Proteomic study participating the enhancement of growth and salt tolerance of bottle gourd rootstock-grafted watermelon seedlings

Saline-Alkaline Stress Resistance of Cabernet Sauvignon Grapes Grafted on Different Rootstocks and Rootstock Combinations

Grafting the wine grape variety Cabernet Sauvignon onto salinity-tolerant rootstocks can improve salinity tolerance and grape yields in regions with high salinity soils. In this experiment, the effects of different rootstocks and rootstock combinations on the saline-alkaline stress (modified Hoagland nutrient solution + 50 mmol L-1 (NaCl + NaHCO3)) of Cabernet Sauvignon were studied. Correlation and principal component analyses were conducted on several physiological indicators of saline-alkaline stress. Salinity limited biomass accumulation, induced damage to the plant membrane, reduced the chlorophyll content and photosynthetic capacity of plants, and increased the content of malondialdehyde, sodium (Na+)/potassium (K+) ratio, and antioxidant enzyme activities (superoxide dismutase, peroxidase, and catalase). Significant differences in several indicators were observed among the experimental groups. The results indicate that the saline-alkaline tolerance of Cabernet Sauvignon after grafting was the same as that of the rootstock, indicating that the increased resistance of Cabernet Sauvignon grapes to saline-alkaline stress stems from the transferability of the saline-alkaline stress resistance of the rootstock to the scion.

Drought stress tolerance mechanisms and their potential common indicators to salinity, insights from the wild watermelon (Citrullus lanatus): A review

Climate change has escalated the effect of drought on crop production as it has negatively altered the environmental condition. Wild watermelon grows abundantly in the Kgalagadi desert even though the environment is characterized by minimal rainfall, high temperatures and intense sunshine during growing season. This area is also characterized by sandy soils with low water holding capacity, thus bringing about drought stress. Drought stress affects crop productivity through its effects on development and physiological functions as dictated by molecular responses. Not only one or two physiological process or genes are responsible for drought tolerance, but a combination of various factors do work together to aid crop tolerance mechanism. Various studies have shown that wild watermelon possess superior qualities that aid its survival in unfavorable conditions. These mechanisms include resilient root growth, timely stomatal closure, chlorophyll fluorescence quenching under water deficit as key physiological responses. At biochemical and molecular level, the crop responds through citrulline accumulation and expression of genes associated with drought tolerance in this species and other plants. Previous salinity stress studies involving other plants have identified citrulline accumulation and expression of some of these genes (chloroplast APX, Type-2 metallothionein), to be associated with tolerance. Emerging evidence indicates that the upstream of functional genes are the transcription factor that regulates drought and salinity stress responses as well as adaptation. In this review we discuss the drought tolerance mechanisms in watermelons and some of its common indicators to salinity at physiological, biochemical and molecular level.

Effect of Cold Stress on Growth, Physiological Characteristics, and Calvin-Cycle-Related Gene Expression of Grafted Watermelon Seedlings of Different Gourd Rootstocks

Recently, grafting has been used to improve abiotic stress resistance in crops. Here, using watermelon ‘Zaojia 8424’ (Citrullus lanatus) as scions, three different gourds (Lagenaria siceraria, 0526, 2505, and 1226) as rootstocks, and non-grafted plants as controls (different plants were abbreviated as 0526, 2505, 1226, and 8424), the effect of cold stress on various physiological and molecular parameters was investigated. The results demonstrate that the improved cold tolerance of gourd-grafted watermelon was associated with higher chlorophyll and proline content, and lower malondialdehyde (MDA) content, compared to 8424 under cold stress. Furthermore, grafted watermelons accumulated fewer reactive oxygen species (ROS), accompanied by enhanced antioxidant activity and a higher expression of enzymes related to the Calvin cycle. In conclusion, watermelons with 2505 and 0526 rootstocks were more resilient compared to 1226 and 8424. These results confirm that using tolerant rootstocks may be an efficient adaptation strategy for improving abiotic stress tolerance in watermelon.

