Grafting triggers differential responses between scion and rootstock

Effects of Substrate Composition on the Growth Traits of Grafted Seedling in Macadamia (Macadamia integrifolia) Nuts

Macadamia nut plantings in China are expanding year by year. In order to breed and promote superior varieties, this study analyzed the effects of different rootstocks and scions on the survival rate of grafted seedlings, and then selected the best substrate composition for plant growth. The results showed that the survival rate of the HAES788 variety as rootstock and Guire No. 1 as scion was the highest, reaching 96%. The optimal grafting time in December was better than that in March. Furthermore, among 16 substrate formulations, T12, T13, T15, and T16 had advantages of agglomerated soil and more well-developed root systems compared to the CK made of loess. The plant height, stem diameter, leaf length, leaf width, and dry weight of the aboveground and underground parts of the grafted seedlings planted in these substrate formulations were significantly higher than those plants planted in the CK. In addition, the substrate formulations T12, T13, T15, and T16 significantly improved the organic matter, total nitrogen, and total potassium content of the substrate soils, but little improvement was observed for total phosphorus content after 13 months. Overall, macadamia grafting times are best in December, with HAES788 and Guire No. 1 being the best rootstock and scion. The optimal substrate formulations are T12, T13, T15, and T16. This study provides a solid foundation for the production of high-quality macadamia plants.

Long distance signalling and epigenetic changes in crop grafting

Humans have used grafting for more than 4000 years to improve plant production, through physically joining two different plants, which can continue to grow as a single organism. Today, grafting is becoming increasingly more popular as a technique to increase the production of herbaceous horticultural crops, where rootstocks can introduce traits such as resistance to several pathogens and/or improving the plant vigour. Research in model plants have documented how long-distance signalling mechanisms across the graft junction, together with epigenetic regulation, can produce molecular and phenotypic changes in grafted plants. Yet, most of the studied examples rely on proof-of-concept experiments or on limited specific cases. This review explores the link between research findings in model plants and crop species. We analyse studies investigating the movement of signalling molecules across the graft junction and their implications on epigenetic regulation. The improvement of genomics analyses and the increased availability of genetic resources has allowed to collect more information on potential benefits of grafting in horticultural crop models. Ultimately, further research into this topic will enhance our ability to use the grafting technique to exploit genetic and epigenetic variation in crops, as an alternative to traditional breeding.

Comparative transcriptomic analyses of citrus cold-resistant vs. sensitive rootstocks might suggest a relevant role of ABA signaling in triggering cold scion adaption

BACKGROUND

The citrus genus comprises a number of sensitive tropical and subtropical species to cold stress, which limits global citrus distribution to certain latitudes and causes major economic loss. We used RNA-Seq technology to analyze changes in the transcriptome of Valencia delta seedless orange in response to long-term cold stress grafted on two frequently used citrus rootstocks: Carrizo citrange (CAR), considered one of the most cold-tolerant accessions; C. macrophylla (MAC), a very sensitive one. Our objectives were to identify the genetic mechanism that produce the tolerant or sensitive phenotypes in citrus, as well as to gain insights of the rootstock-scion interactions that induce the cold tolerance or sensitivity in the scion.

RESULTS

Plants were kept at 1 ºC for 30 days. Samples were taken at 0, 15 and 30 days. The metabolomic analysis showed a significant increase in the concentration of free sugars and proline, which was higher for the CAR plants. Hormone quantification in roots showed a substantially increased ABA concentration during cold exposure in the CAR roots, which was not observed in MAC. Different approaches were followed to analyze gene expression. During the stress treatment, the 0-15-day comparison yielded the most DEGs. The functional characterization of DEGs showed enrichment in GO terms and KEGG pathways related to abiotic stress responses previously described in plant cold adaption. The DEGs analysis revealed that several key genes promoting cold adaption were up-regulated in the CAR plants, and those repressing it had higher expression levels in the MAC samples.

CONCLUSIONS

The metabolomic and transcriptomic study herein performed indicates that the mechanisms activated in plants shortly after cold exposure remain active in the long term. Both the hormone quantification and differential expression analysis suggest that ABA signaling might play a relevant role in promoting the cold hardiness or sensitiveness of Valencia sweet orange grafted onto Carrizo citrange or Macrophylla rootstocks, respectively. Our work provides new insights into the mechanisms by which rootstocks modulate resistance to abiotic stress in the production variety grafted onto them.

