Graft-union development: a delicate process that involves cell-cell communication between scion and stock for local auxin accumulation

Chemical composition determination and transcriptomic analyses provide insight into the differences between wild and grafted Semen Ziziphi Spinosae

Semen Ziziphi Spinosae (SZS) is a traditional Chinese herbal medicine widely used to treat insomnia and anxiety in clinical practice. Currently, the demand for SZS is increasing every year, but the production of wild SZS is unstable due to environmental factors. Grafting sour jujube scions onto sour jujube or jujube tree stocks can achieve a high production rate within a short period of time. However, the effects of grafting on the quality of SZS have not been reported. This study investigated the differences between wild-type and grafted SZS from three aspects: phenotype, chemical composition, and molecular mechanism. The findings revealed that the grafted specimens were generally larger in morphology and lighter in color than the wild-type samples. The dimensions of both the grafted specimens were generally larger than those of the wild specimens. The HPLC-ELSD results revealed that the three main chemical components in the grafted SZS, namely, spinosin, jujuboside A, and jujuboside B, had higher contents than their wild-type counterparts. Comprehensive transcriptome sequencing analysis and KEGG annotation revealed that DEG enrichment between grafted and wild-type SZS occurred mainly during stress resistance and rootstock scion healing. There were 23 DEGs that may encode enzymes involved in the biosynthetic pathway of flavonoids and 21 genes encoding terpenoid saponins. Further investigation revealed that the expression of the genes C4H, CHS, CHI, and F3'5'H in the flavonoid biosynthesis pat.hway and HMGR, MVK, MVD, and FPPS in the saponin biosynthesis pathway accounted for the difference in quality between grafted and wild SZS. Furthermore, WGCNA identified 15 core genes related to medicinal ingredients between grafted and wild SZS. These results provide support for further research on the differences in the quality of medicinal ingredients between grafted and wild SZS.

An Enhanced Interaction of Graft and Exogenous SA on Photosynthesis, Phytohormone, and Transcriptome Analysis in Tomato under Salinity Stress

Salt stress can adversely affect global agricultural productivity, necessitating innovative strategies to mitigate its adverse effects on plant growth and yield. This study investigated the effects of exogenous salicylic acid (SA), grafting (G), and their combined application (GSA) on various parameters in tomato plants subjected to salt stress. The analysis focused on growth characteristics, photosynthesis, osmotic stress substances, antioxidant enzyme activity, plant hormones, ion content, and transcriptome profiles. Salt stress severely inhibits the growth of tomato seedlings. However, SA, G, and GSA improved the plant height by 22.5%, 26.5%, and 40.2%; the stem diameter by 11.0%, 26.0%, and 23.7%; the shoot fresh weight by 76.3%, 113.2%, and 247.4%; the root fresh weight by 150.9%, 238.6%, and 286.0%; the shoot dry weight by 53.5%, 65.1%, and 162.8%; the root dry weight by 150.0%, 150.0%, and 166.7%, and photosynthesis by 4.0%, 16.3%, and 32.7%, with GSA presenting the most pronounced positive effect. Regarding the osmotic stress substances, the proline content increased significantly by more than 259.2% in all treatments, with the highest levels in GSA. Under salt stress, the tomato seedlings accumulated high Na+ levels; the SA, G, and GSA treatments enhanced the K+ and Ca2+ absorption while reducing the Na+ and Al3+ levels, thereby alleviating the ion toxicity. The transcriptome analysis indicated that SA, G, and GSA influenced tomato growth under salt stress by regulating specific signaling pathways, including the phytohormone and MAPK pathways, which were characterized by increased endogenous SA and decreased ABA content. The combined application of grafting and exogenous SA could be a promising strategy for enhancing plant tolerance to salt stress, offering potential solutions for sustainable agriculture in saline environments.

Transcriptional dynamics reveals the asymmetrical events underlying graft union formation in pecan (Carya illinoinensis)

Grafting is a widely used technique for pecan propagation; however, the background molecular events underlying grafting are still poorly understood. In our study, the graft partners during pecan [Carya illinoinensis (Wangenh.) K. Koch] graft union formation were separately sampled for RNA-seq, and the transcriptional dynamics were described via weighted gene co-expression network analysis. To reveal the main events underlying grafting, the correlations between modules and grafting traits were analyzed. Functional annotation showed that during the entire graft process, signal transduction was activated in the scion, while messenger RNA splicing was induced in the rootstock. At 2 days after grafting, the main processes occurring in the scion were associated with protein synthesis and processing, while the primary processes occurring in the rootstock were energy release-related. During the period of 7-14 days after grafting, defense response was a critical process taking place in the scion; however, the main process functioning in the rootstock was photosynthesis. From 22 to 32 days after grafting, the principal processes taking place in the scion were jasmonic acid biosynthesis and defense response, whereas the highly activated processes associated with the rootstock were auxin biosynthesis and plant-type secondary cell wall biogenesis. To further prove that the graft partners responded asymmetrically to stress, hydrogen peroxide contents as well as peroxidase and β-1,3-glucanase activities were detected, and the results showed that their levels were increased in the scion not the rootstock at certain time points after grafting. Our study reveals that the scion and rootstock might respond asymmetrically to grafting in pecan, and the scion was likely associated with stress response, while the rootstock was probably involved in energy supply and xylem bridge differentiation during graft union formation.

Plant grafting: Molecular mechanisms and applications

People have grafted plants since antiquity for propagation, to increase yields, and to improve stress tolerance. This cutting and joining of tissues activates an incredible regenerative ability as different plants fuse and grow as one. For over a hundred years, people have studied the scientific basis for how plants graft. Today, new techniques and a deepening knowledge of the molecular basis for graft formation have allowed a range of previously ungraftable combinations to emerge. Here, we review recent developments in our understanding of graft formation, including the attachment and vascular formation steps. We analyze why plants graft and how biotic and abiotic factors influence successful grafting. We also discuss the ability and inability of plants to graft, and how grafting has transformed both horticulture and fundamental plant science. As our knowledge about plant grafting improves, new combinations and techniques will emerge to allow an expanded use of grafting for horticultural applications and to address fundamental research questions.

Genetic diversity and signature of divergence in the genome of grapevine clones of Southern Italy varieties

Sexual reproduction has contributed to a significant degree of variability in cultivated grapevine populations. However, the additional influence of spontaneous somatic mutations has played a pivotal role in shaping the diverse landscape of grapevine agrobiodiversity. These naturally occurring selections, termed 'clones,' represent a vast reservoir of potentially valuable traits and alleles that hold promise for enhancing grape quality and bolstering plant resilience against environmental and biotic challenges. Despite their potential, many of these clones remain largely untapped.In light of this context, this study aims to delve into the population structure, genetic diversity, and distinctive genetic loci within a collection of 138 clones derived from six Campanian and Apulian grapevine varieties, known for their desirable attributes in viticulture and winemaking. Employing two reduced representation sequencing methods, we extracted Single-Nucleotide Polymorphism (SNP) markers. Population structure analysis and fixation index (FST) calculations were conducted both between populations and at individual loci. Notably, varieties originating from the same geographical region exhibited pronounced genetic similarity.The resulting SNP dataset facilitated the identification of approximately two hundred loci featuring divergent markers (FST ≥ 0.80) within annotated exons. Several of these loci exhibited associations with essential traits like phenotypic adaptability and environmental responsiveness, offering compelling opportunities for grapevine breeding initiatives. By shedding light on the genetic variability inherent in these treasured traditional grapevines, our study contributes to the broader understanding of their potential. Importantly, it underscores the urgency of preserving and characterizing these valuable genetic resources to safeguard their intra-varietal diversity and foster future advancements in grapevine cultivation.

