PlaMoM: a comprehensive database compiles plant mobile macromolecules

Exact Bayesian inference for the detection of graft-mobile transcripts from sequencing data

The long-distance transport of messenger RNAs (mRNAs) has been shown to be important for several developmental processes in plants. A popular method for identifying travelling mRNAs is to perform RNA-Seq on grafted plants. This approach depends on the ability to correctly assign sequenced mRNAs to the genetic background from which they originated. The assignment is often based on the identification of single-nucleotide polymorphisms (SNPs) between otherwise identical sequences. A major challenge is therefore to distinguish SNPs from sequencing errors. Here, we show how Bayes factors can be computed analytically using RNA-Seq data over all the SNPs in an mRNA. We used simulations to evaluate the performance of the proposed framework and demonstrate how Bayes factors accurately identify graft-mobile transcripts. The comparison with other detection methods using simulated data shows how not taking the variability in read depth, error rates and multiple SNPs per transcript into account can lead to incorrect classification. Our results suggest experimental design criteria for successful graft-mobile mRNA detection and show the pitfalls of filtering for sequencing errors or focusing on single SNPs within an mRNA.

Long-distance transport RNAs between rootstocks and scions and graft hybridization

The present review addresses the advances of the identification methods, functions, and transportation mechanism of long-distance transport RNAs between rootstock and scion. In addition, we highlight the cognitive processes and potential mechanisms of graft hybridization. Phloem, the main transport channel of higher plants, plays an important role in the growth and development of plants. Numerous studies have identified a large number of RNAs, including mRNAs, miRNAs, siRNAs, and lncRNAs, in the plant phloem. They can not only be transported to long distances across the grafting junction in the phloem, but also act as signal molecules to regulate the growth, development, and stress resistance of remote cells or tissues, resulting in changes in the traits of rootstocks and scions. Many mobile RNAs have been discovered, but their detection methods, functions, and long-distance transport mechanisms remain to be elucidated. In addition, grafting hybridization, a phenomenon that has been questioned before, and which has an important role in selecting for superior traits, is gradually being recognized with the emergence of new evidence and the prevalence of horizontal gene transfer between parasitic plants. In this review, we outline the species, functions, identification methods, and potential mechanisms of long-distance transport RNAs between rootstocks and scions after grafting. In addition, we summarize the process of recognition and the potential mechanisms of graft hybridization. This study aimed to emphasize the role of grafting in the study of long-distance signals and selection for superior traits and to provide ideas and clues for further research on long-distance transport RNAs and graft hybridization.

Mobile Host mRNAs Are Translated to Protein in the Associated Parasitic Plant Cuscuta campestris

Cuscuta spp. are obligate parasites that connect to host vascular tissue using a haustorium. In addition to water, nutrients, and metabolites, a large number of mRNAs are bidirectionally exchanged between Cuscuta spp. and their hosts. This trans-specific movement of mRNAs raises questions about whether these molecules function in the recipient species. To address the possibility that mobile mRNAs are ultimately translated, we built upon recent studies that demonstrate a role for transfer RNA (tRNA)-like structures (TLSs) in enhancing mRNA systemic movement. C. campestris was grown on Arabidopsis that expressed a β-glucuronidase (GUS) reporter transgene either alone or in GUS-tRNA fusions. Histochemical staining revealed localization in tissue of C. campestris grown on Arabidopsis with GUS-tRNA fusions, but not in C. campestris grown on Arabidopsis with GUS alone. This corresponded with detection of GUS transcripts in Cuscuta on Arabidopsis with GUS-tRNA, but not in C. campestris on Arabidopsis with GUS alone. Similar results were obtained with Arabidopsis host plants expressing the same constructs containing an endoplasmic reticulum localization signal. In C. campestris, GUS activity was localized in the companion cells or phloem parenchyma cells adjacent to sieve tubes. We conclude that host-derived GUS mRNAs are translated in C. campestris and that the TLS fusion enhances RNA mobility in the host-parasite interactions.

