Impact of scion/rootstock reciprocal effects on metabolomics of fruit juice and phloem sap in grafted Citrus reticulata

Effects of rootstocks and developmental time on the dynamic changes of main functional substances in 'Orah' (Citrus reticulata Blanco) by HPLC coupled with UV detection

Introduction

Citrus fruit is rich in important functional constituents such as flavonoids, phenolic acids terpenes and other functional substances that play an important role for treating clinical diseases or controlling major agricultural diseases and pests. Plant secondary metabolites have become one of the most important resources of novel lead compounds, especially young citrus fruits contain multiple functional substances. 'Orah', a type of citrus reticulata, is known for its fine appearance, productivity, delicious sweetness, late-maturing characteristics, and is widely cultivated in China. Fruit thinning and rootstock selection are commonly used agronomic measures in its production to ensure its quality and tree vigor. However, few studies have demonstrated the effects of these agronomic measures on the functional substances of 'Orah'.

Methods

In this study, we used HPLC coupled with UV to detect the dynamic changes of fruit quality, 13 main flavonoids, 7 phenolic acids, 2 terpenes, synephrine and antioxidant capacity in both peel and pulp of citrus fruits grafted on four rootstocks (Red orange Citrus reticulata Blanco cv. red tangerine, Ziyang xiangcheng Citrus junos Sieb. ex Tanaka, Trifoliate orange Poncirus trifoliata L. Raf, and Carrizo citrange Citrus sinensis Osb.×P.trifoliate Raf) at six different developmental stages (from 90 DAF to 240 DAF).

Results

The results indicated that rootstock can significantly affect the contents of functional constituents and antioxidant capacity in 'Orah'. Additionally, it was found that pruning at either 90 DAF (days after flowering) or 150 DAF produced the most favorable outcomes for extracting functional substances. We also identified rootstock 'Trifoliate orange' has the highest total soluble solids (TSS) and 'Ziyang xiangcheng' to be the optimal in terms of comprehensive sensory of fruit quality, while 'Red orange' and 'Ziyang xiangcheng' are optimal in terms of functional substance quality, and 'Red orange' excels in antioxidant capacity.

Discussion

Overall, the findings demonstrate the important role of rootstocks and developmental stage in shaping fruit sensory quality and functional substance synthesis, providing valuable insights for guiding rootstock selection, determining thinning time, and utilizing pruned fruits in a more informed manner.

Widely Targeted Metabolomic Analysis Provides New Insights into the Effect of Rootstocks on Citrus Fruit Quality

The use of different rootstocks has a significant effect on the content of flavor components and overall fruit quality. However, little information is available about the metabolic basis of the nutritional value of citrus plants. In this study, UPLC-MS/MS (ultra-performance liquid chromatography-tandem mass spectrometry) was performed to analyze the metabolites of three late-maturing hybrid mandarin varieties ('Gold Nugget', 'Tango' and 'Orah') grafted on four rootstocks ('Trifoliate orange', 'Carrizo citrange', 'Red tangerine' and 'Ziyang Xiangcheng'). A total of 1006 metabolites were identified through OPLS-DA (Orthogonal Partial Least Squares-Discriminant Analysis) analysis. KEGG (Kyoto Encyclopedia of Genes and Genomes) analysis revealed the most critical pathways among the different pathways associated with genes grafted on the four rootstocks that were differentially activated, including tryptophan metabolism and sphingolipid metabolism in 'Gold Nugget'; tryptophan metabolism, phenylpropanoid biosynthesis and sphingolipid metabolism in 'Tango'; and pantothenate and CoA biosynthesis- and photosynthesis-related biosynthesis in 'Orah'. A considerable difference between the different rootstocks was also observed in the accumulation of lipids, phenolic acids and flavonoids; further analysis revealed that the rootstocks regulated specific metabolites, including deacetylnomylinic acid, sudachinoid A, amoenin evodol, rutaevin, cyclo (phenylalanine-glutamic acid), cyclo (proline-phenylalanine), 2-hydroxyisocaproic acid, and 2-hydroxy-3-phenylpropanoic acid. The results of this study provide a useful foundation for further investigation of rootstock selection for late-maturation hybrid mandarin varieties.

