Resistant Watermelon Research Articles

Zucchini yellow mosaic virus-induced hypersensitive response is associated with pathogenesis-related 1 protein expression and confers resistance in watermelon.

The pathogenesis-related 1 gene of watermelon responds to the infection of ZYMV and contributes to the resistance of its host. Zucchini yellow mosaic virus (ZYMV; family Potyviridae) is a single-stranded positive-sense RNA virus that is a serious threat to cucurbits. Previously, we observed a hypersensitivity response (HR) in the systemic leaves of the 938-16-B watermelon line infected with ZYMV, distinct from the typical HR at infected sites. In this study, we confirmed that ZYMV accumulation in 938-16-B was significantly lower than in the susceptible line H1. Upon inoculation, the entry of ZYMV-eGFP into mesophyll cells is restricted into necrotic spots in leaves, indicating that resistance to ZYMV in 938-16-B is linked to the HR. Further, grafting experiments between 938-16-B and susceptible varieties were performed, and revealed an HR induction in susceptible varieties, suggesting the transfer of resistance signal(s) from 938-16-B to susceptible varieties. Through RNA-sequencing and proteomics analyses of the H1 scions on 938-16-B rootstock, a pathogenesis-related 1 (ClPR1) gene was identified. Specifically, ClPR1 expression is unique to ZYMV-infected 938-16-B. Repression of the expression of ClPR1 prevents an HR in 938-16-B. Conversely, overexpression of ClPR1 in susceptible varieties significantly reduces ZYMV accumulation, but an HR was not induced in susceptible line. Besides the virus, jasmonic acid (JA) can also trigger an HR in 938-16-B. Intriguingly, the expression of ClPR1 (Cla97C02G034020) is induced in both of 938-16-B and H1 by MeJA, rather than salicylic acid. These results suggest that HR is associated with the expression of ClPR1 and contributes to resistance to ZYMV in 938-16-B.

Identification of Genomic Regions Associated with Powdery Mildew Resistance in Watermelon through Genome-Wide Association Study.

Watermelon (Citrullus spp.) is an economically important crop globally, but it is susceptible to various diseases, including powdery mildew. Previous studies have identified genetic factors associated with powdery mildew resistance. However, further research using diverse genetic approaches is necessary to elucidate the underlying genetic mechanisms of this resistance. In this study, the germplasm collection comprising highly homozygous inbred lines was employed, which enabled the accumulation of consistent data and improved the reliability of the genome-wide association study (GWAS) findings. Our investigation identified two significant single-nucleotide polymorphisms (SNPs), pm2.1 and pm3.1, which were strongly associated with disease resistance. Moreover, several candidate genes were revealed within the linkage disequilibrium (LD) blocks surrounding the significant SNPs. In conclusion, the identification of significant SNPs and their additive effects, combined with the discovery of relevant candidate genes, expanded our understanding of the genetic basis of disease resistance and can pave the way for the development of more resilient watermelon cultivars through marker-assisted selection.

Synthetic community derived from grafted watermelon rhizosphere provides protection for ungrafted watermelon against Fusarium oxysporum via microbial synergistic effects

BackgroundPlant microbiota contributes to plant growth and health, including enhancing plant resistance to various diseases. Despite remarkable progress in understanding diseases resistance in plants, the precise role of rhizosphere microbiota in enhancing watermelon resistance against soil-borne diseases remains unclear. Here, we constructed a synthetic community (SynCom) of 16 core bacterial strains obtained from the rhizosphere of grafted watermelon plants. We further simplified SynCom and investigated the role of bacteria with synergistic interactions in promoting plant growth through a simple synthetic community.ResultsOur results demonstrated that the SynCom significantly enhanced the growth and disease resistance of ungrafted watermelon grown in non-sterile soil. Furthermore, analysis of the amplicon and metagenome data revealed the pivotal role of Pseudomonas in enhancing plant health, as evidenced by a significant increase in the relative abundance and biofilm-forming pathways of Pseudomonas post-SynCom inoculation. Based on in vitro co-culture experiments and bacterial metabolomic analysis, we selected Pseudomonas along with seven other members of the SynCom that exhibited synergistic effects with Pseudomonas. It enabled us to further refine the initially constructed SynCom into a simplified SynCom comprising the eight selected bacterial species. Notably, the plant-promoting effects of simplified SynCom were similar to those of the initial SynCom. Furthermore, the simplified SynCom protected plants through synergistic effects of bacteria.ConclusionsOur findings suggest that the SynCom proliferate in the rhizosphere and mitigate soil-borne diseases through microbial synergistic interactions, highlighting the potential of synergistic effects between microorganisms in enhancing plant health. This study provides a novel insight into using the functional SynCom as a promising solution for sustainable agriculture.6UoEKpA3MAKccxe2Gy9v1zVideo

