Lost in Transit: Long-Distance Trafficking and Phloem Unloading of Protein Signals in Arabidopsis Homografts

The biology of grafting and its applications in studying information exchange between plants

Grafting techniques have been used for centuries for trait improvement in agriculture and horticulture. In recent years, technical progress with the grafting of species previously thought to be recalcitrant to the formation of a graft union, and the discovery of new types of information exchange between grafted plants, have stirred renewed interest in the use of grafting as a research tool. In this Review, we describe our current understanding of the molecular and cellular processes involved in the establishment of successful grafts between plants of the same genotype (homografts) or different genotypes (heterografts). We also highlight recent progress with the elucidation of the mechanisms underlying the exchange of macromolecules (small RNAs, messenger RNAs and proteins) across graft junctions as well as the transfer of cell organelles and its role in horizontal gene and genome transfer. Finally, we discuss novel applications of grafting, including new opportunities for transgene-free genetic engineering, and the relevance of grafting in plant evolution.

Identification of Long-Distance Mobile mRNAs Responding to Drought Stress in Heterografted Tomato Plants

Grafting is widely used as an effective strategy to enhance tolerance to biotic and abiotic stresses and improve fruit quality in horticultural crops. However, the molecular mechanisms of transcription and the regulatory functions in response to drought stress of mobile mRNAs remain poorly understood. In this study, we developed a grafting system based on the "one grafted plant-three samples" approach using the cultivated tomato/Solanum pennellii (Heinz 1706/LA 0716) heterografting system. A bioinformatics pipeline was developed based on RNA-seq to identify mobile mRNAs in the heterografting systems. A total of 61 upwardly and 990 downwardly mobile mRNAs were identified. Furthermore, we found that the mobility of mRNAs was not correlated with their abundance. The functional annotation and enrichment analysis indicated that mobile mRNAs were mainly involved in RNA binding, photosynthesis, photosystem, response to heat, and translation processes, and ultimately increased the drought tolerance of grafted plants. In addition, we also analyzed the RNA-binding proteins (RBPs) of downwardly mobile mRNAs and found that RBPs were conserved among species. Further, mobile mRNAs may be degraded during transportation. This study provides a pipeline for detecting mobile mRNAs in plant heterografting systems and offers new insights into future studies on long-distance mRNAs transport and regulatory mechanisms involved in drought stress responses.

Transgenic tomato strategies targeting whitefly eggs from apoplastic or ovary-directed proteins

BACKGROUND

Transgenic plants expressing proteins that target the eggs of the ubiquitous plant pest Bemisia tabaci (whitefly) could be an effective insecticide strategy. Two approaches for protein delivery are assessed using the mCherry reporter gene in transgenic tomato plants, while accommodating autofluorescence in both the plant, phloem-feeding whitefly and pedicle-attached eggs.

RESULTS

Both transgenic strategies were segregated to homozygous genotype using digital PCR. The first strategy uses a glycotransferase secretion signal peptide. Despite bright apoplastic accumulation, mCherry is not evident in the eggs. The second strategy targets in vivo whitefly eggs, where the mCherry transgene was fused to a protein transduction domain (PTD) to facilitate uptake into the whitefly hemolymph as well as a synthetic vitellogenin ovary-targeting sequence. Phloem-specific expression of the mCherry fusion is achieved from a Commelina viral promoter. Accumulation was not sufficient to be observed in females feeding on these ovary-targeting plants nor in their eggs subsequently laid on non-transgenic plants. Egg protection may be mediated by protease activity which is observed in macerated eggs.

CONCLUSIONS

mCherry proved an effective reporter for the desired tissue-specific expression in tomato, but insufficiently sensitive to allow for localization in feeding whiteflies or their eggs. Segregated homozygous transgenic tomato lines were important for drawing these conclusions. The implications of these observations to possible pest-control strategies including preliminary expression of analogous chitinase constructs are discussed.

Dynamic relationships among pathways producing hydrocarbons and fatty acids of maize silk cuticular waxes

The hydrophobic cuticle is the first line of defense between aerial portions of plants and the external environment. On maize (Zea mays L.) silks, the cuticular cutin matrix is infused with cuticular waxes, consisting of a homologous series of very long-chain fatty acids (VLCFAs), aldehydes, and hydrocarbons. Together with VLC fatty-acyl-CoAs (VLCFA-CoAs), these metabolites serve as precursors, intermediates, and end-products of the cuticular wax biosynthetic pathway. To deconvolute the potentially confounding impacts of the change in silk microenvironment and silk development on this pathway, we profiled cuticular waxes on the silks of the inbreds B73 and Mo17, and their reciprocal hybrids. Multivariate interrogation of these metabolite abundance data demonstrates that VLCFA-CoAs and total free VLCFAs are positively correlated with the cuticular wax metabolome, and this metabolome is primarily affected by changes in the silk microenvironment and plant genotype. Moreover, the genotype effect on the pathway explains the increased accumulation of cuticular hydrocarbons with a concomitant reduction in cuticular VLCFA accumulation on B73 silks, suggesting that the conversion of VLCFA-CoAs to hydrocarbons is more effective in B73 than Mo17. Statistical modeling of the ratios between cuticular hydrocarbons and cuticular VLCFAs reveals a significant role of precursor chain length in determining this ratio. This study establishes the complexity of the product-precursor relationships within the silk cuticular wax-producing network by dissecting both the impact of genotype and the allocation of VLCFA-CoA precursors to different biological processes and demonstrates that longer chain VLCFA-CoAs are preferentially utilized for hydrocarbon biosynthesis.