Assessing the Feasibility of Biorefineries for a Sustainable Citrus Waste Management in Korea

Citrus fruits are one of the most widely used fruits around the world and are used as raw fruits, but are also processed into products such as beverages, and large amounts of by-products and waste are generated in this process. Globally, disposal of citrus waste (CW) through simple landfilling or ocean dumping can result in soil and groundwater contamination, which can negatively impact ecosystem health. The case of Korea is not much different in that these wastes are simply buried or recycled wastes are used as livestock feed additives. However, there are many reports that CW, which is a waste, has high potential to produce a variety of products that can minimize environmental load and increase added value through appropriate waste management. In this study, we aim to explore the latest developments in the evaluation and valorization of the growing CW green technologies in an effort to efficiently and environmentally transform these CW for resource recovery, sustainability, and economic benefits. Recent research strategies on integrated biorefinery approaches have confirmed that CW can be converted into various bioproducts such as enzymes, biofuels and biopolymers, further contributing to energy security. It was found that more efforts are needed to scale up green recovery technologies and achieve diverse product profiling to achieve zero waste levels and industrial viability.

A First Omics Data Integration Approach in Hass Avocados to Evaluate Rootstock-Scion Interactions: From Aerial and Root Plant Growth to Fruit Development

Grafting, the careful selection of rootstocks and scions, has played a crucial role maintaining Chilean avocado fruit quality standards in a scenario in which climate change and drought-related issues have considerably decreased avocado fruit production in the last fifteen years. The historical use of seedling rootstocks in Chile has experienced a recent shift towards clonal rootstocks, driven by the potential to produce more consistent and predictable crops. This research aims to compare Hass avocado plants grafted on Mexicola seedling and Dusa® clonal rootstocks in a soilless and protected system using (i) a differential expression analysis of root and leaf samples and (ii) a fruit transcriptomic and metabolomic integration analysis to improve our understanding of rootstock-scion interaction and its impact on avocado tree performance and fruit quality. The results demonstrated that no significant transcriptomic and metabolomic differences were identified at fruit level in the ready-to-eat (RTE) stage for Hass avocado fruit from both rootstocks. However, Hass avocados grafted on the clonal rootstock showed greater aerial growth and slightly increased fruit size than the seedling rootstock due to the enrichment of cell wall-remodeling genes as revealed in leaves and fruit at harvest stage.

Characterization of Almond Scion/Rootstock Communication in Cultivar and Rootstock Tissues through an RNA-Seq Approach

The rootstock genotype plays a crucial role in determining various aspects of scion development, including the scion three-dimensional structure, or tree architecture. Consequently, rootstock choice is a pivotal factor in the establishment of new almond (Prunus amygdalus (L.) Batsch, syn P. dulcis (Mill.)) intensive planting systems, demanding cultivars that can adapt to distinct requirements of vigor and shape. Nevertheless, considering the capacity of the rootstock genotype to influence scion development, it is likely that the scion genotype reciprocally affects rootstock performance. In the context of this study, we conducted a transcriptomic analysis of the scion/rootstock interaction in young almond trees, with a specific focus on elucidating the scion impact on the rootstock molecular response. Two commercial almond cultivars were grafted onto two hybrid rootstocks, thereby generating four distinct combinations. Through RNA-Seq analysis, we discerned that indeed, the scion genotype exerts an influence on the rootstock expression profile. This influence manifests through the modulation of genes associated with hormonal regulation, cell division, root development, and light signaling. This intricate interplay between scion and rootstock communication plays a pivotal role in the development of both scion and rootstock, underscoring the critical importance of a correct choice when establishing new almond orchards.