Sodium Tetraborate Induces Systemic Resistance in Watermelon against Stagonosporopsis cucurbitacearum

Imbibing watermelon seeds in 1 mM sodium tetraborate (Na2B4O7) for 24 h systemically protected plants against foliar infection by Stagonosporopsis cucurbitacearum in detached leaves and under greenhouse conditions. The treatment resulted in both a reduction in the overall percentage of leaf infection as well as in the size of lesions. Studies of the mechanisms by which Na2B4O7 protected watermelon showed that there was no direct effect on the S. cucurbitacearum mycelium growth in vitro. On the other hand, plants raised from seeds primed with Na2B4O7 showed a higher frequency of fluorescent epidermal cells compared to the plants treated with water. This indicates that a higher number of cells expressed the hypersensitive response after Na2B4O7 priming. In addition, there was an increase in peroxidase activity and an enhanced accumulation of a 45 kDa acidic peroxidase isoform during the early stages of infection in plants treated with Na2B4O7 compared to plants treated with water and this was positively correlated to the reduction of leaf infection caused by the pathogen. These results indicate that Na2B4O7 is able to induce systemic resistance in watermelon against S. cucurbitacearum by activating the hypersensitive reaction at penetration sites, increasing peroxidase activity and altering the peroxidase isozyme profile. Although each individual response may only have had a minor effect, their combined effects had a reducing effect on the disease.

Joint transcriptomic and metabolomic analysis provides new insights into drought resistance in watermelon (Citrullus lanatus).

Watermelon is an annual vine of the family Cucurbitaceae. Watermelon plants produce a fruit that people love and have important nutritional and economic value. With global warming and deterioration of the ecological environment, abiotic stresses, including drought, have become important factors that impact the yield and quality of watermelon plants. Previous research on watermelon drought resistance has included analyzing homologous genes based on known drought-responsive genes and pathways in other species. However, identifying key pathways and genes involved in watermelon drought resistance through high-throughput omics methods is particularly important. In this study, RNA-seq and metabolomic analysis were performed on watermelon plants at five time points (0h, 1h, 6h, 12h and 24h) before and after drought stress. Transcriptomic analysis revealed 7829 differentially expressed genes (DEGs) at the five time points. The DEGs were grouped into five clusters using the k-means clustering algorithm. The functional category for each cluster was annotated based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) database; different clusters were associated with different time points after stress. A total of 949 metabolites were divided into 10 categories, with lipids and lipid-like molecules accounting for the most metabolites. Differential expression analysis revealed 22 differentially regulated metabolites (DRMs) among the five time points. Through joint analysis of RNA-seq and metabolome data, the 6-h period was identified as the critical period for watermelon drought resistance, and the starch and sucrose metabolism, plant hormone signal transduction and photosynthesis pathways were identified as important regulatory pathways involved in watermelon drought resistance. In addition, 15 candidate genes associated with watermelon drought resistance were identified through joint RNA-seq and metabolome analysis combined with weighted correlation network analysis (WGCNA). Four of these genes encode transcription factors, including bHLH (Cla97C03G068160), MYB (Cla97C01G002440), HSP (Cla97C02G033390) and GRF (Cla97C02G042620), one key gene in the ABA pathway, SnRK2-4 (Cla97C10G186750), and the GP-2 gene (Cla97C05G105810), which is involved in the starch and sucrose metabolism pathway. In summary, our study provides a theoretical basis for elucidating the molecular mechanisms underlying drought resistance in watermelon plants and provides new genetic resources for the study of drought resistance in this crop.