Control of phloem unloading and root development

Root growth and development need proper carbon partitioning between sources and sinks. Photosynthesis products are unloaded from the phloem and enter the root meristem cell by cell. While sugar transporters play a major role in phloem loading, phloem unloading occurs via the plasmodesmata in growing root tips. The aperture and permeability of plasmodesmata strongly influence symplastic unloading. Recent research has dissected the symplastic path for phloem unloading and identified several genes that regulate phloem unloading in the root. Callose turnover and membrane lipid composition alter the shape of plasmodesmata, allowing fine-tuning to adapt phloem unloading to the environmental and developmental conditions. Unloaded sugars act both as an energy supply and as signals to coordinate root growth and development. Increased knowledge of how phloem unloading is regulated enhances our understanding of carbon allocation in plants. In the future, it may be possible to modulate carbon allocation between sources and sinks in a manner that would contribute to increased plant biomass and carbon fixation.

Discontinuous Translocation of a Luciferase Protein beyond Graft Junction in Tobacco

Transgrafting, a grafting technique that uses both genetically modified (GM) and non-GM plants, is a novel plant breeding technology that can be used to improve the efficiency of crop cultivation without introducing foreign genes into the edible parts of non-GM plants. This technique can facilitate the acquisition of disease resistance and/or increased yield. However, the translocation of low-molecular-weight compounds, ribonucleic acid (RNA), and proteins through graft junctions raises a potential safety risk for food crops. Here, we used a transgenic tobacco plant expressing a firefly luciferase gene (LUC) to examine the translocation of the LUC protein beyond the graft junction in grafted plants. We observed the bi-directional translocation of LUC proteins in transgrafted tobacco plants, i.e., from the rootstock to scion and vice versa. Transcriptomic analysis revealed that transcripts of the LUC gene were undetectable in non-GM plant bodies, indicating that the LUC protein itself was translocated. Moreover, the movement of the LUC protein is an episodic (i.e., non-continuous) event, since non-GM samples showing high LUC activity were flanked by non-GM samples showing no apparent LUC activity. Translocation from the GM to non-GM part depends on the characteristics of GM plant bodies; here, the enhanced translocation of the LUC protein into the non-GM scion was observed when LUC-expressing rootstocks with hairy roots were used. Moreover, the quantity of translocated LUC protein was far below the level that is generally required to induce an allergenic response. Finally, since the LUC protein levels of plants used for transgrafting are moderate and the LUC protein itself is relatively unstable, further investigation is necessary regarding whether the newly expressed protein in GM plants is highly stable, easily translocated, and/or highly expressed.

Communicating Across Cell Walls: Structure, Evolution, and Regulation of Plasmodesmatal Transport in Plants

Plasmodesmata are conduits in plant cell walls that allow neighboring cells to communicate and exchange resources. Despite their central importance to plant development and physiology, our understanding of plasmodesmata is relatively limited compared to other subcellular structures. In recent years, technical advances in electron microscopy, mass spectrometry, and phylogenomics have illuminated the structure, composition, and evolution of plasmodesmata in diverse plant lineages. In parallel, forward genetic screens have revealed key signaling pathways that converge to regulate plasmodesmatal transport, including chloroplast-derived retrograde signaling, phytohormone signaling, and metabolic regulation by the conserved eukaryotic Target of Rapamycin kinase. This review summarizes our current knowledge of the structure, evolution, and regulation of plasmodesmatal transport in plants.

Traveling with purpose: cell-to-cell transport of plant mRNAs

Messenger RNAs (mRNAs) in multicellular organisms can act as signals transported cell-to-cell and over long distances. In plants, mRNAs traffic cell-to-cell via plasmodesmata (PDs) and over long distances via the phloem vascular system to control diverse biological processes - such as cell fate and tissue patterning - in destination organs. Research on long-distance transport of mRNAs in plants has made remarkable progress, including the cataloguing of many mobile mRNAs, characterization of mRNA features important for transport, identification of mRNA-binding proteins involved in their transport, and understanding of the physiological roles of mRNA transport. However, information on short-range mRNA cell-to-cell transport is still limited. This review discusses the regulatory mechanisms and physiological functions of mRNA transport at the cellular and whole plant levels.

The Interplay between Enucleated Sieve Elements and Companion Cells

In order to adapt to sessile life and terrestrial environments, vascular plants have developed highly sophisticated cells to transport photosynthetic products and developmental signals. Of these, two distinct cell types (i.e., the sieve element (SE) and companion cell) are arranged in precise positions, thus ensuring effective transport. During SE differentiation, most of the cellular components are heavily modified or even eliminated. This peculiar differentiation implies the selective disintegration of the nucleus (i.e., enucleation) and the loss of cellular translational capacity. However, some cellular components necessary for transport (e.g., plasmalemma) are retained and specific phloem proteins (P-proteins) appear. Likewise, MYB (i.e., APL) and NAC (i.e., NAC45 and NAC86) transcription factors (TFs) and OCTOPUS proteins play a notable role in SE differentiation. The maturing SEs become heavily dependent on neighboring non-conducting companion cells, to which they are connected by plasmodesmata through which only 20-70 kDa compounds seem to be able to pass. The study of sieve tube proteins still has many gaps. However, the development of a protocol to isolate proteins that are free from any contaminating proteins has constituted an important advance. This review considers the very detailed current state of knowledge of both bound and soluble sap proteins, as well as the role played by the companion cells in their presence. Phloem proteins travel long distances by combining two modes: non-selective transport via bulk flow and selective regulated movement. One of the goals of this study is to discover how the protein content of the sieve tube is controlled. The majority of questions and approaches about the heterogeneity of phloem sap will be clarified once the morphology and physiology of the plasmodesmata have been investigated in depth. Finally, the retention of specific proteins inside an SE is an aspect that should not be forgotten.

