such as the florigen making it indispensable to discriminate specific macromolecules entering the phloem from others which cannot cross this selectivity filter. Nevertheless, several findings have discussed for a non-selective transport via plasmodesmata, or contamination of the phloem sap by degradation products coming from immature still developing young sieve elements undergoing differentiation. Here, we discuss several possibilities, and raise the question how selectivity of the phloem sap composition could be achieved thereby focusing on mobility and dynamics of sucrose transporter mRNA and proteins.

Multiplexed heritable gene editing using RNA viruses and mobile single guide RNAs

An in planta gene editing approach was developed wherein Cas9 transgenic plants are infected with an RNA virus that expresses single guide RNAs (sgRNAs). The sgRNAs are augmented with sequences that promote cell-to-cell mobility. Mutant progeny are recovered in the next generation at frequencies ranging from 65 to 100%; up to 30% of progeny derived from plants infected with a virus expressing three sgRNAs have mutations in all three targeted loci.

Viral Hacks of the Plant Vasculature: The Role of Phloem Alterations in Systemic Virus Infection

For plant viruses, the ability to load into the vascular phloem and spread systemically within a host is an essential step in establishing a successful infection. However, access to the vascular phloem is highly regulated, representing a significant obstacle to virus loading, movement, and subsequent unloading into distal uninfected tissues. Recent studies indicate that during virus infection, phloem tissues are a source of significant transcriptional and translational alterations, with the number of virus-induced differentially expressed genes being four- to sixfold greater in phloem tissues than in surrounding nonphloem tissues. In addition, viruses target phloem-specific components as a means to promote their own systemic movement and disrupt host defense processes. Combined, these studies provide evidence that the vascular phloem plays a significant role in the mediation and control of host responses during infection and as such is a site of considerable modulation by the infecting virus. This review outlines the phloem responses and directed reprograming mechanisms that viruses employ to promote their movement through the vasculature.

Systemic Long-Distance Signaling and Communication Between Rootstock and Scion in Grafted Vegetables

Grafting is widely used in fruit, vegetable, and flower propagation to improve biotic and abiotic stress resistance, yield, and quality. At present, the systemic changes caused by grafting, as well as the mechanisms and effects of long-distance signal transport between rootstock and scion have mainly been investigated in model plants (Arabidopsis thaliana and Nicotiana benthamiana). However, these aspects of grafting vary when different plant materials are grafted, so the study of model plants provides only a theoretical basis and reference for the related research of grafted vegetables. The dearth of knowledge about the transport of signaling molecules in grafted vegetables is inconsistent with the rapid development of large-scale vegetable production, highlighting the need to study the mechanisms regulating the rootstock-scion interaction and long-distance transport. The rapid development of molecular biotechnology and "omics" approaches will allow researchers to unravel the physiological and molecular mechanisms involved in the rootstock-scion interaction in vegetables. We summarize recent progress in the study of the physiological aspects (e.g., hormones and nutrients) of the response in grafted vegetables and focus in particular on long-distance molecular signaling (e.g., RNA and proteins). This review provides a theoretical basis for studies of the rootstock-scion interaction in grafted vegetables, as well as provide guidance for rootstock breeding and selection to meet specific demands for efficient vegetable production.

Detection and in vitro studies of Cucurbita maxima phloem serpin-1 RNA-binding properties

Apart from being a conduit for photoassimilate transport in plants, the phloem serves as a pathway for transport of proteins and RNAs from sites of their synthesis to distant plant parts. As demonstrated for mRNAs and small RNAs such as miRNA and siRNA, their phloem transport is largely involved in responses to environmental cues including stresses and pathogen attacks. RNA molecules are believed to be transported in the phloem in the form of complexes with RNA-binding proteins; however, proteins forming such complexes are generally poorly studied. Here, we demonstrate that the Cucurbita maxima phloem serpin-1 (CmPS1), which has been previously described as a functional protease inhibitor capable of long-distance transport via the phloem, is able to bind RNA in vitro. Among different RNAs tested, CmPS1 exhibits a preference for imperfect RNA duplexes and the highest affinity to tRNA. A characteristic complex formed by CmPS1 with tRNA is not observed upon CmPS1 binding to tRNA-like structures of plant viruses. Mutational analysis demonstrates that the CmPS1 N-terminal region is not involved in RNA binding. Since antithrombin-III, the human protease inhibitor of serpin family most closely sequence-related to CmPS1, is found to be unable to bind RNA, one can suggest that, in its evolution, CmPS1 has gained the RNA binding capability as an additional function likely relevant to its specific activities in the plant phloem.

