Inflorescence stem grafting made easy in Arabidopsis

A conserved graft formation process in Norway spruce and Arabidopsis identifies the PAT gene family as central regulators of wound healing

The widespread use of plant grafting enables eudicots and gymnosperms to join with closely related species and grow as one. Gymnosperms have dominated forests for over 200 million years, and despite their economic and ecological relevance, we know little about how they graft. Here we developed a micrografting method in conifers using young tissues that allowed efficient grafting with closely related species and between distantly related genera. Conifer graft junctions rapidly connected vasculature and differentially expressed thousands of genes including auxin and cell-wall-related genes. By comparing these genes to those induced during Arabidopsis thaliana graft formation, we found a common activation of cambium, cell division, phloem and xylem-related genes. A gene regulatory network analysis in Norway spruce (Picea abies) predicted that PHYTOCHROME A SIGNAL TRANSDUCTION 1 (PAT1) acted as a core regulator of graft healing. This gene was strongly up-regulated during both spruce and Arabidopsis grafting, and Arabidopsis mutants lacking PAT genes failed to attach tissues or successfully graft. Complementing Arabidopsis PAT mutants with the spruce PAT1 homolog rescued tissue attachment and enhanced callus formation. Together, our data show an ability for young tissues to graft with distantly related species and identifies the PAT gene family as conserved regulators of graft healing and tissue regeneration.

Export of defensive glucosinolates is key for their accumulation in seeds

Plant membrane transporters controlling metabolite distribution contribute key agronomic traits1-6. To eliminate anti-nutritional factors in edible parts of crops, the mutation of importers can block the accumulation of these factors in sink tissues7. However, this often results in a substantially altered distribution pattern within the plant8-12, whereas engineering of exporters may prevent such changes in distribution. In brassicaceous oilseed crops, anti-nutritional glucosinolate defence compounds are translocated to the seeds. However, the molecular targets for export engineering of glucosinolates remain unclear. Here we identify and characterize members of the USUALLY MULTIPLE AMINO ACIDS MOVE IN AND OUT TRANSPORTER (UMAMIT) family-UMAMIT29, UMAMIT30 and UMAMIT31-in Arabidopsis thaliana as glucosinolate exporters with a uniport mechanism. Loss-of-function umamit29 umamit30 umamit31 triple mutants have a very low level of seed glucosinolates, demonstrating a key role for these transporters in translocating glucosinolates into seeds. We propose a model in which the UMAMIT uniporters facilitate glucosinolate efflux from biosynthetic cells along the electrochemical gradient into the apoplast, where the high-affinity H+-coupled glucosinolate importers GLUCOSINOLATE TRANSPORTERS (GTRs) load them into the phloem for translocation to the seeds. Our findings validate the theory that two differently energized transporter types are required for cellular nutrient homeostasis13. The UMAMIT exporters are new molecular targets to improve nutritional value of seeds of brassicaceous oilseed crops without altering the distribution of the defence compounds in the whole plant.

Mix-and-match: an improved, fast and accessible protocol for hypocotyl micrografting of Arabidopsis seedlings with systemic ACC responses as a case study

BACKGROUND

Grafting is a technique widely used in horticulture that also has been applied in agriculture. In plant physiology, grafting facilitates the elucidation of mechanisms underlying growth and developmental processes, through the construction of chimeric plants with organs of different genotypes. Despite its small size, the model species Arabidopsis thaliana is very amenable for grafting, which can be useful to investigate transport of nutrients, amino acids or secondary metabolites between different tissues, or to investigate developmental processes depending on root-to-shoot communication, such as shoot branching, root and shoot plasticity upon shade avoidance, or disease resistance. Nevertheless, grafting protocols are usually technically challenging and training is required to achieve a reasonable success rate. Additionally, specialized tools and equipment are often needed, such as chips to accommodate the grafted plantlets or collars to maintain the contact between root and shoot.

RESULTS

In this methodology paper, we provide a fast, easy, accessible, and specialized equipment-free protocol that enables high success ratios. Critical steps and notes are detailed, easing the implementation of the procedure for non-trained researchers. An example of the protocol application by three independent non-trained researchers shows that this method allows to achieve a 90-100% of grafting efficiency after 6 days post-grafting recovery. In addition, the grafting of Col-0 with the acs8x mutant, depleted in 1-aminocyclopropane-1-carboxylic acid (ACC), the biosynthetic precursor of the phytohormone ethylene, provides an example of the application of this optimized protocol, showing the suitability of the process to study long-distance transport processes.