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

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

Noncell-autonomous HSC70.1 chaperone displays homeostatic feedback regulation by binding its own mRNA

The HSC70/HSP70 family of heat shock proteins are evolutionarily conserved chaperones involved in protein folding, protein transport, and RNA binding. Arabidopsis HSC70 chaperones are thought to act as housekeeping chaperones and as such are involved in many growth-related pathways. Whether Arabidopsis HSC70 binds RNA and whether this interaction is functional has remained an open question. We provide evidence that the HSC70.1 chaperone binds its own mRNA via its C-terminal short variable region (SVR) and inhibits its own translation. The SVR encoding mRNA region is necessary for HSC70.1 transcript mobility to distant tissues and that HSC70.1 transcript and not protein mobility is required to rescue root growth and flowering time of hsc70 mutants. We propose that this negative protein-transcript feedback loop may establish an on-demand chaperone pool that allows for a rapid response to stress. In summary, our data suggest that the Arabidopsis HSC70.1 chaperone can form a complex with its own transcript to regulate its translation and that both protein and transcript can act in a noncell-autonomous manner, potentially maintaining chaperone homeostasis between tissues.

Scion-to-Rootstock Mobile Transcription Factor CmHY5 Positively Modulates the Nitrate Uptake Capacity of Melon Scion Grafted on Squash Rootstock

It is generally recognized that the root uptake capacity of grafted plants strongly depends on the rootstocks' well-developed root system. However, we found that grafted plants showed different nitrate uptake capacities when different varieties of oriental melon scion were grafted onto the same squash rootstock, suggesting that the scion regulated the nitrate uptake capacity of the rootstock root. In this study, we estimated the nitrate uptake capacity of grafted plants with the different oriental melon varieties' seedlings grafted onto the same squash rootstocks. The results indicated a significant difference in the nitrate uptake rate and activity of two heterologous grafting plants. We also showed a significant difference in CmoNRT2.1 expression in the roots of two grafting combinations and verified the positive regulation of nitrate uptake by CmoNRT2.1 expression. In addition, the two varieties of oriental melon scion had highly significant differences in CmHY5 expression, which was transported to the rootstock and positively induced CmoHY5-1 and CmoHY5-2 expression in the rootstock roots. Meanwhile, CmHY5 could positively regulate CmoNRT2.1 expression in the rootstock roots. Furthermore, CmoHY5-1 and CmoHY5-2 also positively regulated CmoNRT2.1 expression, respectively, and CmoHY5-1 dominated the positive regulation of CmoNRT2.1, while CmHY5 could interact with CmoHY5-1 and CmoHY5-2, respectively, to jointly regulate CmoNRT2.1 expression. The oriental melon scion regulated the nitrate uptake capacity of the melon/squash grafting plant roots, and the higher expression of CmHY5 in the oriental melon scion leaves, the more substantial the nitrate uptake capacity of squash rootstock roots.

Highly efficient hairy root genetic transformation and applications in citrus

Highly efficient genetic transformation technology is greatly beneficial for crop gene function analysis and precision breeding. However, the most commonly used genetic transformation technology for woody plants, mediated by Agrobacterium tumefaciens, is time-consuming and inefficient, which limits its utility for gene function analysis. In this study, a simple, universal, and highly efficient genetic transformation technology mediated by A. rhizogenes K599 is described. This technology can be applied to multiple citrus genotypes, and only 2-8 weeks were required for the entire workflow. Genome-editing experiments were simultaneously conducted using 11 plasmids targeting different genomic positions and all corresponding transformants with the target knocked out were obtained, indicating that A. rhizogenes-mediated genome editing was highly efficient. In addition, the technology is advantageous for investigation of specific genes (such as ACD2) for obtaining "hard-to-get" transgenic root tissue. Furthermore, A. rhizogenes can be used for direct viral vector inoculation on citrus bypassing the requirement for virion enrichment in tobacco, which facilitates virus-induced gene silencing and virus-mediated gene expression. In summary, we established a highly efficient genetic transformation technology bypassing tissue culture in citrus that can be used for genome editing, gene overexpression, and virus-mediated gene function analysis. We anticipate that by reducing the cost, required workload, experimental period, and other technical obstacles, this genetic transformation technology will be a valuable tool for routine investigation of endogenous and exogenous genes in citrus.

The Phloem as an Arena for Plant Pathogens

Although the phloem is a highly specialized tissue, certain pathogens, including phytoplasmas, spiroplasmas, and viruses, have evolved to access and live in this sequestered and protected environment, causing substantial economic harm. In particular, Candidatus Liberibacter spp. are devastating citrus in many parts of the world. Given that most phloem pathogens are vectored, they are not exposed to applied chemicals and are therefore difficult to control. Furthermore, pathogens use the phloem network to escape mounted defenses. Our review summarizes the current knowledge of phloem anatomy, physiology, and biochemistry relevant to phloem/pathogen interactions. We focus on aspects of anatomy specific to pathogen movement, including sieve plate structure and phloem-specific proteins. Phloem sampling techniques are discussed. Finally, pathogens that cause particular harm to the phloem of crop species are considered in detail.