mRNA Localization in Plant Cells

Localization of mRNAs at the subcellular level is an essential mechanism for specific protein targeting and local control of protein synthesis in both eukaryotes and bacteria. While mRNA localization is well documented in metazoans, somatic cells, and microorganisms, only a handful of well-defined mRNA localization examples have been reported in vascular plants and algae. This review summarizes the function and mechanism of mRNA localization and highlights recent studies of mRNA localization in vascular plants. While the emphasis focuses on storage protein mRNA localization in rice endosperm cells, information on targeting of RNAs to organelles (chloroplasts and mitochondria) and plasmodesmata is also discussed.

In Vivo Imaging of Mobile mRNAs in Plants Using MS2

Multicellular organisms rely on systemic signals to orchestrate diverse developmental and physiological programs. To transmit environmental stimuli that perceived in the leaves, plants recruit many mobile molecules including mobile mRNAs as systemic signals for interorgan communication. The mobile mRNAs provide an efficient and specific remote control system for plants to cope with environmental dynamics. Upon being transcribed in local tissues, mobile mRNAs are selectively targeted to plasmodesmata for cell-to-cell and long-distance translocation. The mRNA labeling system based on the RNA-binding protein MS2 provides a useful tool to investigate intracellular trafficking of mobile mRNAs in plants. Here we describe the detailed protocol to visualize intracellular trafficking of plant mobile mRNAs by using the MS2 live-cell imaging system.

Transcriptome changes in reciprocal grafts involving watermelon and bottle gourd reveal molecular mechanisms involved in increase of the fruit size, rind toughness and soluble solids

Transcriptome landscape reveals the molecular mechanisms involved in the improvement of fruit traits by the grafting of watermelon and bottle gourd. Grafting has been used as a sustainable alternative for watermelon breeding to control soil-borne pathogens and to increase tolerance to various abiotic stresses. However, some reports have shown that grafting can negatively affect the quality of fruits. Despite several field studies on the effects of grafting on fruit quality, the regulation of this process at the molecular level has not been revealed. The aim of this study was to elucidate various molecular mechanisms involved in different tissues of heterografted watermelon and bottle gourd plants. Grafting with bottle gourd rootstock increased the size and rind thickness of watermelon fruits, whereas that with watermelon rootstock produced bottle gourd fruits with higher total soluble solid content and thinner rinds. Correspondingly, genes related to ripening, softening, cell wall strengthening, stress response and disease resistance were differentially expressed in watermelon fruits. Moreover, genes associated mainly with sugar metabolism were differentially expressed in bottle gourd fruits. RNA-seq revealed more than 400 mobile transcripts across the heterografted sets. More than half of these were validated from PlaMoM, a database for plant mobile macromolecules. In addition, some of these mobile transcripts contained a transfer RNA-like structure. Other RNA motifs were also enriched in these transcripts, most with a biological role based on GO analysis. This transcriptome study provided a comprehensive understanding of various molecular mechanisms underlying grafted tissues in watermelon.