Granger-causal testing for irregularly sampled time series with application to nitrogen signalling in Arabidopsis

MOTIVATION

Identification of system-wide causal relationships can contribute to our understanding of long-distance, intercellular signalling in biological organisms. Dynamic transcriptome analysis holds great potential to uncover coordinated biological processes between organs. However, many existing dynamic transcriptome studies are characterized by sparse and often unevenly spaced time points that make the identification of causal relationships across organs analytically challenging. Application of existing statistical models, designed for regular time series with abundant time points, to sparse data may fail to reveal biologically significant, causal relationships. With increasing research interest in biological time series data, there is a need for new statistical methods that are able to determine causality within and between time series data sets. Here, a statistical framework was developed to identify (Granger) causal gene-gene relationships of unevenly spaced, multivariate time series data from two different tissues of Arabidopsis thaliana in response to a nitrogen signal.

RESULTS

This work delivers a statistical approach for modelling irregularly sampled bivariate signals which embeds functions from the domain of engineering that allow to adapt the model's dependence structure to the specific sampling time. Using maximum-likelihood to estimate the parameters of this model for each bivariate time series, it is then possible to use bootstrap procedures for small samples (or asymptotics for large samples) in order to test for Granger-Causality. When applied to the A.thaliana data, the proposed approach produced 3078 significant interactions, in which 2012 interactions have root causal genes and 1066 interactions have shoot causal genes. Many of the predicted causal and target genes are known players in local and long-distance nitrogen signalling, including genes encoding transcription factors, hormones and signalling peptides. Of the 1007 total causal genes (either organ), 384 are either known or predicted mobile transcripts, suggesting that the identified causal genes may be directly involved in long-distance nitrogen signalling through intercellular interactions. The model predictions and subsequent network analysis identified nitrogen-responsive genes that can be further tested for their specific roles in long-distance nitrogen signalling.

AVAILABILITY AND IMPLEMENTATION

The method was developed with the R statistical software and is made available through the R package 'irg' hosted on the GitHub repository https://github.com/SMAC-Group/irg where also a running example vignette can be found (https://smac-group.github.io/irg/articles/vignette.html). A few signals from the original data set are made available in the package as an example to apply the method and the complete A.thaliana data can be found at: https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE97500.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

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.

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.

Reshaping of the Arabidopsis thaliana Proteome Landscape and Co-regulation of Proteins in Development and Immunity

Proteome remodeling is a fundamental adaptive response, and proteins in complexes and functionally related proteins are often co-expressed. Using a deep sampling strategy we define core proteomes of Arabidopsis thaliana tissues with around 10 000 proteins per tissue, and absolutely quantify (copy numbers per cell) nearly 16 000 proteins throughout the plant lifecycle. A proteome-wide survey of global post-translational modification revealed amino acid exchanges pointing to potential conservation of translational infidelity in eukaryotes. Correlation analysis of protein abundance uncovered potentially new tissue- and age-specific roles of entire signaling modules regulating transcription in photosynthesis, seed development, and senescence and abscission. Among others, the data suggest a potential function of RD26 and other NAC transcription factors in seed development related to desiccation tolerance as well as a possible function of cysteine-rich receptor-like kinases (CRKs) as ROS sensors in senescence. All of the components of ribosome biogenesis factor (RBF) complexes were found to be co-expressed in a tissue- and age-specific manner, indicating functional promiscuity in the assembly of these less-studied protein complexes in Arabidopsis.Furthermore, we characterized detailed proteome remodeling in basal immunity by treating Arabidopsis seeldings with flg22. Through simultaneously monitoring phytohormone and transcript changes upon flg22 treatment, we obtained strong evidence of suppression of jasmonate (JA) and JA-isoleucine (JA-Ile) levels by deconjugation and hydroxylation by IAA-ALA RESISTANT3 (IAR3) and JASMONATE-INDUCED OXYGENASE 2 (JOX2), respectively, under the control of JASMONATE INSENSITIVE 1 (MYC2), suggesting an unrecognized role of a new JA regulatory switch in pattern-triggered immunity. Taken together, the datasets generated in this study present extensive coverage of the Arabidopsis proteome in various biological scenarios, providing a rich resource available to the whole plant science community.

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.