Phloem Loading and Unloading of Sucrose: What a Long, Strange Trip from Source to Sink

Sucrose is transported from sources (mature leaves) to sinks (importing tissues such as roots, stems, fruits, and seeds) through the phloem tissues in veins. In many herbaceous crop species, sucrose must first be effluxed to the cell wall by a sugar transporter of the SWEET family prior to being taken up into phloem companion cells or sieve elements by a different sugar transporter, called SUT or SUC. The import of sucrose into these cells is termed apoplasmic phloem loading. In sinks, sucrose can similarly exit the phloem apoplasmically or, alternatively, symplasmically through plasmodesmata into connecting parenchyma storage cells. Recent advances describing the regulation and manipulation of sugar transporter expression and activities provide stimulating new insights into sucrose phloem loading in sources and unloading processes in sink tissues. Additionally, new breakthroughs have revealed distinct subpopulations of cells in leaves with different functions pertaining to phloem loading. These and other discoveries in sucrose transport are discussed.

Long-Distance Transported RNAs: From Identity to Function

There is now a wealth of data, from different plants and labs and spanning more than two decades, which unequivocally demonstrates that RNAs can be transported over long distances, from the cell where they are transcribed to distal cells in other tissues. Different types of RNA molecules are transported, including micro- and messenger RNAs. Whether these RNAs are selected for transport and, if so, how they are selected and transported remain, in general, open questions. This aspect is likely not independent of the biological function and relevance of the transported RNAs, which are in most cases still unclear. In this review, we summarize the experimental data supporting selectivity or nonselectivity of RNA translocation and review the evidence for biological functions. After discussing potential issues regarding the comparability between experiments, we propose criteria that need to be critically evaluated to identify important signaling RNAs.

Grafting: a potential method to reveal the differential accumulation mechanism of secondary metabolites

Plant secondary metabolites make a great contribution to the agricultural and pharmaceutical industries. Their accumulation is determined by the integrated transport of target compounds and their biosynthesis-related RNA, protein, or DNA. However, it is hard to track the movement of these biomolecules in vivo. Grafting may be an ideal method to solve this problem. The differences in genetic and metabolic backgrounds between rootstock and scion, coupled with multiple omics approaches and other molecular tools, make it feasible to determine the movement of target compounds, RNAs, proteins, and DNAs. In this review, we will introduce methods of using the grafting technique, together with molecular biological tools, to reveal the differential accumulation mechanism of plant secondary metabolites at different levels. Details of the case of the transport of one diterpene alkaloid, fuziline, will be further illustrated to clarify how the specific accumulation model is shaped with the help of grafting and multiple molecular biological tools.

A correlative light electron microscopy approach reveals plasmodesmata ultrastructure at the graft interface

Despite recent progress in our understanding of graft union formation, we still know little about the cellular events underlying the grafting process. This is partially due to the difficulty of reliably targeting the graft interface in electron microscopy to study its ultrastructure and three-dimensional architecture. To overcome this technological bottleneck, we developed a correlative light electron microscopy (CLEM) approach to study the graft interface with high ultrastructural resolution. Grafting hypocotyls of Arabidopsis thaliana lines expressing yellow FP or monomeric red FP in the endoplasmic reticulum (ER) allowed efficient targeting of the grafting interface for examination under light and electron microscopy. To explore the potential of our method to study sub-cellular events at the graft interface, we focused on the formation of secondary plasmodesmata (PD) between the grafted partners. We showed that four classes of PD were formed at the interface and that PD introgression into the cell wall was initiated equally by both partners. Moreover, the success of PD formation appeared not systematic with a third of PD not spanning the cell wall entirely. Characterizing the ultrastructural characteristics of these incomplete PD gives us insights into the process of secondary PD biogenesis. We found that the establishment of successful symplastic connections between the scion and rootstock occurred predominantly in the presence of thin cell walls and ER-plasma membrane tethering. The resolution reached in this work shows that our CLEM method advances the study of biological processes requiring the combination of light and electron microscopy.

Quantifying Plasmodesmatal Transport with an Improved GFP Movement Assay

Plasmodesmata (PD) are membrane-lined channels that cross the cell wall to connect the cytosol of adjacent plant cells, permitting diverse cytosolic molecules to move between cells. PD are essential for plant multicellularity, and the regulation of PD transport contributes to metabolism, developmental patterning, abiotic stress responses, and pathogen defenses, which has sparked broad interest in PD among diverse plant biologists. Here, we present a straightforward method to reproducibly quantify changes in the rate of PD transport in leaves. Individual cells are transformed with Agrobacterium to express fluorescent proteins, which then move beyond the transformed cell via PD. Forty-eight to 72 h later, the extent of GFP movement is monitored by confocal fluorescence microscopy. This assay is versatile and may be combined with transient gene overexpression, virus-induced gene silencing, physiological treatments, or pharmaceutical treatments to test how PD transport responds to specific conditions. We expect that this improved method for monitoring PD transport in leaves will be broadly useful for plant biologists working in diverse fields.

An update on passive transport in and out of plant cells

Transport across membranes is critical for plant survival. Membranes are the interfaces at which plants interact with their environment. The transmission of energy and molecules into cells provides plants with the source material and power to grow, develop, defend, and move. An appreciation of the physical forces that drive transport processes is thus important for understanding the plant growth and development. We focus on the passive transport of molecules, describing the fundamental concepts and demonstrating how different levels of abstraction can lead to different interpretations of the driving forces. We summarize recent developments on quantitative frameworks for describing diffusive and bulk flow transport processes in and out of cells, with a more detailed focus on plasmodesmata, and outline open questions and challenges.