CONCLUSIONS

We present an optimized protocol for hypocotyl grafting of 4-day-old Arabidopsis thaliana seedlings. The combination of conditions yields a grafting success of 90-100% and provides an easy and accessible methodology, reducing the time frame, and without the necessity of acquiring specialized equipment. The presented protocol is simple, fast and highly efficient, easing the inclusion of hypocotyl grafting assays in any research project. In addition, the description of the protocol is detailed to a level ensuring that even non-trained researchers, are sufficiently prepared to adopt the grafting methodology.

ZRT-IRT-Like PROTEIN 6 expression perturbs local ion homeostasis in flowers and leads to anther indehiscence and male sterility

Metallic micronutrients are essential throughout the plant life cycle. Maintaining metal homeostasis in plant tissues requires a highly complex and finely tuned network controlling metal uptake, transport, distribution and storage. Zinc and cadmium hyperaccumulation, such as observed in the model plant Arabidopsis halleri, represents an extreme evolution of this network. Here, non-ectopic overexpression of the A. halleri ZIP6 (AhZIP6) gene, encoding a zinc and cadmium influx transporter, in Arabidopsis thaliana enabled examining the importance of zinc for flower development and reproduction. We show that AhZIP6 expression in flowers leads to male sterility resulting from anther indehiscence in a dose-dependent manner. The sterility phenotype is associated to delayed tapetum degradation and endothecium collapse, as well as increased magnesium and potassium accumulation and higher expression of the MHX gene in stamens. It is rescued by the co-expression of the zinc efflux transporter AhHMA4, linking the sterility phenotype to zinc homeostasis. Altogether, our results confirm that AhZIP6 is able to transport zinc in planta and highlight the importance of fine-tuning zinc homeostasis in reproductive organs. The study illustrates how the characterization of metal hyperaccumulation mechanisms can reveal key nodes and processes in the metal homeostasis network.

Use of Micrografting to Study the Role Played by Peptide Signals in ABA Biosynthesis

Plants survive by deploying appropriate stress responses under rapidly changing environmental conditions. Vascular plants transmit environmental information among distant organs. Long-distance signaling plays a crucial role in plant adaptation and subsequent survival under severe environmental conditions. In model plants, the micrografting method has recently emerged as an important method to elucidate tissue-to-tissue communication via hormones, RNAs, and peptides. In this chapter, I describe a micrografting method for Arabidopsis thaliana to graft shoots onto roots of plants with different genotypes. This grafting method may facilitate further research investigating how vascular plants integrate environmental information among distant organs via long-distance signaling.

Genome-wide analysis of the AINTEGUMENTA-like (AIL) transcription factor gene family in pumpkin (Cucurbita moschata Duch.) and CmoANT1.2 response in graft union healing

AINTEGUMENTA-like (AIL) proteins are members of the APETALA 2/ETHYLENE RESPONSE FACTOR (AP2/ERF) domain family of transcription factors involved in plant growth, development, and abiotic stress responses. However, the biological functions of AIL members in pumpkin (Cucurbita moschata Duch.) remain unknown. In this study, we identified 12 AIL genes in the pumpkin genome encoding proteins predicted to be localized in the nucleus. Phylogenetic analysis showed that the AIL gene family could be classified into six major subfamilies, with each member encoding two AP2/ERF domains separated by a linker region. CmoAIL genes were expressed at varying levels in the examined tissues, and CmoANT genes showed different expression patterns under auxin (IAA), 1-naphthylphthalamic acid (NPA), and abscisic acid (ABA) treatments. Ectopic overexpression of CmoANT1.2 in Arabidopsis increased organ size and promoted growth of grafted plants by accelerating graft union formation. However, there was no significant difference at the graft junction for WT/WT and WT/ANT under IAA or NPA treatments. Taken together, the results of this study provide critical information about CmoAIL genes and their encoded proteins, and suggest future work should investigate the functions of CmoANT1.2 in the grafting process in pumpkin.

Model systems for regeneration: Arabidopsis

Plants encompass unparalleled multi-scale regenerative potential. Despite lacking specialized cells that are recruited to injured sites, and despite their cells being encased in rigid cell walls, plants exhibit a variety of regenerative responses ranging from the regeneration of specific cell types, tissues and organs, to the rebuilding of an entire organism. Over the years, extensive studies on embryo, shoot and root development in the model plant species Arabidopsis thaliana have provided insights into the mechanisms underlying plant regeneration. These studies highlight how Arabidopsis, with its wide array of refined molecular, genetic and cell biological tools, provides a perfect model to interrogate the cellular and molecular mechanisms of reprogramming during regeneration.