Development of Parasitic Organs of a Stem Holoparasitic Plant in Genus Cuscuta

Parasitic plants infect a broad range of plant species including economically important crops. They survive by absorbing water, minerals, and photosynthates from their hosts. To support their way of life, parasitic plants generally establish parasitic organs that allow them to attach to their hosts and to efficiently absorb substances from the vascular system of the host. Here, we summarize the recent progress in understanding the mechanisms underlying the formation of these parasitic organs, focusing on the process depicted in the stem holoparasitic genus, Cuscuta. An attachment structure called "holdfast" on the stem surface is induced by the light and contact stimuli. Concomitantly with holdfast formation, development of an intrusive structure called haustorium initiates in the inner cortex of the Cuscuta stem, and it elongates through apoplastic space of the host tissue. When haustoria reaches to host vascular tissues, they begin to form vascular conductive elements to connect vascular tissue of Cuscuta stem to those of host. Recent studies have shown parasite-host interaction in the interfacial cell wall, and regulation of development of these parasitic structures in molecular level. We also briefly summarize the role of host receptor in the control of compatibility between Cuscuta and hosts, on which occurrence of attachment structure depends, and the role of plant-to-plant transfer of long-distance signals after the establishment of conductive structure.

Small RNA Mobility: Spread of RNA Silencing Effectors and its Effect on Developmental Processes and Stress Adaptation in Plants

Plants are exposed every day to multiple environmental cues, and tight transcriptome reprogramming is necessary to control the balance between responses to stress and processes of plant growth. In this context, the silencing phenomena mediated by small RNAs can drive transcriptional and epigenetic regulatory modifications, in turn shaping plant development and adaptation to the surrounding environment. Mounting experimental evidence has recently pointed to small noncoding RNAs as fundamental players in molecular signalling cascades activated upon exposure to abiotic and biotic stresses. Although, in the last decade, studies on stress responsive small RNAs increased significantly in many plant species, the physiological responses triggered by these molecules in the presence of environmental stresses need to be further explored. It is noteworthy that small RNAs can move either cell-to-cell or systemically, thus acting as mobile silencing effectors within the plant. This aspect has great importance when physiological changes, as well as epigenetic regulatory marks, are inspected in light of plant environmental adaptation. In this review, we provide an overview of the categories of mobile small RNAs in plants, particularly focusing on the biological implications of non-cell autonomous RNA silencing in the stress adaptive response and epigenetic modifications.

Molecular Responses during Plant Grafting and Its Regulation by Auxins, Cytokinins, and Gibberellins

Plant grafting is an important horticulture technique used to produce a new plant after joining rootstock and scion. This is one of the most used techniques by horticulturists to enhance the quality and production of various crops. Grafting helps in improving the health of plants, their yield, and the quality of plant products, along with the enhancement of their postharvest life. The main process responsible for successful production of grafted plants is the connection of vascular tissues. This step determines the success rate of grafts and hence needs to be studied in detail. There are many factors that regulate the connection of scion and stock, and plant hormones are of special interest for researchers in the recent times. These phytohormones act as signaling molecules and have the capability of translocation across the graft union. Plant hormones, mainly auxins, cytokinins, and gibberellins, play a major role in the regulation of various key physiological processes occurring at the grafting site. In the current review, we discuss the molecular mechanisms of graft development and the phytohormone-mediated regulation of the growth and development of graft union.

Systemic signalling through translationally controlled tumour protein controls lateral root formation in Arabidopsis

The plant body plan and primary organs are established during embryogenesis. However, in contrast to animals, plants have the ability to generate new organs throughout their whole life. These give them an extraordinary developmental plasticity to modulate their size and architecture according to environmental constraints and opportunities. How this plasticity is regulated at the whole-organism level is elusive. Here we provide evidence for a role for translationally controlled tumour protein (TCTP) in regulating the iterative formation of lateral roots in Arabidopsis. AtTCTP1 modulates root system architecture through a dual function: as a general constitutive growth promoter enhancing root elongation and as a systemic signalling agent via mobility in the vasculature. AtTCTP1 encodes mRNAs with long-distance mobility between the shoot and roots. Mobile shoot-derived TCTP1 gene products act specifically to enhance the frequency of lateral root initiation and emergence sites along the primary root pericycle, while root elongation is controlled by local constitutive TCTP1 expression and scion size. These findings uncover a novel type for an integrative signal in the control of lateral root initiation and the compromise for roots between branching more profusely or elongating further. They also provide the first evidence in plants of an extracellular function of the vital, highly expressed ubiquitous TCTP1.