Effects of rootstocks on the flavor quality of huanglongbing-affected sweet orange juices using targeted flavoromics strategy

Citrus greening disease or Huanglongbing (HLB) is one of the most destructive diseases affecting all varieties of citrus worldwide. Aimed at optimizing the scion/rootstock combination to improve HLB-affected orange juice quality, a flavoromics strategy was used to investigate the effects of six different rootstocks (CH, blue, 1804, FG, SW, and Volk) on flavor quality of HLB affected orange juices. A sensory quality test was conducted by a panel to evaluate the sensory attributes of different orange juices. The orange juice from rootstock CH had the best flavor quality with highest sweetness, low sourness and bitterness, while rootstocks Volk and FG produced the poorest quality orange juices. Chemical profile analysis resulted in semi-quantification of 89 metabolites including 57 nonvolatile compounds and 32 volatile compounds using UHPLC-MS and GC-MS, respectively. Canonical correlation analysis indicated that some specific sugar and sugar alcohols including raffinose, xylose, rhamnose, glucose, sorbitol, and myo-inositol made a strong positive contribution to sweetness. Meanwhile, several amino acids including alanine, glutamic acid, proline, arginine, serine, asparagine, as well as aspartic acid were responsible for positive flavor quality. On the other hand, some nucleotides and limonin increased bitterness. In addition, KEGG pathway enrichment analysis demonstrated different rootstocks could affect aminoacyl-tRNA biosynthesis, ABC transporters, and monoterpenoid biosynthesis. These results indicated different rootstocks can change specific metabolites and thus affect the flavor quality of orange juices. This study also provides reference for optimizing the scion/rootstock combination to improve HLB-affected orange juice quality.

Comparative study on comprehensive quality of Xinhui chenpi by two main plant propagation techniques

Xinhui chenpi (XHCP), the sun-dried peel of the mandarin orange, Citrus reticulata "Chachi," is the most famous crude drug, as well as a traditional seasoning in Chinese cooking. The main cultivation methods of XHCP are cutting and grafting, but it is generally considered that the quality of XHCP after cutting is superior to that obtained from plants propagated by graftings, which had a negative impact on the marketing of the finished product. In our study, a total of 25 samples of XHCP obtained from plants cultivated by either traditional methods (i.e., from cuttings) or by grafting were collected to compare the contents of four types of metabolites (essential oils, flavonoids, synephrine, and total polysaccharides) as well as antioxidant activity. The results revealed that the quality of XHCP did not decline after cutting, and marked individual differences between XHCP samples, even when prepared from plants grown in the same way. In general, grafting had no significant effect on the most essential oils components, total polysaccharides, synephrine, total flavonoids, total polymethoxylated flavones, hesperidin, nobiletin, tangeretin content, and antioxidant activity. Nevertheless, five volatile compounds can be used as potential chemical markers (p < 0.05) to distinguish between cutting XHCP and grafted XHCP, while four volatile compounds showed high content in grafted XHCP. Our study is expected to provide a theoretical basis for XHCP breeding and cultivation, and thereby further standardize the market of XHCP.

Transcriptome Analysis of the Effects of Grafting Interstocks on Apple Rootstocks and Scions

Apples are a major horticultural crop worldwide. Grafting techniques are widely utilized in apple production to keep the varieties pure. Interstocks are frequently used in Northern China to achieve intensive apple dwarfing cultivation. High-throughput sequencing was used to investigate differentially expressed genes in the phloem tissues of two different xenograft systems, M ('Gala'/'Mac 9'/Malus baccata (L.) Borkh.) and B ('Gala'/Malus baccata (L.) Borkh.). The results showed that dwarfing interstocks could significantly reduce the height and diameters of apple trees while have few effects on the growth of annual branches. The interstocks were found to regulate the expression of genes related to hormone metabolism and tree body control (GH3.9, PIN1, CKI1, ARP1, GA2ox1 and GA20ox1), these effects may attribute the dwarf characters for apple trees with interstocks. Besides, the interstocks reduce photosynthesis-related genes (MADH-ME4 and GAPC), promote carbon (C) metabolism gene expression (AATP1, GDH and PFK3), promote the expression of nitrogen (N)-metabolism-related genes (NRT2.7, NADH and GDH) in rootstocks, and improve the expression of genes related to secondary metabolism in scions (DX5, FPS1, TPS21 and SRG1). We also concluded that the interstocks acquired early blooming traits due to promotion of the expression of flowering genes in the scion (MOF1, FTIP7, AGL12 and AGL24). This study is a valuable resource regarding the molecular mechanisms of dwarf interstocks' influence on various biological processes and transplantation systems in both scions and rootstocks.