Root-to-Shoot Long-Distance Mobile miRNAs Identified from Nicotiana Rootstocks

Root-derived mobile signals play critical roles in coordinating a shoot's response to underground conditions. However, the identification of root-to-shoot long-distance mobile signals has been scant. In this study, we aimed to characterize root-to-shoot endogenous mobile miRNAs by using an Arabidopsis/Nicotiana interfamilial heterograft in which these two taxonomically distant species with clear genetic backgrounds had sufficient diversity in differentiating miRNA sources. Small RNA deep sequencing analysis revealed that 82 miRNAs from the Arabidopsis scion could travel through the graft union to reach the rootstock, whereas only a very small subset of miRNA (6 miRNAs) preferred the root-to-shoot movement. We demonstrated in an ex vivo RNA imaging experiment that the root-to-shoot mobile Nb-miR164, Nb-miR395 and Nb-miR397 were targeted to plasmodesmata using the bacteriophage coat protein MS2 system. Furthermore, the Nb-miR164 was shown to move from the roots to the shoots to induce phenotypic changes when its overexpressing line was used as rootstock, strongly supporting that root-derived Nb-miR164 was able to modify the scion trait via its long-distance movement.

Cell-to-Cell Connection in Plant Grafting-Molecular Insights into Symplasmic Reconstruction

Grafting is a means to connect tissues from two individual plants and grow a single chimeric plant through the establishment of both apoplasmic and symplasmic connections. Recent molecular studies using RNA-sequencing data have provided genetic information on the processes involved in tissue reunion, including wound response, cell division, cell-cell adhesion, cell differentiation and vascular formation. Thus, studies on grafting increase our understanding of various aspects of plant biology. Grafting has also been used to study systemic signaling and transport of micromolecules and macromolecules in the plant body. Given that graft viability and molecular transport across graft junctions largely depend on vascular formation, a major focus in grafting biology has been the mechanism of vascular development. In addition, it has been thought that symplasmic connections via plasmodesmata are fundamentally important to share cellular information among newly proliferated cells at the graft interface and to accomplish tissue differentiation correctly. Therefore, this review focuses on plasmodesmata formation during grafting. We take advantage of interfamily grafts for unambiguous identification of the graft interface and summarize morphological aspects of de novo formation of plasmodesmata. Important molecular events are addressed by re-examining the time-course transcriptome of interfamily grafts, from which we recently identified the cell-cell adhesion mechanism. Plasmodesmata-associated genes upregulated during graft healing that may provide a link to symplasm establishment are described. We also discuss future research directions.

A Sequential Three-Phase Pathway Constitutes Tracheary Element Connection in the Arabidopsis/Nicotiana Interfamilial Grafts

Scion-rootstock union formation is a critical step toward the functional assemblage of heterogeneous plants. Interfamilial scion-rootstock interaction often results in graft incompatibility during the assemblage process, and the underlying mechanisms are largely unknown. In this study, we reported that tracheary element (TE) remodeling, including TE segmentation and deformation, rather than de novo formation from callus or adjacent tissues, took place at the early stage of grafting interface between Arabidopsis thaliana and Nicotiana benthamiana (At/Nb). Following cellular deposits, the short TEs from both partners were overlapping, dependent on the homogeneity of contacting TEs, with each other. Without overlapping, the TEs at the interface would grow laterally, and the TEs above and below the interface would undergo self-fusion to form insulating spiraling bundles. Finally, the overlapping TEs constituted a continuous network through alignment. Our results provide a definitive framework for the critical process of TE behavior in the At/Nb distant grafts, including (1) segmentation and/or deformation, (2) matching, overlapping, and cellular deposits, and (3) aligning or spiraling. These insights might guide us in the future into constructing more compatible distant grafts from the perspective of TE homogeneity.

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.

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.

Modulation of plant root traits by nitrogen and phosphate: transporters, long-distance signaling proteins and peptides, and potential artificial traps

As sessile organisms, plants rely on their roots for anchorage and uptake of water and nutrients. Plant root is an organ showing extensive morphological and metabolic plasticity in response to diverse environmental stimuli including nitrogen (N) and phosphorus (P) nutrition/stresses. N and P are two essential macronutrients serving as not only cell structural components but also local and systemic signals triggering root acclimatory responses. Here, we mainly focused on the current advances on root responses to N and P nutrition/stresses regarding transporters as well as long-distance mobile proteins and peptides, which largely represent local and systemic regulators, respectively. Moreover, we exemplified some of the potential pitfalls in experimental design, which has been routinely adopted for decades. These commonly accepted methods may help researchers gain fundamental mechanistic insights into plant intrinsic responses, yet the output might lack strong relevance to the real situation in the context of natural and agricultural ecosystems. On this basis, we further discuss the established-and yet to be validated-improvements in experimental design, aiming at interpreting the data obtained under laboratory conditions in a more practical view.

Unraveling the Roles of Vascular Proteins Using Proteomics

Vascular bundles play important roles in transporting nutrients, growth signals, amino acids, and proteins between aerial and underground tissues. In order to understand these sophisticated processes, a comprehensive analysis of the roles of the components located in the vascular tissues is required. A great deal of data has been obtained from proteomic analyses of vascular tissues in plants, which mainly aim to identify the proteins moving through the vascular tissues. Here, different aspects of the phloem and xylem proteins are reviewed, including their collection methods, and their main biological roles in growth, and biotic and abiotic stress responses. The study of vascular proteomics shows great potential to contribute to our understanding of the biological mechanisms related to development and defense in plants.