Morphological, physio-biochemical and nutritional status as potential markers for grafting compatibility in Kalamata olive cultivar

BACKGROUND

Choosing the most compatible and desirable rootstock for Kalamata olive cultivar is an important decision due to the longevity of the orchard and the difficulty rooting of Kalamata cuttings. Therefore, the goal of this study was to examine the morphological, physio-biochemical, and nutritional status as ppotential markers for grafting compatibility between Kalamata olive cultivar and three olive rootstocks (Coratina, Picual, Manzanillo) during two seasons (2020-2021) as well as follow up physio-bichemical and nutritional status of one-year-old Kalamata plants (2022).

RESULTS

The results indicated that, Picual rootstock recorded the highest significant grafting success which was associated with increasing number of leaves, leaf area and SPAD value in Kalamata scions by 22.15%, 36.86% and 14.64% compared to Manzanillo rootstock as mean of both seasons, respectively. While, Manzanillo rootstock recorded the highest significant activity for peroxidase and catalase by 51.41% and 60.1% at grafting union compared to Picual rootstock. Moreover, Picual rootstock for Kalamata scions had the highest acid invertase and sucrose synthase activities by 67.23% and 57.94% compared to Manzanillo rootstock. Furthermore, Picual rootstock recorded the highest significant Gibberellic acid by 52.8% and 18.6% compared to Coratina and Manzanillo rootstocks. Meanwhile, Picual rootstock recorded the lowest significant Abscisic acid by 68.17% and 63.15% as well as the lowest total phenols by 14.36% and 23.47% compared to Coratina and Manzanillo rootstocks.

CONCLUSIONS

This study sheds light for the importance of choosing the suitable rootstock for Kalamata cultivar. Also, sucrose synthase and acid invertase may have a novel role in determining grafting compatibility in olives. Increasing growth promoters (Gibberellic, Nitrogen) and decreasing both growth inhibitors (Abscisic, phenols) and oxidative enzyme (catalase, peroxidase) required for better graft compatibility.

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.

Comparative physiological and biochemical mechanisms in diploid, triploid, and tetraploid watermelon (Citrullus lanatus L.) grafted by branches

Seed production for polyploid watermelons is costly, complex, and labor-intensive. Tetraploid and triploid plants produce fewer seeds/fruit, and triploid embryos have a harder seed coat and are generally weaker than diploid seeds. In this study, we propagated tetraploid and triploid watermelons by grafting cuttings onto gourd rootstock (C. maxima × C. mochata). We used three different scions: the apical meristem (AM), one-node (1N), and two-node (2N) branches of diploid, triploid, and tetraploid watermelon plants. We then evaluated the effects of grafting on plant survival, some biochemical traits, oxidants, antioxidants, and hormone levels at different time points. We found significant differences between the polyploid watermelons when the 1N was used as a scion. Tetraploid watermelons had the highest survival rates and the highest levels of hormones, carbohydrates, and antioxidant activity compared to diploid watermelons, which may explain the high compatibility of tetraploid watermelons and the deterioration of the graft zone in diploid watermelons. Our results show that hormone production and enzyme activity with high carbohydrate content, particularly in the 2-3 days after transplantation, contribute to a high survival rate. Sugar application resulted in increased carbohydrate accumulation in the grafted combination. This study also presents an alternative and cost-effective approach to producing more tetraploid and triploid watermelon plants for breeding and seed production by using branches as sprouts.

Metabolite profiling and transcriptome analyses provide insight into the regulatory network of graft incompatibility in litchi

Litchi is an important commercial fruit crop widely grown in the world. Graft incompatibility between rootstocks and scions is a major constraint for large-scale cultivation of litchi orchards, popularization of new and excellent litchi varieties, and associated industrial development. Further, the genetic mechanism of graft incompatibility is still unclear in litchi. To reduce the incompatibility problems, this study investigated metabolic and transcriptomic differences between graft compatible and incompatible rootstock-scion combinations of litchi. The result of metabolomics analysis showed that incompatible rootstock-scion interaction modified the profiles of several metabolic substances. However, various compounds of flavonoids, phenolic acids, and lignin predominantly exhibited significantly altered abundance in graft incompatible combinations. Transcriptome analysis identified that graft incompatibility induces dynamic gene differences. The majority of these differentially expressed genes were enriched in biosynthetic pathways of phenylpropanoids. The differential expressions of genes in these pathways could be linked to the differential abundance levels of flavonoids, phenolic acids, and lignin compounds. Integrated metabolomic and transcriptomic analyses revealed a strong relationship between differential genes and differential metabolites identified in this study. In addition, identified hub genes and metabolites were closely associated with graft incompatibility of litchi. This study characterized the abundance of metabolites and genes in graft incompatible combinations and further discussed the genetic mechanism of graft incompatibility in litchi. Our results provide a platform to dissect the molecular mechanisms of graft incompatibility in the litchi fruit.

Integrated DNA methylation, transcriptome and physiological analyses reveal new insights into superiority of poplars formed by interspecific grafting

Plant grafting has a long history and it is extensively employed to improve plant performance. In our previous research, reciprocal grafts of Populus cathayana Rehder (C) and Populus deltoides Bart. Ex Marsh (D) were generated. The results showed that interspecific grafting combinations (scion/rootstock: C/D and D/C) grew better than intraspecific grafting combinations (C/C and D/D). To further understand differences in molecular mechanisms between interspecific and intraspecific grafting, we performed an integrated analysis, including bisulfite sequencing, RNA sequencing and measurements of physiological indicators, to investigate leaves of different grafting combinations. We found that the difference at the genome-wide methylation level was greater in D/C vs D/D than in C/D vs C/C, but no difference was detected at the transcription level in D/C vs D/D. Furthermore, the grafting superiority of D/C vs D/D was not as strong as that of C/D vs C/C. These results may be associated with the different methylation forms, mCHH (71.76%) and mCG (57.16%), that accounted for the highest percentages in C/D vs C/C and D/C vs D/D, respectively. In addition, the interspecific grafting superiority was found mainly related to the process of photosynthesis, phytohormone signal transduction, biosynthesis of secondary metabolites, cell wall and transcriptional regulation based on both physiological and molecular results. Overall, the results indicated that the physiological and molecular phenotypes of grafted plants are affected by the interaction between scion and rootstock. Thus, our study provides a theoretical basis for developing suitable scion-rootstock combinations for grafted plants.