Untargeted metabolism approach reveals difference of varieties of bud and relation among characteristics of grafting seedlings in Camellia oleifera

Camellia oleifera is one of the essential wood oil trees in the world. C.oleifera was propagated by nurse seedling grafting. Since the scion of C.oleifera had a significant regulated effect on the properties of rootstock after grafting and impacted on the growth of the grafted seedlings, it was necessary to understand the characteristics of buds among varieties to cultivate high-quality grafted seedlings. The metabolome was thought to be a powerful tool for understanding connecting phenotype-genotype interactions, which has an important impact on plant growth and development. In this study, UPLC-MS was used to determine the metabolites of the apical buds of CL3, CL4, CL40, and CL53 spring shoots after 30 days of sprout and to measure the growth characteristics of roots and stems after grafting. Metabolomics analysis revealed 554 kinds of metabolites were significant differences among four varieties, and 29 metabolic pathways were identified to have significant changes (p< 0.05), including carboxylic acids and derivatives, fatty Acyls, organooxygen compounds, and prenol lipids metabolites. The metabolites appeared in all varieties, including phenethyl rutinoside in glycosyl compounds and hovenidulcioside A1 in terpene glycosides. Metabolite-metabolite correlations in varieties revealed more complex patterns in relation to bud and enabled the recognition of key metabolites (e.g., Glutamate, (±)Catechin, GA₅₂, ABA, and cs-Zeatin) affecting grafting and growth ability. Each variety has a unique metabolite type and correlation network relationship. Differentiated metabolites showed different growth trends for development after grafting. Many metabolites regulate the growth of scions in buds before grafting, which plays a crucial role in the growth of seedlings after grafting. It not only regulates the growth of roots but also affects the development of this stem. Finally, those results were associated with the genetic background of each cultivar, showing that metabolites could be potentially used as indicators for the genetic background, indicating that metabolites could potentially be used as indicators for seedling growth characteristics. Together, this study will enrich the theoretical basis of seedling growth and lay a foundation for further research on the molecular regulation mechanism interaction between rootstock and scion, rootstock growth, and the development of grafted seedlings after grafting.

Effect of methionine hydroxy analog feed supplements: Significant alteration and enrichment of rumen microbiota and metabolome in Hu sheep

Methionine hydroxy analogs (MHA) are widely used as the main sources of methionine in ruminant feed production. The purpose of this study was to explore the effect of using MHA supplements such as MHA as a salt of calcium (MHA-Ca) and 2-hydroxy-4-(methylthio)-butanoic acid isopropyl ester (HMBi) as sources of methionine on the rumen microbiota and metabolome in Hu sheep. Seventy-two healthy Hu sheep were randomly assigned to three dietary treatment groups: control, MHA-Ca, and HMBi groups. The results showed that the concentrations of total volatile fatty acids, acetate, and propionate were higher in the HMBi group than in the control group. The HMBi and MHA-Ca groups had higher alpha diversity values than those in control group. We compared the rumen microbiota by using 16S rRNA gene sequencing. At the phylum level, the HMBi group had a higher relative abundance of Firmicutes and a lower relative abundance of Synergistetes than did the control group. At the genus level, the control group had a higher relative abundance of Treponema_2 than did the HBMi group and a higher relative abundance of Prevotellaceae_UCG_004 than did the MHA-Ca group. Metabolomic analyses revealed that fatty acids, amino acids, lipids, organic acids, sugars, amines, and nucleosides were significantly altered in both MHA-Ca and HMBi groups. Metabolites with significant differences were enriched in amino acid and carbohydrate metabolisms, such as phenylalanine metabolism, biosynthesis of amino acids, tryptophan metabolism, galactose metabolism, and tyrosine metabolism. Above all, the findings presented in this study indicate that MHA alter the rumen microbiota and metabolites and that different forms of MHA have different impacts. The results of our study contribute to a better understanding of the effects of MHA.

