Graft Incompatibility Research Articles

The emergence of spiraling tracheary element bundles in incompatible grafts.

In distantly-related plant grafting, incompatibility often occurs between scion and rootstock, resulting in growth stagnation, and eventually graft failure. In this study, we found that an emergent structure, or the spiraling tracheary element (TE) bundles consisting of TE masses occurring at the graft interface, was extensively present in the highly incompatible interfamilial graft of Brassica napus/Portulaca oleracea (Bn/Po) and Nicotiana benthamiana/Portulaca oleracea (Nb/Po). This special structure mostly appeared in the local area near the grafting union, and the frequency and quantity of the spiraling tracheary element bundles were much higher in the scion than in the rootstock. Nevertheless, only a small portion of Arabidopsis thaliana/Portulaca oleracea (At/Po) interfamilial grafts showed a less spiraled TEs at the grafting union (usually a circular TE), which is consistent with its growth performance. This study consolidated that spiraling TE bundles were an important indicator for graft incompatibility. The possible reason for the formation of spiraling TE bundles in interfamilial grafts was discussed.

Micrografting: An Old Dog Plays New Tricks in Obligate Plant Pathogens

Micrografting, which was developed almost 50 years ago, has long been used for virus eradication, micropropagation, regeneration, rejuvenation, and graft compatibility. Recently, micrografting has been used for studies of long-distance trafficking and signaling of molecules between scions and rootstocks. The graft transmissiveness of obligate plant pathogens, such as viruses, viroids, and phytoplasmas, facilitated the use of micrografting to study biological indexing and pathogen transmission, pathogen-induced graft incompatibility, and screening for the pathogen resistance during the past 20 years. The present study provides comprehensive information on the latter subjects. Finally, prospects are proposed to direct further studies.

Determination of The Grafting Performance of Some Hybrid Chestnut Genotypes and Cultivars

This study has been carried out to determine the graft compatibility of newly registered some hybrid chestnut cultivars and genotype. The study was conducted between 2012-2018 at the Ondokuz Mayıs University and the Black Sea Agricultural Research Institute. In the study, ‘Marigoule’, Akyüz’, ‘Ali Nihat’ and ‘Macit 55’ cultivars were tested as scion and rootstock, and A41 genotype was tested only as rootstock. Grafting studies were carried out on generative rootstocks in 2014 and mother plants in 2015 using the chip budding method. Graft success (%) was determined 30 days after grafting and survival ratio (%) at the end of the vegetation period. Also rootstock, graft area and scion diameter were measured at the end of the vegetation period by digital caliper. As a result of the study, graft success and survival ratio were low in combinations created with ‘Ali Nihat’ and A41 generative rootstocks. Among the plants, the best generative rootstock was the ‘Macit 55’ cultivar in terms of graft success and survival ratio. ‘Macit 55’ rootstock had good graft compatibility with both itself (‘Macit 55’), ‘Akyüz’ and ‘Ali Nihat’ cultivars. ‘Marigoule’ cultivar has generally shown severe graft incompatibility with complex hybrids tested in the study. In the grafts made on clone plants, the ‘Ali Nihat’ rootstock was determined as a good rootstock for itself (‘Ali Nihat’), ‘Akyüz’ and ‘Macit 55’ cultivars.

Survival Rate of Young Pear Trees in Different Rootstock and Cultivar Combinations under Field Conditions: Preliminary Results

This study was carried out to determine the effects of 7 different rootstocks on rootstock diameter (mm), stem diameter (mm), and plant survival ratio (%) of some pear cultivars between 2019-2021. Rootstocks, cultivars, research years, and their interactions significantly affected all examined parameters in the study, except for the interaction of year x rootstock x cultivar. The ‘Deveci’/BA29 and ‘Deveci’/OHxF333 had the largest rootstock and trunk diameters, while the 'Williams'/MC had the smallest. The Fox11 and OHxF333 rootstocks had the highest plant survival ratios, whereas the MC rootstocks had the lowest. In terms of cultivar, the maximum plant survival ratio was found in the ‘Deveci’. At the end of the first 3 years after planting, there was a decrease in the survival ratio of the plants. ‘Williams’ had the lowest rootstock diameter, trunk diameter and survival ratio. The highest trunk diameter was in the OHxF333, BA29, Fox11 and Farold40 rootstocks, and the cultivar was in the ‘Deveci’. Plant survival ratios in quince clonal rootstocks were slightly lower than in pear rootstocks. This is due to the graft incompatibility between quince rootstocks and some pear cultivars. Compatible inter-stock with rootstock and cultivar should be employed to overcome this incompatibility problem caused by localized graft incompatibility. According to the findings of this study on young plants, it is required to extend the research and conduct additional observations in order to give more precise recommendations.

