Adaptation of a seedling micro-grafting technique to the study of long-distance signaling in flowering of Arabidopsis thaliana

Grafting in plants: recent discoveries and new applications

Grafting is a traditional horticultural technique that makes use of plant wound healing mechanisms to join two different genotypes together to form one plant. In many agricultural systems, grafting with rootstocks controls the vigour of the scion and/or provides tolerance to deleterious soil conditions such as the presence of soil pests or pathogens or limited or excessive water or mineral nutrient supply. Much of our knowledge about the limits to grafting different genotypes together comes from empirical knowledge of horticulturalists. Until recently, researchers believed that grafting monocotyledonous plants was impossible, because they lack a vascular cambium, and that graft compatibility between different scion/rootstock combinations was restricted to closely related genotypes. Recent studies have overturned these ideas and open up the possibility of new research directions and applications for grafting in agriculture. The objective of this review is to describe and assess these recent advances in the field of grafting and, in particular, the molecular mechanisms underlining graft union formation and graft compatibility between different genotypes. The challenges of characterizing the different stages of graft union formation and phenotyping graft compatibility are examined.

Micrografting in Arabidopsis Using a Silicone Chip

The micrografting technique in the model plant Arabidopsis has been widely used in the field of plant science. Grafting experiments have demonstrated that signal transductions are systematically regulated in many plant characteristics, including defense mechanisms and responses to surrounding environments such as soil and light conditions. Hypocotyl micrografting is a powerful tool for the analysis of signal transduction between shoots and roots; however, the requirement for a high level of skill for micrografting, during which small seedlings are microdissected and micromanipulated, has limited its use. Here, we developed a silicone-made microdevice, called a micrografting chip, to perform Arabidopsis micrografting easily and uniformly. The micrografting chip has tandemly arrayed units, each of which consists of a seed pocket for seed germination and a micro-path to hold hypocotyl. All micrografting procedures are performed on the chip. This method using a micrografting chip will avoid the need for training and promote studies of systemic signaling in plants. Graphic abstract: A silicone chip for easy grafting.

Non-sterile Grafting Methods for Arabidopsis

Mobile signals play pivotal roles in coordinating interorgan communication. Grafting provides an effective strategy to identify and explore the movement of the mobile signals. The mutant collection of Arabidopsis offers background-free living materials for examining the transport of mobile signals in vivo. In the past few years, many grafting methods have been developed to overcome the limitations of rosette-type growth and small size in Arabidopsis. Here we describe a non-sterile grafting method involving an insect pin to secure the scion to the rootstock. The scions can be grafted onto epicotyls or hypocotyls of soil-grown Arabidopsis rootstocks at a wide range of developmental stages. This grafting method provides a useful tool to analyze leaf-derived mobile signals in Arabidopsis.

Cell-cell adhesion in plant grafting is facilitated by β-1,4-glucanases

Plant grafting is conducted for fruit and vegetable propagation, whereby a piece of living tissue is attached to another through cell-cell adhesion. However, graft compatibility limits combinations to closely related species, and the mechanism is poorly understood. We found that Nicotiana is capable of graft adhesion with a diverse range of angiosperms. Comparative transcriptomic analyses on graft combinations indicated that a subclade of β-1,4-glucanases secreted into the extracellular region facilitates cell wall reconstruction near the graft interface. Grafting was promoted by overexpression of the β-1,4-glucanase. Using Nicotiana stem as an interscion, we produced tomato fruits on rootstocks from other plant families. These findings demonstrate that the process of cell-cell adhesion is a potential target to enhance plant grafting techniques.

Micrografting device for testing systemic signaling in Arabidopsis

Grafting techniques have been applied in studies of systemic, long-distance signaling in several model plants. Seedling grafting in Arabidopsis, known as micrografting, enables investigation of the molecular mechanisms of systemic signaling between shoots and roots. However, conventional micrografting requires a high level of skill, limiting its use. Thus, an easier user-friendly method is needed. Here, we developed a silicone microscaled device, the micrografting chip, to obviate the need for training and to generate less stressed and more uniformly grafted seedlings. The chip has tandemly arrayed units, each of which consists of a seed pocket for seed germination and a micro-path with pairs of pillars for hypocotyl holding. Grafting, including seed germination, micrografting manipulation and establishment of tissue reunion, is performed on the chip. Using the micrografting chip, we evaluated the effect of temperature and the carbon source on grafting, and showed that a temperature of 27°C and a sucrose concentration of 0.5% were optimal. We also used the chip to investigate the mechanism of systemic signaling of iron status using a quadruple nicotianamine synthase (nas) mutant. The constitutive iron-deficiency response in the nas mutant because of iron accumulation in shoots was significantly rescued by grafting of wild-type shoots or roots, suggesting that shoot- and root-ward translocation of nicotianamine-iron complexes and/or nicotianamine is essential for iron mobilization. Thus, our micrografting chip will promote studies of long-distance signaling in plants.