Progress on Genetic Improvement and Analysis of Bottle Gourd [Lagenaria siceraria (Molina) Standl.] for Agronomic Traits, Nutrient Compositions, and Stress Tolerance: A Review

Bottle gourd [Lagenaria siceraria (Molina) Standl.] is an important multi-purpose cucurbit crop grown for its leaf, fruit, and seed. It is widely cultivated and used for human consumption in sub-Saharan Africa (SSA) providing vital human nutrition and serving as food security crop. There is wide genetic variation among bottle gourd genetic resources in Africa for diverse qualitative and quantitative attributes for effective variety design, product development, and marketing. However, the crop is under- researched and -utilized, and improved varieties are yet to be developed and commercialized in the region. Therefore, the objective of this review is to provide the progress on bottle gourd genetic improvement and genetic analysis targeting agronomic and horticultural attributes, nutritional composition, biotic, and abiotic stress tolerance to guide current and future cultivar development, germplasm access, and conservation in SSA. The first section of the paper presents progress on breeding of bottle gourd for horticultural traits, agronomic performance, nutritional and anti-nutritional composition, and biotic and abiotic stress tolerance. This is followed by important highlights on key genetic resources of cultivated and wild bottle gourd for demand driven breeding. Lastly, the review summaries advances in bottle gourd genomics, genetic engineering and genome editing. Information presented in this paper should aid bottle gourd breeders and agronomists to develop and deploy new generation and promising varieties with farmer- and market -preferred attributes.

Proteomic Analysis of Fusarium oxysporum-Induced Mechanism in Grafted Watermelon Seedlings

Grafting can improve the resistance of watermelon to soil-borne diseases. However, the molecular mechanism of defense response is not completely understood. Herein, we used a proteomic approach to investigate the molecular basis involved in grafted watermelon leaf defense against Fusarium oxysporum f.sp. niveum (FON) infection. The bottle gourd rootstock-grafted (RG) watermelon seedlings were highly resistant to FON compared with self-grafted (SG) watermelon plants, with a disease incidence of 3.4 and 89%, respectively. Meanwhile, grafting significantly induced the activity of pathogenesis-related proteases under FON challenge. Proteins extracted from leaves of RG and SG under FON inoculation were analyzed using two-dimensional gel electrophoresis. Thirty-nine differentially accumulated proteins (DAPs) were identified and classified into 10 functional groups. Accordingly, protein biosynthetic and stress- and defense-related proteins play crucial roles in the enhancement of disease resistance of RG watermelon seedlings, compared with that of SG watermelon seedlings. Proteins involved in signal transduction positively regulated the defense process. Carbohydrate and energy metabolism and photosystem contributed to energy production in RG watermelon seedlings under FON infection. The disease resistance of RG watermelon seedlings may also be related to the improved scavenging capacity of reactive oxygen species (ROS). The expression profile of 10 randomly selected proteins was measured using quantitative real-time PCR, among which, 7 was consistent with the results of the proteomic analysis. The functional implications of these proteins in regulating grafted watermelon response against F. oxysporum are discussed.

The influence of grafting on crops' photosynthetic performance

In a near scenario of climate change where stress-derived limitations on crops' yield by affecting plant gas-exchange are expected, grafting may become a cheap and easy technique to improve crops photosynthetic performance and water-use efficiency. Inconsistent data of the effect of rootstocks over gas-exchange can be found in literature, being necessary an integrative analysis of the effect of grafting over photosynthetic parameters. With this aim, we present a compilation of the effect of graft on the net CO₂ assimilation rate (A_N) and other photosynthetic parameters across different species with agronomic interest. No differences were observed in any photosynthetic parameter between non-grafted and self-grafted plants under non-stress conditions. However, differences were found depending on the used rootstock, particularly for the intrinsic water-use efficiency (WUE_i). We observed that variations in A_N induced by rootstocks were related to changes in both diffusive and biochemical parameters. Under drought or salt stress, different photosynthetic performances were observed depending on the rootstock, although the high variability among studies promted to remarkable results. Overall, we observed that grafting can be a useful technique to improve plant photosynthetic performance, and therefore, crop yield and WUE, and that the rootstock selection for a target environment is determinant for the variations in photosynthesis.