Comparisons of Anatomical Characteristics and Transcriptomic Differences between Heterografts and Homografts in Pyrus L.

Pear (Pyrus L.) is an important temperate fruit worldwide, and grafting is widely used in pear vegetative propagation. However, the mechanisms of graft healing or incompatibility remain poorly understood in Pyrus. To study the differences in graft healing in Pyrus, the homograft “Qingzhen D1/Qingzhen D1” and the heterograft “QAUP-1/Qingzhen D1” as compatibility and incompatibility combinations were compared. Anatomical differences indicated the healing process was faster in homografts than in heterografts. During the healing process, four critical stages in graft union formation were identified in the two types of grafts. The expression of the genes associated with hormone signaling (auxin and cytokinins), and lignin biosynthesis was delayed in the healing process of heterografts. In addition, the PbBglu13 gene, encoded β-glucosidase, was more highly up-regulated in heterografts than in homografts to promote healing. Meanwhile, the most of DEGs related starch and sucrose metabolism were found to be up-regulated in heterografts; those results indicated that cellulose and sugar signals were also involved in graft healing. The results of this study improved the understanding of the differences in the mechanisms of graft healing between homografts and heterografts.

Comparative transcriptomic analysis on compatible/incompatible grafts in citrus.

Grafting is a useful cultivation technology to resist abiotic and biotic stresses and is an integral part of citrus production. However, some widely utilized rootstocks may still exhibit graft incompatibility in the orchard. ‘Hongmian miyou’ (Citrus maxima (Burm.) Merrill) is mutated from ‘Guanxi miyou’, but these two scions showed different compatibility with available Poncirus trifoliata rootstock. Foliage etiolation is an observed symptom of graft incompatibility, but its mechanism remains poorly understood. This study is the first to investigate the morphological, physiological, and anatomical differences between compatible and incompatible grafts, and perform transcriptome profiling at crucial stages of the foliage etiolation process. Based on comprehensive analyses, hormonal balance was disordered, and two rate-limiting genes, NCED3 (9-cis-epoxycarotenoid dioxygenase 3) and NCED5, being responsible for ABA (abscisic acid) accumulation, were highlighted. Further correlation analysis indicated that IAA (indole-3-acetic acid) and ABA were the most likely inducers of the expression of stress-related genes. In addition, excessive starch accumulation was observed in the lamina and midribs of leaves of incompatible grafts. These results provide a new insight into the role of hormonal balance and ABA biosynthesis genes in regulating and contributing to graft incompatibility, and will further define and deploy candidate genes to explore the mechanisms underlying citrus rootstock–scion interactions.

Interfamily grafting capacity of petunia.

In grafting, an agricultural technique for propagating flower species and fruit trees, two plants are combined to exploit their beneficial characteristics, such as rootstock disease tolerance and vigor. Grafting incompatibility has been observed, however, between distantly related plant combinations, which limits the availability of plant resources. A high grafting capacity has been found in Nicotiana, belonging to Solanaceae, but not in Ipomoea nil, a Convolvulaceae species. Here, we found that Petunia hybrida, another solanaceous species, has similar ability of interfamily grafting, which indicates that interfamily grafting capability in Solanaceae is not limited to the genus Nicotiana. RNA sequencing-based comparative time-series transcriptomic analyses of Nicotiana benthamiana, I. nil, and P. hybrida revealed that N. benthamiana and P. hybrida share a common gene expression pattern, with continued elevated expression of the β-1,4-glucanase subclade gene GH9B3 observed after interfamily grafting. During self-grafting, GH9B3 expression in each species was similarly elevated, thus suggesting that solanaceous plants have altered regulatory mechanisms for GH9B3 gene expression that allow tissue fusion even with other species. Finally, we tested the effect of the β-1,4-glucanase inhibitor D-glucono-1,5-lactone, using glucose as a control, on the interfamily grafting usability of P. hybrida with Arabidopsis rootstock. Strong inhibition of graft establishment was observed only with D-glucono-1,5-lactone, thus suggesting the important role of GH9B3 in P. hybrida grafting. The newly discovered grafting compatibility of Petunia with different families enhances the propagation techniques and the production of flower plants.