High temperature perception in leaves promotes vascular regeneration and graft formation in distant tissues

Cellular regeneration in response to wounding is fundamental to maintain tissue integrity. Various internal factors including hormones and transcription factors mediate healing, but little is known about the role of external factors. To understand how the environment affects regeneration, we investigated the effects of temperature upon the horticulturally relevant process of plant grafting. We found that elevated temperatures accelerated vascular regeneration in Arabidopsis thaliana and tomato grafts. Leaves were crucial for this effect, as blocking auxin transport or mutating PHYTOCHROME INTERACTING FACTOR 4 (PIF4) or YUCCA2/5/8/9 in the cotyledons abolished the temperature enhancement. However, these perturbations did not affect grafting at ambient temperatures, and temperature enhancement of callus formation and tissue adhesion did not require PIF4, suggesting leaf-derived auxin specifically enhanced vascular regeneration in response to elevated temperatures. We also found that elevated temperatures accelerated the formation of inter-plant vascular connections between the parasitic plant Phtheirospermum japonicum and host Arabidopsis, and this effect required shoot-derived auxin from the parasite. Taken together, our results identify a pathway whereby local temperature perception mediates long distance auxin signaling to modify regeneration, grafting and parasitism.

This article has an associated ‘The people behind the papers’ interview.

Physiological, biochemical, and molecular aspects of grafting in fruit trees

Grafting is a widely used practice for asexual propagation of fruit trees. Many physiological, biochemical, and molecular changes occur upon grafting that can influence important horticultural traits. This technology has many advantages, including avoidance of juvenility, modifying the scion architecture, improving productivity, adapting scion cultivars to unfavourable environmental conditions, and developing traits in resistance to insect pests, bacterial and fungal diseases. A limitation of grafting is scion-rootstock incompatibility. It may be caused by many factors, including insufficient genetic proximity, physiological or biochemical factors, lignification at the graft union, poor graft architecture, insufficient cell recognition between union tissues, and metabolic differences in the scion and the rootstock. Plant hormones, like auxin, ethylene (ET), cytokinin (CK), gibberellin (GA), abscisic acid (ABA), and jasmonic acid (JA) orchestrate several crucial physiological and biochemical processes happening at the site of the graft union. Additionally, epigenetic changes at the union affect chromatin architecture by DNA methylation, histone modification, and the action of small RNA molecules. The mechanism triggering these effects likely is affected by hormonal crosstalk, protein and small molecules movement, nutrients uptake, and transport in the grafted trees. This review provides an overview of the basis of physiological, biochemical, and molecular aspects of fruit tree grafting between scion and rootstock.

Transcriptomic analysis of melon/squash graft junction reveals molecular mechanisms potentially underlying the graft union development

Oriental melon (Cucumis melo var. makuwa Makino) has become a widely planted horticultural crop in China especially in recent years and has been subjected to the grafting technique for the improvement of cultivation and stress resistance. Although grafting has a long history in horticulture, there is little known about the molecular mechanisms of the graft healing process in oriental melon. This study aims to reveal the molecular changes involved in the graft healing process. In the present work, anatomical observations indicated that the 2, 6, and 9 DAG were three critical stages for the graft healing and therefore, were selected for the subsequent high-throughput RNA-seq analysis. A total of 1,950 and 1,313 DEGs were identified by comparing IL vs. CA and CA vs. VB libraries, respectively. More DEGs in the melon scion exhibited abundant transcriptional changes compared to the squash rootstock, providing increased metabolic activity and thus more material basis for the graft healing formation in the scion. Several DEGs were enriched in the plant hormone signal transduction pathway, phenylpropanoid biosynthesis, and carbon metabolism. In addition, the results showed that concentrations of IAA, GA_3, and ZR were induced in the graft junctions. In conclusion, our study determined that genes involved in the hormone-signaling pathway and lignin biosynthesis played the essential roles during graft healing. These findings expand our current understandings of the molecular basis of the graft junction formation and facilitate the improvement and success of melon grafting in future production.

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.

Comparative Transcriptome Analysis in Homo- and Hetero-Grafted Cucurbit Seedlings

Watermelon (Citrullus lanatus) is a valuable horticultural crop with nutritional benefits grown worldwide. It is almost exclusively cultivated as grafted scions onto interspecific squash rootstock (Cucurbita maxima × Cucurbita moschata) to improve the growth and yield and to address the problems of soilborne diseases and abiotic stress factors. This study aimed to examine the effect of grafting (homo- and hetero-grafting) on the transcriptome level of the seedlings. Therefore, we compared homo-grafted watermelon (WW) with non-grafted watermelon control (W), homo-grafted squash (SS) with non-grafted squash control (S), hetero-grafted watermelon onto squash (WS) with SS, and WS with WW. Different numbers of differentially expressed genes (DEGs) were identified in each comparison. In total, 318 significant DEGs were detected between the transcriptomes of hetero-grafts and homo-grafts at 16 h after grafting. Overall, a significantly higher number of downregulated transcripts was detected among the DEGs. Only one gene showing increased expression related to the cytokinin synthesis was common in three out of four comparisons involving WS, SS, and S. The highest number of differentially expressed (DE) transcripts (433) was detected in the comparison between SS and S, followed by the 127 transcripts between WW and W. The study provides a description of the transcriptomic nature of homo- and hetero-grafted early responses, while the results provide a start point for the elucidation of the molecular mechanisms and candidate genes for the functional analyses of hetero-graft and homo-graft systems in Cucurbitaceae and generally in the plants.

Phloem unloading via the apoplastic pathway is essential for shoot distribution of root-synthesized cytokinins

Root-synthesized cytokinins are transported to the shoot and regulate the growth, development, and stress responses of aerial tissues. Previous studies have demonstrated that Arabidopsis (Arabidopsis thaliana) ATP binding cassette (ABC) transporter G family member 14 (AtABCG14) participates in xylem loading of root-synthesized cytokinins. However, the mechanism by which these root-derived cytokinins are distributed in the shoot remains unclear. Here, we revealed that AtABCG14-mediated phloem unloading through the apoplastic pathway is required for the appropriate shoot distribution of root-synthesized cytokinins in Arabidopsis. Wild-type rootstocks grafted to atabcg14 scions successfully restored trans-zeatin xylem loading. However, only low levels of root-synthesized cytokinins and induced shoot signaling were rescued. Reciprocal grafting and tissue-specific genetic complementation demonstrated that AtABCG14 disruption in the shoot considerably increased the retention of root-synthesized cytokinins in the phloem and substantially impaired their distribution in the leaf apoplast. The translocation of root-synthesized cytokinins from the xylem to the phloem and the subsequent unloading from the phloem is required for the shoot distribution and long-distance shootward transport of root-synthesized cytokinins. This study revealed a mechanism by which the phloem regulates systemic signaling of xylem-mediated transport of root-synthesized cytokinins from the root to the shoot.

