Grafting Cucumber Onto Pumpkin Induced Early Stomatal Closure by Increasing ABA Sensitivity Under Salinity Conditions

Pumpkin CmoDREB2A enhances salt tolerance of grafted cucumber through interaction with CmoNAC1 to regulate H2O2 and ABA signaling and K+/Na+ homeostasis

Pumpkin CmoNAC1 enhances salt tolerance in grafted cucumbers. However, the potential interactions with other proteins that may co-regulate salt tolerance alongside CmoNAC1 have yet to be explored. In this study, we identified pumpkin CmoDREB2A as a pivotal transcription factor that interacts synergistically with CmoNAC1 in the co-regulation of salt tolerance. Both transcription factors were observed to bind to each other's promoters, forming a positive regulatory loop of their transcription. Knockout of CmoDREB2A in the root resulted in reduced salt tolerance in grafted cucumbers, whereas overexpression demonstrated the opposite effect. Multiple assays in our study provided evidence of the protein interaction between CmoDREB2A and CmoNAC1. Exploiting this interaction, CmoDREB2A facilitated the binding of CmoNAC1 to the promoters of CmoRBOHD1, CmoNCED6, CmoAKT1;2, and CmoHKT1;1, inducing H2O2 and ABA synthesis and increasing the K+/Na+ ratio in grafted cucumbers under salt stress. Additionally, CmoNAC1 also promoted the binding of CmoDREB2A to CmoHAK5;1/CmoHAK5;2 promoters, further contributing to the K+/Na+ homeostasis. In summary, these findings reveal a crucial mechanism of CmoNAC1 and CmoDREB2A forming a complex enhancing salt tolerance in grafted cucumbers.

CmoNAC1 in pumpkin rootstocks improves salt tolerance of grafted cucumbers by binding to the promoters of CmoRBOHD1, CmoNCED6, CmoAKT1;2 and CmoHKT1;1 to regulate H2O2, ABA signaling and K+/Na+ homeostasis

The NAC transcription factor is a type of plant-specific transcription factor that can regulate plant salt tolerance, but the underlying mechanism is unclear in grafted vegetables. H2O2 and ABA in pumpkin rootstocks can be transported to cucumber scion leaves, promoting stomatal closure to improve salt tolerance of grafted cucumbers. Despite these observations, the regulatory mechanism is unknown. Here, our research revealed that CmoNAC1 is a key transcription factor that regulates H2O2 and ABA signaling in pumpkin roots under salt stress. The function of CmoNAC1 was analyzed using root transformation and RNA-seq, and we found that pumpkin CmoNAC1 promoted the production of H2O2 and ABA via CmoRBOHD1 and CmoNCED6, respectively, and regulated K+/Na+ homeostasis via CmoAKT1;2, CmoHKT1;1, and CmoSOS1 to improve salt tolerance of grafted cucumbers. Root knockout of CmoNAC1 resulted in a significant decrease in H2O2 (52.9% and 32.1%) and ABA (21.8% and 42.7%) content and K+/Na+ ratio (81.5% and 56.3%) in leaf and roots of grafted cucumber, respectively, while overexpression showed the opposite effect. The root transformation experiment showed that CmoNCED6 could improve salt tolerance of grafted cucumbers by regulating ABA production and K+/Na+ homeostasis under salt stress. Finally, we found that CmoNAC1 bound to the promoters of CmoRBOHD1, CmoNCED6, CmoAKT1;2, and CmoHKT1;1 using yeast one-hybrid, luciferase, and electrophoretic mobility shift assays. In conclusion, pumpkin CmoNAC1 not only binds to the promoters of CmoRBOHD1 and CmoNCED6 to regulate the production of H2O2 and ABA signals in roots, but also binds to the promoters of CmoAKT1;2 and CmoHKT1;1 to increase the K+/Na+ ratio, thus improving salt tolerance of grafted cucumbers.

Study on the Applications and Regulatory Mechanisms of Grafting on Vegetables

Grafting can overcome problems with soil sensitivity, enhance plant stress tolerance, improve product quality, and increase crop yield and value. This paper reviews the various mechanisms of vegetable grafting, the graft survival process and its influencing factors, the practical applications of grafting, and the molecular regulation of grafting in vegetables. The importance of germplasm and rootstock interactions, the mechanization of vegetable grafting, and future aspects, including intelligence and digitalization, are discussed.