Identifying early metabolite markers of successful graft union formation in grapevine.

Grafting is an important horticultural technique used for many crop species. However, some scion/rootstock combinations are considered as incompatible due to poor graft union formation and subsequently high plant mortality. The early identification of graft incompatibility could allow the selection of non-viable plants before planting and would have a beneficial impact on research and development in the nursery sector. In general, visible phenotypes of grafted plants (size, root number, etc.) are poorly correlated with grafting success, but some studies have suggested that some polyphenols could be used as markers of graft incompatibility several months or years after grafting. However, much of the previous studies into metabolite markers of grafting success have not included all the controls necessary to unequivocally validate the markers proposed. In this study, we quantified 73 primary and secondary metabolites in nine hetero-grafts and six homo-grafted controls 33 days after grafting at the graft interface and in both the scion and rootstock woody tissues. Certain biomarker metabolites typical of a high stress status (such as proline, GABA and pallidol) were particularly accumulated at the graft interface of the incompatible scion/rootstock combination. We then used correlation analysis and generalized linear models to identify potential metabolite markers of grafting success measured one year after grafting. Here we present the first attempt to quantitatively predict graft compatibility and identify marker metabolites (especially asparagine, trans-resveratrol, trans-piceatannol and α-viniferin) 33 days after grafting, which was found to be particularly informative for homo-graft combinations.

A Roadmap for Participatory Chestnut Breeding for Nut Production in the Eastern United States.

Chestnut cultivation for nut production is increasing in the eastern half of the United States. Chinese chestnuts (Castanea mollissima Blume), or Chinese hybrids with European (C. sativa Mill.) and Japanese chestnuts (C. crenata Sieb. & Zucc.), are cultivated due to their high kernel quality, climatic adaptation, and disease resistance. Several hundred thousand pounds of high-quality fresh nuts are taken to market every fall, and several hundred additional orchards are entering bearing years. Grower-led on-farm improvement has largely facilitated this growth. A lack of significant investments in chestnut breeding in the region, paired with issues of graft incompatibility, has led many growers to cultivate seedlings of cultivars rather than grafted cultivars. After decades of evaluation, selection, and sharing of plant materials, growers have reached a threshold of improvement where commercial seedling orchards can be reliably established by planting offspring from elite selected parents. Growers recognize that if cooperation persists and university expertise and resources are enlisted, improvement can continue and accelerate. To this end, the University of Missouri Center for Agroforestry (UMCA) and chestnut growers throughout the eastern United States are partnering to formalize a participatory breeding program – the Chestnut Improvement Network. This partnership entails the UMCA providing an organizational structure and leadership to coordinate on-farm improvement, implement strategic crossing schemes, and integrate genetic tools. Chestnut growers offer structural capacity by cultivating seedling production orchards that provide financial support for the grower but also house segregating populations with improved individuals, in situ repositories, and selection trials, creating great value for the industry.

Effect of Grafting on Growth and Shelf Life of Tomatoes (<i>Solanum lycopersicum</i> L.) Grafted on Two Local <i>Solanum</i> Species

The Rio Grande variety of tomato is widely grown because of its high productivity during the cold and dry seasons, and its resistance to Verticillium wilt (caused by Fusarium oxysporium) and to stem canker (Alternaria). Grafting tomato onto compatible rootstocks resistant to these diseases offers a better potential to overcome soil-borne diseases, abiotic stresses, improve growth, yield and fruit quality. However, in Cameroon, there is little or no information on grafting between Rio Grande tomato and selected eggplant rootstocks. The objectives of this study were: 1) To determine the compatibility between Rio tomato grafting and a popular local eggplant (Nkeya) rootstock; 2) To verify the effect of grafting on flowering time; 3) To evaluate the effect of eggplant rootstocks on growth, fruit shelf life and fruit quality of Rio tomatoes. The trial was conducted in a randomized complete block design with 3 replications. Rio Grande (To) was the ungrafted treatment used as a control. To/Ko, To/To and To/Nk were the grafted treatments eventually transplanted to the field. Growth data were subjected to analysis of variance using SPSS software. Descriptive analyses were performed for the other parameters. The results revealed that, 1) The cleft grafting method used was successful with success rate varying between 90 and 100%; 2) Grafting influenced flowering date (DAT, p = 0.05) as well as tomato growth parameters including stem height (H, 1.49 × 10-10 p 0.00014) and collar diameter (SD, 4 × 10-14 p 0.009). The To/To treatment was significantly different from the ungrafted cultivar To, which had no significant difference in stem diameter. A significant difference in plant height was also observed between the ungrafted treatment To and the To/Ko and To/Nk treatments. In addition, only the collar diameter of To/Nk was different from To. Also, there was no significant difference between To/To and To, but a significant difference between To/Ko and To/Nk compared to To. Conversely, grafting improved the lifespan of To/Ko. Ultimately, the grafting method used was successful, but further studies are needed to overcome the problem of graft incompatibility in order to improve the agronomic performance of grafted plants.