Comparative proteomic analysis of two sesame genotypes with contrasting salinity tolerance in response to salt stress

Sesame is one of the most important oilseed crops and has high nutritional value. The yield and quality of sesame are severely affected by high salinity in coastal and semi-arid/arid regions. In this study, the phenotypic, physiological, and proteomic changes induced by salt treatment were analyzed in salt-tolerant (G441) and salt-sensitive (G358) seedlings. Phenotypic and physiological results indicated that G441 had an enhanced capacity to withstand salinity stress compared to G358. Proteomic analysis revealed a strong induction of salt-responsive protein species in sesame, mainly related to catalytic, hydrolase, oxidoreductase, and binding activities. Pathway enrichment analysis showed that more salt-responsive proteins in G441 were involved in tyrosine metabolism, carbon fixation in photosynthetic organisms, carbon metabolism, alpha-linolenic acid metabolism, biosynthesis of amino acids, photosynthesis, and glutathione metabolism. Furthermore, G441 displayed unique differentially accumulated proteins in seedlings functioning as heat shock proteins, abscisic acid receptor PYL2-like, calcium-dependent protein kinases, serine/threonine-protein phosphatases, nucleoredoxin, and antioxidant enzymes. Quantitative real-time PCR analysis revealed that some of the proteins were also regulated by salinity stress at the transcript level. Our findings provide important information on salinity responses in plants and may constitute useful resources for enhancing salinity tolerance in sesame. SIGNIFICANCE: Our study identified potential biological pathways and salt-responsive protein species related to transducing stress signals and scavenging reactive oxygen species under salt stress. These findings will provide possible participants/pathways/proteins that contribute to salt tolerance and may serve as the basis for improving salinity tolerance in sesame and other plants.

Identification of the Genetic Variation and Gene Exchange between Citrus Trifoliata and Citrus Clementina

To identify the genetic variation between Citrus trifoliata and Citrus clementina, we performed genome resequencing on the two citrus species. Compared with the citrus reference genome, a total of 9,449,204 single-nucleotide polymorphisms (SNPs) and 846,615 insertion/deletion polymorphisms (InDels) were identified in the two citrus species, while 1,868,115 (19.77%) of the SNPs and 190,199 (22.47%) of the InDels from the two citrus species were located in the genic regions. Meanwhile, a total of 8,091,407 specific SNPs and 692,654 specific InDels were identified in the two citrus genotypes, yielding an average of 27.32 SNPs/kb and 2.34 InDels/kb. We identified and characterized the patterns of gene exchanges in the grafted citrus plants by using specific genetic variation from genome resequencing. A total of 4396 transporting genes across graft junctions was identified. Some specific genetic variation and mobile genes was also confirmed by Sanger sequencing. Furthermore, these mobile genes could move directionally or bidirectionally between the scions and the rootstocks. In addition, a total of 1581 and 2577 differentially expressed genes were found in the scions and the rootstocks after grafting compared with the control, respectively. These genetic variations provide fundamental information on the genetic basis of important traits between C. trifoliata and C. clementina, as the transport of genes would be applicable to horticulture crops.

Proteomic analysis of heat stress resistance of cucumber leaves when grafted onto Momordica rootstock