Evaluation of differential miRNA expression between Fusarium wilt-resistant and -susceptible watermelon varieties

Watermelon (Citrullus lanatus) has been cultivated for nearly one thousand years and remains a commercially important crop. However, continuous planting often leads to the aggravation of many diseases. Watermelon Fusarium wilt (FW) is one of them, which is a fungal soil-borne disease caused by Fusarium oxysporum f. sp. niveum (Fon) that damages plant roots by invading vascular bundles. Therefore, it is imperative to comprehensively characterize the mechanism mediating disease resistance or susceptibility, as well as identify effective resistance genes, in order to breed improved disease-resistant watermelon varieties. miRNAs are endogenous non-coding RNAs that are widely involved in plant growth, development, metabolism, transport, stress response, and pathogen defense. However, there are few reports of disease-associated miRNAs, or their mechanisms of action, in Cucurbitaceae crops. In this study, the roots of both Fon-susceptible and resistant watermelon varieties were infected with GFP-labeled Fon. Three key infection time points were identified. Illumina sequencing was used to obtain Fon-resistance related miRNAs, and degradome sequencing was used to obtain miRNA target genes. A total of 23 differentially expressed miRNAs were identified at the three key infection time points, the degradation group sequencing found their only target gene. The results of this study provide a theoretical basis for studies of miRNA function and regulation during the interaction between Fon and watermelon plants and clues for the screening of watermelon resistance genes. In addition, this information can be used to breed disease-resistant watermelon varieties.

Identification of watermelon genes involved in the ZYMV interaction through a miRNA bio-informatics analysis and characterization of ATRIP and RBOHB

Our objective was to identify differentially expressed watermelon genes in its interaction with ZYMV through a bioinformatics analysis of differentially expressed miRNAs. Small RNAseq data analysis of healthy and ZYMV-infected watermelon (21 dpi) identified 353 miRNAs from which 22 known and 331 new miRNAs. Important information about their precursors, their length, the loci of which they originated on watermelon genome are provided. The ZYMV genome could be a target for mir396a-3p, miR8706a, and miR1886i-5p from the miRbase, but none of them was identified in the watermelon miRNAome. Furthermore, watermelon miRNAome does not contain a miRNA targeting ZYMV genome with an expectation score ≤3.5. In healthy watermelon bioinformatically predicted targets of 32 miRNAs were 34 resistance genes, as CC-NBS-LRR, TIR-NBS-LRR, TIR-NBS. For nine differentially expressed miRNAs the respective target genes (10 in total) were bioinformatically predicted. For cla-new_miR307 (upregulated upon ZYMV infection) and cla-miR166h-3p (downregulated upon ZYMV infection) the targets were predicted to be ClaATRIP and ClaRBOHB, respectively, with ClaATRIP downregulated and ClaRBOHB upregulated upon ZYMV infection. ALSV-mediated VIGS of ClaATRIP rendered watermelon plants more resistant to ZYMV, whereas VIGS of ClaRBOHB resulted in higher levels of ZYMV titer in watermelon. These data suggest that ClaATRIP and ClaRBOHB are a susceptibility and resistant gene, respectively. Our results provide new insights in watermelon miRNAome and could propose new strategies for generating resistant watermelon to ZYMV.

Mapping and validation of Fusarium wilt race 2 resistance QTL from Citrullus amarus line USVL246-FR2