Comparative metabolomics analysis of pericarp from four varieties of Zanthoxylum bungeanum Maxim

A qualitative and quantitative analysis of metabolites was performed by metabolomics comparation on the pericarps of four varieties of Zanthoxylum bungeanum Maxim. The Zanthoxylum bunganum as scion combined with three rootstock varieties of Zanthoxylum piasezkii Maxim (YJ), July Zanthoxylum bunganum Maxim (QJ), and August Zanthoxylum bunganum Maxim (BJ), at the same time Zanthoxylum bungeanum seedlings breeding were compared as control (MJ). A total of 1429 metabolites were identified in Zanthoxylum bungeanum Maxim pericarps based on chromatography and mass spectrometry dual detection platform. While the metabolites between four varieties of Z. bungeanum varied, there was identified 31, 15, 7, 79, 42, 19 down-regulated and 55, 50, 13, 75, 43, 27 up-regulated differential metabolites between MJ and BJ, MJ and QJ, MJ and YJ, QJ and BJ, YJ and BJ, YJ and QJ. Meanwhile, the differential metabolites composition was distinct among various varieties of Z. bungeanum and dominant by phenolic compounds flavonoid and phenolic acids, especially highest in varieties July Zanthoxylum bunganum Maxim. Highlight A comparative metabolomics analyzed in four varieties of Zanthoxylum bungeanum pericarp.Total 1429 metabolites were identified and mainly in flavonoid and phenolic acid.July and August Zanthoxylum bunganum Maxim has highest antioxidant capacity.The rootstock July Zanthoxylum bunganum Maxim was recommended in Loess Plateau.

Leveraging a graft collection to develop metabolome-based trait prediction for the selection of tomato rootstocks with enhanced salt tolerance

Grafting has been demonstrated to significantly enhance the salt tolerance of crops. However, breeding efforts to develop enhanced graft combinations are hindered by knowledge-gaps as to how rootstocks mediate scion-response to salt stress. We grafted the scion of cultivated M82 onto rootstocks of 254 tomato accessions and explored the morphological and metabolic responses of grafts under saline conditions (EC = 20 dS m^-1) as compared to self-grafted M82 (SG-M82). Correlation analysis and Least Absolute Shrinkage and Selection Operator were performed to address the association between morphological diversification and metabolic perturbation. We demonstrate that grafting the same variety onto different rootstocks resulted in scion phenotypic heterogeneity and emphasized the productivity efficiency of M82 irrespective of the rootstock. Spectrophotometric analysis to test lipid oxidation showed largest variability of malondialdehyde (MDA) equivalents across the population, while the least responsive trait was the ratio of fruit fresh weight to total fresh weight (FFW/TFW). Generally, grafts showed greater values for the traits measured than SG-M82, except for branch number and wild race-originated rootstocks; the latter were associated with smaller scion growth parameters. Highly responsive and correlated metabolites were identified across the graft collection including malate, citrate, and aspartate, and their variance was partly related to rootstock origin. A group of six metabolites that consistently characterized exceptional graft response was observed, consisting of sorbose, galactose, sucrose, fructose, myo-inositol, and proline. The correlation analysis and predictive modelling, integrating phenotype- and leaf metabolite data, suggest a potential predictive relation between a set of leaf metabolites and yield-related traits.

Mobility of FLOWERING LOCUS T protein as a systemic signal in trifoliate orange and its low accumulation in grafted juvenile scions