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.

Insights Into Plant Surgery: An Overview of the Multiple Grafting Techniques for Arabidopsis thaliana

Plant grafting, the ancient practice of cutting and joining different plants, is gaining popularity as an elegant way to generate chimeras that combine desirable traits. Grafting was originally developed in woody species, but the technique has evolved over the past century to now encompass a large number of herbaceous species. The use of plant grafting in science is accelerating in part due to the innovative techniques developed for the model plant Arabidopsis thaliana. Here, we review these developments and discuss the advantages and limitations associated with grafting various Arabidopsis tissues at diverse developmental stages.

Importers Drive Leaf-to-Leaf Jasmonic Acid Transmission in Wound-Induced Systemic Immunity

The transmission of mobile wound signals along the phloem pathway is essential to the activation of wound-induced systemic response/resistance, which requires an upsurge of jasmonic acid (JA) in the distal undamaged leaves. Among these mobile signals, the electrical signal mediated by the glutamate-dependent activation of several clade three GLUTAMATE RECEPTOR-LIKE (GLR3) proteins is involved in the stimulation of JA production in distal leaves. However, whether JA acts as a mobile wound signal and, if so, how it is transmitted and interacts with the electrical signal remain unclear. Here, we show that JA was translocated from the local to distal leaves in Arabidopsis, and this process was predominantly regulated by two phloem-expressed and plasma membrane-localized jasmonate transporters, AtJAT3 and AtJAT4. In addition to the cooperation between AtJAT3/4 and GLR3.3 in the regulation of long-distance JA translocation, our findings indicate that importer-mediated cell-cell JA transport is important for driving the loading and translocation of JA in the phloem pathway in a self-propagating manner.

The Peptide Hormone Receptor CEPR1 Functions in the Reproductive Tissue to Control Seed Size and Yield

The interaction of C-TERMINALLY ENCODED PEPTIDES (CEPs) with CEP RECEPTOR1 (CEPR1) controls root growth and development, as well as nitrate uptake, but has no known role in determining yield. We used physiological, microscopic, molecular, and grafting approaches to demonstrate a reproductive tissue-specific role for CEPR1 in controlling yield and seed size. Independent Arabidopsis (Arabidopsis thaliana) cepr1 null mutants showed disproportionately large reductions in yield and seed size relative to their decreased vegetative growth. These yield defects correlated with compromised reproductive development predominantly in female tissues, as well as chlorosis, and the accumulation of anthocyanins in cepr1 reproductive tissues. The thinning of competing reproductive organs to improve source-to-sink ratios in cepr1, along with reciprocal bolt-grafting experiments, demonstrated that CEPR1 acts locally in the reproductive bolt to control yield and seed size. CEPR1 is expressed throughout the vasculature of reproductive organs, including in the chalazal seed coat, but not in other seed tissues. This expression pattern implies that CEPR1 controls yield and seed size from the maternal tissue. The complementation of cepr1 mutants with transgenic CEPR1 rescued the yield and other phenotypes. Transcriptional analyses of cepr1 bolts showed alterations in the expression levels of several genes of the CEP-CEPR1 and nitrogen homeostasis pathways. This transcriptional profile was consistent with cepr1 bolts being nitrogen deficient and with a reproductive tissue-specific function for CEP-CEPR1 signaling. The results reveal a local role for CEPR1 in the maternal reproductive tissue in determining seed size and yield, likely via the control of nitrogen delivery to the reproductive sinks.

Cut and paste: temperature-enhanced cotyledon micrografting for Arabidopsis thaliana seedlings

BACKGROUND

Cotyledon micrografting represents a useful tool for studying the central role of cotyledons during early plant development, especially their interplay with other plant organs with regard to long distance transport. While hypocotyl micrografting methods are well-established, cotyledon micrografting is still inefficient. By optimizing cotyledon micrografting, we aim for higher success rates and increased throughput in the model species Arabidopsis thaliana.

RESULTS

We established a cut and paste cotyledon surgery procedure on a flat and solid but moist surface which improved handling of small seedlings. By applying a specific cutting and joining pattern, throughput was increased up to 40 seedlings per hour. The combination of short-day photoperiods and low light intensities for germination and long days plus high light intensities, elevated temperature and vertical plate positioning after grafting significantly increased 'ligation' efficiency. In particular high temperatures affected success rates favorably. Altogether, we achieved up to 92% grafting success in A. thaliana. Reconnection of vasculature was demonstrated by transport of a vasculature-specific dye across the grafting site. Phloem and xylem reconnection were completed 3-4 and 4-6 days after grafting, respectively, in a temperature-dependent manner. We observed that plants with grafted cotyledons match plants with intact cotyledons in biomass production and rosette development.