Efficient Establishment of Interfamily Heterograft of Nicotiana benthamiana and Arabidopsis thaliana

The grafting technique has been applied to study systemic signaling in plants, especially to investigate whether gene action is graft transmissible and/or gene products such as RNAs and proteins are transported systemically. Here we describe an interfamily heterograft system between Nicotiana benthamiana scion plants and Arabidopsis stock plants for the identification of systemic phloem-mobile signals. Since these plants belong to evolutionary distant families and genome databases are available for both, we can reliably identify mobile substances transported from one to the other plant.

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.

The Use of Grafting to Study Systemic Signaling in Plants

Grafting has long been an important technique in agriculture. Nowadays, grafting is a widely used technique also to study systemic long-distance signaling in plants. Plants respond to their surrounding environment, and at that time many aspects of their physiology are regulated systemically; these start from local input signals and are followed by the transmission of information to the rest of the plant. For example, soil nutrient conditions, light/photoperiod, and biotic and abiotic stresses affect plants heterogeneously, and plants perceive such information in specific plant tissues or organs. Such environmental cues are crucial determinants of plant growth and development, and plants drastically change their morphology and physiology to adapt to various events in their life. Hitherto, intensive studies have been conducted to understand systemic signaling in plants, and grafting techniques have permitted advances in this field. The breakthrough technique of micrografting in Arabidopsis thaliana was established in 2002 and led to the development of molecular genetic tools in this field. Thereafter, various phenomena of systemic signaling have been identified at the molecular level, including nutrient fixation, flowering, circadian clock and defense against pathogens. The significance of grafting is that it can clarify the transmission of the stimulus and molecules. At present, many micro- and macromolecules have been identified as mobile signals, which are transported through plant vascular tissues to co-ordinate their physiology and development. In this review, we introduce the various grafting techniques that have been developed, we report on the recent advances in the field of plant systemic signaling where grafting techniques have been applied and provide insights for the future.

Protocol: optimisation of a grafting protocol for oilseed rape (Brassica napus) for studying long-distance signalling

BACKGROUND

Grafting is a well-established technique for studying long-distance transport and signalling processes in higher plants. While oilseed rape has been the subject of comprehensive analyses of xylem and phloem sap to identify macromolecules potentially involved in long-distance information transfer, there is currently no standardised grafting method for this species published.

RESULTS

We developed a straightforward collar-free grafting protocol for Brassica napus plants with high reproducibility and success rates. Micrografting of seedlings was done on filter paper. Grafting success on different types of regeneration media was measured short-term after grafting and as the long-term survival rate (>14 days) of grafts after the transfer to hydroponic culture or soil.

CONCLUSIONS

We compared different methods for grafting B. napus seedlings. Grafting on filter paper with removed cotyledons, a truncated hypocotyl and the addition of low levels of sucrose under long day conditions allowed the highest grafting success. A subsequent long-term hydroponic cultivation of merged grafts showed highest survival rates and best reproducibility.

Transcriptome analysis reveals the effects of sugar metabolism and auxin and cytokinin signaling pathways on root growth and development of grafted apple

BACKGROUND

The root architecture of grafted apple (Malus spp.) is affected by various characteristics of the scions. To provide information on the molecular mechanisms underlying this influence, we examined root transcriptomes of M. robusta rootstock grafted with scions of wild-type (WT) apple (M. spectabilis) and a more-branching (MB) mutant at the branching stage.

RESULTS

The growth rate of rootstock grafted MB was repressed significantly, especially the primary root length and diameter, and root weight. Biological function categories of differentially expressed genes were significantly enriched in processes associated with hormone signal transduction and intracellular activity, with processes related to the cell cycle especially down-regulated. Roots of rootstock grafted with MB scions displayed elevated auxin and cytokinin contents and reduced expression of MrPIN1, MrARF, MrAHP, most MrCRE1 genes, and cell growth-related genes MrGH3, MrSAUR and MrTCH4. Although auxin accumulation and transcription of MrPIN3, MrALF1 and MrALF4 tended to induce lateral root formation in MB-grafted rootstock, the number of lateral roots was not significantly changed. Sucrose, fructose and glucose contents were not decreased in MB-grafted roots compared with those bearing WT scions, but glycolysis and tricarboxylic acid cycle metabolic activities were repressed. Root resistance and nitrogen metabolism were reduced in MB-grafted roots as well.