Various biotic and abiotic stresses threaten the cultivation of future agricultural crops. Among these stresses, heat stress is a major abiotic stress that substantially reduces agricultural productivity. Many strategies to enhance heat stress tolerance of crops have been developed, among which is grafting. Here, we show that Momordica-grafted cucumber scions have intrinsically enhanced chlorophyll content, leaf area, and net photosynthetic rate under heat stress compared to plants grafted onto cucumber rootstock. To investigate the mechanisms by which Momordica rootstock enhanced cucumber scions heat stress tolerance, comparative proteomic analysis of cucumber leaves in response to rootstock-grafting and/or heat stress was conducted. Seventy-seven differentially accumulated proteins involved in diverse biological processes were identified by two-dimensional electrophoresis (2-DE) in conjunction with matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF MS). The following four main categories of proteins were involved: photosynthesis (42.8%), energy and metabolism (18.2%), defense response (14.3%), and protein and nucleic acid biosynthesis (11.7%). Proteomic analysis revealed that scions grafted onto Momordica rootstocks upregulated more proteins involved in photosynthesis compared to scions grafted onto cucumber rootstocks under heat stress and indicated enhanced photosynthetic capacity when seedlings were exposed to heat stress. Furthermore, the expression of photosynthesis-related genes in plants grafted onto Momordica rootstocks significantly increased in response to heat stress. In addition, increased high-temperature tolerance of plants grafted onto Momordica rootstock was associated with the accumulation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and oxygen-evolving enhancer protein 1 (OEE1). Taken together, the data indicated that Momordica rootstock might alleviate growth inhibition caused by heat stress by improving photosynthesis, providing valuable insight into enhancing heat stress tolerance in the global warming epoch.

Comparative Proteomic Analysis of the Graft Unions in Hickory (Carya cathayensis) Provides Insights into Response Mechanisms to Grafting Process

Hickory (Carya cathayensis), a tree with high nutritional and economic value, is widely cultivated in China. Grafting greatly reduces the juvenile phase length and makes the large scale cultivation of hickory possible. To reveal the response mechanisms of this species to grafting, we employed a proteomics-based approach to identify differentially expressed proteins in the graft unions during the grafting process. Our study identified 3723 proteins, of which 2518 were quantified. A total of 710 differentially expressed proteins (DEPs) were quantified and these were involved in various molecular functional and biological processes. Among these DEPs, 341 were up-regulated and 369 were down-regulated at 7 days after grafting compared with the control. Four auxin-related proteins were down-regulated, which was in agreement with the transcription levels of their encoding genes. The Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that the 'Flavonoid biosynthesis' pathway and 'starch and sucrose metabolism' were both significantly up-regulated. Interestingly, five flavonoid biosynthesis-related proteins, a flavanone 3-hyfroxylase, a cinnamate 4-hydroxylase, a dihydroflavonol-4-reductase, a chalcone synthase, and a chalcone isomerase, were significantly up-regulated. Further experiments verified a significant increase in the total flavonoid contents in scions, which suggests that graft union formation may activate flavonoid biosynthesis to increase the content of a series of downstream secondary metabolites. This comprehensive analysis provides fundamental information on the candidate proteins and secondary metabolism pathways involved in the grafting process for hickory.

Comparative transcriptome profiling of chilling stress responsiveness in grafted watermelon seedlings

Rootstock grafting may improve the resistance of watermelon plants to low temperatures. However, information regarding the molecular responses of rootstock grafted plants to chilling stress is limited. To elucidate the molecular mechanisms of chilling tolerance in grafted plants, the transcriptomic responses of grafted watermelon under chilling stress were analyzed using RNA-seq analysis. Sequencing data were used for digital gene expression (DGE) analysis to characterize the transcriptomic responses in grafted watermelon seedlings. A total of 702 differentially-expressed genes (DEGs) were found in rootstock grafted (RG) watermelon relative to self-grafted (SG) watermelon; among these genes, 522 genes were up-regulated and 180 were down-regulated. Additionally, 164 and 953 genes were found to specifically expressed in RG and SG seedlings under chilling stress, respectively. Functional annotations revealed that up-regulated DEGs are involved in protein processing, plant-pathogen interaction and the spliceosome, whereas down-regulated DEGs are associated with photosynthesis. Moreover, 13 DEGs were randomly selected for quantitative real time PCR (qRT-PCR) analysis. The expression profiles of these 13 DEGs were consistent with those detected by the DGE analysis, supporting the reliability of the DGE data. This work provides additional insight into the molecular basis of grafted watermelon responses to chilling stress.