CONCLUSIONS

This cut and paste cotyledon-to-petiole micrografting protocol simplifies the handling of plant seedlings in surgery, increases the number of grafted plants per hour and greatly improves success rates for A. thaliana seedlings. The developed cotyledon micrografting method is also suitable for other plant species of comparable size.

Methods for grafting Arabidopsis thaliana and Eutrema salsugineum

BACKGROUND

Grafting, an ancient agronomic technique, is an artificial mode of asexual reproduction in plants. Recently, grafting research has gradually shifted from modifying agronomic traits to the study of molecular mechanism. Grafting is an excellent tool to study long-range signaling processes in plants. And the grafting between species will help elucidate the molecular mechanisms underlying contrasting differences between different species. Arabidopsis thaliana is a salt-sensitive model glycophyte and Eutrema salsugineum (previously Thellungiella salsuginea, salt cress) is a salt-tolerant model halophyte. Successful grafting of these two model plants will help further study the physiological and molecular mechanisms underlying salt tolerance in plants. The aim of this study was to demonstrate two sterile micro-grafting methods for Arabidopsis and salt cress.

RESULTS

We developed the methods for sterile grafting between A. thaliana and E. salsugineum; this is the first report on inter-generic grafting between Arabidopsis and Eutrema. The method involves cut-in grafting under sterile conditions. The grafted plant part was placed in half strength Murashige and Skoog medium with 1% agar and 1% sugar, and then cultured vertically with 22 °C/18 °C short-day/night cycles. The plants were then transferred to half strength Hoagland nutrient solution for hydroponics. The reported method is simple and easy to operate. Self-grafted Arabidopsis-Arabidopsis and Eutrema-Eutrema plants were used as controls, which were obtained with an improved hypocotyl-cutting grafting method. Ion contents in grafted plants were detected by inductively coupled plasma optical emission spectroscopy. The results showed that the ion content in salt cress and Arabidopsis changed to different degrees after grafting.

CONCLUSIONS

The inter-species grafting technique described here makes it possible to study hybrid plants between Arabidopsis and Eutrema and will contribute to further understanding of long-distance communications in plants. This technique also provides a reference for improving plant varieties using grafting, such as gardening plants, as well as fruit and vegetable crops.

Grafting with Arabidopsis thaliana

Generating chimeric organisms is an invaluable way to study cell-to-cell movement and non-cell-autonomous actions of molecules. Plant grafting is an ancient method of generating chimeric organisms and recently has been used to study the movement of hormones, proteins, and RNAs. Here, I describe a simple and efficient way to graft Arabidopsis thaliana at the seedling stage to generate plants with roots and shoots of different genotypes. Using this protocol, success rates of over 80 % with up to 80 grafts assembled per hour can be achieved.

Monitoring Vascular Regeneration and Xylem Connectivity in Arabidopsis thaliana

Plants have a remarkable ability to regenerate vascular tissue after damage or wounding. A striking example of this phenomenon is the cutting and rejoining of plants during the process of grafting, which humans have used for millennia. Here, I describe how to graft Arabidopsis seedlings and how to monitor the vascular reconnection process during wound healing using fluorescent dyes and fluorescent proteins. Furthermore, I describe how to visualize xylem formation during graft healing. These techniques are useful for studying how the vascular system regenerates and for better understanding the process of plant grafting.

Plant grafting: insights into tissue regeneration

For millennia, people have cut and joined different plants together through a process known as grafting. The severed tissues adhere, the cells divide and the vasculature differentiates through a remarkable process of regeneration between two genetically distinct organisms as they become one. Grafting is becoming increasingly important in horticulture where it provides an efficient means for asexual propagation. Grafting also combines desirable roots and shoots to generate chimeras that are more vigorous, more pathogen resistant and more abiotic stress resistant. Thus, it presents an elegant and efficient way to improve plant productivity in vegetables and trees using traditional techniques. Despite this horticultural importance, we are only beginning to understand how plants regenerate tissues at the graft junction. By understanding grafting better, we can shed light on fundamental regeneration pathways and the basis for self/non‐self recognition. We can also better understand why many plants efficiently graft whereas others cannot, with the goal of improving grafting so as to broaden the range of grafted plants to create even more desirable chimeras. Here, I review the latest findings describing how plants graft and provide insight into future directions in this emerging field.