Exogenous Treatments to Enhance Splice-Grafted Watermelon Survival

This study evaluated the use of splice grafting as a propagation strategy for watermelon. In experiment 1, the treatments consisted of sucrose, antitranspirant A, antitranspirant B, auxin (indole-3-butyric acid (IBA)) at two concentrations (10 and 20 mg·L−1), plus a water control. The survival (%) of splice-grafted watermelon plants differed due to the number of days after grafting and treatment (p < 0.0001, for both). At 21 days after grafting, plants treated with sucrose and antitranspirant A, and sucrose and antitranspirant A with 10 mg·L−1 auxin had 90% and 88% survival, respectively, whereas the graft survival was 18% for plants treated with water. Experiment 2 included the three top performing treatments from experiment 1 and a water control treatment, applied to both root-intact and root-excised rootstocks. There was a significant difference in survival (%) of splice-grafted watermelon due to root treatments, exogenous treatments, and the number of days after grafting (p < 0.0001, for all). At 21 days after grafting, survival for root-excised grafted plants was 11% lower compared to root-intact plants. Plants treated with sucrose and antitranspirant A, and sucrose and antitranspirant A with 10 mg·L−1 auxin had 87% and 86% survival, respectively, whereas plants treated with water had 14% survival. The external application of auxin applied to rootstock seedlings does not appear to be cost-effective; however, other products should be evaluated.

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.

Sugars promote graft union development in the heterograft of cucumber onto pumpkin

The use of heterografts is widely applied for the production of several important commercial crops, but the molecular mechanism of graft union formation remains poorly understood. Here, cucumber grafted onto pumpkin was used to study graft union development, and genome-wide tempo-spatial gene expression at the graft interface was comprehensively investigated. Histological analysis suggested that resumption of the rootstock growth occurred after both phloem and xylem reconnection, and the scion showed evident callus production compared with the rootstock 3 days after grafting. Consistently, transcriptome data revealed specific responses between the scion and rootstock in the expression of genes related to cambium development, the cell cycle, and sugar metabolism during both vascular reconnection and healing, indicating distinct mechanisms. Additionally, lower levels of sugars and significantly changed sugar enzyme activities at the graft junction were observed during vascular reconnection. Next, we found that the healing process of grafted etiolated seedlings was significantly delayed, and graft success, xylem reconnection, and the growth of grafted plants were enhanced by exogenous glucose. This demonstrates that graft union formation requires the correct sugar content. Furthermore, we also found that graft union formation was delayed with a lower energy charge by the target of rapamycin (TOR) inhibitor AZD-8055, and xylem reconnection and the growth of grafted plants were enhanced under AZD-8055 with exogenous glucose treatment. Taken together, our results reveal that sugars play a positive role in graft union formation by promoting the growth of cucumber/pumpkin and provide useful information for understanding graft union healing and the application of heterografting in the future.

Effect of Transgenic Rootstock Grafting on the Omics Profiles in Tomato

Grafting of non-transgenic scion onto genetically modified (GM) rootstocks provides superior agronomic traits in the GM rootstock, and excellent fruits can be produced for consumption. In such grafted plants, the scion does not contain any foreign genes, but the fruit itself is likely to be influenced directly or indirectly by the foreign genes in the rootstock. Before market release of such fruit products, the effects of grafting onto GM rootstocks should be determined from the perspective of safety use. Here, we evaluated the effects of a transgene encoding β-glucuronidase (GUS) on the grafted tomato fruits as a model case. An edible tomato cultivar, Stella Mini Tomato, was grafted onto GM Micro-Tom tomato plants that had been transformed with the GUS gene. The grafted plants showed no difference in their fruit development rate and fresh weight regardless of the presence or absence of the GUS gene in the rootstock. The fruit samples were subjected to transcriptome (NGS-illumina), proteome (shotgun LC-MS/MS), metabolome (LC-ESI-MS and GC-EI-MS), and general food ingredient analyses. In addition, differentially detected items were identified between the grafted plants onto rootstocks with or without transgenes (more than two-fold). The transcriptome analysis detected approximately 18,500 expressed genes on average, and only 6 genes were identified as differentially expressed. Principal component analysis of 2,442 peaks for peptides in proteome profiles showed no significant differences. In the LC-ESI-MS and GC-EI-MS analyses, a total of 93 peak groups and 114 peak groups were identified, respectively, and only 2 peak groups showed more than two-fold differences. The general food ingredient analysis showed no significant differences in the fruits of Stella scions between GM and non-GM Micro-Tom rootstocks. These multiple omics data showed that grafting on the rootstock harboring the GUS transgene did not induce any genetic or metabolic variation in the scion.

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.

Molecular and physiological characterization of the effects of auxin-enriched rootstock on grafting

Grafting is a highly useful technique, and its success largely depends on graft union formation. In this study, we found that root-specific expression of the auxin biosynthetic gene iaaM in tobacco, when used as rootstock, resulted in more rapid callus formation and faster graft healing. However, overexpression of the auxin-inactivating iaaL gene in rootstocks delayed graft healing. We observed increased endogenous auxin levels and auxin-responsive DR5::GUS expression in scions of WT/iaaM grafts compared with those found in WT/WT grafts, which suggested that auxin is transported upward from rootstock to scion tissues. A transcriptome analysis showed that auxin enhanced graft union formation through increases in the expression of genes involved in graft healing in both rootstock and scion tissues. We also observed that the ethylene biosynthetic gene ACS1 and the ethylene-responsive gene ERF5 were upregulated in both scions and rootstocks of the WT/iaaM grafts. Furthermore, exogenous applications of the ethylene precursor ACC to the junction of WT/WT grafts promoted graft union formation, whereas application of the ethylene biosynthesis inhibitor AVG delayed graft healing in WT/WT grafts, and the observed delay was less pronounced in the WT/iaaM grafts. These results demonstrated that elevated auxin levels in the iaaM rootstock in combination with the increased auxin levels in scions caused by upward transport/diffusion enhanced graft union formation and that ethylene was partially responsible for the effects of auxin on grafting. Our findings showed that grafting success can be enhanced by increasing the auxin levels in rootstocks using transgenic or gene-editing techniques.