Proteomics analysis of compatibility and incompatibility in grafted cucumber seedlings

Graft compatibility between rootstock and scion is the most important factor influencing the survival of grafted plants. In this study, we used two-dimensional gel electrophoresis (2-DE) and matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry (MALDI-TOF/TOF MS) to investigate differences in leaf proteomes of graft-compatible and graft-incompatible cucumber (Cucumis sativus L.)/pumpkin (Cucurbita L.) combinations. Cucumber seedlings were used as the scions and two pumpkin cultivars with strongly contrasting grafting compatibilities were used as the rootstocks. Non-grafted and self-grafted cucumber seedlings served as control groups. An average of approximately 500 detectable spots were observed on each 2-DE gel. A total of 50 proteins were differentially expressed in response to self-grafting, compatible-rootstock grafting, and incompatible-rootstock grafting and were all successfully identified by MALDI-TOF/TOF MS. The regulation of Calvin cycle, photosynthetic apparatus, glycolytic pathway, energy metabolism, protein biosynthesis and degradation, and reactive oxygen metabolism will probably contribute to intensify the biomass and photosynthetic capacity in graft-compatible combinations. The improved physiological and growth characteristics of compatible-rootstock grafting plants are the result of the higher expressions of proteins involved in photosynthesis, carbohydrate and energy metabolism, and protein metabolism. At the same time, the compatible-rootstock grafting regulation of stress defense, amino acid metabolism, and other metabolic functions also plays important roles in improvement of plant growth.

Rootstock-scion interaction affecting citrus response to CTV infection: a proteomic view

Citrus tristeza virus (CTV) is the causal agent of various diseases with dramatic effects on citrus crops worldwide. Most Citrus species, grown on their own roots, are symptomless hosts for many CTV isolates. However, depending on different scion-rootstock combination, CTV infection should result in distinct syndromes, being 'tristeza' the more severe one, leading to a complete decline of the susceptible plants in a few weeks. Transcriptomic analyses revealed several genes involved either in defense response, or systemic acquired resistance, as well as transcription factors and components of the phosphorylation cascades, to be differentially regulated during CTV infection in Citrus aurantifolia species. To date little is known about the molecular mechanism of this host-pathogen interaction, and about the rootstock effect on citrus response to CTV infection. In this work, the response to CTV infection has been investigated in tolerant and susceptible scion-rootstock combinations by two-dimensional gel electrophoresis (2DE). A total of 125 protein spots have been found to be differently accumulated and/or phosphorylated between the two rootstock combinations. Downregulation in tolerant plants upon CTV infection was detected for proteins involved in reactive oxygen species (ROS) scavenging and defense response, suggesting a probable acclimation response able to minimize the systemic effects of virus infection. Some of these proteins resulted to be modulated also in absence of virus infection, revealing a rootstock effect on scion proteome modulation. Moreover, the phospho-modulation of proteins involved in ROS scavenging and defense response, further supports their involvement either in scion-rootstock crosstalk or in the establishment of tolerance/susceptibility to CTV infection.

Proteomics, photosynthesis and salt resistance in crops: An integrative view

Salinity is a stressful condition that causes a significant decrease in crop production worldwide. Salt stress affects several photosynthetic reactions, including the modulation of several important proteins. Despite these effects, few molecular-biochemical markers have been identified and evaluated for their importance in improving plant salt resistance. Proteomics is a powerful tool that allows the analysis of multigenic events at the post-translational level that has been widely used to evaluate protein modulation changes in plants exposed to salt stress. However, these studies are frequently fragmented and the results regarding photosynthesis proteins in response to salinity are limited. These constraints could be related to the low number of important photosynthetic proteins differently modulated in response to salinity, as has been commonly revealed by conventional proteomics. In this review, we present an evaluation and perspective on the integrated application of proteomics for the identification of photosynthesis proteins to improve salt resistance. We propose the use of phospho-, thiol- and redox-proteomics, associated with the utilization of isolated chloroplasts or photosynthetic sub-organellar components. This strategy may allow the characterization of essential proteins, providing a better understanding of photosynthesis regulation. Furthermore, this may contribute to the selection of molecular markers to improve salt resistance in crops.