Quali-Quantitative Study on Phenol Compounds as Early Predictive Markers of Graft Incompatibility: A Case Study on Chestnut (Castanea spp.)

In recent years, research has focused on phenolic compounds and their putative role as markers of graft incompatibility. Thus far, no studies have been conducted on the role of phenolic compounds in chestnut (Castanea spp.). The present study investigated the content of phenolic compounds in different combinations of Castanea spp. cultivars and rootstocks. Analyses were performed on the inner and outer tissues of chestnut grafts at two phenological sampling stages. The separation, identification and quantification of the phenolic markers via HPLC were preceded by an ultrasonic green extraction. Two chromatographic methods were tested for a total of 15 phenol compounds. Flavonol compounds were not detected, while cinnamic acids were found in low concentrations. The amount of gallic acid turned out to be higher at the graft union of the incompatible combination (20.11 ± 1.47 mg/100 gFW vs. 8.94 ± 1.08 mg/100 gFW). The same pattern was observed for catechin (15.79 ± 1.83 mg/100 gFW vs. 9.63 ± 1.98 mg/100 gFW). Differences in tannin concentrations seemed to be species-specific, and were apparently not related to graft incompatibility. The present work underlines the potential application of certain phenol compounds for the early prediction of graft incompatibility in Castanea spp.

Analysis of Phenolic Compounds for the Determination of Grafts (in) Compatibility Using In Vitro Callus Cultures of Sato-Zakura Cherries.

The aim of this study was to prove that under in vitro conditions, the adhesiveness of the callus between rootstock and scion, the development of callus cells at the points of fusion, and the presence of phenolic components are closely related to the level of (in) compatibility of the grafting combinations between Sato-zakura cherry cultivars (‘Amanogawa’, ‘Kanzan’, and ‘Kiku-shidare-zakura’) and commercial rootstocks. Prunus avium, Prunus ‘Colt’, Prunus mahaleb and Prunus serrulata were used as compatible and Prunus serotina and Pyrus communis ‘Pyrodwarf’ were used as two potentially incompatible rootstocks. The results indicated the significant manifestations of the early signs of the incompatibility on the callus junction. Phenols, as well as tissue senescence, were very precisely localized by toluidine blue and alcian blue as well as safranin staining, which can indicate the early signs of the callus incompatibility in some grafting unions. In the callus unions of Prunus avium with ‘Amanogawa’ and ‘Kiku-shidare-zakura’ the results of chemical analyses indicated that the existence of several flavonols, flavones and phenol acids could be involved in the incompatibility process in grafted combination. The detection of flavonol astragalin in the unions can be a biomarker of compatibility between scion and the rootstock, while some polyphenols, such as neochlorogenic acid, sinapic acid, ellagic acid, caffeic acid, baicalein, naringenin, apigenin and luteolin can be used as the indicators of graft incompatibility. p-coumaric acid and ferulic acid could be used for detection of delayed incompatibility.

Tree Growth and Production of Rainfed Valencia Sweet Orange Grafted onto Trifoliate Orange Hybrid Rootstocks under Aw Climate