CONCLUSIONS

Our findings suggest that root growth and development of rootstock are mainly influenced by sugar metabolism and auxin and cytokinin signaling pathways. This study provides a basis that the characteristics of scions are related to root growth and development, resistance and activity of rootstocks.

Structure, function, and evolution of plant NIMA-related kinases: implication for phosphorylation-dependent microtubule regulation

Microtubules are highly dynamic structures that control the spatiotemporal pattern of cell growth and division. Microtubule dynamics are regulated by reversible protein phosphorylation involving both protein kinases and phosphatases. Never in mitosis A (NIMA)-related kinases (NEKs) are a family of serine/threonine kinases that regulate microtubule-related mitotic events in fungi and animal cells (e.g. centrosome separation and spindle formation). Although plants contain multiple members of the NEK family, their functions remain elusive. Recent studies revealed that NEK6 of Arabidopsis thaliana regulates cell expansion and morphogenesis through β-tubulin phosphorylation and microtubule destabilization. In addition, plant NEK members participate in organ development and stress responses. The present phylogenetic analysis indicates that plant NEK genes are diverged from a single NEK6-like gene, which may share a common ancestor with other kinases involved in the control of microtubule organization. On the contrary, another mitotic kinase, polo-like kinase, might have been lost during the evolution of land plants. We propose that plant NEK members have acquired novel functions to regulate cell growth, microtubule organization, and stress responses.

Strigolactone Regulates Leaf Senescence in Concert with Ethylene in Arabidopsis

Leaf senescence is not a passive degenerative process; it represents a process of nutrient relocation, in which materials are salvaged for growth at a later stage or to produce the next generation. Leaf senescence is regulated by various factors, such as darkness, stress, aging, and phytohormones. Strigolactone is a recently identified phytohormone, and it has multiple functions in plant development, including repression of branching. Although strigolactone is implicated in the regulation of leaf senescence, little is known about its molecular mechanism of action. In this study, strigolactone biosynthesis mutant strains of Arabidopsis (Arabidopsis thaliana) showed a delayed senescence phenotype during dark incubation. The strigolactone biosynthesis genes MORE AXIALLY GROWTH3 (MAX3) and MAX4 were drastically induced during dark incubation and treatment with the senescence-promoting phytohormone ethylene, suggesting that strigolactone is synthesized in the leaf during leaf senescence. This hypothesis was confirmed by a grafting experiment using max4 as the stock and Columbia-0 as the scion, in which the leaves from the Columbia-0 scion senesced earlier than max4 stock leaves. Dark incubation induced the synthesis of ethylene independent of strigolactone. Strigolactone biosynthesis mutants showed a delayed senescence phenotype during ethylene treatment in the light. Furthermore, leaf senescence was strongly accelerated by the application of strigolactone in the presence of ethylene and not by strigolactone alone. These observations suggest that strigolactone promotes leaf senescence by enhancing the action of ethylene. Thus, dark-induced senescence is regulated by a two-step mechanism: induction of ethylene synthesis and consequent induction of strigolactone synthesis in the leaf.

Molecular and physiological mechanisms regulating tissue reunion in incised plant tissues

Interactions among the functionally specialized organs of higher plants ensure that the plant body develops and functions properly in response to changing environmental conditions. When an incision or grafting procedure interrupts the original organ or tissue connection, cell division is induced and tissue reunion occurs to restore physiological connections. Such activities have long been observed in grafting techniques, which are advantageous not only for agriculture and horticulture but also for basic research. To understand how this healing process is controlled and how this process is initiated and regulated at the molecular level, physiological and molecular analyses of tissue reunion have been performed using incised hypocotyls of cucumber (Cucumis sativus) and tomato (Solanum lycopersicum) and incised flowering stems of Arabidopsis thaliana. Our results suggest that leaf gibberellin and microelements from the roots are required for tissue reunion in the cortex of the cucumber and tomato incised hypocotyls. In addition, the wound-inducible hormones ethylene and jasmonic acid contribute to the regulation of the tissue reunion process in the upper and lower parts, respectively, of incised Arabidopsis stems. Ethylene and jasmonic acid modulate the expression of ANAC071 and RAP2.6L, respectively, and auxin signaling via ARF6/8 is essential for the expression of these transcription factors. In this report, we discuss recent findings regarding molecular and physiological mechanisms of the graft union and the tissue reunion process in wounded tissues of plants.