Key messageFon race 2 resistant QTLs were identified on chromosomes 8 and 9. Families homozygous for resistance alleles at a haplotype of three KASP markers had 42% lower disease severity than those with susceptible alleles in an independent, interspecific validation population confirming their utility for introgression of Fusarium wilt resistance.Fusarium oxysporum f. sp. niveum (Fon) race 2 causes Fusarium wilt in watermelon and threatens watermelon production worldwide. Chemical management options are not effective, and no resistant edible watermelon cultivars have been released. Implementation of marker-assisted selection to develop resistant cultivars requires identifying sources of resistance and the underlying quantitative trait loci (QTL), developing molecular markers associated with the QTL, and validating marker-phenotype associations with an independent population. An intraspecific Citrullus amarus recombinant inbred line population from a cross of resistant USVL246-FR2 and susceptible USVL114 was used for mapping Fon race 2 resistance QTL. KASP markers were developed (N = 51) for the major QTL on chromosome 9 and minor QTL on chromosomes 1, 6, and 8. An interspecific F2:3 population was developed from resistance donor USVL246-FR2 (C. amarus) and a susceptible cultivar ‘Sugar Baby’ (Citrullus lanatus) to validate the utility of the markers for introgression of resistance from the wild crop relative into cultivated watermelon. Only 16 KASP markers segregated in the interspecific C. amarus/lanatus validation population. Four markers showed significant differences in the separation of genotypes based on family-mean disease severity, but together explained only 16% of the phenotypic variance. Genotypes that inherited homozygous resistant parental alleles at three KASP markers had 42% lower family-mean disease severity than homozygous susceptible genotypes. Thus, haplotype analysis was more effective at predicting the mean disease severity of families than single markers. The haplotype identified in this study will be valuable for developing Fon race 2 resistant watermelon cultivars.

Fine mapping of ClLOX, a QTL for powdery mildew resistance in watermelon (Citrullus lanatus L.).

ClLOX, is located on chromosome 2 and encodes a lipoxygenase gene, which induced watermelon powdery mildew resistance by inhibiting pathogen spread. Powdery mildew is one of the most severe fungal diseases reducing yield and quality of watermelon (Citrullus lanatus L.) and other cucurbit crops. Genes responsible for powdery mildew resistance in watermelon are highly valuable. In this study, we first identified the QTL pm-lox for powdery mildew resistance in watermelon, located within a 0.93Mb interval of chromosome 2, via XP-GWAS method using two F2 populations. The F2:3 families from one of the F2 populations were then used for fine-mapping the pm-lox locus into a 9,883bp physical region between 29,581,906 and 29,591,789, containing only two annotated genes. Of these, only ClG42_02g0161300 showed a significant differential expression between the resistant and susceptible lines after powdery mildew inoculation based on RNA sequencing (RNA-seq) and qRT-PCR analysis, and is designated ClLOX. Derived Cleaved Amplified Polymorphic Sequence (dCAPs) markers were developed and validated. In addition, our tests showed that the resistance was anti-spread rather than anti-infection of the pathogen. This study identified a new resistance gene (ClLOX), provided insights into the mechanism of powdery mildew resistance, and developed a molecular marker for watermelon breeding.

Genome-Wide Identification and Expression Analysis of Chitinase Genes in Watermelon under Abiotic Stimuli and Fusarium oxysporum Infection.

Chitinases, which catalyze the hydrolysis of chitin, the primary components of fungal cell walls, play key roles in defense responses, symbiotic associations, plant growth, and stress tolerance. In this study, 23 chitinase genes were identified in watermelon (Citrullus lanatus [Thunb.]) and classified into five classes through homology search and phylogenetic analysis. The genes with similar exon-intron structures and conserved domains were clustered into the same class. The putative cis-elements involved in the responses to phytohormone, stress, and plant development were identified in their promoter regions. A tissue-specific expression analysis showed that the ClChi genes were primarily expressed in the roots (52.17%), leaves (26.09%), and flowers (34.78%). Moreover, qRT-PCR results indicate that ClChis play multifaceted roles in the interaction between plant/environment. More ClChi members were induced by Race 2 of Fusarium oxysporum f. sp. niveum, and eight genes were expressed at higher levels on the seventh day after inoculation with Races 1 and 2, suggesting that these genes play a key role in the resistance of watermelon to Fusarium wilt. Collectively, these results improve knowledge of the chitinase gene family in watermelon species and help to elucidate the roles played by chitinases in the responses of watermelon to various stresses.

Bacillus velezensis WB induces systemic resistance in watermelon against Fusarium wilt.