Vegetable Grafting From a Molecular Point of View: The Involvement of Epigenetics in Rootstock-Scion Interactions

Vegetable grafting is extensively used today in agricultural production to control soil-borne pathogens, abiotic and biotic stresses and to improve phenotypic characteristics of the scion. Commercial vegetable grafting is currently practiced in tomato, watermelon, melon, eggplant, cucumber, and pepper. It is also regarded as a rapid alternative to the relatively slow approach of breeding for increased environmental-stress tolerance of fruit vegetables. However, even though grafting has been used for centuries, until today, there are still many issues that have not been elucidated. This review will emphasize on the important mechanisms taking place during grafting, especially the genomic interactions between grafting partners and the impact of rootstocks in scion's performance. Special emphasis will be drawn on the relation between vegetable grafting, epigenetics, and the changes in morphology and quality of the products. Recent advances in plant science such as next-generation sequencing provide new information regarding the molecular interactions between rootstock and scion. It is now evidenced that genetic exchange is happening across grafting junctions between rootstock and scion, potentially affecting grafting-mediated effects already recorded in grafted plants. Furthermore, significant changes in DNA methylation are recorded in grafted scions, suggesting that these epigenetic mechanisms could be implicated in grafting effects. In this aspect, we also discuss the process and the molecular aspects of rootstock scion communication. Finally, we provide with an extensive overview of gene expression changes recorded in grafted plants and how these are related to the phenotypic changes observed. Τhis review finally seeks to elucidate the dynamics of rootstock-scion interactions and thus stimulate more research on grafting in the future. In a future where sustainable agricultural production is the way forward, grafting could play an important role to develop products of higher yield and quality in a safe and "green" way.

Conserved Regulatory Pathways for Stock-Scion Healing Revealed by Comparative Analysis of Arabidopsis and Tomato Grafting Transcriptomes

Many plants can successfully join root and shoot sections at cut surfaces when severed at the stem. Graft healing is complex and conserved in diverse taxonomic groups with different vascular structures. Herein, we compared transcriptome data from autografted and separated stem sections of Arabidopsis thaliana and tomato (Solanum lycopersicum) to explore changes related to graft healing. Using orthologous gene pairs identified between the two species, temperal expression patterns of evolutionary associated genes in grafted top and bottom, separated top and bottom, and intact stems were exhibited. Genes with expression preference indicate functional diversification of genes related to anatomical structure and cellular development in the two species. Expression profiles of the variable genes revealed common pathways operating during graft healing, including phenylpropanoid metabolism, response to oxygen-containing compounds, xylan, and cell wall biogenesis, mitosis and the cell cycle, carboxylic acid catabolism, and meristem structural organization. In addition, vascular differentiation related NAC domain transcription factors and genome-wide members in Arabidopsis and tomato were used for phylogenetic and expression analysis. Expression differences were largely consistent with sequence differences, reflecting high similarity for protein-coding and regulatory regions of individual clades. NAC proteins mainly clustered in accordance with their reported functions in xylem differentiation or cambium formation. The putative conserved mechanisms suggested by conserved genes and functions could help to expand graft healing theory to a wider range of species, and temporal fluctuations in common pathways imply conserved biological processes during graft healing.

An in vitro grafting method to quantify mechanical forces of adhering tissues

Grafting is an indispensable agricultural technology for propagating useful tree varieties and obtaining beneficial traits of two varieties/species-as stock and scion-at the same time. Recent studies of molecular events during grafting have revealed dynamic physiological and transcriptomic changes. Strategies focused on specific grafting steps are needed to further associate each physiological and molecular event with those steps. In this study, we developed a method to investigate the tissue adhesion event, an early grafting step, by improving an artificial in vitro grafting system in which two pieces of 1.5-mm thick Nicotiana benthamiana cut stem sections were combined and cultured on medium. We prepared a silicone sheet containing five special cutouts for adhesion of cut stem slices. We quantitatively measured the adhesive force at these grafting interfaces using a force gauge and found that graft adhesion started 2 days after grafting, with the adhesive force gradually increasing over time. After confirming the positive effect of auxin on grafting by this method, we tested the effect of cellulase treatment and observed significant enhancement of graft tissue adhesion. Compared with the addition of auxin or cellulase individually, the adhesive force was stronger when both auxin and cellulase were added simultaneously. The in vitro grafting method developed in this study is thus useful for examining the process of graft adhesion.

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.

Breaking Bad News: Dynamic Molecular Mechanisms of Wound Response in Plants

Recognition and repair of damaged tissue are an integral part of life. The failure of cells and tissues to appropriately respond to damage can lead to severe dysfunction and disease. Therefore, it is essential that we understand the molecular pathways of wound recognition and response. In this review, we aim to provide a broad overview of the molecular mechanisms underlying the fate of damaged cells and damage recognition in plants. Damaged cells release the so-called damage associated molecular patterns to warn the surrounding tissue. Local signaling through calcium (Ca2+), reactive oxygen species (ROS), and hormones, such as jasmonic acid, activates defense gene expression and local reinforcement of cell walls to seal off the wound and prevent evaporation and pathogen colonization. Depending on the severity of damage, Ca2+, ROS, and electrical signals can also spread throughout the plant to elicit a systemic defense response. Special emphasis is placed on the spatiotemporal dimension in order to obtain a mechanistic understanding of wound signaling in plants.

Plant grafting relieves asymmetry of jasmonic acid response induced by wounding between scion and rootstock in tomato hypocotyl

Plant grafting is a sequential wound healing process. However, whether wounding induces a different jasmonic acid (JA) response within half a day (12 h) after grafting or non-grafting remains unclear. Using the tomato hypocotyl grafting method, we show that grafting alleviates the asymmetrical accumulation of JA and jasmonic acid isoleucine conjugate (JA-Ile) in scion and rootstock caused by wounding, and from 2 h after tomato micrografting, grafting obviously restored the level of JA-Ile in the scion and rootstock. Meanwhile, five JA-related genes, SlLOX11, SlAOS, SlCOI1, SlLAPA and SlJA2L, are detected and show significant changes in transcriptional expression patterns within 12 h of grafting, from asymmetrical to symmetrical, when the expression of 30 JA- and defense-related genes were analyzed. The results indicated that grafting alleviates the asymmetrical JA and defense response between scion and rootstock of the tomato hypocotyl within 12 h as induced by wounding. Moreover, we demonstrate that in the very early hours after grafting, JA-related genes may be involved in a molecular mechanism that changes asymmetrical expression as induced by wounding between scion and rootstock, thereby promoting wound healing and grafting success.

Histological Changes Associated with the Graft Union Development in Tomato

Despite the importance of grafting in horticultural crops such as tomato (Solanum lycopersicum L.), the structural changes that occur during the graft establishment are little understood. Using histological techniques, the present work examines the time course of changes on the anatomical structure of the graft junction in functional tomato homografts and compares it to that of heterografts and non-functional grafts. No apparent differences were detected between homo- and heterografts, showing similar tissue development. At 10 days after grafting, the cell walls of the scion and rootstock in the area of the graft junction were thicker than usual. Undifferentiated cells and new vascular tissue emerged from the pre-existing vasculature. Adventitious roots appeared mainly on the scion, arising from the pre-existing vasculature. At 20 days, more pronounced vascular tissue was visible, along with large areas showing vascular connection. At 210 days, vestiges of the changes undergone in graft development were still visible. Generally, non-functional grafts presented layers of necrotic remains and deposition of cell wall material in the cut edges, impeding the suitable scion-rootstock connection. Our results show that accurate changes in pre-existing vasculature and the cell walls of the adhesion line are crucial to the development of functional grafts.