Low red to far-red light ratio promotes salt tolerance by improving leaf photosynthetic capacity in cucumber

Soil salinity severely inhibits leaf photosynthesis and limits agricultural production. Red to far-red light ratio (R/FR) affects leaf photosynthesis under salt stress, however, its regulation mechanism is still largely unknown. This study investigated the effects of different R/FR on plant growth, gas exchange parameters, photosynthetic electron transport, Calvin cycle and key gene expression under salt stress. Cucumber seedlings were exposed to four treatments including 0 mM NaCl and R/FR=7 (L7, control), 0 mM NaCl and R/FR=0.7 (L0.7), 80 mM NaCl and R/FR=7 (H7) and 80 mM NaCl and R/FR=0.7 (H0.7) for 9 days in an artificial climate chamber. The results showed that compared to L7 treatment, H7 treatment significantly reduced relative growth rate (RGR), CO₂ assimilation rate (P _n), maximum photochemical efficiency PSII (F _v/F _m), most JIP-test parameters and total Rubisco activity, indicating that salt stress severely inhibited photosynthetic electron transport from PSII to PSI and blocked Calvin cycle in cucumber leaves. However, these suppressions were effectively alleviated by low R/FR addition (H0.7 treatment). Compared to H7 treatment, H0.7 treatment significantly increased RGR and P _n by 209.09% and 7.59%, respectively, enhanced F _v/F _m, maximum quantum yield for primary photochemistry (φ _Po), quantum yield for electron transport (φ _Eo) and total Rubisco activity by 192.31%, 17.6%, 36.84% and 37.08%, respectively, and largely up-regulated expressions of most key genes involved in electron transport and Calvin cycle. In conclusion, low R/FR effectively alleviated the negative effects of salt stress on leaf photosynthesis by accelerating photosynthetic electron transport from PSII to PQ pool and promoting Calvin cycle in cucumber plants. It provides a novel environmentally friendly light-quality regulation technology for high efficiency salt-resistant vegetable production.

Plant Hormone and Inorganic Ion Concentrations in the Xylem Exudate of Grafted Plants Depend on the Scion-Rootstock Combination

In grafted plants, inorganic ions and plant hormones in the xylem exudate transported from the rootstock to the scion directly or indirectly affect the scion, thereby improving the traits. Therefore, the concentration of these components in the xylem exudate of grafted plants may be an indicator for rootstock selection. On the other hand, few reports have presented a comprehensive analysis of substances transferred from the rootstock to the scion in plants grafted onto different rootstocks, primarily commercial cultivars. In this study, we measured inorganic ions and plant hormones in the xylem exudate from the rootstock to the scion in various grafted plants of tomato and eggplant. The results revealed that the concentrations of inorganic ions and plant hormones in the xylem exudate significantly differed depending on the type of rootstock. In addition, we confirmed the concentration of the inorganic ions and plant hormones in the xylem exudate of plants grafted onto the same tomato rootstock cultivars as rootstock with tomato or eggplant as the scions. As a result, the concentrations of inorganic ions and plant hormones in the xylem exudate were significantly different in the grafted plants with eggplant compared with tomato as the scion. These results suggest that signals from the scion (shoot) control the inorganic ions and plant hormones transported from the rootstock (root).

WRKY41/WRKY46-miR396b-5p-TPR module mediates abscisic acid-induced cold tolerance of grafted cucumber seedlings

Grafting is one of the key agronomic measures to enhance the tolerance to environmental stresses in horticultural plants, but the specific molecular regulation mechanism in this tolerance largely remains unclear. Here, we found that cucumber grafted onto figleaf gourd rootstock increased cold tolerance through abscisic acid (ABA) activating WRKY41/WRKY46-miR396b-5p-TPR (tetratricopeptide repeat-like superfamily protein) module. Cucumber seedlings grafted onto figleaf gourd increased cold tolerance and induced the expression of miR396b-5p. Furthermore, overexpression of cucumber miR396b-5p in Arabidopsis improved cold tolerance. 5' RNA ligase-mediated rapid amplification of cDNA ends (5' RLM-RACE) and transient transformation experiments demonstrated that TPR was the target gene of miR396b-5p, while TPR overexpression plants were hypersensitive to cold stress. The yeast one-hybrid and dual-luciferase assays showed that both WRKY41 and WRKY46 bound to MIR396b-5p promoter to induce its expression. Furthermore, cold stress enhanced the content of ABA in the roots and leaves of figleaf gourd grafted cucumber seedlings. Exogenous application of ABA induced the expression of WRKY41 and WRKY46, and cold tolerance of grafted cucumber seedlings. However, figleaf gourd rootstock-induced cold tolerance was compromised when plants were pretreated with ABA biosynthesis inhibitor. Thus, ABA mediated figleaf gourd grafting-induced cold tolerance of cucumber seedlings through activating the WRKY41/WRKY46-miR396b-5p-TPR module.