Transgenic Citrange troyer rootstocks overexpressing antimicrobial potato Snakin-1 show reduced citrus canker disease symptoms

Citrus canker is a major disease caused by Xanthomonas citri pv. citri. Snakin-1 is an antimicrobial peptide, which was previously shown to be effective against different bacterial and fungal diseases in potato, wheat and lettuce when expressed in transgenic plants. We generated transgenic Citrange Troyer citrus rootstocks constitutively expressing this peptide and 5 different transgenic lines were challenged against virulent X. citri isolates. Challenge assays conducted in vitro using detached leaves and in planta by infiltration revealed a significant reduction of the number and size of canker lesions in some of the transgenic lines.

Long-distance regulation of shoot gravitropism by Cyclophilin 1 in tomato (Solanum lycopersicum) plants

The phloem-mobile protein SlCyp1 traffics to distant parts of the shoot to regulate its gravitropic response. In addition, SlCyp1 targets specific cells in the root to promote lateral root development. The tomato (Solanum lycopersicum) Cyclophilin 1 (SlCyp1) gene encodes a peptidyl-prolyl isomerase required for auxin response, lateral root development and gravitropic growth. The SlCyp1 protein is a phloem-mobile signal that moves from shoot to root to regulate lateral root development (Spiegelman et al., Plant J 83:853-863, 2015; J Exp Bot 68:953-964, 2017a). Here, we explored the mechanism of SlCyp1 movement by fusing it to the fluorescent protein mCherry. We found that, once trafficked to the root, SlCyp1 is unloaded from the phloem to the surrounding tissues, including the pericycle and lateral root primordia. Interestingly, SlCyp1 not only moves to the root system, but also to distant parts of the shoot. Grafting of the SlCyp1 mutant diageotropica (dgt) scions on VFN8 control rootstocks resulted in recovery of dgt shoot gravitropism, which was associated with the restoration of auxin-response capacity. Application of the cyclophilin inhibitor cyclosporine A suppressed gravitropic recovery, indicating that SlCyp1 must be active in the target tissue to affect the gravitropic response. These results provide new insights on the mechanism of SlCyp1 transport and functioning as a long-distance signal regulating shoot gravitropism.

Local HY5 Activity Mediates Hypocotyl Growth and Shoot-to-Root Communication

Plants optimize their growth in fluctuating environments using information acquired by different organs. This information is then transmitted through the rest of the plant using both short- and long-distance signals, including hormones and mobile proteins. Although a few of these signals have been characterized, long-distance signaling is not well understood in plants. Recently, the light-regulated transcription factor HY5 was reported to move from the shoot to the root to regulate root growth. We generated a cell-type specifically expressed HY5 fusion protein that could not be detected outside the tissue in which it was targeted. By expressing this DOF-HY5 protein in specific cell types of the hypocotyl, we showed that its local activity was sufficient to regulate hypocotyl growth. We also found that, although DOF-HY5 was expressed specifically in the shoot and not detected in the roots, it could rescue hy5 growth defects in primary roots but not in lateral roots. We therefore conclude that HY5 protein mobility is not required in the hypocotyl or for shoot-to-root communication. Our results indicate that a signal downstream of, or in parallel with, HY5 in the shoot is mobile and links shoot and root growth.

iTRAQ and RNA-Seq analyses revealed the effects of grafting on fruit development and ripening of oriental melon (Cucumis melo L. var. makuwa)

Rootstocks are among the primary factors that influence fruit yield and quality as well as melon development. To understand the differences in the molecular mechanisms and gene expression networks of fruit development between grafted and nongrafted plants in oriental melon, we performed a comprehensive analysis of the transcriptome and proteome dynamic gene/protein expression profiles during fruit development in oriental melon (Cucumis melo L. var. makuwa). Using pairwise comparisons between grafted and nongrafted samples by transcriptome analysis, we identified a large number of candidate genes involved in hormonal signaling pathways, transcription factors, resistance-related biosynthetic pathways and photosynthesis-related metabolic pathways. Many transcription factor-encoded genes were significantly more strongly expressed in the grafted samples, for example, AP2/ERF, C2H2, MYB, bHLH, and AUX/IAA, which are well-known participants in the regulation of developmental processes and hormonal signaling metabolism. Some differentially expressed genes (DEGs) were enriched in flavonoid biosynthesis and phenylpropanoid biosynthesis and determined plant resistance. In addition, some differentially expressed proteins (DEPs) were enriched in photosynthesis-related pathways, which could improve fruit quality and yield. Moreover, through weighted gene coexpression network analyses, we identified modules of coexpressed genes and hub genes specifically related to grafting for different fruit developmental stages. The results suggested that graft-related modules and hub genes were primarily associated with photosynthate metabolism and hormonal signaling pathways. The results obtained in this study provide a valuable resource for dissecting the role of candidate genes governing graft-related metabolism in oriental melon fruit, suggesting an interesting correlation with the effects of rootstock on fruit development.

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.