What drives interspecies graft union success? Exploring the role of phylogenetic relatedness and stem anatomy

The underlying mechanisms that determine whether two species can form a successful graft union (graft compatibility) remain obscure. Two prominent hypotheses are (1) the more closely related species are, the higher the graft success and (2) the vascular anatomy at the graft junction influences graft success. In this paper these two hypotheses are examined in a systematic way using graft combinations selected from a range of (a) phylogenetically close and more distant legume species, (b) species displaying different germination patterns and (c) scions and rootstocks possessing contrasting stem tissues and vascular patterns. Relatedness of species was not a good predictor of graft compatibility, as vascular reconnection can occur between distantly related species and can fail to occur in some more closely related species. Similarly, neither the stem tissues present at the graft junction nor the vascular anatomy correlated with the success of vascular reconnection. Relatedness and stem anatomy therefore do not appear to be the determining factors in successful vascular reconnection after grafting in legumes. These results are discussed in conjunction with other hypotheses such as the role of auxin.

Transcription profiles reveal sugar and hormone signaling pathways mediating tree branch architecture in apple (Malus domestica Borkh.) grafted on different rootstocks

Apple trees grafted on different rootstock types, including vigorous rootstock (VR), dwarfing interstock (DIR), and dwarfing self-rootstock (DSR), are widely planted in production, but the molecular determinants of tree branch architecture growth regulation induced by rootstocks are still not well known. In this study, the branch growth phenotypes of three combinations of ‘Fuji’ apple trees grafted on different rootstocks (VR: Malus baccata; DIR: Malus baccata/T337; DSR: T337) were investigated. The VR trees presented the biggest branch architecture. The results showed that the sugar content, sugar metabolism-related enzyme activities, and hormone content all presented obvious differences in the tender leaves and buds of apple trees grafted on these rootstocks. Transcriptomic profiles of the tender leaves adjacent to the top buds allowed us to identify genes that were potentially involved in signaling pathways that mediate the regulatory mechanisms underlying growth differences. In total, 3610 differentially expressed genes (DEGs) were identified through pairwise comparisons. The screened data suggested that sugar metabolism-related genes and complex hormone regulatory networks involved the auxin (IAA), cytokinin (CK), abscisic acid (ABA) and gibberellic acid (GA) pathways, as well as several transcription factors, participated in the complicated growth induction process. Overall, this study provides a framework for analysis of the molecular mechanisms underlying differential tree branch growth of apple trees grafted on different rootstocks.

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.

The influence of grafting on crops' photosynthetic performance

In a near scenario of climate change where stress-derived limitations on crops' yield by affecting plant gas-exchange are expected, grafting may become a cheap and easy technique to improve crops photosynthetic performance and water-use efficiency. Inconsistent data of the effect of rootstocks over gas-exchange can be found in literature, being necessary an integrative analysis of the effect of grafting over photosynthetic parameters. With this aim, we present a compilation of the effect of graft on the net CO₂ assimilation rate (A_N) and other photosynthetic parameters across different species with agronomic interest. No differences were observed in any photosynthetic parameter between non-grafted and self-grafted plants under non-stress conditions. However, differences were found depending on the used rootstock, particularly for the intrinsic water-use efficiency (WUE_i). We observed that variations in A_N induced by rootstocks were related to changes in both diffusive and biochemical parameters. Under drought or salt stress, different photosynthetic performances were observed depending on the rootstock, although the high variability among studies promted to remarkable results. Overall, we observed that grafting can be a useful technique to improve plant photosynthetic performance, and therefore, crop yield and WUE, and that the rootstock selection for a target environment is determinant for the variations in photosynthesis.

Identification and expression analysis of auxin-responsive GH3 family genes in Chinese hickory (Carya cathayensis) during grafting

The GH3 genes play vital roles in auxin homeostasis by conjugating excess auxin to amino acids. However, how GH3 genes function during grafting in Chinese hickory (Carya cathayensis) is largely unknown. Here, based on the transcriptome database, a comprehensive identification and expression profiling analysis of 12 GH3 genes in Chinese hickory were performed. Phylogenetic analysis indicated that CcGH3-x exists in a specific subfamily. To understand the roles of CcGH3 genes, tissue-specific expression and the response to different phytohormones were determined. Expression profiles of GH3 genes of Chinese hickory during grafting were analysed. The data suggested that 10 CcGH3 genes were down-regulated at an early stage of grafting, indicating that auxin homeostasis regulated by the CcGH3 family might be inhibited at initial stages. At the completion of grafting, expression levels of members of the CcGH3 family were restored to normal levels. Endogenous auxin levels were also measured, and the data showed that free auxin decreased to the lowest level at an early stage of grafting, and then increased during grafting. Auxin amino acid conjugation increased at an early stage of grafting in rootstock, and then decreased with progression of the graft union. Our results demonstrate that the reduced expression of CcGH3 family genes during grafting might contribute to the release of free auxin, making an important contribution to the recovery of auxin levels after grafting.

Specification and regulation of vascular tissue identity in the Arabidopsis embryo

Development of plant vascular tissues involves tissue identity specification, growth, pattern formation and cell-type differentiation. Although later developmental steps are understood in some detail, it is still largely unknown how the tissue is initially specified. We used the early Arabidopsis embryo as a simple model to study this process. Using a large collection of marker genes, we found that vascular identity was specified in the 16-cell embryo. After a transient precursor state, however, there was no persistent uniform tissue identity. Auxin is intimately connected to vascular tissue development. We found that, although an AUXIN RESPONSE FACTOR5/MONOPTEROS (ARF5/MP)-dependent auxin response was required, it was not sufficient for tissue specification. We therefore used a large-scale enhanced yeast one-hybrid assay to identify potential regulators of vascular identity. Network and functional analysis of candidate regulators suggest that vascular identity is under robust, complex control. We found that one candidate regulator, the G-class bZIP transcription factor GBF2, can modulate vascular gene expression by tuning MP output through direct interaction. Our work uncovers components of a gene regulatory network that controls the initial specification of vascular tissue identity.

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.

Identifying Molecular Markers of Successful Graft Union Formation and Compatibility

Grafting is a technique used for millennia for vegetative propagation, especially in perennial fruit crops. This method, used on woody and herbaceous plants, can improve several agronomic characteristics, such as yield or vigor, as well as tolerance to biotic and abiotic stresses. However, some scion/rootstock combinations suffer from poor graft compatibility, i.e., they are unable to form and/or sustain a successful graft union. Identifying symptoms of graft incompatibility is difficult because they are not always present in the first years after grafting and in most cases the causes of incompatibility are still poorly understood. Studies of changes in transcript abundance during graft union formation indicate that grafting responses are similar to responses to wounding and include the differential expression of genes related to hormone signaling, oxidative stress, formation of new vascular vessels, cell development, and secondary metabolites, in particular polyphenols. This review summarizes current knowledge of the changes in transcript abundance, redox status and metabolites accumulation during graft union formation and in cases of graft incompatibility. The goal of this review is to discuss the possibility of identifying marker transcripts, enzyme activities and/or metabolites of grafting success and graft compatibility which could be used to score grafting success for genetic research and in breeding programs. We highlight gaps in current knowledge and potential research directions in this field.