Grafting-enhanced tolerance of cucumber to toxic stress is associated with regulation of phenolic and other aromatic acids metabolism

Toxic stress caused by autotoxins is a common phenomenon for cucumber under monoculture condition. A previous study demonstrated that grafting could enhance the resistance of cucumber to cinnamic acid (CA) stress, but the underlying mechanism behind this enhanced resistance is still unclear. In the present study, we reconfirmed the stronger resistance of grafted rootstock (RG) compared to the non-grafted (NG) cucumber as measured though plant biomass accumulation. In addition, we focused on the phenolic and other aromatic acids metabolism in hydroponic culture model system using a combination of qRT-PCR (to measure gene expression of relevant genes) and HPLC (to detect the presence of phenolic and other aromatic acids). The results showed that the exogenous CA lead to the expression of four enzymes involved in phenolic and other aromatic acids biosynthesis, and a larger increase was observed in grafted rootstock (RG). Specifically, expression of six genes, involved in phenolic and other aromatic acids biosynthesis (PAL, PAL1, C4H, 4CL1, 4CL2 and COMT), with the exception of 4CL2, were significantly up-regulated in RG but down-regulated in NG when exposed to CA. Furthermore, six kinds of phenolic and other aromatic acids were detected in leaves and roots of NG and RG cucumber, while only benzoic acid and cinnamic acid were detected in root exudate of all samples. The CA treatment resulted in an increase of p-hydroxybenzonic acid, benzoic acid and cinnamic acid contents in RG cucumber, but decrease of p-coumaric acid and sinapic acid contents in NG cucumber. Surprisingly, the type and amount of phenolic and other aromatic acids in root exudate was improved by exogenous CA, particularly for RG cucumber. These results suggest that a possible mechanism for the stronger resistance to CA of RG than NG cucumber could involve the up-regulation of key genes involved in phenolic and other aromatic acids metabolism, and that the excessive phenolic compounds released to surroundings is a result of the accumulation of phenolic compounds in a short time by the plant under stress.

Multiple Stressors in Vegetable Production: Insights for Trait-Based Crop Improvement in Cucurbits

Vegetable production is a key determinant of contribution from the agricultural sector toward national Gross Domestic Product in a country like India, the second largest producer of fresh vegetables in the world. This calls for a careful scrutiny of the threats to vegetable farming in the event of climate extremes, environmental degradation and incidence of plant pests/diseases. Cucurbits are a vast group of vegetables grown almost throughout the world, which contribute to the daily diet on a global scale. Increasing food supply to cater to the ever-increasing world population, calls for intensive, off-season and year-round cultivation of cucurbits. Current situation predisposes these crops to a multitude of stressors, often simultaneously, under field conditions. This scenario warrants a systematic understanding of the different stress specific traits/mechanisms/pathways and their crosstalk that have been examined in cucurbits and identification of gaps and formulation of perspectives on prospective research directions. The careful dissection of plant responses under specific production environments will help in trait identification for genotype selection, germplasm screens to identify superior donors or for direct genetic manipulation by modern tools for crop improvement. Cucurbits exhibit a wide range of acclimatory responses to both biotic and abiotic stresses, among which a few like morphological characters like waxiness of cuticle; primary and secondary metabolic adjustments; membrane thermostability, osmoregulation and, protein and reactive oxygen species homeostasis and turnover contributing to cellular tolerance, appear to be common and involved in cross talk under combinatorial stress exposures. This is assumed to have profound influence in triggering system level acclimation responses that safeguard growth and metabolism. The possible strategies attempted such as grafting initiatives, molecular breeding, novel genetic manipulation avenues like gene editing and ameliorative stress mitigation approaches, have paved way to unravel the prospects for combined stress tolerance. The advent of next generation sequencing technologies and big data management of the omics output generated have added to the mettle of such emanated concepts and ideas. In this review, we attempt to compile the progress made in deciphering the biotic and abiotic stress responses of cucurbits and their associated traits, both individually and in combination.