Identification of mRNAs that move over long distances using an RNA-Seq analysis of Arabidopsis/Nicotiana benthamiana heterografts

Phloem is a conductive tissue that allocates nutrients from mature source leaves to sinks such as young developing tissues. Phloem also delivers proteins and RNA species, such as small RNAs and mRNAs. Intensive studies on plant systemic signaling revealed the essential roles of proteins and RNA species. However, many of their functions are still largely unknown, with the roles of transported mRNAs being particularly poorly understood. A major difficulty is the absence of an accurate and comprehensive list of mobile transcripts. In this study, we used a hetero-graft system with Nicotiana benthamiana as the recipient scion and Arabidopsis as the donor stock, to identify transcripts that moved long distances across the graft union. We identified 138 Arabidopsis transcripts as mobile mRNAs, which we collectively termed the mRNA mobilome. Reverse transcription-PCR, quantitative real-time PCR and droplet digital PCR analyses confirmed the mobility. The transcripts included potential signaling factors and, unexpectedly, more general factors. In our investigations, we found no preferred transcript length, no previously known sequence motifs in promoter or transcript sequences and no similarities between the level of the transcripts and that in the source leaves. Grafting experiments regarding the function of ERECTA, an identified transcript, showed that no function of the transcript mobilized. To our knowledge, this is the first report identifying transcripts that move over long distances using a hetero-graft system between different plant taxa.

A pin-fasten grafting method provides a non-sterile and highly efficient method for grafting Arabidopsis at diverse developmental stages

BACKGROUND

Higher plants have evolved sophisticated communication systems to integrate environmental stimuli into their developmental programs. Grafting provides a powerful technique to examine transportation and systemic effects of mobile molecules. In Arabidopsis, many grafting approaches have been developed to investigate systemic molecules. However, these methods are usually limited to specific developmental stages or require sterilized conditions. To broaden the application of grafting for examining systemic signals at diverse developmental stages, we developed an Arabidopsis pin-fasten grafting method with insect pins used to assemble stocks and scions.

RESULTS

We report the step-by-step protocol of Arabidopsis pin-fasten grafting. Arabidopsis wild-type or gl1-1 plants were grown under long- or short-day conditions. Insect pins were inserted into gl1-1 scions at different developmental stages for grafting onto epicotyls or hypocotyls of stocks. Successfully grafted scions with newly developed glabrous leaves were observed at 14 days after grafting. Further longitudinal sections of the graft union showed well-connected vascular tissues between grafted plants. Use of fluorescent phloem-limited dye carboxyfluorescein diacetate in grafted plants demonstrated a symplastic connection established at 6 days after grafting and almost fully developed at 8 days.

CONCLUSIONS

Our method provides a simple and robust approach to grafting Arabidopsis at different developmental stages. Sterilized conditions are not required, which greatly improves the success of grafting and plant growth.

The cotyledons produce sufficient FT protein to induce flowering: evidence from cotyledon micrografting in Arabidopsis

In Arabidopsis, long-distance movement of FLOWERING LOCUS T (FT) protein from the leaf to the shoot apex triggers flower development. In wild-type Arabidopsis plants under long-day conditions, FT is mainly expressed in the cotyledon but is weakly expressed in the first true leaf prior to floral induction. To test the importance of the cotyledon in floral induction, we developed a cotyledon micrografting (Cot-grafting) method that, unlike other grafting methods, allows the FT protein from the graft to be transported via its native route from leaves to the shoot apex. By using Cot-grafting, we found that grafting a single wild-type cotyledon onto an ft-10 mutant strongly suppressed the ft-10 late flowering phenotype. Neither Y-grafting wild-type shoots nor butt-grafting wild-type roots to ft-10 plants resulted in comparably accelerated flowering in the ft-10 recipient plants. ft-10 mutants grafted with a 35S::FT cotyledon flowered as early as wild-type plants. When phloem-specific tracers were applied to a donor cotyledon, the tracers were detected in the vein of the true leaf of recipient plants 6 d after Cot-grafting. Also, macromolecule trafficking of an FT:yellow fluorescent protein:hemagglutinin fusion occurred across the graft junction 6 d after Cot-grafting. These results suggest that Cot-grafting, which allows protein movement in a manner consistent with the natural flow of FT protein from the leaf to the shoot apex, can efficiently suppress the late flowering of ft-10 mutants. Our results further suggest that in Arabidopsis, the cotyledon is an important organ for producing FT protein to induce flowering.