tRNA-Related Sequences Trigger Systemic mRNA Transport in Plants

In plants, protein-coding mRNAs can move via the phloem vasculature to distant tissues, where they may act as non-cell-autonomous signals. Emerging work has identified many phloem-mobile mRNAs, but little is known regarding RNA motifs triggering mobility, the extent of mRNA transport, and the potential of transported mRNAs to be translated into functional proteins after transport. To address these aspects, we produced reporter transcripts harboring tRNA-like structures (TLSs) that were found to be enriched in the phloem stream and in mRNAs moving over chimeric graft junctions. Phenotypic and enzymatic assays on grafted plants indicated that mRNAs harboring a distinctive TLS can move from transgenic roots into wild-type leaves and from transgenic leaves into wild-type flowers or roots; these mRNAs can also be translated into proteins after transport. In addition, we provide evidence that dicistronic mRNA:tRNA transcripts are frequently produced in Arabidopsis thaliana and are enriched in the population of graft-mobile mRNAs. Our results suggest that tRNA-derived sequences with predicted stem-bulge-stem-loop structures are sufficient to mediate mRNA transport and seem to be necessary for the mobility of a large number of endogenous transcripts that can move through graft junctions.

Grafting triggers differential responses between scion and rootstock

Grafting is a well-established practice to facilitate asexual propagation in horticultural and agricultural crops. It has become a method for studying molecular aspects of root-to-shoot and/or shoot-to-root signaling events. The objective of this study was to investigate differences in gene expression between the organs of the scion and rootstock of a homograft (Arabidopsis thaliana). MapMan and Gene Ontology enrichment analysis revealed differentially expressed genes from numerous functional categories related to stress responses in the developing flower buds and leaves of scion and rootstock. Meta-analysis suggested induction of drought-type responses in flower buds and leaves of the scion. The flower buds of scion showed over-representation of the transcription factor genes, such as Homeobox, NAC, MYB, bHLH, B3, C3HC4, PLATZ etc. The scion leaves exhibited higher accumulation of the regulatory genes for flower development, such as SEPALLATA 1-4, Jumonji C and AHL16. Differential transcription of genes related to ethylene, gibberellic acid and other stimuli was observed between scion and rootstock. The study is useful in understanding the molecular basis of grafting and acclimation of scion on rootstock.

Arabidopsis MSH1 mutation alters the epigenome and produces heritable changes in plant growth

Plant phenotypes respond to environmental change, an adaptive capacity that is at least partly transgenerational. However, epigenetic components of this interplay are difficult to measure. Depletion of the nuclear-encoded protein MSH1 causes dramatic and heritable changes in plant development, and here we show that crossing these altered plants with isogenic wild type produces epi-lines with heritable, enhanced growth vigour. Pericentromeric DNA hypermethylation occurs in a subset of msh1 mutants, indicative of heightened transposon repression, while enhanced growth epi-lines show large chromosomal segments of differential CG methylation, reflecting genome-wide reprogramming. When seedlings are treated with 5-azacytidine, root growth of epi-lines is restored to wild-type levels, implicating hypermethylation in enhanced growth. Grafts of wild-type floral stems to mutant rosettes produce progeny with enhanced growth and altered CG methylation strikingly similar to epi-lines, indicating a mobile signal when MSH1 is downregulated, and confirming the programmed nature of methylome and phenotype changes.

Suppression of MutS HOMOLOGUE 1 (MSH1), a plant protein targeted to mitochondria and plastids, causes a variety of phenotypes. Here Virdi et al. show that MSH1 depletion in Arabidopsis results in heritable changes in nuclear DNA methylation, which can lead to enhanced growth vigour.

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.

An efficient flat-surface collar-free grafting method for Arabidopsis thaliana seedlings

BACKGROUND

Grafting procedures are an excellent tool to study long range signalling processes within a plant. In the last decade, suitable flat-surface grafting procedures for young Arabidopsis seedlings using a collar to support the graft have been developed, allowing the study of long-range signals from a molecular perspective.

RESULTS

In the modification presented here, scion and stock are put together on the medium without supporting elements, while cotyledons are removed from the scion, resulting in increased grafting success that can reach up to 100%. At the same time, the protocol enables to process as many as 36 seedlings per hour, which combined with the high success percentage represents increased efficiency per time unit.

CONCLUSIONS

Growing cotyledons usually push the scion and the rootstock away in the absence of a supporting element. Removing them at the grafting step greatly improved success rate and reduced post-grafting manipulations.