The long juvenile period of perennial woody plants is a major constraint in breeding programs. FLOWERING LOCUS T (FT) protein is an important mobile florigen signal that induces plant flowering. However, whether FT can be transported in woody plants to shorten the juvenile period is unknown, and its transport mechanism remains unclear. In this study, trifoliate orange FT (ToFT) and Arabidopsis FT (AtFT, which has been confirmed to be transportable in Arabidopsis) as a control were transformed into tomato and trifoliate orange, and early flowering was induced in the transgenic plants. Long-distance and two-way (upward and downward) transmission of ToFT and AtFT proteins was confirmed in both tomato and trifoliate orange using grafting and western blot analysis. However, rootstocks of transgenic trifoliate orange could not induce flowering in grafted wild-type juvenile scions because of the low accumulation of total FT protein in the grafted scions. It was further confirmed that endogenous ToFT protein was reduced in trifoliate orange, and the accumulation of the transported ToFT and AtFT proteins was lower than that in grafted juvenile tomato scions. Furthermore, the trifoliate orange FT-INTERACTING PROTEIN1 homolog (ToFTIP1) was isolated by yeast two-hybrid analysis. The FTIP1 homolog may regulate FT transport by interacting with FT in tomato and trifoliate orange. Our findings suggest that FT transport may be conserved between the tomato model and woody plants. However, in woody plants, the transported FT protein did not accumulate in significant amounts in the grafted wild-type juvenile scions and induce the scions to flower.

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.

Galactinol synthase 1 improves cucumber performance under cold stress by enhancing assimilate translocation

Cucumber (Cucumis sativus L.) predominately translocates raffinose family oligosaccharides (RFOs) in the phloem and accumulates RFOs in leaves. Galactinol synthase (GolS) catalyzes the critical step of RFO biosynthesis, and determining the functional diversity of multiple GolS isoforms in cucumber is of scientific significance. In this study, we found that all four isoforms of CsGolS in the cucumber genome were upregulated by different abiotic stresses. β-glucuronidase staining and tissue separation experiments suggested that CsGolS1 is expressed in vascular tissues, whereas the other three CsGolSs are located in mesophyll cells. Further investigation indicates that CsGolS1 plays double roles in both assimilate loading and stress response in minor veins, which could increase the RFO concentration in the phloem sap and then improve assimilate transport under adverse conditions. Cold-induced minor vein-specific overexpression of CsGolS1 enhanced the assimilate translocation efficiency and accelerated the growth rates of sink leaves, fruits and whole plants under cold stress. Finally, our results demonstrate a previously unknown response to adverse environments and provide a potential biotechnological strategy to improve the stress resistance of cucumber.

Reinvigoration/Rejuvenation Induced through Micrografting of Tree Species: Signaling through Graft Union

Trees have a distinctive and generally long juvenile period during which vegetative growth rate is rapid and floral organs do not differentiate. Among trees, the juvenile period can range from 1 year to 15–20 years, although with some forest tree species, it can be longer. Vegetative propagation of trees is usually much easier during the juvenile phase than with mature phase materials. Therefore, reversal of maturity is often necessary in order to obtain materials in which rooting ability has been restored. Micrografting has been developed for trees to address reinvigoration/rejuvenation of elite selections to facilitate vegetative propagation. Generally, shoots obtained after serial grafting have increased rooting competence and develop juvenile traits; in some cases, graft-derived shoots show enhanced in vitro proliferation. Recent advances in graft signaling have shown that several factors, e.g., plant hormones, proteins, and different types of RNA, could be responsible for changes in the scion. The focus of this review includes (1) a discussion of the differences between the juvenile and mature growth phases in trees, (2) successful restoration of juvenile traits through micrografting, and (3) the nature of the different signals passing through the graft union.

The Impact of Metabolic Scion-Rootstock Interactions in Different Grapevine Tissues and Phloem Exudates

In viticulture, grafting is used to propagate Phylloxera-susceptible European grapevines, thereby using resistant American rootstocks. Although scion–rootstock reciprocal signaling is essential for the formation of a proper vascular union and for coordinated growth, our knowledge of graft partner interactions is very limited. In order to elucidate the scale and the content of scion–rootstock metabolic interactions, we profiled the metabolome of eleven graft combination in leaves, stems, and phloem exudate from both above and below the graft union 5–6 months after grafting. We compared the metabolome of scions vs. rootstocks of homografts vs. heterografts and investigated the reciprocal effect of the rootstock on the scion metabolome. This approach revealed that (1) grafting has a minor impact on the metabolome of grafted grapevines when tissues and genotypes were compared, (2) heterografting affects rootstocks more than scions, (3) the presence of a heterologous grafting partner increases defense-related compounds in both scion and rootstocks in shorter and longer distances from the graft, and (4) leaves were revealed as the best tissue to search for grafting-related metabolic markers. These results will provide a valuable metabolomics resource for scion–rootstock interaction studies and will facilitate future efforts on the identification of metabolic markers for important agronomic traits in grafted grapevines.