Novel features of radiation-induced bystander signaling in Arabidopsis thaliana demonstrated using root micro-grafting

Radiation-induced bystander effects (RIBE) have been well demonstrated in whole organisms, as well as in single-cell culture models in vitro and multi-cellular tissues models in vitro, however, the underlying mechanisms remain unclear, including the temporal and spatial course of bystander signaling. The RIBE in vivo has been shown to exist in the model plant Arabidopsis thaliana (A. thaliana). Importantly, the unique plant grafting provides a delicate approach for studying the temporal and spatial course of bystander signaling in the context of whole plants. In our previous study, the time course of bystander signaling in plants has been well demonstrated using the root micro-grafting technique. In this study, we further investigated the temporal cooperation pattern of multiple bystander signals, the directionality of bystander signaling, and the effect of bystander tissues on the bystander signaling. The results showed that the bystander response could also be induced efficiently when the asynchronously generated bystander signals reached the bystander tissues in the same period, but not when they entered into the bystander tissues in an inversed sequence. The absence of bystander response in root-inversed grafting indicated that the bystander signaling along roots might be of directionality. The bystander signaling was shown to be independent of the bystander tissues.

Phloem-mobile Aux/IAA transcripts target to the root tip and modify root architecture

In plants, the phloem is the component of the vascular system that delivers nutrients and transmits signals from mature leaves to developing sink tissues. Recent studies have identified proteins, mRNA, and small RNA within the phloem sap of several plant species. It is now of considerable interest to elucidate the biological functions of these potential long-distance signal agents, to further our understanding of how plants coordinate their developmental programs at the whole-plant level. In this study, we developed a strategy for the functional analysis of phloem-mobile mRNA by focusing on IAA transcripts, whose mobility has previously been reported in melon (Cucumis melo cv. Hale's Best Jumbo). Indoleacetic acid (IAA) proteins are key transcriptional regulators of auxin signaling, and are involved in a broad range of developmental processes including root development. We used a combination of vasculature-enriched sampling and hetero-grafting techniques to identify IAA18 and IAA28 as phloem-mobile transcripts in the model plant Arabidopsis thaliana. Micro-grafting experiments were used to confirm that these IAA transcripts, which are generated in vascular tissues of mature leaves, are then transported into the root system where they negatively regulate lateral root formation. Based on these findings, we present a model in which auxin distribution, in combination with phloem-mobile Aux/IAA transcripts, can determine the sites of auxin action.

Write 'systemic small RNAs': read 'systemic immunity'

About 50 years ago, it was reported that pathogen-infected plants are less susceptible to a broad spectrum of the subsequent pathogen attacks. This form of induced resistance, which resembles the immunisation in mammalian cells, is called systemic acquired resistance (SAR). In the last 10 years, plant molecular biology has been revolutionised by the discovery of RNA silencing, which is also a systemic phenomenon and also contributes to plant immunity. Here, I review these two systemic phenomena in a comparative way to highlight the possibility that systemic silencing contributes to systemic immunity. This potential contribution could be in the process of gene expression reprogramming, which is needed for SAR induction, and/or in SAR signal complex, and/or in establishing SAR in remote tissues and forming priming status.

The time course of long-distance signaling in radiation-induced bystander effect in vivo in Arabidopsis thaliana demonstrated using root micro-grafting

The radiation-induced bystander effect has been demonstrated in whole organisms as well as in multicellular tissues in vitro and single-cell culture systems in vitro. However, the time course of bystander signaling, especially in whole organisms, is not clear. Long-distance bystander/abscopal effects in vivo in plants have been demonstrated by our group. Plant grafting is a useful experimental tool for studying the root-shoot signaling of plants. In the present study, we developed a root micro-grafting technique with young seedlings of Arabidopsis thaliana in which the bystander signaling communication of root-to-shoot could easily be stopped or started at specific times after root irradiation. Using this methodology, we demonstrated the time course of long-distance signaling in radiation-induced bystander effects at the level of the organism using the expression level of the AtRAD54 gene as a biological end point. Briefly, an 8-h accumulation of damage signals in bystander parts after irradiation was essential for eliciting a bystander response. The protraction of signal accumulation was not related to the transmission speed of signaling molecules in plants and did not result from the delayed initiation of bystander signals in targeted root cells. It was suggested that the bystander effect might be induced jointly by multiple bystander signals initiated at different stages after irradiation. Moreover, reactive oxygen species (ROS) were shown to be implicated in the response process of bystander cells to radiation damage signals rather than in the generation of bystander signals in targeted cells.