Sieve element biology provides leads for research on phytoplasma lifestyle in plant hosts

Phytoplasmas reside exclusively in sieve tubes, tubular arrays of sieve element-companion cell complexes. Hence, the cell biology of sieve elements may reveal (ultra)structural and functional conditions that are of significance for survival, propagation, colonization, and effector spread of phytoplasmas. Electron microscopic images suggest that sieve elements offer facilities for mobile and stationary stages in phytoplasma movement. Stationary stages may enable phytoplasmas to interact closely with diverse sieve element compartments. The unique, reduced sieve element outfit requires permanent support by companion cells. This notion implies a future focus on the molecular biology of companion cells to understand the sieve element-phytoplasma inter-relationship. Supply of macromolecules by companion cells is channelled via specialized symplasmic connections. Ca2+-mediated gating of symplasmic corridors is decisive for the communication within and beyond the sieve element-companion cell complex and for the dissemination of phytoplasma effectors. Thus, Ca2+ homeostasis, which affects sieve element Ca2+ signatures and induces a range of modifications, is a key issue during phytoplasma infection. The exceptional physical and chemical environment in sieve elements seems an essential, though not the only factor for phytoplasma survival.

Key checkpoints in the movement of plant viruses through the host

Plant viruses cannot exploit any of the membrane fusion-based routes of entry described for animal viruses. In addition, one of the distinctive structures of plant cells, the cell wall, acts as the first barrier against the invasion of pathogens. To overcome the rigidity of the cell wall, plant viruses normally take advantage of the way of life of different biological vectors. Alternatively, the physical damage caused by environmental stresses can facilitate virus entry. Once inside the cell and taking advantage of the characteristic symplastic continuity of plant cells, viruses need to remodel and/or modify the restricted pore size of the plasmodesmata (channels that connect plant cells). In a successful interaction for the virus, it can reach the vascular tissue to systematically invade the plant. The connections between the different cell types in this path are not designed to allow the passage of molecules with the complexity of viruses. During this process, viruses face different cell barriers that must be overcome to reach the distal parts of the plant. In this review, we highlight the current knowledge about how plant RNA viruses enter plant cells, move between them to reach vascular cells and overcome the different physical and cellular barriers that the phloem imposes. Finally, we update the current research on cellular organelles as key regulator checkpoints in the long-distance movement of plant viruses.

m5C Methylation Guides Systemic Transport of Messenger RNA over Graft Junctions in Plants

In plants, transcripts move to distant body parts to potentially act as systemic signals regulating development and growth. Thousands of messenger RNAs (mRNAs) are transported across graft junctions via the phloem to distinct plant parts. Little is known regarding features, structural motifs, and potential base modifications of transported transcripts and how these may affect their mobility. We identified Arabidopsis thaliana mRNAs harboring the modified base 5-methylcytosine (m⁵C) and found that these are significantly enriched in mRNAs previously described as mobile, moving over graft junctions to distinct plant parts. We confirm this finding with graft-mobile methylated mRNAs TRANSLATIONALLY CONTROLLED TUMOR PROTEIN 1 (TCTP1) and HEAT SHOCK COGNATE PROTEIN 70.1 (HSC70.1), whose mRNA transport is diminished in mutants deficient in m⁵C mRNA methylation. Together, our results point toward an essential role of cytosine methylation in systemic mRNA mobility in plants and that TCTP1 mRNA mobility is required for its signaling function.