Brazil is the largest producer of sweet orange and its juice in the world. Extensive cultivated area is located under an Aw climate in the North–Northwest of the state of São Paulo and the Triângulo of Minas Gerais state, being subjected to severe drought events. Although 56% of the orchards are irrigated in these regions, there is a need for drought tolerant rootstocks as an alternative to traditional genotypes such as Rangpur lime and Volkamer lemon, which are susceptible to the endemic citrus sudden death disease (CSD). In this sense, the tree size and production of Valencia sweet orange grafted onto 23 rootstock genotypes were evaluated over a ten-year period in rainfed cultivation at 7.0 m × 3.0 m spacing. Most evaluated types resulted from the cross of Poncirus trifoliata with Citrus, but two interspecific hybrids of Citrus (Sunki mandarin × Rangpur lime hybrids), the Barnes trifoliate orange and a tetraploid selection of Swingle citrumelo were also tested. Tropical Sunki mandarin was used as the reference control. Those hybrids coming from the cross of Sunki × Flying Dragon induced large tree sizes to Valencia sweet orange as well as the other citrandarins, Tropical Sunki mandarin and the Sunki mandarin × Rangpur lime hybrids, whereas only the tetraploid Swingle citrumelo behaved as a dwarfing rootstock, decreasing the canopy volume by 77% compared to that induced by the most vigorous citrandarin 535. The citrandarins 543 and 602 and the citrange C38 induced the highest mean fruit production, 67.2 kg·tree−1, but they also caused pronounced alternate bearing and only the hybrid 543 led to a high production efficiency consistently. Graft incompatibility symptoms were not observed over the evaluation period, and the canopy shape of Valencia sweet orange was also influenced by the rootstocks tested. Two citrandarins and one citrange were selected as the most promising alternative rootstocks for Valencia sweet orange grown under an Aw climate, even though productivity would likely benefit from supplementary irrigation.

Development of biotic stress resistant F1 interspecific hybrid rootstock derived from Solanum lycopersicum and Solanum habrochaites

Tomato is major horticultural plant consumed worldwide. Biotic stress (nematodes, fungus and bacteria) has negative effect on tomato production due to causing reduced yield or plant death. Rootstocks confer resistance to soil-borne pathogen are considered the most effective and environment friendly approach for such a stress management. Thus, development of genetic resources having multiple resistance genes is essential for sustainable tomato breeding. Solanum habrochaites is one of the most studied wild tomato species due to its high genetic potential for biotic and abiotic stresses. In the present study, rootstock potential of an interspecific F1 hybrid derived from S. habrochaites was evaluated as using resistance genes (Frl, I-2, I-3, Mi-3, Pto Ty-1, Ty-3 and Sw-5) specific molecular markers for 6 major tomato diseases and 31 fruit quality traits. The study reported that F1 hybrid had resistance alleles for 5 genes (Frl, I-2, I-3, Pto and Sw-5) confer resistance to fusarium crown rot disease, crown – root rot disease, race 2 and 3 of Fusarium oxysporum f. sp. radicis lycopersici, bacterial speck and tomato spotted wilt virus (TSWV), respectively. Despite high performance of F1 hybrid for biotic stress, the study pointed S. habrochaites specific graft incompatibility due to poor rate of grafting efficiency, small fruit formation and low yield. This is the first comprehensive study evaluated the horticultural performance of an interspecific hybrid in tomato.

Compatibility Evaluation and Anatomical Observation of Melon Grafted Onto Eight Cucurbitaceae Species.

Melon (Cucumis melo) is one of the top 10 fruits in the world, and its production often suffers due to soil-borne diseases. Grafting is an effective way to solve this problem. However, graft incompatibility between scion and rootstock limits the application of melon grafting. In this study, the melon was grafted onto eight Cucurbitaceae species (cucumber, pumpkin, melon, luffa, wax gourd, bottle gourd, bitter gourd, and watermelon), and graft compatibility evaluation and anatomical observation were conducted. Taking melon homo-grafted plants as control, melon grafted onto cucumber and pumpkin rootstocks was compatible, while melon grafted onto luffa, wax gourd, bottle gourd, bitter gourd, and watermelon rootstocks was incompatible based on the scion dry weight on day 42 after grafting. Meanwhile, we found that starch–iodine staining of scion stem base is an index to predict graft compatibility earlier, on day 14 after grafting. Further, microsection observations showed that there was more cell proliferation at graft junction of melon hetero-grafted combinations; vascular reconnection occurred in all graft combinations. However, excess callose deposited at graft junction resulted in the blockage of photosynthate transport, thus, leading to starch accumulation in scion stem base, and finally graft incompatibility. In addition, undegraded necrotic layer fragments were observed at graft junctions of melon grafted onto incompatible bitter gourd and watermelon rootstocks. The above results provide clues for the selection and breeding of compatible Cucurbitaceae rootstocks of melon and demonstrate that starch accumulation in scion base and callose deposition at graft junction is associated with melon graft compatibility.

The molecular biology of grafting: Recent research may provide new applications for a millennia-old agricultural technology.