Rootstock-scion exchanging mRNAs participate in the pathways of amino acids and fatty acid metabolism in cucumber under early chilling stress

Cucumber (Cucumis sativus L.) often experiences chilling stress that limits their growth and productivity. Grafting is widely used to improve abiotic stress resistance by alternating a vigorous root system, suggesting there exists systemic signals communication between distant organs. mRNAs are reported to be evolving in fortification strategies by long-distance signaling when plants suffering from chilling stress. However, the potential function of mobile mRNAs alleviating chilling stress in grafted cucumber is still unknown. Here, the physiological changes, mobile mRNAs profiling, transcriptomic and metabolomic changes in above- and underground tissues of all graft combinations of cucumber and pumpkin responding to chilling stress were established and analyzed comprehensively. The co-relationship between the cluster of chilling-induced pumpkin mobile mRNAs with Differentially Expressed Genes (DEGs) and Differentially Intensive Metabolites (DIMs) revealed that four key chilling-induced pumpkin mobile mRNAs were highly related to glycine, serine and threonine synthesis and fatty acid β-oxidative degradation metabolism in cucumber tissues of heterografts. The verification of mobile mRNAs, potential transport of metabolites and exogenous application of key metabolites of glycerophospholipid metabolism pathway in cucumber seedlings confirmed that the role of mobile mRNAs in regulating chilling responses in grafted cucumber. Our results build a link between the long-distance mRNAs of chilling-tolerant pumpkin and the fatty acid β-oxidative degradation metabolism of chilling-sensitive cucumber. It helps to uncover the mechanism of signaling interaction between scion and stock responding to chilling tolerant in grafted cucumber.

Long-distance control of pumpkin rootstock over cucumber scion under drought stress as revealed by transcriptome sequencing and mobile mRNAs identifications

Grafting with pumpkin rootstock is commonly used not only to improve the quality of cucumber fruits but also to confer biotic or abiotic stress tolerance. However, the molecular mechanism of grafted cucumbers to drought stress and the possible roles of mobile mRNAs to improve stress tolerance have remained obscure. Hence, we conducted transcriptome sequencing and combined it with morpho-physiological experiments to compare the response of homografts (cucumber as scion and rootstock) (C) and heterografts (cucumber as scion and pumpkin as rootstock) (P) to drought stress. After applying drought stress, homografts and heterografts expressed 2960 and 3088 genes in response to drought stress, respectively. The identified DEGs in heterografts under drought stress were categorized into different stress-responsive groups, such as carbohydrate metabolism (involved in osmotic adjustment by sugar accumulation), lipid and cell wall metabolism (involved in cell membrane integrity by a reduction in lipid peroxidation), redox homeostasis (increased antioxidant enzymes activities), phytohormone (increased ABA content), protein kinases and transcription factors (TFs) using MapMan software. Earlier and greater H2O2 accumulation in xylem below the graft union was accompanied by leaf ABA accumulation in heterografts in response to drought stress. Greater leaf ABA helped heterografted cucumbers to sense and respond to drought stress earlier than homografts. The timely response of heterografts to drought stress led to maintain higher water content in the leaves even in the late stage of drought stress. The identified mobile mRNAs (mb-mRNAs) in heterografts were mostly related to photosynthesis which would be the possible reason for improved chlorophyll content and maximum photochemical efficiency of PSII (Fv/Fm). The existence of some stress-responsive pumpkin (rootstock) mRNAs in cucumber (scion), such as heat shock protein (HSP70, a well-known stress-responsive gene), led to the higher proline accumulation than homografts. The expression of the mobile and immobile stress-responsive mRNAs and timely response of heterografts to drought stress could improve drought tolerance in pumpkin-rooted plants.

Transcriptomic and Physiological Analysis Reveal That α-Linolenic Acid Biosynthesis Responds to Early Chilling Tolerance in Pumpkin Rootstock Varieties