Our previous findings indicated that Bacillus velezensis WB could control Fusarium wilt by changing the structure of the microbial community in the watermelon rhizosphere. However, there are few studies on its mechanism in the pathogen resistance of watermelon. Therefore, in this study, we determined the mechanism of B. velezensis WB-induced systemic resistance in watermelon against Fusarium wilt through glasshouse pot experiments. The results showed that B. velezensis WB significantly reduced the incidence and disease index of Fusarium wilt in watermelon. B. velezensis WB can enhance the basal immunity of watermelon plants by: increasing the activity of phenylalanine ammonia-lyase (PAL), peroxidase (POD), superoxide dismutase (SOD) and β-1,3-glucanase; accumulating lignin, salicylic acid (SA) and jasmonic acid (JA); reducing malondialdehyde (MDA) concentrations; and inducing callus deposition in watermelon plant cells. RNA-seq analysis showed that 846 watermelon genes were upregulated and 612 watermelon genes were downregulated in the WF treatment. This process led to the activation of watermelon genes associated with auxin, gibberellin, SA, ethylene and JA, and the expression of genes in the phenylalanine biosynthetic pathway was upregulated. In addition, transcription factors involved in plant resistance to pathogens, such as MYB, NAC and WRKY, were induced. Gene correlation analysis showed that Cla97C10G195840 and Cla97C02G049930 in the phenylalanine biosynthetic pathway, and Cla97C02G041360 and Cla97C10G197290 in the plant hormone signal transduction pathway showed strong correlations with other genes. Our results indicated that B. velezensis WB is capable of inducing systemic resistance in watermelon against Fusarium wilt. © 2023 Society of Chemical Industry.

Control effect of root exudates from mycorrhizal watermelon seedlings on Fusarium wilt and the bacterial community in continuously cropped soil.

Watermelon (Citrullus lanatus) is susceptible to wilt disease caused by Fusarium oxysporum f. sp niveum (FON). AMF colonization alleviates watermelon wilt and regulates the composition of root exudates, but the effects of mycorrhizal watermelon root exudates on watermelon Fusarium wilt is not well understood. Root exudates of watermelon inoculated with AMF (Funeliformis mosseae or Glomus versiformme) were collected in this study. Then the root exudates of control plants and mycorrhizal plants were used to irrigate watermelon in continuous cropping soil, respectively. Meanwhile, the watermelon growth, antioxidant enzyme activity, rhizosphere soil enzyme activities and bacterial community composition, as well as the control effect on FON were analyzed. The results indicated that mycorrhizal watermelon root exudates promoted the growth of watermelon seedlings and increased soil enzyme activities, actinomyces, and the quantity of bacteria in rhizosphere soil. The proportion of Proteobacteria and Bacteroides was decreased, and the proportion of Actinobacteria, Firmicutes, and Chloroflexi in rhizosphere soil was increased when the seedlings were watered with high concentrations of mycorrhizal root exudates. The dominant bacterial genera in rhizosphere soil were Kaistobacter, Rhodanobacter, Thermomonas, Devosia, and Bacillus. The root exudates of mycorrhizal watermelon could reduce the disease index of Fusarium wilt by 6.7-30%, and five ml/L of watermelon root exudates inoculated with F. mosseae had the strongest inhibitory effect on watermelon Fusarium wilt. Our results suggest mycorrhizal watermelon root exudates changed the composition of bacteria and soil enzyme activities in rhizosphere soil, which increase the resistance of watermelon to Fusarium wilt and promoted the growth of plants in continuous cropping soil.

Methyl jasmonate‐dependent hydrogen sulfide mediates melatonin‐induced aphid resistance of watermelon