Identification of Long-Distance Transmissible mRNA between Scion and Rootstock in Cucurbit Seedling Heterografts

Grafting has been widely used to improve plant growth and tolerance in crop production, as well as for clarifying systemic mRNA signaling from donor to recipient tissues in organ-to-organ communication. In this study, we investigated graft partner interaction mechanisms of Cucumis sativus (Csa) and Cucurbita moschata (Cmo) using a large-scale endogenous mRNA transport. The results indicated that most mobile transcripts followed an allocation pathway from source to sink. Gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that mRNA mobility functions are universally common and individually specific. Identification of mRNA mobility between distant tissues in heterografts with RT-PCR (reverse transcription PCR), RT-qPCR (reverse transcriptional quantitative real time PCR), and clone sequencing were used to estimate 78.75% of selected mobile transcripts. Integration of bioinformatic analysis and RT-qPCR identification allowed us to hypothesize a scion-to-rootstock-to-scion feedback signal loop of Csa move-down and Cmo move-up mRNAs, where Csa scion move-down mRNAs were involved in carbon fixation and biosynthesis of amino acid pathways, and Cmo root received Csa move-down mRNA and then delivered the corresponding Cmo upward mRNA to scion to improve photosynthesis of cucumber scion. This formed a feedback signal loop of scion-to-rootstock-to scion to explain why pumpkin rootstock enhanced cucumber production in the industry, which was utilized for organ communication and mediates photosynthesis processes in heterograft cucurbit crops.

Tissue-specific isolation of Arabidopsis/plant mitochondria - IMTACT (isolation of mitochondria tagged in specific cell types)

Plant cells contain numerous subcompartments with clearly delineated metabolic functions. Mitochondria represent a very small fraction of the total cell volume and yet are the site of respiration and thus crucial for cells throughout all developmental stages of a plant's life. As such, their isolation from the rest of the cellular components is a basic requirement for numerous biochemical and physiological experiments. Although procedures exist to isolate plant mitochondria from different organs (i.e. leaves, roots, tubers, etc.), they are often tedious and do not provide resolution at the tissue level (i.e. phloem, mesophyll or pollen). Here, we present a novel method called IMTACT (isolation of mitochondria tagged in specific cell types), developed in Arabidopsis thaliana (Arabidopsis) that involves biotinylation of mitochondria in a tissue-specific manner using transgenic lines expressing a synthetic version of the OM64 (Outer Membrane 64) gene combined with BLRP and the BirA biotin ligase gene. Tissue specificity is achieved with cell-specific promoters (e.g. CAB3 and SUC2). Labeled mitochondria from crude extracts are retained by magnetic beads, allowing the simple and rapid isolation of highly pure and intact organelles from organs or specific tissues. For example, we could show that the mitochondrial population from mesophyll cells was significantly larger in size than the mitochondrial population isolated from leaf companion cells. To facilitate the applicability of this method in both wild-type and mutant Arabidopsis plants we generated a set of OM64-BLRP one-shot constructs with different selection markers and tissue-specific promoters.

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.

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.

Extensive Inter-plant Protein Transfer between Cuscuta Parasites and Their Host Plants

Cuscuta species (dodders) are holoparasites that totally rely on host plants to survive. Although various mobile proteins have been identified to travel within a plant, whether and to what extent protein transfer between Cuscuta and host plants remain unclear. We found that hundreds to more than 1500 proteins were transferred between Cuscuta and the host plants Arabidopsis and soybean, and hundreds of inter-plant mobile proteins were even detected in the seeds of Cuscuta and the host soybean. Different hosts bridge-connected by dodder were also found to exchange hundreds of proteins. Quantitatively, the mobile proteins represent a few to more than 10% of the proteomes of foreign plants. Using Arabidopsis plants expressing different reporter proteins, we further showed that these reporter proteins could travel between plants and, importantly, retained their activity in the foreign plants. Comparative analysis between the inter-plant mobile proteins and mRNAs indicated that the majority of mobile proteins were not de novo synthesized from the translocated mRNAs, but bona fide mobile proteins. We propose that large-scale inter-plant protein translocation may play an important role in the interactions between host plants and dodder and even among the dodder bridge-connected hosts.

Genome-wide identification and phylogenetic analysis of rice FTIP gene family

FT-INTERACTING PROTEIN (FTIP) gene family in rice are the members of multiple C2 domain and transmembrane region proteins (MCTPs). There are many homologs of OsFTIPs in plants; however, the bioinformatics of them remains unclear. In the studies, 13 OsFTIP genes are identified in rice. OsFTIPs are unevenly located in 12 chromosomes. The OsFTIPs are phylogenetically divided into three clades. Cis-elements respond to abiotic stress, light, and hormones are found in the promoter region of OsFTIPs which are induced by the stimuli. All OsFTIPs are expressed with different profiles. Syntenic analysis of 128 OsFTIPs and FTIP-like homologs reveals that various number of gene pairs are identified between rice and other species. The 128 FTIP-like homologs are divided into six groups which fall into three classes. Ten motifs are shared by most OsFTIPs and their homologs. The studies provide a theoretical basis for further elucidating the functions of OsFTIP gene family.

TOR dynamically regulates plant cell-cell transport

The coordinated redistribution of sugars from mature "source" leaves to developing "sink" leaves requires tight regulation of sugar transport between cells via plasmodesmata (PD). Although fundamental to plant physiology, the mechanisms that control PD transport and thereby support development of new leaves have remained elusive. From a forward genetic screen for altered PD transport, we discovered that the conserved eukaryotic glucose-TOR (TARGET OF RAPAMYCIN) metabolic signaling network restricts PD transport in leaves. Genetic approaches and chemical or physiological treatments to either promote or disrupt TOR activity demonstrate that glucose-activated TOR decreases PD transport in leaves. We further found that TOR is significantly more active in mature leaves photosynthesizing excess sugars than in young, growing leaves, and that this increase in TOR activity correlates with decreased rates of PD transport. We conclude that leaf cells regulate PD trafficking in response to changing carbohydrate availability monitored by the TOR pathway.