Does scion-rootstock compatibility modulate photoassimilate and hormone trafficking through the graft junction in melon-pumpkin graft combinations?

The effect of the rootstock on the acropetal and basipetal transport of photoassimilates and hormones was studied in the 'Kiran' (Ki) melon cultivar grafted onto pumpkin rootstocks with different degrees of compatibility. A complementary experiment was performed to compare the incompatible combination (as evidenced by plant collapse at the fruit ripening stage), designated Ki/r53, with self-grafted r53/r53 as a model compatible combination. Both experiments showed the accumulation of a number of amino acids, sugars, and sugar alcohols in the scion of the incompatible Ki/r53 grafts. Additionally, they showed a marked reduction of trans-zeatin-type cytokinins and an elevated content of cis-zeatin-type cytokinins in the rootstock, and the opposite pattern in the scion, hinting at the possible involvement of a hormonal signal for graft compatibility. There was no direct evidence of a blockage at the graft union, since hormone acropetal and basipetal trafficking was demonstrated for all combinations. Dye uptake experiments did not show xylem flow impairment. A possibly significant finding in the incompatible combination was the deposition of undifferentiated cells in the hollow space that replaces the pith region in melon and pumpkin. The link between the above findings and the collapse of the plants of the incompatible combination remains unclear.

High-Throughput LC-ESI-MS/MS Metabolomics Approach Reveals Regulation of Metabolites Related to Diverse Functions in Mature Fruit of Grafted Watermelon

Grafting has been reported as a factor regulating the metabolome of a plant. Therefore, a comprehensive metabolic profile and comparative analysis of metabolites were conducted from fully mature fruit of pumpkin-grafted watermelon (PGW) and a self-rooted watermelon (SRW). Widely targeted LC-ESI-MS/MS metabolomics approach facilitated the simultaneous identification and quantification of 339 metabolites across PGW and SRW. Regardless of grafting, delta-aminolevulinic acid hydrochloride, sucrose, mannose-6-phosphate (carbohydrates), homocystine, 2-phenylglycine, s-adenosyl-L-homocysteine (amino acids and derivatives), malic, azelaic, H-butanoic acid ethyl ester-hexoside isomer 1, (organic acids), MAG (18:3) isomer1, LysoPC 16:0, LysoPC 18:2 2n isomer (lipids) p-coumaric acid, piperidine, and salicylic acid-o-glycoside (secondary metabolites) were among the dominant metabolite. Dulcitol, mono-, and disaccharide sugars were higher in PGW, while polysaccharides showed complex behavior. In PGW, most aromatic and nitrogen-rich amino acids accumulated greater than 1.5- and 1-fold, respectively. Intermediates of the tricarboxylic acid cycle (TCA), stress-related metabolites, vitamin B5, and several flavonoids were significantly more abundant in PGW. Most lipids were also significantly higher in grafted watermelon. This is the first report providing a comprehensive picture of watermelon metabolic profile and changes induced by grafting. Hence, the untargeted high-throughput LC-ESI-MS/MS metabolomics approach could be suitable to provide significant differences in metabolite contents between grafted and ungrafted plants.