AbstractAs one of the most destructive pests, aphids cause significant damage on various agricultural and horticultural crops. Recently, melatonin has been shown to enhance plant resistance to aphids; however, the underlying mechanisms remain unclear. In this study, our results showed that melatonin, MeJA, and H2S enhanced aphid resistance of watermelon in a dose‐dependent manner, accompanied by increases in the defense‐related enzyme activities and lignin accumulation. On the plants pretreated with 100 μM melatonin, 100 μM MeJA, and 50 μM NaHS, the numbers of aphids were 86.0%, 59.6%, and 47.9% lower than that on control plants, respectively, after aphid infestation for 7 days. Melatonin application induced MeJA and H2S accumulation in response to aphid infestation, while inhibition of MeJA and H2S accumulation attenuated or abolished melatonin‐induced defense response and aphid resistance, suggestive of the involvement of MeJA and H2S in melatonin‐induced aphid resistance. MeJA also increased H2S accumulation, but inhibition of MeJA biosynthesis prevented melatonin‐enhanced H2S accumulation, suggesting that MeJA mediates melatonin‐induced H2S accumulation. Furthermore, inhibition of H2S production attenuated MeJA‐induced defense response and aphid resistance. Taken together, the current study reveals a novel mechanism in which MeJA‐dependent H2S signaling is involved in melatonin‐induced defense response and subsequent aphid resistance. The increasing concern to minimize the use of pesticides and to switch onto sustainable and natural control strategies indicates the great exploitation of such a mechanism in combating aphids.

Distinct Genomic Loci Underlie Quantitative Resistance to Meloidogyne enterolobii Galling and Reproduction in Citrullus amarus.

Meloidogyne enterolobii is a virulent species of root-knot nematode that threatens watermelon (Citrullus lanatus) production in the southeastern United States. There are no known sources of root-knot nematode resistance in cultivated C. lanatus. Specific genotypes of a wild watermelon relative, C. amarus, are resistant against M. incognita but the genetics that underly this resistance are still unknown and it is not clear that this same resistance will be effective against M. enterolobii. To identify and characterize new sources of resistance to M. enterolobii, we screened 108 diverse C. amarus lines alongside a susceptible C. lanatus cultivar (Charleston Gray) for resistance against M. enterolobii. Different C. amarus genotypes ranged from resistant to susceptible for the three resistance phenotypes measured; mean percent root system galled ranged from 10 to 73%, mean egg mass counts ranged from 0.3 to 64.5, and mean eggs per gram of root ranged from 326 to 146,160. We used each of these three resistance phenotypes combined with whole-genome resequencing data to conduct a genome-wide association scan that identified significant associations between M. enterolobii resistance and 11 single-nucleotide polymorphisms (SNPs) within the C. amarus genome. Interestingly, SNPs associated with reduced galling and egg masses were located within a single quantitative trait locus (QTL) on chromosome Ca03, while reductions in nematode eggs per gram of root were associated with separate QTL on chromosomes Ca04 and Ca08. The results of this study suggest that multiple genes are involved with M. enterolobii resistance in C. amarus and the SNPs identified will assist with efforts to breed for M. enterolobii resistance in watermelon.

Transcriptomic and metabolic profiling of watermelon uncovers the role of salicylic acid and flavonoids in the resistance to cucumber green mottle mosaic virus.

Understanding the mechanisms underlying plant resistance to virus infections is crucial for viral disease management in agriculture. However, the defense mechanism of watermelon (Citrullus lanatus) against cucumber green mottle mosaic virus (CGMMV) infection remains largely unknown. In this study, we performed transcriptomic, metabolomic, and phytohormone analyses of a CGMMV susceptible watermelon cultivar 'Zhengkang No.2' ('ZK') and a CGMMV resistant wild watermelon accession PI 220778 (PI) to identify the key regulatory genes, metabolites, and phytohormones responsible for CGMMV resistance. We then tested several phytohormones and metabolites for their roles in watermelon CGMMV resistance via foliar application, followed by CGMMV inoculation. Several phenylpropanoid metabolism-associated genes and metabolites, especially those involved in the flavonoid biosynthesis pathway, were found to be significantly enriched in the CGMMV-infected PI plants compared with the CGMMV-infected 'ZK' plants. We also identified a gene encoding UDP-glycosyltransferase (UGT) that is involved in kaempferol-3-O-sophoroside biosynthesis and controls disease resistance, as well as plant height. Additionally, salicylic acid (SA) biogenesis increased in the CGMMV-infected 'ZK' plants, resulting in the activation of a downstream signaling cascade. SA levels in the tested watermelon plants correlated with that of total flavonoids, and SA pre-treatment up-regulated the expression of flavonoid biosynthesis genes, thus increasing the total flavonoid content. Furthermore, application of exogenous SA or flavonoids extracted from watermelon leaves suppressed CGMMV infection. In summary, our study demonstrates the role of SA-induced flavonoid biosynthesis in plant development and CGMMV resistance, which could be used to breed for CGMMV resistance in watermelon.

