The Role of Grafting in the Resistance of Tomato to Viruses

Grafting seedling rootstock strengthens tolerance to drought stress in polyploid mulberry (Morus alba L.)

The economically adaptable mulberry (Morus alba L.) has a long history of grafting in China, yet the physiological mechanisms and advantages in drought tolerance remain unexplored. In our study, we investigated the responses of self-rooted 2X (diploid), 3X (triploid), and 4X (tetraploid) plants, as well as polyploid plants grafted onto diploid seedling rootstocks (2X/2X, 3X/2X, and 4X/2X) under drought stress. We found that self-rooted diploid plants exhibited the most severe phenotypic damage, lowest water retention, photosynthetic capacity, and the least effective osmotic stress adjustment compared to tetraploid and triploid plants. However, grafted diploid and triploid plants showed effective mitigation of drought-induced damage, with higher relative water content and improved soil water retention. Grafted plants also improved the photosystem response to drought stress through elevated photosynthetic potential, closed stomatal aperture, and faster recovery of chlorophyll biosynthesis in the leaves. Additionally, grafted plants altered osmotic protective compound levels, including starch, soluble sugar, and proline content, thereby enhancing drought resistance. Absolute quantification PCR indicated that the expression levels of proline synthesis-related genes in grafted plants were not influenced after drought stress, whereas they were significantly increased in self-rooted plants. Consequently, our findings support that self-rooted triploid and tetraploid mulberries exhibited superior drought resistance compared to diploid plants. Moreover, grafting onto seedling rootstocks enhanced tolerance against drought stress in diploid and triploid mulberry, but not in tetraploid. Our study provides valuable insights for a comprehensive analysis of physiological effects in response to drought stress between stem-roots and seedling rootstocks.

Integrated transcriptome and DNA methylome analysis reveal the biological base of increased resistance to gray leaf spot and growth inhibition in interspecific grafted tomato scions

BACKGROUND

Grafting is widely used as an important agronomic approach to deal with environmental stresses. However, the molecular mechanism of grafted tomato scions in response to biotic stress and growth regulation has yet to be fully understood.

RESULTS

This study investigated the resistance and growth performance of tomato scions grafted onto various rootstocks. A scion from a gray leaf spot-susceptible tomato cultivar was grafted onto tomato, eggplant, and pepper rootstocks, creating three grafting combinations: one self-grafting of tomato/tomato (TT), and two interspecific graftings, namely tomato/eggplant (TE) and tomato/pepper (TP). The study utilized transcriptome and DNA methylome analyses to explore the regulatory mechanisms behind the resistance and growth traits in the interspecific graftings. Results indicated that interspecific grafting significantly enhanced resistance to gray leaf spot and improved fruit quality, though fruit yield was decreased compared to self-grafting. Transcriptome analysis demonstrated that, compared to self-grafting, interspecific graftings triggered stronger wounding response and endogenous immune pathways, while restricting genes related to cell cycle pathways, especially in the TP grafting. Methylome data revealed that the TP grafting had more hypermethylated regions at CHG (H = A, C, or T) and CHH sites than the TT grafting. Furthermore, the TP grafting exhibited increased methylation levels in cell cycle related genes, such as DNA primase and ligase, while several genes related to defense kinases showed decreased methylation levels. Notably, several kinase transcripts were also confirmed among the rootstock-specific mobile transcripts.

CONCLUSIONS

The study concludes that interspecific grafting alters gene methylation patterns, thereby activating defense responses and inhibiting the cell cycle in tomato scions. This mechanism is crucial in enhancing resistance to gray leaf spot and reducing growth in grafted tomato scions. These findings offer new insights into the genetic and epigenetic contributions to agronomic trait improvements through interspecific grafting.

Evaluation of Grafting for Management of Southern Blight in Processing Tomatoes in California

Options for managing southern blight of processing tomato (caused by Athelia rolfsii) in California are limited. The objectives of this study were to: (i) evaluate grafting with the resistant rootstock Maxifort for southern blight management in processing tomato and (ii) evaluate increasing the height of the graft union to further reduce incidence of southern blight in grafted plants. We evaluated two cultivars (Heinz 5608 or Heinz 8504) and a grafting factor with three levels (grafted to Maxifort rootstock with standard scion height, grafted to Maxifort rootstock at a tall height, and nongrafted) in a field study with natural inoculum or in inoculated greenhouse experiments. Southern blight severity was low in both greenhouse experiments in 2018 and 2019, and no consistent trends were observed. In field experiments in 2018 and 2019, mean incidence in nongrafted plots was 6.2 to 17.0 times higher when compared with either the standard or tall grafted treatments. Southern blight was numerically lower in tall grafted plots compared with standard, but the magnitude was small and not statistically significant. Based on our studies, grafting can reduce losses of processing tomato in California to southern blight, but increasing the height of the graft union does not offer a tangible benefit.