Climate changes especially chilling stress affects cucurbit crops during winter seasonal production. Grafting to pumpkin rootstocks is widely used to improve the vigor of cucurbits, especially cucumber (Cucumis sativus L.) plants, in the face of chilling stress. In our study, multi-disciplinary aspect approaches were used to investigate growth changes of pumpkin under chilling stress. Firstly, the morphological and physiological characteristics of 14 pumpkin (Cucurbita moschata) varieties following different periods of chilling stress was analyzed by using physiological means. Mathematical results of principal component analysis (PCA) with chlorophyll-a, chlorophyll-b, carotenoid contents, chilling injury index and relative electrolyte permeability indicated that relative electrolyte permeability as the primary judgment index was best associated with the comparison of chilling tolerance in pumpkin rootstock varieties. Then, transcriptomic and DCMU (Diuron) application and chlorophyll fluorescence examination analysis of pumpkin leaves revealed that 390 Cucurbita moschata differentially expressed genes (CmoDEGs) that affect photosynthesis were upregulated in leaves. 127 CmoDEGs both in leaves and roots were enriched for genes involved in unsaturated fatty acid metabolism, suggesting that plasma membrane lipids are involved in chilling perception. The results of increased composition of unsaturated fatty acid in leaves and qRT-PCR analysis of relative mRNA abundance confirmed that α-linolenic acid biosynthesis was responding to pumpkin chilling tolerance. The integration of physiological, mathematical bioinformatical and biological analysis results contributes to our understanding of the molecular mechanisms underlying chilling tolerance and its improvement in cucumber grafted on pumpkin rootstocks. It provided an important theoretical basis and reference for further understanding on the impact of climate change on plant physiological changes.

Contrasting Rootstock-Mediated Growth and Yield Responses in Salinized Pepper Plants (Capsicum annuum L.) Are Associated with Changes in the Hormonal Balance

Salinity provokes an imbalance of vegetative to generative growth, thus impairing crop productivity. Unlike breeding strategies, grafting is a direct and quick alternative to improve salinity tolerance in horticultural crops, through rebalancing plant development. Providing that hormones play a key role in plant growth and development and stress responses, we hypothesized that rootstock-mediated reallocation of vegetative growth and yield under salinity was associated with changes in the hormonal balance. To test this hypothesis, the hybrid pepper variety (Capsicum annuum L. “Gacela F1”) was either non-grafted or grafted onto three commercial rootstocks (Creonte, Atlante, and Terrano) and plants were grown in a greenhouse under control (0 mM NaCl) and moderate salinity (35 mM NaCl) conditions. Differential vegetative growth versus fruit yield responses were induced by rootstock and salinity. Atlante strongly increased shoot and root fresh weight with respect to the non-grafted Gacela plants associated with improved photosynthetic rate and K⁺ homeostasis under salinity. The invigorating effect of Atlante can be explained by an efficient balance between cytokinins (CKs) and abscisic acid (ABA). Creonte improved fruit yield and maintained the reproductive to vegetative ratio under salinity as a consequence of its capacity to induce biomass reallocation and to avoid Na⁺ accumulation in the shoot. The physiological responses associated with yield stability in Creonte were mediated by the inverse regulation of CKs and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid. Finally, Terrano limited the accumulation of gibberellins in the shoot thus reducing plant height. Despite scion compactness induced by Terrano, both vegetative and reproductive biomass were maintained under salinity through ABA-mediated control of water relations and K⁺ homeostasis. Our data demonstrate that the contrasting developmental and physiological responses induced by the rootstock genotype in salinized pepper plants were critically mediated by hormones. This will be particularly important for rootstock breeding programs to improve salinity tolerance by focusing on hormonal traits.

Identification of Long-Distance Transmissible mRNA between Scion and Rootstock in Cucurbit Seedling Heterografts

Grafting has been widely used to improve plant growth and tolerance in crop production, as well as for clarifying systemic mRNA signaling from donor to recipient tissues in organ-to-organ communication. In this study, we investigated graft partner interaction mechanisms of Cucumis sativus (Csa) and Cucurbita moschata (Cmo) using a large-scale endogenous mRNA transport. The results indicated that most mobile transcripts followed an allocation pathway from source to sink. Gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that mRNA mobility functions are universally common and individually specific. Identification of mRNA mobility between distant tissues in heterografts with RT-PCR (reverse transcription PCR), RT-qPCR (reverse transcriptional quantitative real time PCR), and clone sequencing were used to estimate 78.75% of selected mobile transcripts. Integration of bioinformatic analysis and RT-qPCR identification allowed us to hypothesize a scion-to-rootstock-to-scion feedback signal loop of Csa move-down and Cmo move-up mRNAs, where Csa scion move-down mRNAs were involved in carbon fixation and biosynthesis of amino acid pathways, and Cmo root received Csa move-down mRNA and then delivered the corresponding Cmo upward mRNA to scion to improve photosynthesis of cucumber scion. This formed a feedback signal loop of scion-to-rootstock-to scion to explain why pumpkin rootstock enhanced cucumber production in the industry, which was utilized for organ communication and mediates photosynthesis processes in heterograft cucurbit crops.