Genome-Wide Association Study of Resistance to Pseudomonas syringae in the USDA Collection of Citrullus amarus.

Pseudomonas leaf spot (PLS), caused by Pseudomonas syringae pv. syringae, is an emerging disease of watermelon in the United States with the potential to severely reduce yield under humid conditions. The genetic basis of resistance to this disease is not known and no resistant germplasm is available. Because Citrullus amarus is an important reservoir of resistance genes for the cultivated watermelon, C. lanatus, we screened the United States Department of Agriculture plant introduction collection of C. amarus for resistance to PLS. Accessions (n = 117) were phenotyped for their level of resistance to PLS in two separate tests. Accession means of percent leaf area affected ranged from 1.5 to 99.4%. The broad-sense heritability for the trait was 0.51. Whole-genome resequencing generated 2,126,759 single-nucleotide polymorphisms (SNPs) which were used to perform a genome-wide association study (GWAS) aimed at discovering molecular markers for resistance. Three different models-BLINK, FarmCPU, and MLM-were included in the GWAS analyses. BLINK and FarmCPU, which are multilocus models, found eight SNPs, located on chromosomes Ca01, Ca05, Ca06, Ca08, and Ca10, that were significantly associated with resistance to PLS. Two of these SNPs were found by both BLINK and FarmCPU. The MLM model did not detect any significant associations. BLINK and FarmCPU estimated an explained phenotypic variance of 43.6 and 28.5%, respectively, for SNP S6_19327000 and 25.0 and 26.0%, respectively, for SNP S1_33362258, the two most significant SNPs found. In total, 43 candidate genes with known involvement in disease resistance were discovered within the genomic intervals of seven of the eight peak SNPs. Eleven of the candidate genes that were found have been reported to be involved in resistance to P. syringae in other plant species. Two significant SNPs were within resistance genes previously documented to play important roles of plant resistance specific to P. syringae in other pathosystems. The SNPs identified in this study will be instrumental in finding causal genes involved in PLS resistance in watermelon and developing resistant germplasm through breeding.

Evaluation of rootstocks resistant to gummy stem blight and their effect on the fruit yield and quality traits of grafted watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai)

ABSTRACT Three gummy stem blight (GSB) resistant watermelon (Citrullus lanatus var. citroides) genotypes (IIHR-82, IIHR-617 and BIL-53), two resistant bottle gourd (Lagenaria siceraria) genotypes (BG-95, BG-77-6-1) and three hybrid combinations based on watermelon genotypes (IIHR-617 × Arka Manik, IIHR-82 × Arka Manik and IIHR-82 × IIHR-617) were evaluated as rootstocks for yield, quality and GSB incidence in a sick plot during spring 2019, spring 2020 and spring 2021 at Indian Institute of Horticultural Research, Bangalore. The commercial watermelon hybrid NS-295 was used as a scion for grafting onto these rootstocks. Significant differences were observed among the grafted entries and checks in terms of disease incidence (PDI). Among agronomic traits, grafting significantly affected plant growth, earliness and yield in a desirable direction. The grafted scion onto the hybrid rootstocks and BG-95 could maintain its oblong fruit shape compared to those onto IIHR-82 and BIL-53, whereas BG-77-6-1 produced spherical fruits. Quality traits showed an improvement in terms of total soluble solids (TSS), flesh colour, total carotenoids and lycopene content among the grafted entries as compared to non-grafted controls. Overall, the use of hybrid rootstocks i.e. IIHR-82 × IIHR-617 and IIHR-82 × Arka Manik was found to be advantageous in managing GSB apart from imparting desirable agronomic and fruit quality traits.