Rootstock-induced scion resistance against tobacco mosaic virus is associated with the induction of defence-related transcripts and graft-transmissible mRNAs

Grafting is a common horticultural practice used to confer desirable traits between rootstock and scion, including disease resistance. To investigate graft-conferred resistance against viral diseases a novel heterografting system was developed using Nicotiana benthamiana scions grafted onto different tomato rootstocks. N. benthamiana is normally highly susceptible to tobacco mosaic virus (TMV) infection. However, specific tomato rootstock varieties were found to confer a range of resistance levels to N. benthamiana scions inoculated with TMV. Conferred resistance was associated with delays in virus accumulation and the reduction in virus spread. RNA sequencing analysis showed the enrichment of transcripts associated with disease resistance and plant stress in N. benthamiana scions grafted onto resistance-inducing tomato rootstocks. Genome sequencing of resistance- and nonresistance-conferring rootstocks was used to identify mobile tomato transcripts within N. benthamiana scions. Within resistance-induced N. benthamiana scions, enriched mobile tomato transcripts were predominantly associated with defence, stress, and abscisic acid signalling when compared to similar scions grafted onto nonresistance-inducing rootstock. Combining these findings suggests that graft-induced resistance is modulated by rootstock scion transcriptional responses and rootstock-specific mobile transcripts.

Grafting to Manage Infections of the Emerging Tomato Leaf Curl New Delhi Virus in Cucurbits

Tomato leaf curl New Delhi virus (ToLCNDV) is an emerging begomovirus (Geminiviridae family) listed in the EPPO Alert List 2, present in the Mediterranean area and in Italy, where it was reported in 2015 in Sicilian courgette. The virus is widespread in cucurbits where it causes up to 100% production losses. In 2018, ToLCNDV was isolated in Apulia (southern Italy) in commercial fields of zucchini squash and since then its recurrent outbreaks generated justified concern among growers. Thus, a sustainable and environmentally friendly approach must be adopted. Genetic resistances have been identified in Cucurbita moschata and Luffa cylindrica but, compared to genetic resistance, grafting could provide a faster and more flexible solution because the graft wounding induces tolerance rather than resistance against airborne virus infection. Compared to tolerance, the up-regulation of resistance genes requires energy resources mobilized at the expense of primary metabolism, plant growth, and development. Results of screening among twenty-one local cucurbit cvs. ecotypes and accessions to evaluate tolerance levels against rub-inoculation of ToLCNDV led to the identification of potential rootstocks to attain suitable levels of tolerance against the virus in commercial cucurbit varieties. Cucurbit plants were challenged by a ToLCNDV isolated in Apulia denoted ToLCNDV-Le and evaluated for disease symptoms development and viral DNA accumulation up to 28 days after inoculation. On the basis of disease symptoms developed, plants were classified as tolerant, moderately tolerant, moderately susceptible, and susceptible. Cucumis melo cv. Barattiere did not show any detectable disease symptoms and very low levels of viral DNA accumulation was recorded; thus, it was used as rootstock for some of the remaining cucurbit genotypes that were used as scions. The tolerance trait was transmitted to the otherwise susceptible and moderately susceptible cucurbit genotypes grafted onto the cv. Barattiere. The results of this study suggest practical implications of the approach described.

Biotechnological Advances to Improve Abiotic Stress Tolerance in Crops

The major challenges that agriculture is facing in the twenty-first century are increasing droughts, water scarcity, flooding, poorer soils, and extreme temperatures due to climate change. However, most crops are not tolerant to extreme climatic environments. The aim in the near future, in a world with hunger and an increasing population, is to breed and/or engineer crops to tolerate abiotic stress with a higher yield. Some crop varieties display a certain degree of tolerance, which has been exploited by plant breeders to develop varieties that thrive under stress conditions. Moreover, a long list of genes involved in abiotic stress tolerance have been identified and characterized by molecular techniques and overexpressed individually in plant transformation experiments. Nevertheless, stress tolerance phenotypes are polygenetic traits, which current genomic tools are dissecting to exploit their use by accelerating genetic introgression using molecular markers or site-directed mutagenesis such as CRISPR-Cas9. In this review, we describe plant mechanisms to sense and tolerate adverse climate conditions and examine and discuss classic and new molecular tools to select and improve abiotic stress tolerance in major crops.

Rootstock-Mediated Genetic Variance in Cadmium Uptake by Juvenile Cacao (Theobroma cacao L.) Genotypes, and Its Effect on Growth and Physiology