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.

Expression of stilbene synthase VqSTS6 from wild Chinese Vitis quinquangularis in grapevine enhances resveratrol production and powdery mildew resistance

In grape (Vitis), stilbene phytoalexins can either be in situ synthesized or transported to the site of response during powdery mildew infection, enhancing disease resistance. Resveratrol is a phytoprotective stilbenoid compound that is synthesized by stilbene synthase (STS) in response to biotic and abiotic stresses, and is also known to have health benefits in the human diet. We have previously shown that transgenic Vitis vinifera cv. Thompson Seedless plants overexpressing a stilbene synthase gene, VqSTS6, from wild Chinese Vitis quinquangularis had a higher stilbenoid content, leading to an enhanced resistance to powdery mildew (Uncinula necator (Schw.) Burr). However, the biosynthesis and transportation in the plant tissue under powdery mildew infection are still unclear. Here, inhibitor and micro-grafting technologies were used to study the accumulation of resveratrol following powdery mildew infection. We observed that the levels of STS expression and stilbenoids increased in response to powdery mildew infection. Powdery mildew and inhibitor treatment on detached grape branches showed that resveratrol was in situ synthesized. Experiments with grafted plantlets showed that the abundance of stilbenoid compounds increased in the shoot during VqSTS6 overexpression in the root, while VqSTS6-Flag fusion was not tranported to the scions and only expressed in the transgenic rootstocks. Compared with wild-type Thompson Seedless plants, the non-transgenic/VqSTS6 transgenic (scion/rootstock) grafted Thompson Seedless plantlets exhibited increased resistance to powdery mildew. In addition, overexpression of VqSTS6 in roots led to increased levels of stilbenoid compounds in five other European grape varieties (V. vinifera cvs. Chardonnay, Perlette, Cabernet Sauvignon, Riesling and Muscat Hamburg). In conclusion, stilbenoid compounds can be either in situ synthesized or transported to the site of powdery mildew infection, and overexpression of VqSTS6 in the root promotes stilbenoids accumulation and disease resistance in European grapevine varieties.

Understanding the molecular mechanisms underlying graft success in grapevine

BACKGROUND

Grafting is an intensive commercial practice required to protect the European grapevine against the Phylloxera pest. Rootstocks resistant to this pest are hybrids of American vine species with different levels of compatibility with European Vitis vinifera varieties. Aiming to understand what drives grafting compatibility in grapevine, a transcriptomic approach was used to search for master regulators of graft success. Two scion/rootstock combinations, with different levels of compatibility, were compared in a nursery-grafting context at two stages, at 21 and 80 days after grafting.

RESULTS

In the most compatible combination, an earlier and higher expression of genes signaling the metabolic and hormonal pathways as well as a reduced expression of genes of the phenolic metabolism and of the oxidative stress response was observed. At 80 days after grafting a higher expression of transcription factors regulating vascular maintenance, differentiation and proliferation was obtained in the most compatible combination. Moreover, lower expression levels of microRNAs potentially targeting important transcription factors related to plant development was observed in the more compatible combination when compared to the less compatible one.

CONCLUSION

In this context, a set of regulators was selected as potential expression markers for early prediction of a compatible grafting.

Understanding the molecular mechanisms underlying graft success in grapevine

BACKGROUND

Grafting is an intensive commercial practice required to protect the European grapevine against the Phylloxera pest. Rootstocks resistant to this pest are hybrids of American vine species with different levels of compatibility with European Vitis vinifera varieties. Aiming to understand what drives grafting compatibility in grapevine, a transcriptomic approach was used to search for master regulators of graft success. Two scion/rootstock combinations, with different levels of compatibility, were compared in a nursery-grafting context at two stages, at 21 and 80 days after grafting.

RESULTS

In the most compatible combination, an earlier and higher expression of genes signaling the metabolic and hormonal pathways as well as a reduced expression of genes of the phenolic metabolism and of the oxidative stress response was observed. At 80 days after grafting a higher expression of transcription factors regulating vascular maintenance, differentiation and proliferation was obtained in the most compatible combination. Moreover, lower expression levels of microRNAs potentially targeting important transcription factors related to plant development was observed in the more compatible combination when compared to the less compatible one.

CONCLUSION

In this context, a set of regulators was selected as potential expression markers for early prediction of a compatible grafting.

Gene co-expression network analysis reveals pathways associated with graft healing by asymmetric profiling in tomato

BACKGROUND

The ability of severed rootstocks and shoots to re-establish vascular connections is used to generate grafted plants that combine desirable traits from both scions and rootstocks. Clarifying the mechanisms of graft healing is essential for its further application. We performed RNA sequencing of internodes near the cut position, making a distinction between separated or grafted tissues above and below the cut, in order to obtain a genetic description of graft union formation.

RESULTS

Using weighted gene co-expression analysis, variable transcripts were clustered into 10 distinct co-expression networks (modules) based on expression profiles, and genes with the most "hubness" ("hub" genes show the most connections in a network) within each module were predicted. A large proportion of modules were related to Position, and represent asymmetric expression networks from different pathways. Expression of genes involved in auxin and sugar transport and signaling, and brassinosteroid biosynthesis was increased above the cut, while stress response genes were up-regulated below the cut. Some modules were related to graft union formation, among which oxidative detoxification genes were co-expressed along with both wounding response and cell wall organization genes.

CONCLUSIONS

The present work provides a comprehensive understanding of graft healing-related gene networks in tomato. Also, the candidate pathways and hub genes identified here will be valuable for future studies of grafting in tomato.

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.

Living with Two Genomes: Grafting and Its Implications for Plant Genome-to-Genome Interactions, Phenotypic Variation, and Evolution

Plant genomes interact when genetically distinct individuals join, or are joined, together. Individuals can fuse in three contexts: artificial grafts, natural grafts, and host-parasite interactions. Artificial grafts have been studied for decades and are important platforms for studying the movement of RNA, DNA, and protein. Yet several mysteries about artificial grafts remain, including the factors that contribute to graft incompatibility, the prevalence of genetic and epigenetic modifications caused by exchanges between graft partners, and the long-term effects of these modifications on phenotype. Host-parasite interactions also lead to the exchange of materials, and RNA exchange actively contributes to an ongoing arms race between parasite virulence and host resistance. Little is known about natural grafts except that they can be frequent and may provide opportunities for evolutionary innovation through genome exchange. In this review, we survey our current understanding about these three mechanisms of contact, the genomic interactions that result, and the potential evolutionary implications.