RNA phloem transport mediated by pre-miRNA and viral tRNA-like structures

Phloem-mobile mRNAs are assumed to contain sequence elements directing RNA to the phloem translocation pathway. One of such elements is represented by tRNA sequences embedded in untranslated regions of many mRNAs, including those proved to be mobile. Genomic RNAs of a number of plant viruses possess a 3'-terminal tRNA-like structures (TLSs) only distantly related to genuine tRNAs, but nevertheless aminoacylated and capable of interaction with some tRNA-binding proteins. Here, we elaborated an experimental system for analysis of RNA phloem transport based on an engineered RNA of Potato virus X capable of replication, but not encapsidation and movement in plants. The TLSs of Brome mosaic virus, Tobacco mosaic virus and Turnip yellow mosaic virus were demonstrated to enable the phloem transport of foreign RNA. A miRNA precursor, pre-miR390b, was also found to render RNA competent for the phloem transport. In line with this, sequences of miRNA precursors were identified in a Cucurbita maxima phloem transcriptome, supporting the hypothesis that, at least in some cases, miRNA phloem signaling can involve miRNA precursors. Collectively, the data presented here suggest that RNA molecules can be directed into the phloem translocation pathway by structured RNA elements such as those of viral TLSs and miRNA precursors.

Analyzing and Predicting Phloem Mobility of Macromolecules with an Online Database

Phloem, a specialized plant tissue, serves as a superhighway for macromolecular exchanges between different organs or tissues in plants. These mobile macromolecules may function as signaling molecules to sense intrinsic developmental cues or environmental inputs. Among these mobile molecules, RNAs generally need non-cell-autonomous pathway proteins (NCAPPs) to bind to and help them move along the symplasmic passage (through plasmodesmata) into the phloem stream. Grafting experiments combined with next-generation sequencing discovered that around 11.4% of identified Arabidopsis mobile mRNAs have a tRNA-like structure (TLS) motif. Adding an artificial tRNA-like structure at the 5' end of cell-autonomous RNAs (e.g., GUS transcript) can trigger its mobility and movement across a grafting junction to distant organs. Based on the accumulated data and the role of the TLS motif in RNA mobility, we built a web server in our database PLaMoM (a database for plant mobile macromolecules) to enable plant biologists to predict and analyze the transcripts they are interested in. In this chapter, we describe how to use our built-in web server to investigate RNA mobility.

Small RNA Functions as a Trafficking Effector in Plant Immunity

Small RNAs represent a class of small but powerful agents that regulate development and abiotic and biotic stress responses during plant adaptation to a constantly challenging environment. Previous findings have revealed the important roles of small RNAs in diverse cellular processes. The recent discovery of bidirectional trafficking of small RNAs between different kingdoms has raised many interesting questions. The subsequent demonstration of exosome-mediated small RNA export provided a possible tool for further investigating how plants use small RNAs as a weapon during the arms race between plant hosts and pathogens. This review will focus on discussing the roles of small RNAs in plant immunity in terms of three aspects: the biogenesis of extracellular small RNAs and the transportation and trafficking small RNA-mediated gene silencing in pathogens.

Plant lncRNAs are enriched in and move systemically through the phloem in response to phosphate deficiency

In response to phosphate (Pi) deficiency, it has been shown that micro-RNAs (miRNAs) and mRNAs are transported through the phloem for delivery to sink tissues. Growing evidence also indicates that long non-coding RNAs (lncRNAs) are critical regulators of Pi homeostasis in plants. However, whether lncRNAs are present in and move through the phloem, in response to Pi deficiency, remains to be established. Here, using cucumber as a model plant, we show that lncRNAs are enriched in the phloem translocation stream and respond, systemically, to an imposed Pi-stress. A well-known lncRNA, IPS1, the target mimic (TM) of miRNA399, accumulates to a high level in the phloem, but is not responsive to early Pi deficiency. An additional 24 miRNA TMs were also detected in the phloem translocation stream; among them miRNA171 TMs and miR166 TMs were induced in response to an imposed Pi stress. Grafting studies identified 22 lncRNAs which move systemically into developing leaves and root tips. A CU-rich PTB motif was further identified in these mobile lncRNAs. Our findings revealed that lncRNAs respond to Pi deficiency, non-cell-autonomously, and may act as systemic signaling agents to coordinate early Pi deficiency signaling, at the whole-plant level.