Salicylic Acid Is Involved in Rootstock-Scion Communication in Improving the Chilling Tolerance of Grafted Cucumber

Salicylic acid (SA) has been proven to be a multifunctional signaling molecule that participates in the response of plants to abiotic stresses. In this study, we used cold-sensitive cucumber and cold-tolerant pumpkin as experimental materials to examine the roles of SA in root-shoot communication responses to aerial or/and root-zone chilling stress in own-root and hetero-root grafted cucumber and pumpkin plants. The results showed that pumpkin (Cm) rootstock enhanced the chilling tolerance of grafted cucumber, as evidenced by the observed lower levels of electrolyte leakage (EL), malondialdehyde (MDA), and higher photosynthetic rate (Pn) and gene expression of Rubisco activase (RCA). However, cucumber (Cs) rootstock decreased the chilling tolerance of grafted pumpkins. Cs/Cm plants showed an increase in the mRNA expression of C-repeat-binding factor (CBF1), an inducer of CBF expression (ICE1), and cold-responsive (COR47) genes and CBF1 protein levels in leaves under 5/25 and 5/5°C stresses, or in roots under 25/5 and 5/5°C stresses, respectively, compared with the Cs/Cs. Chilling stress increased the endogenous SA content and the activity of phenylalanine ammonia-lyase (PAL), and the increase in SA content and activity of PAL in Cs/Cm plants was much higher than in Cs/Cs plants. Transcription profiling analysis revealed the key genes of SA biosynthesis, PAL, ICS, and SABP2 were upregulated, while SAMT, the key gene of SA degradation, was downregulated in Cs/Cm leaves, compared with Cs/Cs leaves under chilling stress. The accumulation of SA in the Cs/Cm leaves was mainly attributed to an increase in SA biosynthesis in leaves and that in transport from roots under aerial and root-zone chilling stress, respectively. In addition, exogenous SA significantly upregulated the expression level of cold-responsive (COR) genes, enhanced actual photochemical efficiency (Φ _PSII), maximum photochemical efficiency (F _v/F _m), and Pn, while decreased EL, MDA, and CI in grafted cucumber. These results suggest that SA is involved in rootstock-scion communication and grafting-induced chilling tolerance by upregulating the expression of COR genes in cucumber plants under chilling stress.

Endodormancy release in Norway spruce grafts representing trees of different ages

Studies addressing endodormancy release in adult trees are usually carried out using twigs detached from the trees in the experiments. Potential problems caused by cutting the root-shoot connection when detaching the twigs can be avoided by using grafts as the experimental material. We studied the effects of chilling on the endodormancy release in Norway spruce (Picea abies (L.) Karst.) grafts where twigs of 16-, 32- and 80-year-old trees were used as the scions. The grafts were first exposed to chilling in natural conditions and then samples of them were transferred at intervals to a regrowth test in forcing conditions in a greenhouse. The bud burst percentage, BB%, in the forcing conditions generally increased from zero to near 100% with increasing previous chilling accumulation from mid-October until mid-November, indicating that endodormancy was released in almost all of the grafts by mid-November. The days to bud burst, DBB, decreased in the forcing conditions with successively later transfers until the next spring. Neither BB% nor DBB was dependent on the age of the scion. However, in the early phase of ecodormancy release, the microscopic internal development of the buds was more advanced in the grafts representing the 16-year-old than in those representing the 32- or 80-year-old trees. In conclusion, our findings suggest that no major change in the environmental regulation of endodormancy release in Norway spruce takes place when the trees get older. Taken together with earlier findings with Norway spruce seedlings, our results suggest that regardless of the seedling or tree age, the chilling requirement of endodormancy release is met in late autumn. The implications of our findings for Norway spruce phenology under climatic warming and the limitations of our novel method of using grafts as a proxy of trees of different ages are discussed.

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.

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.

Comprehensive transcriptome profiling and phenotyping of rootstock and scion in a tomato/potato heterografting system

Tomato/potato heterografting-triggered phenotypic variations are well documented, yet the molecular mechanisms underlying grafting-induced phenotypic processes remain unknown. To investigate the phenotypic and transcriptomic responses of grafting parents in heterografting in comparison with self-grafting, tomato (Sl) was grafted onto potato rootstocks (St), and comparative phenotyping and transcriptome profiling were performed. Phenotypic analysis showed that Sl/St heterografting induced few phenotypic changes in the tomato scion. A total of 209 upregulated genes were identified in the tomato scion, some of which appear to be involved in starch and sucrose biosynthesis. Sl/St heterografting induced several modifications in the potato rootstocks (St-R), stolon number, stolon length and tuber number decreased significantly, together with an increase in GA₃ content of stolon and tuber, compared with self-grafted potato (St-WT). These results indicate that the tomato scion is less effective at producing substances or signals to induce tuberization but promotes stolon development into aerial stems and sprouting. RNA-Seq data analysis showed that 1529 genes were upregulated and 1329 downregulated between St-WT and St-R; some of these genes are involved in plant hormone signal transduction, with GID1-like gibberellin receptor (StGID1) and DELLA protein (StDELLA) being upregulated. Several genes in auxin, abscisic acid and ethylene pathways were differentially expressed as well. Various hormone signals engage in crosstalk to regulate diverse phenotypic events after grafting. This work provides abundant transcriptome profile data and lays a foundation for further research on the molecular mechanisms underlying RNA-based interactions between rootstocks and scions after tomato/potato heterografting.