NAMS webserver: coding potential assessment and functional annotation of plant transcripts

Recent advances in transcriptomics have uncovered lots of novel transcripts in plants. To annotate such transcripts, dissecting their coding potential is a critical step. Computational approaches have been proven fruitful in this task; however, most current tools are designed/optimized for mammals and only a few of them have been tested on a limited number of plant species. In this work, we present NAMS webserver, which contains a novel coding potential classifier, NAMS, specifically optimized for plants. We have evaluated the performance of NAMS using a comprehensive dataset containing more than 3 million transcripts from various plant species, where NAMS demonstrates high accuracy and remarkable performance improvements over state-of-the-art software. Moreover, our webserver also furnishes functional annotations, aiming to provide users informative clues to the functions of their transcripts. Considering that most plant species are poorly characterized, our NAMS webserver could serve as a valuable resource to facilitate the transcriptomic studies. The webserver with testing dataset is freely available at http://sunlab.cpy.cuhk.edu.hk/NAMS/.

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.

A constitutive and drought-responsive mRNA undergoes long-distance transport in pear (Pyrus betulaefolia) phloem

Pear is one of the most commercially important fruit trees worldwide and is widely cultivated in temperate zones. Drought stress can greatly limit pear fruit yield and quality. Pyrus betulaefolia Bunge, a drought-resistant pear rootstock that is commonly used in northern China, confers favourable characteristics to pear scions, allowing them to respond rapidly to drought stress via the transport of macromolecules such as phloem-mobile mRNAs. How drought-responsive mRNAs function as phloem-mobile signals remains unknown, however. Here, we used RNA sequencing (RNA-seq) combined with SNP analysis to identify mobile mRNAs in P. betulaefolia. We focused on mobile mRNAs that respond to drought stress and found that the abundance of a novel mRNA named PbDRM (P. betulaefoliaDROUGHT-RESPONSIVE MOBILE GENE) significantly increased in several different scion cultivars when they were grafted onto P. betulaefolia rootstock under drought conditions. In addition, downregulating PbDRM by virus-induced gene silencing (VIGS) increased the drought sensitivity of P. betulaefolia. CAPS RT-PCR analysis confirmed that PbDRM mRNA moves from rootstock to scion in micrografting systems. Therefore, PbDRM mRNA acts as a phloem-mobile signal in pear under drought stress.

Connecting the pieces: uncovering the molecular basis for long-distance communication through plant grafting

Vascular plants are wired with a remarkable long-distance communication system. This network can span from as little as a few centimeters (or less) in species like Arabidopsis, up to 100 m in the tallest giant sequoia, linking distant organ systems into a unified, multicellular organism. Grafting is a fundamental technique that allows researchers to physically break apart and reassemble the long-distance transport system, enabling the discovery of molecular signals that underlie intraorganismal communication. In this review, we highlight how plant grafting has facilitated the discovery of new long-distance signaling molecules that function in coordinating developmental transitions, abiotic and biotic responses, and cross-species interactions. This rapidly expanding area of research offers sustainable approaches for improving plant performance in the laboratory, the field, the orchard, and beyond.

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.

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.

Transcriptomics and co-expression networks reveal tissue-specific responses and regulatory hubs under mild and severe drought in papaya (Carica papaya L.)

Plants respond to drought stress through the ABA dependent and independent pathways, which in turn modulate transcriptional regulatory hubs. Here, we employed Illumina RNA-Seq to analyze a total of 18 cDNA libraries from leaves, sap, and roots of papaya plants under drought stress. Reference and de novo transcriptomic analyses identified 8,549 and 6,089 drought-responsive genes and unigenes, respectively. Core sets of 6 and 34 genes were simultaneously up- or down-regulated, respectively, in all stressed samples. Moreover, GO enrichment analysis revealed that under moderate drought stress, processes related to cell cycle and DNA repair were up-regulated in leaves and sap; while responses to abiotic stress, hormone signaling, sucrose metabolism, and suberin biosynthesis were up-regulated in roots. Under severe drought stress, biological processes related to abiotic stress, hormone signaling, and oxidation-reduction were up-regulated in all tissues. Moreover, similar biological processes were commonly down-regulated in all stressed samples. Furthermore, co-expression network analysis revealed three and eight transcriptionally regulated modules in leaves and roots, respectively. Seventeen stress-related TFs were identified, potentially serving as main regulatory hubs in leaves and roots. Our findings provide insight into the molecular responses of papaya plant to drought, which could contribute to the improvement of this important tropical crop.

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.