Genome of a citrus rootstock and global DNA demethylation caused by heterografting

Grafting is an ancient technique used for plant propagation and improvement in horticultural crops for at least 1,500 years. Citrus plants, with a seed-to-seed cycle of 5-15 years, are among the fruit crops that were probably domesticated by grafting. Poncirus trifoliata, a widely used citrus rootstock, can promote early flowering, strengthen stress tolerance, and improve fruit quality via scion-rootstock interactions. Here, we report its genome assembly using PacBio sequencing. We obtained a final genome of 303 Mb with a contig N50 size of 1.17 Mb and annotated 25,680 protein-coding genes. DNA methylome and transcriptome analyses indicated that the strong adaptability of P. trifoliata is likely attributable to its special epigenetic modification and expression pattern of resistance-related genes. Heterografting by using sweet orange as scion and P. trifoliata as rootstock and autografting using sweet orange as both scion and rootstock were performed to investigate the genetic effects of the rootstock. Single-base methylome analysis indicated that P. trifoliata as a rootstock caused DNA demethylation and a reduction in 24-nt small RNAs (sRNAs) in scions compared to the level observed with autografting, implying the involvement of sRNA-mediated graft-transmissible epigenetic modifications in citrus grafting. Taken together, the assembled genome for the citrus rootstock and the analysis of graft-induced epigenetic modifications provide global insights into the genetic effects of rootstock-scion interactions and grafting biology.

Fruit Ripening Development of ‘Valencia’ Orange Trees Grafted on Different ‘Trifoliata’ Hybrid Rootstocks

The development of fruit ripening of ‘Valencia’ orange trees [Citrus sinensis (L.) Osb.] grafted on the following ‘Trifoliata’ hybrid rootstocks: ‘US-852’, IPEACS-256 and IPEACS-264 citrandarins, and F.80-3, ‘W-2’ citrumelo, and ‘Swingle’ citrumelo (control), was assessed in three different subtropical locations: Rancho Alegre (RA); São Sebastião da Amoreira (SSA); and São Jerônimo da Serra (SJS), Parana state, Brazil. The climate of the RA and SSA locations was classified as Cfa with hot summers, whereas that of the SJS location was Cfb with temperate summers, which are located at 380, 650, and 835 m a.s.l., respectively. A completely randomized block design with four replications and four trees per plot was used as a statistical model for each location. The soluble solids (SS) content, titratable acidity (TA), and the maturation index (MI) or ratio (SS/TA) of the juice, as well as the citrus color index (CCI) of fruit skin, were assessed monthly, beginning 200 days after flowering until harvest, totaling seven sampling dates. The data sets of each location were analyzed independently through a two-way analysis of variance (ANOVA) involving rootstocks in a split plot array in time (days) to allow for the assessment of the significance of the main effects, complemented by regression analysis. In general, the ripening of ‘Valencia’ orange fruits was influenced by the different ‘Trifoliata’ hybrid rootstocks. At the RA location, trees on IPEACS-256 and’ US-852’ citrandarins had the highest SS, and on ‘US-852’ citrandarin had the highest MI, reaching the MIim earlier than the other rootstocks. The highest CCI was achieved when trees were on IPEACS-256 citrandarin. At the SSA location, trees on ‘US-852’, IPEACS-256, and IPEACS-264 citrandarins had the highest SS, but on ‘US–852’ had the highest MI, reaching the earliest MIim among the rootstocks. The highest CCI was achieved when trees were on ‘US-852’ and IPEACS-256 citrandarins. In the SSJ location, there was no significant effect of the rootstocks on and of the variables of ‘Valencia’ orange fruit. This assessment can be useful in the planning of new orchards producing high-quality fruit with desirable features for the orange juice industry.

Special Issue on "Fruit Metabolism and Metabolomics"

Over the past 10 years, knowledge about several aspects of fruit metabolism has been greatly improved. Notably, high-throughput metabolomic technologies have allowed quantifying metabolite levels across various biological processes, and identifying the genes that underly fruit development and ripening. This Special Issue is designed to exemplify the current use of metabolomics studies of temperate and tropical fruit for basic research as well as practical applications. It includes articles about different aspects of fruit biochemical phenotyping, fruit metabolism before and after harvest, including primary and specialized metabolisms, and bioactive compounds involved in growth and environmental responses. The effect of genotype, stages of development or fruit tissue on metabolomic profiles and corresponding metabolism regulations are addressed, as well as the combination of other omics with metabolomics for fruit metabolism studies.

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.