Bottle gourd rootstock-grafting promotes photosynthesis by regulating the stomata and non-stomata performances in leaves of watermelon seedlings under NaCl stress

Previously, we found that the amelioration of photosynthetic capacity by bottle gourd (Lagenaria siceraria Standl.) rootstock in watermelon seedlings (Citrullus lanatus [Thunb.] Mansf.) with salt treatment might be closely related to the enzymes in Calvin cycle such as ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) (Yang et al., 2012). We confirmed this and showed more details in this study that improved photosynthesis of watermelon plants by bottle gourd rootstock was associated with the decreased stomata resistance and the increased photochemical activity and photosynthetic metabolism with or without 100mM NaCl stress for 3 days. The analysis of gas exchange parameters showed that self-grafted plants suffered serious non-stomatal limitation to photosynthesis under salt stress while rootstock-grafted plants were mainly affected by stomata limitation in stress conditions. Further, results showed that NaCl stress markedly reduced the chlorophyll content, damaged the structure of photosynthetic apparatus, and inhibited photochemical activity and CO2 assimilation in self-grafted plants. In contrast, rootstock-grafting increased the chlorophyll content, especially chlorophyll b, and minimized the harmful effects on photosystem II (PSII) reaction center and the thylakoids structure induced by NaCl stress. Furthermore, rootstock-grafting enhanced the content and activity of Rubisco and thus elevated carbon fixation in the leaves of watermelon scions under salt stress. The gene expressions of enzymes related to ribulose-1,5-bisphosphate (RuBP) regeneration were also up-regulated by rootstock and this probably guaranteed the sufficient supply of RuBP for the operation of Calvin cycle in watermelon scions under salt stress. Thus, bottle gourd rootstock promoted photosynthesis by the activation of stomatal and non-stomatal abilities, especially the regulation of a variety of photosynthetic enzymes, including Rubisco in grafted watermelon plants under NaCl stress.

Proteomic study related to vascular connections in watermelon scions grafted onto bottle-gourd rootstock under different light intensities

Although grafting is broadly used in the production of crops, no information is available about the proteins involved in vascular connections between rootstock and scion. Similarly, proteome changes under the light intensities widely used for grafted seedlings are of practical use. The objective of this study was to determine the proteome of vascular connections using watermelon (Citrullus vulgaris Schrad.) ‘Sambok Honey’ and ‘Speed’ as the scion and bottle gourd (Lagenaria siceraria Stanld.) ‘RS Dongjanggun’ as the rootstock grown under different light intensities (25, 50, 75 and 100 μmol m⁻² s⁻¹). Our proteomic analysis revealed 24 and 27 differentially expressed proteins in ‘Sambok Honey’ and ‘Speed’, respectively, under different light intensities. The identified proteins were largely involved in ion binding, amino acid metabolism, transcriptional regulation and defense response. The enhancement of ion-binding, transcriptional regulation, amino acid metabolism, and defense response proteins suggests a strengthening of the connection between the rootstock and scion under high light intensity. Indeed, the accumulation of key enzymes in the biological processes described above appears to play an important role in the vascular connections of grafted seedlings. Moreover, it appears that 100 μmol m⁻² s⁻¹ results in better protein expression responses in grafted seedlings.

Deciphering the protective role of spermidine against saline-alkaline stress at physiological and proteomic levels in tomato

In this research, the protective effect of spermidine (Spd) in mitigating saline-alkaline stress in tomato (Solanum lycopersicum L.) at physiological and proteomic levels were examined. The results showed that saline-alkaline stress induced accumulation of H2O2 and O2(-*), and increased the activities of antioxidase (SOD, CAT, and POD). Spermidine efficiently alleviated the inhibitory role of saline-alkaline on plant growth and inhibited saline-alkaline stress-induced H2O2 and O2(-*) accumulation. Proteomics investigations of the leaves of tomato seedlings, responding to a 75 mM saline-alkaline solution and 0.25 mM Spd, were performed. Maps of the proteome of leaf extracts were obtained by two-dimensional gel electrophoresis. An average of 49, 47 and 34 spots, which appeared repeatedly and that significantly altered the relative amounts of polypeptides by more than twofold, were detected for seedlings treated with saline-alkaline solution (S) compared to normal solution (CK), saline-alkaline plus spermidine (MS) compared to CK, or S versus MS, respectively. Thirty-nine of these proteins were identified by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry and were classified into five functional categories, including energy and metabolism, signal transduction, amino acid metabolism, protein metabolism, and stress-defense response. Proteomics analysis coupled with bioinformatics indicated that Spd treatment helps tomato seedlings combat saline-alkaline stress by modulating the defense mechanism of plants and activating cellular detoxification, which protect plants from oxidative damage induced by saline-alkaline stress.