Genome-wide probing RNA structure with the modified DMS-MaPseq in Arabidopsis

Transcripts have intrinsic propensity to form stable secondary structure that is fundamental to regulate RNA transcription, splicing, translation, RNA localization and turnover. Numerous methods that integrate chemical reactions with next-generation sequencing (NGS) have been applied to study in vivo RNA structure, providing new insights into RNA biology. Dimethyl sulfate (DMS) probing coupled with mutational profiling through NGS (DMS-MaPseq) is a newly developed method for revealing genome-wide or target-specific RNA structure. Herein, we present our experimental protocol of a modified DMS-MaPseq method for plant materials. The DMS treatment condition was optimized, and library preparation procedures were simplified. We also provided custom scripts for bioinformatic analysis of genome-wide DMS-MaPseq data. Bioinformatic results showed that our method could generate high-quality and reproducible data. Further, we assessed sequencing depth and coverage for genome-wide RNA structure profiling in Arabidopsis, and provided two examples of in vivo structure of mobile RNAs. We hope that our modified DMS-MaPseq method will serve as a powerful tool for analyzing in vivo RNA structurome in plants.

Intercellular and systemic trafficking of RNAs in plants

Plants have evolved dynamic and complex networks of cell-to-cell communication to coordinate and adapt their growth and development to a variety of environmental changes. In addition to small molecules, such as metabolites and phytohormones, macromolecules such as proteins and RNAs also act as signalling agents in plants. As information molecules, RNAs can move locally between cells through plasmodesmata, and over long distances through phloem. Non-cell-autonomous RNAs may act as mobile signals to regulate plant development, nutrient allocation, gene silencing, antiviral defence, stress responses and many other physiological processes in plants. Recent work has shed light on mobile RNAs and, in some cases, uncovered their roles in intercellular and systemic signalling networks. This review summarizes the current knowledge of local and systemic RNA movement, and discusses the potential regulatory mechanisms and biological significance of RNA trafficking in plants.

Mobility of Antiflorigen and PEBP mRNAs in Tomato-Tobacco Heterografts

Photoperiodic floral induction is controlled by the leaf-derived and antagonistic mobile signals florigen and antiflorigen. In response to photoperiodic variations, florigen and antiflorigen are produced in leaves and translocated through phloem to the apex, where they counteract floral initiation. Florigen and antiflorigen are encoded by a pair of homologs belonging to FLOWERING LOCUS T (FT)- or TERMINAL FLOWER1 (TFL1)-like clades in the phosphatidylethanolamine-binding domain protein (PEBP) family. The PEBP gene family contains FT-, TFL1-, and MOTHER OF FT AND TFL1 (MFT)-like clades. Evolutionary analysis suggests that FT- and TFL1-like clades arose from an ancient MFT-like clade. The protein movement of the PEBP family is an evolutionarily conserved mechanism in many plants; however, the mRNA movement of the PEBP family remains controversial. Here, we examined the mRNA movement of PEBP genes in different plant species. We identified a tobacco (Nicotiana sylvestris) CENTRORADIALIS-like1 gene, denoted NsCET1, and showed that NsCET1 is an ortholog of the Arabidopsis (Arabidopsis thaliana) antiflorigen ATC In tobacco, NsCET1 acts as a mobile molecule that non-cell-autonomously inhibits flowering. Grafting experiments showed that endogenous and ectopically expressed NsCET1 mRNAs move long distances in tobacco and Arabidopsis. Heterografts of tobacco and tomato (Solanum lycopersicum) showed that, in addition to NsCET1, multiple members of the FT-, TFL1-, and MFT-like clades of tobacco and tomato PEBP gene families are mobile mRNAs. Our results suggest that the mRNA mobility is a common feature of the three clades of PEBP-like genes among different plant species.

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.

Long-distance movement of helical multipartite phytoviruses: keep connected or die?