Transcriptome analysis reveals the effects of grafting on sweetpotato scions during the full blooming stages

BACKGROUND

Sweetpotato (Ipomoea batatas) is a hexaploid plant and generally most genotypes do not flower at all in sub-tropics. Heterografting was carried out between sweetpotato cultivar 'Xushu 18' and Japanese morning glory (Ipomoea nil). With sweetpotato as 'scion' and I. nil as 'rootstock', sweetpotato was induced flowering in the autumn. However, little is known about the molecular mechanisms underlying sweetpotato responses to grafting, especially during the full blooming stages.

OBJECTIVES

To investigate the poorly understood molecular responses underlying the grafting-induced phenotypic processes in sweetpotato at full anthesis.

METHODS

In this study, to explore the transcriptome diversity and complexity of sweetpotato, PacBio Iso-Seq and Illumina RNA-seq analysis were combined to obtain full-length transcripts and to profile the changes in gene expression of five tissues: scion flowers (SF), scion leaves (SL), scion stems (SS), own-rooted leaves (OL) and own-rooted stems (OS).

RESULTS

A total of 138,151 transcripts were generated with an average length of 2255 bp, and more than 72% (100,396) of the transcripts were full-length. During full blooming, to examine the difference in gene expression of sweetpotato under grafting and natural growth conditions, 7905, 7795 and 15,707 differentially expressed genes were detected in pairwise comparisons of OS versus SS, OL versus SL and SL versus SF, respectively. Moreover, differential transcription of genes associated with anthocyanin biosynthesis, light pathway and photosynthesis, ethylene signal transduction pathway was observed in scion responses to grafting.

CONCLUSION

Our study is useful in understanding the molecular basis of grafting-induced flowering in grafted sweetpotatoes, and will lay a foundation for further research on sweetpotato breeding in the future.

Involvement of metabolic, physiological and hormonal responses in the graft-compatible process of cucumber/pumpkin combinations was revealed through the integrative analysis of mRNA and miRNA expression

Grafting is a widely used technique, and graft compatibility between the rootstock and scion is a prerequisite for grafting. To date, the underlying causes of graft compatibility/incompatibility remain largely unknown. Here, using cucumber (Cucumis sativus L.) grafted onto pumpkin (Cucurbita L.) rootstocks with different degrees of graft compatibility, and both self-grafting and non-grafting as controls, an integrative analysis of mRNA and miRNA expression and regulatory networks was conducted by using RNA-Seq and sRNA-Seq at 25 days after grafting (DAG). A total of 223 differentially expressed genes (DEGs) and 30 differentially expressed miRNAs (DEMs) related to graft compatibility were identified based on their fold change. Using a combination of GO annotations and KEGG pathway data, the functional annotations and pathways of DEGs and DEM targets showed that a number of metabolic, physiological and hormonal responses are involved in graft compatibility in cucumber leaves including metabolic processes (e.g., "carbohydrate metabolic processes"), nutrient transport (e.g., "sugar transport"), signal transduction (e.g., "MAPK cascade"), plant hormone signal transduction (e.g., "abscisic acid-activated signaling pathway"), transcription factors (e.g., MYB, NAC and bHLH), oxidation-reduction processes, and defense responses. The results of our comprehensive analysis suggested that compatible rootstocks might possess a greater ability for cell proliferation and a more efficient carbohydrate metabolism that promotes plant growth. In contrast, incompatible grafts induced multiple defense response-related genes and various transcription factors, likely in response to stress. Additionally, they consumed large amounts of energy, which ultimately restrained the plants normal development. This study advances our understanding of the molecular mechanisms underlying plant graft compatible/incompatible responses and provides numerous mRNA and miRNA candidates for more in-depth studies into the graft compatibility process.

Dissection of the Mechanism for Compatible and Incompatible Graft Combinations of Citrus grandis (L.) Osbeck ('Hongmian Miyou')

'Hongmian miyou' (Citrus grandis L. Osbeck) is mutated from 'Guanxi miyou', with a different spongy layer coloration. Trifoliate orange (Poncirus trifoliata) is widely used as rootstocks in 'Guanxi miyou' grafting, whereas 'Hongmian miyou' is incompatible with available trifoliate orange rootstocks. To explore the reasons for the etiolation of leaves of 'Hongmian miyou'/trifoliate orange, anatomical differences among different graft unions, gene expression profiles, and auxin levels of scion were investigated in this study. A histological assay indicated that there was no significant difference in anatomical structure between the compatible and incompatible combinations. A total of 1950 significant differentially-expressed genes (DEGs) were identified and analyzed. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis revealed that genes involved in carbohydrate metabolism, energy metabolism, amino acid metabolism, and plant hormone signal transduction were significantly enriched. Moreover, the expression of nine genes in the auxin pathway were upregulated and three were downregulated in compatible combinations compared with those in the incompatible group. Further experiments verified that indole-3-acetic acid (IAA) content increases in the compatible graft combination, which suggests that IAA might promote graft compatibility.