Effects of grafting with pumpkin rootstock on carbohydrate metabolism in cucumber seedlings under Ca(NO3)2 stress

This study investigated the effects of grafting on the carbohydrate status and the enzymes of carbohydrate metabolism in self-grafted and grafted cucumber seedlings using the salt-tolerant pumpkin rootstock 'Qingzhen 1' (Cucurbita maxima × Cucurbita moschata) under 80 mM Ca(NO3)2 stress for 6 d. The growth of self-grafted seedlings was significantly inhibited after the treatment of Ca(NO3)2 stress, whereas the inhibition of growth was alleviated in pumpkin rootstock-grafted seedlings. Ca(NO3)2 stress increased the contents of the total soluble sugar, sucrose and fructose, but decreased the starch content in rootstock-grafted leaves. However, compared with self-grafted plants, rootstock-grafted seedlings were observed with a higher content of sucrose and total soluble sugar (TSS) under salt stress. Rootstock-grafted seedlings exhibited higher activities of acid invertase (AI), neutral invertase (NI) and phosphate sucrose synthase (SPS) of sucrose metabolism in leaves than that of self-grafted seedlings under salinity. Moreover, the activities of fructokinase (FK), hexokinase (HK), phosphofructokinase (PFK) and pyruvate kinase (PK) of glycolysis were maintained at a higher level in leaves of rootstock-grafted seedlings after Ca(NO3)2 stress. Additionally, rootstock-grafting decrease the high percentage enhancement of key enzymes gene expression in glycolysis in the scion leaves of cucumber seedlings induced by salt stress. These results suggest that the rootstock-grafting improved salt tolerance, which might play a role in elevated sucrose metabolism and a glycolytic pathway regulated by the pumpkin rootstock.

Proteomic analysis of salt stress and recovery in leaves of Vigna unguiculata cultivars differing in salt tolerance

Cowpea cultivars differing in salt tolerance reveal differences in protein profiles and adopt different strategies to overcome salt stress. Salt-tolerant cultivar shows induction of proteins related to photosynthesis and energy metabolism. Salinity is a major abiotic stress affecting plant cultivation and productivity. The objective of this study was to examine differential proteomic responses to salt stress in leaves of the cowpea cultivars Pitiúba (salt tolerant) and TVu 2331 (salt sensitive). Plants of both cultivars were subjected to salt stress (75 mM NaCl) followed by a recovery period of 5 days. Proteins extracted from leaves of both cultivars were analyzed by two-dimensional electrophoresis (2-DE) under salt stress and after recovery. In total, 22 proteins differentially regulated by both salt and recovery were identified by LC-ESI-MS/MS. Our current proteome data revealed that cowpea cultivars adopted different strategies to overcome salt stress. For the salt-tolerant cultivar (Pitiúba), increase in abundance of proteins involved in photosynthesis and energy metabolism, such as rubisco activase, ribulose-5-phosphate kinase (Ru5PK) (EC 2.7.1.19), glycine decarboxylase (EC 1.4.4.2) and oxygen-evolving enhancer (OEE) protein 2, was observed. However, these vital metabolic processes were more profoundly affected in salt-sensitive cultivar (TVu), as indicated by the down-regulation of OEE protein 1, Mn-stabilizing protein-II, carbonic anhydrase (EC 4.2.1.1) and Rubisco (EC 4.1.1.39), leading to energy reduction and a decline in plant growth. Other proteins differentially regulated in both cultivars corresponded to different physiological responses. Overall, our results provide information that could lead to a better understanding of the molecular basis of salt tolerance and sensitivity in cowpea plants.