Grafting typically offers a shortcut to breed tree orchards throughout a multidimensional space of traits. Despite an overwhelming spectrum of rootstock-mediated effects on scion traits observed across several species, the exact nature and mechanisms underlying the rootstock-mediated effects on scion traits in cacao (Theobroma cacao L.) plants often remain overlooked. Therefore, we aimed to explicitly quantify rootstock-mediated genetic contributions in recombinant juvenile cacao plants across target traits, specifically cadmium (Cd) uptake, and its correlation with growth and physiological traits. Content of chloroplast pigments, fluorescence of chlorophyll a, leaf gas exchange, nutrient uptake, and plant biomass were examined across ungrafted saplings and target rootstock × scion combinations in soils with contrasting levels of Cd. This panel considered a total of 320 progenies from open-pollinated half-sib families and reciprocal full-sib progenies (derived from controlled crosses between the reference genotypes IMC67 and PA121). Both family types were used as rootstocks in grafts with two commercial clones (ICS95 and CCN51) commonly grown in Colombia. A pedigree-based best linear unbiased prediction (A-BLUP) mixed model was implemented to quantify rootstock-mediated narrow-sense heritability (h 2) for target traits. A Cd effect measured on rootstocks before grafting was observed in plant biomass, nutrient uptake, and content of chloroplast pigments. After grafting, damage to the Photosystem II (PSII) was also evident in some rootstock × scion combinations. Differences in the specific combining ability for Cd uptake were mostly detected in ungrafted rootstocks, or 2 months after grafting with the clonal CCN51 scion. Moderate rootstock effects (h 2> 0.1) were detected before grafting for five growth traits, four nutrient uptake properties, and chlorophylls and carotenoids content (h 2 = 0.19, 95% CI 0.05-0.61, r = 0.7). Such rootstock effects faded (h 2< 0.1) when rootstock genotypes were examined in soils without Cd, or 4 months after grafting. These results suggest a pervasive genetic conflict between the rootstock and the scion genotypes, involving the triple rootstock × scion × soil interaction when it refers to Cd and nutrient uptake, early growth, and photosynthetic process in juvenile cacao plants. Overall, deepening on these findings will harness early breeding schemes of cacao rootstock genotypes compatible with commercial clonal scions and adapted to soils enriched with toxic levels of Cd.

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

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

Effect of Transgenic Rootstock Grafting on the Omics Profiles in Tomato

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

Monitoring of Salinity, Temperature, and Drought Stress in Grafted Watermelon Seedlings Using Chlorophyll Fluorescence

Chlorophyll fluorescence (CF) is used to measure the physiological status of plants affected by biotic and abiotic stresses. Therefore, we aimed to identify the changes in CF parameters in grafted watermelon seedlings exposed to salt, drought, and high and low temperatures. Grafted watermelon seedlings at the true three-leaf stage were subjected to salinity levels (0, 50, 100, 150, and 200 mM) and temperature [low (8°C), moderate (24°C), and high (40°C)] stresses for 12 days under controlled environmental conditions independently. Eight CF parameters were measured at 2-day intervals using the FluorCam machine quenching protocol of the FluorCam machine. The seedlings were also exposed to drought stress for 3 days independent of salinity and temperature stress; CF parameters were measured at 1-day intervals. In addition, growth parameters, proline, and chlorophyll content were evaluated in all three experiments. The CF parameters were differentially influenced depending on the type and extent of the stress conditions. The results showed a notable effect of salinity levels on CF parameters, predominantly in maximum quantum yield (Fv/Fm), non-photochemical quenching (NPQ), the ratio of the fluorescence decrease (Rfd), and quantum yield of non-regulated energy dissipation in PSII [Y(NO)]. High temperature had significant effects on Rfd and NPQ, whereas low temperature showed significant results in most CF parameters: Fv/Fm, Y(NO), NPQ, Rfd, the efficiency of excitation capture of open photosystem II (PSII) center (Fv'/Fm'), and effective quantum yield of photochemical energy conversion in PSII [Y(PSII)]. Only NPQ and Rfd were significantly influenced by severe drought stress. Approximately, all the growth parameters were significantly influenced by the stress level. Proline content increased with an increase in stress levels in all three experiments, whereas the chlorophyll (a and b) content either decreased or increased depending upon the stressor. The results provided here may be useful for understanding the effect of abiotic stresses on CF parameters and the selection of index CF parameters to detect abiotic stresses in grafted watermelon seedlings.

Molecular Dissection of Cucumis metuliferus Resistance against Papaya Ringspot Virus by Grafting

Vegetable crops of the genus Cucumis are very popular worldwide and have great market value. However, their fruit quality and yield are hindered by viral diseases. C. metuliferus is considered a wild species with resistance to viral diseases that is lacking in cultivated crops of the Cucumis genus, such as melon. The C. metuliferus line L37 shows extreme resistance against Papaya ringspot virus (PRSV-HA), whereas line L35 is a susceptible line. In this study, reciprocal grafting experiments between L35 and L37 were performed, and the PRSV-HA strain was pre-inoculated in the rootstock leaves. The results revealed that the resistance signal in the L37 rootstock could transmit and provide resistance to the L35 scion. Subsequently, double sandwich grafting was performed using the pre-inoculated L35 as the rootstock, which was then grafted onto the L37 intermediate and the L35 scion. The results showed that PRSV-HA RNA accumulated in the L35 rootstock leaf, petiole, and stem tissues, whereas PRSV-HA RNA accumulated in some intermediate and scion petiole and stem tissues. No HCPro RNA was detected in the L35 scion leaves. The results showed that the suppression of the virus occurred in the leaves, and the resistance effect spread from the rootstock in the scion direction. Hence, this study has demonstrated that RNA silencing of systemic signals is responsible for L37 resistance against PRSV. C. metuliferus L37 could provide a valuable resistance source for crops of the Cucumis species against viral diseases through grafting.