Molecular Mechanisms of Plant Regeneration

Plants reprogram somatic cells following injury and regenerate new tissues and organs. Upon perception of inductive cues, somatic cells often dedifferentiate, proliferate, and acquire new fates to repair damaged tissues or develop new organs from wound sites. Wound stress activates transcriptional cascades to promote cell fate reprogramming and initiate new developmental programs. Wounding also modulates endogenous hormonal responses by triggering their biosynthesis and/or directional transport. Auxin and cytokinin play pivotal roles in determining cell fates in regenerating tissues and organs. Exogenous application of these plant hormones enhances regenerative responses in vitro by facilitating the activation of specific developmental programs. Many reprogramming regulators are epigenetically silenced during normal development but are activated by wound stress and/or hormonal cues.

Unmethyl-esterified homogalacturonan and extensins seal Arabidopsis graft union

BACKGROUND

Grafting is a technique widely used in horticulture. The processes involved in grafting are diverse, and the technique is commonly employed in studies focusing on the mechanisms that regulate cell differentiation or response of plants to abiotic stress. Information on the changes in the composition of the cell wall that occur during the grafting process is scarce. Therefore, this study was carried out for analyzing the composition of the cell wall using Arabidopsis hypocotyls as an example. During the study, the formation of a layer that covers the surface of the graft union was observed. So, this study also aimed to describe the histological and cellular changes that accompany autografting of Arabidopsis hypocotyls and to perform preliminary chemical and structural analyses of extracellular material that seals the graft union.

RESULTS

During grafting, polyphenolic and lipid compounds were detected, along with extracellular deposition of carbohydrate/protein material. The spatiotemporal changes observed in the structure of the extracellular material included the formation of a fibrillar network, polymerization of the fibrillar network into a membranous layer, and the presence of bead-like structures on the surface of cells in established graft union. These bead-like structures appeared either "closed" or "open". Only three cell wall epitopes, namely: LM19 (un/low-methyl-esterified homogalacturonan), JIM11, and JIM20 (extensins), were detected abundantly on the cut surfaces that made the adhesion plane, as well as in the structure that covered the graft union and in the bead-like structures, during the subsequent stages of regeneration.

CONCLUSIONS

To the best of our knowledge, this is the first report on the composition and structure of the extracellular material that gets deposited on the surface of graft union during Arabidopsis grafting. The results showed that unmethyl-esterified homogalacturonan and extensins are together involved in the adhesion of scion and stock, as well as taking part in sealing the graft union. The extracellular material is of importance not only due to the potential pectin-extensin interaction but also due to its origin. The findings presented here implicate a need for studies with biochemical approach for a detailed analysis of the composition and structure of the extracellular material.

Auxin transport and stem vascular reconnection - has our thinking become canalized?

BACKGROUND

The presence of a polar auxin transport stream has long been correlated with the differentiation and patterning of vascular cells across vascular plants. As our understanding of auxin transport and vascular development has grown, so too has evidence for the correlation between these processes. However, a clear understanding of the cellular and molecular mechanisms driving this correlation has not been elucidated.

SCOPE

This article examines the hypothesis that canalization via polar auxin transport regulates vascular reconnection and patterning in the stem after wounding or grafting. We examine the evidence for the causal nature of the relationship and the suggested role that other hormones may play. Data are presented indicating that in grafted plants the degree of auxin transport may not always correlate with vascular reconnection. Furthermore, data on grafting success using plants with a range of hormone-related mutations indicate that these hormones may not be critical for vascular reconnection.

CONCLUSIONS

In the past, excellent work examining elements of auxin synthesis, transport and response in relation to vascular development has been carried out. However, new experimental approaches are required to test more directly the hypothesis that auxin transport regulates stem vascular reconnection after wounding or grafting. This could include studies on the timing of the re-establishment of auxin transport and vascular reconnection after grafting and the influence of auxin transport mutants and inhibitors on these processes using live imaging.

The AtSUC2 Promoter: A Powerful Tool to Study Phloem Physiology and Development

The sucrose carrier AtSUC2 of Arabidopsis thaliana is localized in the phloem, where it catalyzes the uptake of sucrose from the apoplast into companion cells. Imported sucrose moves passively via plasmodesmata from the companion cells into the neighboring sieve elements that distribute this disaccharide to the different sink organs. Phloem loading of sucrose by the AtSUC2 protein is an essential process, and mutants lacking this protein stay tiny, develop no or only few flowers, and have a strongly reduced root system. The promoter of the AtSUC2 gene is active exclusively in companion cells of the phloem. Moreover, it drives very strong expression not only in Arabidopsis, but also in all plant species tested so far, including monocot species. Due to these features, the AtSUC2 promoter has become an important tool in diverse areas of plant research during the last two decades. It was used to study phloem development and function including phloem loading and unloading. Furthermore, it was helpful in analyzing the pathways of posttranscriptional silencing by RNA interference, the regulation of flowering, mechanisms of nutrient withdrawal by phloem-feeding pathogens, and other physiological functions that are related to long distance transport. The present paper gives an overview of different approaches in plant research that utilized the strong and companion cell-specific expression of own or foreign genes driven by the AtSUC2 promoter.

Plasma membrane H+ -ATPase activity and graft success of breadfruit (Artocarpus altilis) onto interspecific rootstocks of marang (A. odoratissimus) and pedalai (A. sericicarpus)

Breadfruit (Artocarpus altilis) is primarily grown as a staple tree crop for food security in Oceania. Significant wind damage has driven interest in developing its dwarfing rootstocks. Due to the predominantly vegetative propagation of the species, grafting onto interspecific seedlings is an approach to identifying dwarfing rootstocks. However, grafting of breadfruit onto unrelated Artocarpus species has not been investigated. Here we first report the success of breadfruit grafting onto interspecific rootstocks, marang (A. odoratissimus) and pedalai (A. sericicarpus). To address the low graft survival, we investigated the relationship of plasma membrane (PM) H⁺ -ATPase activity to graft success. We provide the first evidence for a positive correlation between PM H⁺ -ATPase activity and graft survival. The graft unions of successful grafts had higher PM H⁺ -ATPase activity compared to those of failed grafts. Rootstocks with low PM H⁺ -ATPase activity in leaf microsomes before grafting had lower graft survival than those with high enzyme activity, with graft success of 10% versus 60% and 0% versus 30% for marang and pedalai rootstocks, respectively. There was a positive correlation between graft success and the PM H⁺ -ATPase activity measured from the rootstock stem microsomes 2 months after grafting [marang, r(7) = 0.9203, P = 0.0004; pedalai (r(7) = 0. 8820, P = 0.0017]. Removal of scion's own roots decreased the leaf PM H⁺ -ATPase activity of grafted plants regardless of the final graft outcome. Recovery of the enzyme activity was only found in the successful grafts. The function of PM H⁺ -ATPase in graft union development and graft success improvement is discussed.