Root exudates from grafted-root watermelon showed a certain contribution in inhibiting Fusarium oxysporum f. sp. niveum

Grafting watermelon onto bottle gourd rootstock is commonly used method to generate resistance to Fusarium oxysporum f. sp. niveum (FON), but knowledge of the effect of the root exudates of grafted watermelon on this soil-borne pathogen in rhizosphere remains limited. To investigate the root exudate profiles of the own-root bottle gourd, grafted-root watermelon and own-root watermelon, recirculating hydroponic culture system was developed to continuously trap these root exudates. Both conidial germination and growth of FON were significantly decreased in the presence of root exudates from the grafted-root watermelon compared with the own-root watermelon. HPLC analysis revealed that the composition of the root exudates released by the grafted-root watermelon differed not only from the own-root watermelon but also from the bottle gourd rootstock plants. We identified salicylic acid in all 3 root exudates, chlorogenic acid and caffeic acid in root exudates from own-root bottle gourd and grafted-root watermelon but not own-root watermelon, and abundant cinnamic acid only in own-root watermelon root exudates. The chlorogenic and caffeic acid were candidates for potentiating the enhanced resistance of the grafted watermelon to FON, therefore we tested the effects of the two compounds on the conidial germination and growth of FON. Both phenolic acids inhibited FON conidial germination and growth in a dose-dependent manner, and FON was much more susceptible to chlorogenic acid than to caffeic acid. In conclusion, the key factor in attaining the resistance to Fusarium wilt is grafting on the non-host root stock, however, the root exudates profile also showed some contribution in inhibiting FON. These results will help to better clarify the disease resistance mechanisms of grafted-root watermelon based on plant-microbe communication and will guide the improvement of strategies against Fusarium-mediated wilt of watermelon plants.

Bottle gourd rootstock-grafting affects nitrogen metabolism in NaCl-stressed watermelon leaves and enhances short-term salt tolerance

The plant growth, nitrogen absorption, and assimilation in watermelon (Citrullus lanatus [Thunb.] Mansf.) were investigated in self-grafted and grafted seedlings using the salt-tolerant bottle gourd rootstock Chaofeng Kangshengwang (Lagenaria siceraria Standl.) exposed to 100mM NaCl for 3d. The biomass and NO3(-) uptake rate were significantly increased by rootstock while these values were remarkably decreased by salt stress. However, compared with self-grafted plants, rootstock-grafted plants showed higher salt tolerance with higher biomass and NO3(-) uptake rate under salt stress. Salinity induced strong accumulation of nitrate, ammonium and protein contents and a significant decrease of nitrogen content and the activities of nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), and glutamate synthase (GOGAT) in leaves of self-grafted seedlings. In contrast, salt stress caused a remarkable decrease in nitrate content and the activities of GS and GOGAT, and a significant increase of ammonium, protein, and nitrogen contents and NR activity, in leaves of rootstock-grafted seedlings. Compared with that of self-grafted seedlings, the ammonium content in leaves of rootstock-grafted seedlings was much lower under salt stress. Glutamate dehydrogenase (GDH) activity was notably enhanced in leaves of rootstock-grafted seedlings, whereas it was significantly inhibited in leaves of self-grafted seedlings, under salinity stress. Three GDH isozymes were isolated by native gel electrophoresis and their expressions were greatly enhanced in leaves of rootstock-grafted seedlings than those of self-grafted seedlings under both normal and salt-stress conditions. These results indicated that the salt tolerance of rootstock-grafted seedlings might (be enhanced) owing to the higher nitrogen absorption and the higher activities of enzymes for nitrogen assimilation induced by the rootstock. Furthermore, the detoxification of ammonium by GDH when the GS/GOGAT pathway was inhibited under salt stress might play an important role in the release of salt stress in rootstock-grafted seedlings.