On the selectivity, specificity and signalling potential of the long-distance movement of messenger RNA

Messenger RNA (mRNA) can move through the vascular system in plants. Until recently the transport of mRNA had been demonstrated only for a few well-documented cases, leading to the suggestion that transport was selective and specific. The extent of this long-distance transport has now been shown to be on the genomic scale with thousands of transcripts covering broad regions of gene ontological space. In light of this recent data, I revisit proposed mechanisms of transport of mRNA and critically assess their potential role in signalling.

Elucidation of the Mechanisms of Long-Distance mRNA Movement in a Nicotiana benthamiana/Tomato Heterograft System

Recent heterograft analyses showed that large-scale messenger RNA (mRNA) movement takes place in the phloem, but the number of mobile transcripts reported varies widely. However, our knowledge of the mechanisms underlying large-scale mRNA movement remains limited. In this study, using a Nicotiana benthamiana/tomato (Solanum lycopersicum) heterograft system and a transgenic approach involving potato (Solanum tuberosum), we found that: (1) the overall mRNA abundance in the leaf is not a good indicator of transcript mobility to the root; (2) increasing the expression levels of nonmobile mRNAs in the companion cells does not promote their mobility; (3) mobile mRNAs undergo degradation during their movement; and (4) some mRNAs arriving in roots move back to shoots. These results indicate that mRNA movement has both regulated and unregulated components. The cellular origins of mobile mRNAs may differ between herbaceous and woody species. Taken together, these findings suggest that the long-distance movement of mRNAs is a complex process and that elucidating the physiological roles associated with this movement is challenging but remains an important task for future research.

Long distance RNA movement

Contents Summary 29 I. Introduction 29 II. Phloem as a conduit for macromolecules 30 III. Classes of phloem transported RNAs and their function 32 IV. Mode of RNA transport 35 V. Conclusions 37 Acknowledgements 37 References 37 SUMMARY: In higher plants, small noncoding RNAs and large messenger RNA (mRNA) molecules are transported between cells and over long distances via the phloem. These large macromolecules are thought to get access to the sugar-conducting phloem vessels via specialized plasmodesmata (PD). Analyses of the phloem exudate suggest that all classes of RNA molecules, including silencing-induced RNAs (siRNAs), micro RNAs (miRNAs), transfer RNAs (tRNAs), ribosomal RNA (rRNAs) and mRNAs, are transported via the vasculature to distant tissues. Although the functions of mobile siRNAs and miRNAs as signalling molecules are well established, we lack a profound understanding of mobile mRNA function(s) in recipient cells and tissues, and how they are selected for transport. A surprisingly high number of up to thousands of mRNAs were described in diverse plant species such as cucumber, pumpkin, Arabidopsis and grapevine to move long distances over graft junctions to distinct body parts. In this review, we present an overview of the classes of mobile RNAs, the potential mechanisms facilitating RNA long-distance transport, and the roles of mobile RNAs in regulating transcription and translation. Furthermore, we address potential function(s) of mobile protein-encoding mRNAs with respect to their characteristics and evolutionary constraints.

An update on phloem transport: a simple bulk flow under complex regulation

The phloem plays a central role in transporting resources and signalling molecules from fully expanded leaves to provide precursors for, and to direct development of, heterotrophic organs located throughout the plant body. We review recent advances in understanding mechanisms regulating loading and unloading of resources into, and from, the phloem network; highlight unresolved questions regarding the physiological significance of the vast array of proteins and RNAs found in phloem saps; and evaluate proposed structure/function relationships considered to account for bulk flow of sap, sustained at high rates and over long distances, through the transport phloem.

RNA mobility in parasitic plant - host interactions

The parasitic plant Cuscuta exchanges mRNAs with its hosts. Systemic mobility of mRNAs within plants is well documented, and has gained increasing attention as studies using grafted plant systems have revealed new aspects of mobile mRNA regulation and function. But parasitic plants take this phenomenon to a new level by forming seamless connections to a wide range of host species, and raising questions about how mRNAs might function after transfer to a different species. Cuscuta and other parasitic plant species also take siRNAs from their hosts, indicating that multiple types of RNA are capable of trans-specific movement. Parasitic plants are intriguing systems for studying RNA mobility, in part because such exchange opens new possibilities for control of parasitic weeds, but also because they provide a fresh perspective into understanding roles of RNAs in inter-organismal communication.