Priming Watermelon Resistance by Activating Physiological Response and Defense Gene Expression to Alleviate Fusarium Wilt in Wheat-Watermelon Intercropping

Wheat intercropping in watermelon could provide relief from the occurrence of Fusarium wilting of watermelon, a severe soil-borne disease caused by the fungus Fusarium oxysporum f. sp. niveum (FON). The current study aims to investigate the effect of root exudates from three wheat cultivars and one watermelon cultivar on the growth of FON and the responses of Fusarium wilt in watermelon to intercropping with wheat. The results revealed the contrasting effects of root exudates on the mycelial growth of FON; the wheat root exudates inhibited the mycelial growth of FON, and watermelon root exudates promoted the mycelial growth of FON. Watermelon plants suffered less Fusarium wilt in the intercropping system than in the monocropping system. Wheat intercropping reduced the incidence of Fusarium wilt in watermelon, and this effect was associated with the role of wheat root exudates that inhibited the growth of FON. Malondialdehyde (MDA) contents decreased in the intercropping system compared with the monocropping system after FON inoculation. The catalase (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), and polyphenol oxidase (PPO) activities, and total phenolics and flavonoid contents in the roots of watermelon in the intercropping system were significantly higher than those in the monocropping system. Real-time PCR analysis showed that ClCAT, ClSOD, ClAPX, and ClPPO defensive enzymes and ClPDF2.1 and ClPDF2.4 defensin-like gene expression were significantly induced during the early stage after FON inoculation in the intercropping system compared to the monocropping system, while peroxidases did not show a significant response to FON infection. It is suggested that intercropping with wheat alleviates Fusarium wilt of watermelon by reducing the population of FON in rhizospheric soil and activating physiological responses and defense gene expression to protect watermelon from FON infection and improve the resistance of watermelon to FON in the intercropping system.

QTL associated with resistance to Stagonosporopsis citrulli in Citrullus amarus

Gummy stem blight (GSB) is a fungal disease affecting cucurbit crops, including watermelon (Citrullus lanatus), leading to significant yield losses. The disease is caused by three Stagonosporopsis species, of which Stagonosporopsis citrulli is the most common in the southeastern United States. Currently no gummy stem blight-resistant watermelon cultivars are available to growers. In this study, QTL-seq in an interspecific population developed from Sugar Baby × PI 189225 (Citrullus amarus) identified QTL on chromosomes 2, 5, 9 and 11. A novel QTL on chromosome 5 (Qgsb5.2) associated with resistance to S. citrulli (PVE = 13.3%) was confirmed by genetic mapping. KASP marker assays were developed for selection of Qgsb5.2 to allow breeders to track the allele contributing resistance to GSB, reducing the need for laborious phenotyping. Pyramiding different GSB resistance QTL could be a useful strategy to develop GSB resistant watermelon cultivars.

Reactive oxygen species-related genes participate in resistance to cucumber green mottle mosaic virus infection regulated by boron in Nicotiana benthamiana and watermelon.

Cucumber green mottle mosaic virus (CGMMV) infection causes acidification and rot of watermelon flesh, resulting in serious economic losses. It is widely reported the interaction relationship between boron and reactive oxygen species (ROS) in regulating normal growth and disease resistance in plants. Our previous results demonstrated that exogenous boron could improve watermelon resistance to CGMMV infection. However, the roles of ROS-related genes regulated by boron in resistance to CGMMV infection are unclear. Here, we demonstrated that CGMMV symptoms were alleviated, and viral accumulations were decreased by boron application in Nicotiana benthamiana, indicating that boron contributed to inhibiting CGMMV infection. Meanwhile, we found that a number of differentially expressed genes (DEGs) associated with inositol biosynthesis, ethylene synthesis, Ca2+ signaling transduction and ROS scavenging system were up-regulated, while many DEGs involved in ABA catabolism, GA signal transduction and ascorbic acid metabolism were down-regulated by boron application under CGMMV infection. Additionally, we individually silenced nine ROS-related genes to explore their anti-CGMMV roles using a tobacco rattle virus (TRV) vector. The results showed that NbCat1, NbGME1, NbGGP and NbPrx Q were required for CGMMV infection, while NbGST and NbIPS played roles in resistance to CGMMV infection. The similar results were obtained in watermelon by silencing of ClCat, ClPrx or ClGST expression using a pV190 vector. This study proposed a new strategy for improving plant resistance to CGMMV infection by boron-regulated ROS pathway and provided several target genes for watermelon disease resistance breeding.