A universal pipeline for mobile mRNA detection and insights into heterografting advantages under chilling stress

Heterografting has long been used to enhance the chilling tolerance of temperature-sensitive crops, including watermelon, whose mechanism is known to involve bidirectional long-distance mRNA movements. Despite several studies reporting on mobile mRNA (mb-mRNA) profiles in plants, accurate identification of mb-mRNAs is challenging owing to an array of technical problems. Here, we developed a bioinformatical pipeline that took most of the known technical concerns into consideration and is considered to be a universal tool for mb-mRNA detection in heterografts. By applying this pipeline to a commercial watermelon-bottle gourd heterografting system, we detected 130 and 1144 mb-mRNAs upwardly and 167 and 1051 mb-mRNAs downwardly transmitted under normal and chilling-stress conditions, respectively. Quantitative real-time PCR indicated a high accuracy rate (88.2%) of mb-mRNA prediction with our pipeline. We further revealed that the mobility of mRNAs was not associated with their abundance. Functional annotation and classification implied that scions may convey the stress signal to the rootstock, subsequently triggering energy metabolism reprogramming and abscisic acid-mediated stress responses by upward movement of effective mRNAs, ultimately leading to enhanced chilling tolerance. This study provides a universal tool for mb-mRNA detection in plant heterografting systems and novel insights into heterografting advantages under chilling stress.

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.

Mechanisms Underlying Graft Union Formation and Rootstock Scion Interaction in Horticultural Plants

Grafting is a common practice for vegetative propagation and trait improvement in horticultural plants. A general prerequisite for successful grafting and long term survival of grafted plants is taxonomic proximity between the root stock and scion. For the success of a grafting operation, rootstock and scion should essentially be closely related. Interaction between the rootstock and scion involves complex physiological-biochemical and molecular mechanisms. Successful graft union formation involves a series of steps viz., lining up of vascular cambium, generation of a wound healing response, callus bridge formation, followed by vascular cambium formation and subsequent formation of the secondary xylem and phloem. For grafted trees compatibility between the rootstock/scion is the most essential factor for their better performance and longevity. Graft incompatibility occurs on account of a number of factors including of unfavorable physiological responses across the graft union, transmission of virus or phytoplasma and anatomical deformities of vascular tissue at the graft junction. In order to avoid the incompatibility problems, it is important to predict the same at an early stage. Phytohormones, especially auxins regulate key events in graft union formation between the rootstock and scion, while others function to facilitate the signaling pathways. Transport of macro as well as micro molecules across long distances results in phenotypic variation shown by grafted plants, therefore grafting can be used to determine the pattern and rate of recurrence of this transport. A better understanding of rootstock scion interactions, endogenous growth substances, soil or climatic factors needs to be studied, which would facilitate efficient selection and use of rootstocks in the future. Protein, hormones, mRNA and small RNA transport across the junction is currently emerging as an important mechanism which controls the stock/scion communication and simultaneously may play a crucial role in understanding the physiology of grafting more precisely. This review provides an understanding of the physiological, biochemical and molecular basis underlying grafting with special reference to horticultural plants.

Identification of miRNA precursors in the phloem of Cucurbita maxima

Plant development and responses to environmental cues largely depend on mobile signals including microRNAs (miRNAs) required for post-transcriptional silencing of specific genes. Short-range cell-to-cell transport of miRNA in developing tissues and organs is involved in transferring positional information essential for determining cell fate. Among other RNA species, miRNAs are found in the phloem sap. Long-distance transport of miRNA via the phloem takes a part in regulation of physiological responses to changing environmental conditions. As shown for regulation of inorganic phosphorus and sulfate homeostasis, mature miRNAs rather than miRNAs precursors are transported in the phloem as signaling molecules. Here, a bioinformatics analysis of transcriptomic data for Cucurbita maxima phloem exudate RNAs was carried out to elucidate whether miRNA precursors could also be present in the phloem. We demonstrated that the phloem transcriptome contained a subset of C. maxima pri-miRNAs that differed from a subset of pri-miRNA sequences abundant in a leaf transcriptome. Differential accumulation of pri-miRNA was confirmed by PCR analysis of C. maxima phloem exudate and leaf RNA samples. Therefore, the presented data indicate that a number of C. maxima pri-miRNAs are selectively recruited to the phloem translocation pathway. This conclusion was validated by inter-species grafting experiments, in which C. maxima pri-miR319a was found to be transported across the graft union via the phloem, confirming the presence of pri-miR319a in sieve elements and showing that phloem miRNA precursors could play a role in long-distance signaling in plants.

Plant Cell-Cell Transport via Plasmodesmata Is Regulated by Light and the Circadian Clock

Plasmodesmata (PD) are essential for plant development, but little is known about their regulation. Several studies have linked PD transport to chloroplast-centered signaling networks, but the physiological significance of this connection remains unclear. Here, we show that PD transport is strongly regulated by light and the circadian clock. Light promotes PD transport during the day, but light is not sufficient to increase rates of PD transport at night, suggesting a circadian gating mechanism. Silencing expression of the core circadian clock gene, LHY/CCA1, allows light to strongly promote PD transport during subjective night, confirming that the canonical plant circadian clock controls the PD transport light response. We conclude that PD transport is dynamically regulated during the day/night cycle. Due to the many roles of PD in plant biology, this discovery has strong implications for plant development, physiology, and pathogenesis.

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.