Plant biologists have begun to unravel the molecular mechanisms behind grafting, yielding insights into plant evolution and expanding the scope of this technique.

Gene regulatory networks for compatible versus incompatible grafts identify a role for SlWOX4 during junction formation.

Grafting has been adopted for a wide range of crops to enhance productivity and resilience; for example, grafting of Solanaceous crops couples disease-resistant rootstocks with scions that produce high-quality fruit. However, incompatibility severely limits the application of grafting and graft incompatibility remains poorly understood. In grafts, immediate incompatibility results in rapid death, but delayed incompatibility can take months or even years to manifest, creating a significant economic burden for perennial crop production. To gain insight into the genetic mechanisms underlying this phenomenon, we developed a model system using heterografting of tomato (Solanum lycopersicum) and pepper (Capsicum annuum). These grafted plants express signs of anatomical junction failure within the first week of grafting. By generating a detailed timeline for junction formation, we were able to pinpoint the cellular basis for this delayed incompatibility. Furthermore, we inferred gene regulatory networks for compatible self-grafts and incompatible heterografts based on these key anatomical events, which predict core regulators for grafting. Finally, we examined the role of vascular development in graft formation and uncovered SlWOX4 as a potential regulator of graft compatibility. Following this predicted regulator up with functional analysis, we show that Slwox4 homografts fail to form xylem bridges across the junction, demonstrating that indeed, SlWOX4 is essential for vascular reconnection during grafting, and may function as an early indicator of graft failure.

Endoparasitic plants and fungi show evolutionary convergence across phylogenetic divisions

Endoparasitic plants are the most reduced flowering plants, spending most of their lives as a network of filaments within the tissues of their hosts. Despite their extraordinary life form, we know little about their biology. Research into a few species has revealed unexpected insights, such as the total loss of plastome, the reduction of the vegetative phase to a proembryonic stage, and elevated information exchange between host and parasite. To consolidate our understanding, we review life history, anatomy, and molecular genetics across the four independent lineages of endoparasitic plants. We highlight convergence across these clades and a striking trans-kingdom convergence in life history among endoparasitic plants and disparate lineages of fungi at the molecular and physiological levels. We hypothesize that parasitism of woody plants preselected for the endoparasitic life history, providing parasites a stable host environment and the necessary hydraulics to enable floral gigantism and/or high reproductive output. Finally, we propose a broader view of endoparasitic plants that connects research across disciplines, for example, pollen-pistil and graft incompatibility interactions and plant associations with various fungi. We shine a light on endoparasitic plants and their hosts as under-explored ecological microcosms ripe for identifying unexpected biological processes, interactions and evolutionary convergence.

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.

Localized graft incompatibility in kiwifruit: Analysis of homografts and heterografts with different rootstock & scion combinations

Kiwifruit is a popular type of edible and nutritious fruit of Actinidia chinensis Planchon. Extensive use of rootstocks is needed to control the traits of grafted cultivars, including vigor, early flowering, fruit quality and yield. However, this practice remains limited to a few Actinidia species, mainly due to inconsistent graft success and difficulties in predicting graft compatibility responses of proposed scion–rootstock combinations. Aiming to identify graft (in)compatibility and its potential mechanisms, we grafted 44 kiwifruit (Actinidia chinensis Planchon) cultivars and 18 different Actinidia species onto commercialized kiwifruit (Miliang) rootstocks to study the compatibility behavior of homo- and heterografts in the genus Actinidia. Based on phenotypic and histological observations, the kiwifruit homografts demonstrated high affinity between rootstocks and scions, whereas the heterografts exhibited obviously higher translocated graft incompatibility. In addition, there was no significant difference in graft success rate within the different homograft groups (diploid, tetraploid and hexaploid cultivars), but compared with the other species, the species belonging to Sect. Leiocarpae and used in heterografts presented an obviously lower success rate. When the internal anatomy of the graft unions was observed, the compatible rootstock and scion combinations presented consistent cell and vascular connections at the graft union, whereas the incompatible combinations were separated in many places by clusters of necrotic layers and abnormal vascular complexes. Correlation analysis further confirmed that the phylogenetic relatedness between rootstocks and scions was negatively correlated with grafting survival rate. This work not only indicates the relationship between genetic affinity coefficients and graft incompatibility but also is of practical interest to growers and breeders in terms of their choice of selecting optimal combinations of rootstocks and scions in the genus Actinidia.