All living organisms have to preserve genome integrity to ensure the survival of progeny generations. Viruses, though often regarded as 'non living', protect their nucleic acids from biotic and abiotic stresses, ranging from nuclease action to radiation-induced adducts. When the viral genome is split into multiple segments, preservation of at least one copy of each segment is required. While segmented and monopartite viruses use an all-in-one strategy, multipartite viruses have to address in the cell at least one of each viral particle in which the split positive stranded RNA genome is individually packaged. Here, we review and discuss the biology of multipartite helical RNA phytoviruses to outline our current hypothesis on a coordinated genomic RNA network RNP complex that preserves an all-in-one strategy and genome integrity.

Conceptual and Methodological Considerations on mRNA and Proteins as Intercellular and Long-Distance Signals

High-throughput studies identified approximately one-fifth of Arabidopsis protein-encoding transcripts to be graft transmissible and to move over long distances in the phloem. In roots, one-fifth of transcription factors were annotated as non-cell autonomous, moving between cells. Is this massive transport a way of interorgan and cell-cell communication or does it serve different purposes? On the tissue level, many microRNAs (miRNAs) and all small interfering RNAs (siRNAs) act non-cell autonomously. Why are these RNAs and proteins not just expressed in cells where they exert their function? Short- and long-distance transport of these macromolecules raises the question of whether all mobile mRNAs and transcription factors could be defined as signaling molecules. Since the answer is not clear yet, we will discuss in this review conceptual approaches to this phenomenon using a single mobile signaling macromolecule, FLOWERING LOCUS T, which has been characterized extensively. We conclude that careful individual studies of mobile macromolecules are necessary to uncover their biological function and the observed massive mobility. To stimulate such studies, we provide a review summarizing the resourceful wealth of experimental approaches to this intriguing question and discuss methodological scopes and limits.

Selective Targeting of Mobile mRNAs to Plasmodesmata for Cell-to-Cell Movement

Many plant mRNAs move from cell to cell or long distance to execute non-cell-autonomous functions. These mobile mRNAs traffic through the phloem to regulate many developmental processes, but despite the burgeoning discovery of mobile mRNAs, little is known about the mechanism underlying the intracellular sorting of these mRNAs. Here, we exploited a fluorescence-based mRNA labeling system, using the bacteriophage coat protein MS2, fused to GFP (MS2-GFP) and an MS2 recognition site in the RNA of interest, to visualize the intracellular trafficking of mobile mRNAs in living plant cells of Nicotiana benthamiana We first improved this system by using the nuclear localization sequence from FD, which substantially reduced the fluorescent background of MS2-GFP in the cytoplasm. The modified system allowed us to observe the cytoplasmic fluorescent foci dependent on MS2-binding sites. Coexpressing the MS2-GFP system with a virus movement protein, which is a plasmodesmata (PD)-localized nonspecific RNA-binding protein, targeted cytoplasmic fluorescent foci to the PD, suggesting that the cytoplasmic fluorescent foci contain mRNA and MS2-GFP. Our ex vivo RNA imaging revealed that mobile but not nonmobile mRNAs were selectively targeted to PD. Real-time images of intracellular translocation revealed that the translocation of mRNA and organelles in the transvacuolar strands may be governed by the same mechanism. Our study suggests that PD targeting of mRNA is a selective step in determining mRNA cell-to-cell movement of mRNAs.

Plasmodesmata in phloem: different gateways for different cargoes

The long-distance transport of sugars and nutrients through the phloem is essential for the proper function and growth of vascular plants. However, in addition to essential nutrients and sugars, phloem sap also contains small molecules (e.g. hormones) as well as a diverse population of macromolecules (i.e. proteins small RNAs, and mRNAs), the endogenous functions of which remain largely unknown. Understanding the cellular origins of these mobile macromolecules, their path into and out of the phloem translocation stream, and their fate at their new destination is essential for characterizing their presumptive function. Specialized plasmodesmal connections that regulate phloem entry and exit are central to all of these processes. Here, we highlight new discoveries underscoring plasmodesmal structure and function during unloading of various molecules in the sink, and discuss how these findings shape a new view for the potential function of phloem-mobile macromolecules.