Physiological and Proteomic Investigations to Study the Response of Tomato Graft Unions under Temperature Stress

BACKGROUND

Grafting is an established practice for asexual propagation in horticultural and agricultural crops. The study on graft unions has become of interest for horticulturists using proteomic and genomic techniques to observe transfer of genetic material and signal transduction pathways from root to shoot and shoot to root. Another reason to study the graft unions was potentially to observe resistance against abiotic stresses. Using physiological and proteomic analyses, we investigated graft unions (rootstock and scions) of tomato genotypes exposed to standard-normal (23/23 and 25/18°C day/night) and high-low temperatures (30/15°C day/night).

RESULTS

Graft unions had varied responses to the diverse temperatures. High-low temperature, but not standard-normal temperature, induced the production of reactive oxygen species (ROS) in the form of H2O2 and O2-1 in rootstock and scions. However, the expression of many cell protection molecules was also induced, including antioxidant enzymes and their immunoblots, which also show an increase in their activities such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). The graft interfaces thus actively defend against stress by modifying their physiological and proteomic responses to establish a new cellular homeostasis. As a result, many proteins for cellular defense were regulated in graft unions under diverse temperature, in addition to the regulation of photosynthetic proteins, ion binding/transport proteins, and protein synthesis. Moreover, biomass, hardness, and vascular transport activity were evaluated to investigate the basic connectivity between rootstock and scions.

CONCLUSIONS

Our study provides physiological evidence of the grafted plants' response to diverse temperature. Most notably, our study provides novel insight into the mechanisms used to adapt the diverse temperature in graft unions (rootstock/scion).

Root-to-shoot signalling: integration of diverse molecules, pathways and functions

Plant adaptive potential is critically dependent upon efficient communication and co-ordination of resource allocation and signalling between above- and below-ground plant parts. Plant roots act as gatekeepers that sense and encode information about soil physical, chemical and biological factors, converting them into a sophisticated network of signals propagated both within the root itself, and also between the root and shoot, to optimise plant performance for a specific set of conditions. In return, plant roots receive and decode reciprocal information coming from the shoot. The communication modes are highly diverse and include a broad range of physical (electric and hydraulic signals, propagating Ca2+ and ROS waves), chemical (assimilates, hormones, peptides and nutrients), and molecular (proteins and RNA) signals. Further, different signalling systems operate at very different timescales. It remains unclear whether some of these signalling systems operate in a priming mode(s), whereas others deliver more specific information about the nature of the signal, or whether they carry the same 'weight'. This review summarises the current knowledge of the above signalling mechanisms, and reveals their hierarchy, and highlights the importance of integration of these signalling components, to enable optimal plant functioning in a dynamic environment.

Grapevine Rootstocks Differentially Affect the Rate of Ripening and Modulate Auxin-Related Genes in Cabernet Sauvignon Berries

In modern viticulture, grafting commercial grapevine varieties on interspecific rootstocks is a common practice required for conferring resistance to many biotic and abiotic stresses. Nevertheless, the use of rootstocks to gain these essential traits is also known to impact grape berry development and quality, although the underlying mechanisms are still poorly understood. In grape berries, the onset of ripening (véraison) is regulated by a complex network of mobile signals including hormones such as auxins, ethylene, abscisic acid, and brassinosteroids. Recently, a new rootstock, designated M4, was selected based on its enhanced tolerance to water stress and medium vigor. This study investigates the effect of M4 on Cabernet Sauvignon (CS) berry development in comparison to the commercial 1103P rootstock. Physical and biochemical parameters showed that the ripening rate of CS berries is faster when grafted onto M4. A multifactorial analysis performed on mRNA-Seq data obtained from skin and pulp of berries grown in both graft combinations revealed that genes controlling auxin action (ARF and Aux/IAA) represent one of main categories affected by the rootstock genotype. Considering that the level of auxin tightly regulates the transcription of these genes, we investigated the behavior of the main gene families involved in auxin biosynthesis and conjugation. Molecular and biochemical analyses confirmed a link between the rate of berry development and the modulation of auxin metabolism. Moreover, the data indicate that this phenomenon appears to be particularly pronounced in skin tissue in comparison to the flesh.