Genetic Architecture of Resistance to Alternaria brassicae in Arabidopsis thaliana: QTL Mapping Reveals Two Major Resistance-Conferring Loci

Fungal effectors versus defense-related genes of B. juncea and the status of resistant transgenics against fungal pathogens

Oilseed brassica has become instrumental in securing global food and nutritional security. B. juncea, colloquially known as Indian mustard, is cultivated across tropics and subtropics including Indian subcontinent. The production of Indian mustard is severely hampered by fungal pathogens which necessitates human interventions. Chemicals are often resorted to as they are quick and effective, but due to their economic and ecological unsustainability, there is a need to explore their alternatives. The B. juncea-fungal pathosystem is quite diverse as it covers broad-host range necrotrophs (Sclerotinia sclerotiorum), narrow-host range necrotrophs (Alternaria brassicae and A. brassicicola) and biotrophic oomycetes (Albugo candida and Hyaloperonospora brassica). Plants ward off fungal pathogens through two-step resistance mechanism; PTI which involves recognition of elicitors and ETI where the resistance gene (R gene) interacts with the fungal effectors. The hormonal signalling is also found to play a vital role in defense as the JA/ET pathway is initiated at the time of necrotroph infection and SA pathway is induced when the biotrophs attack plants. The review discuss the prevalence of fungal pathogens of Indian mustard and the studies conducted on effectoromics. It covers both pathogenicity conferring genes and host-specific toxins (HSTs) that can be used for a variety of purposes such as identifying cognate R genes, understanding pathogenicity and virulence mechanisms, and establishing the phylogeny of fungal pathogens. It further encompasses the studies on identifying resistant sources and characterisation of R genes/quantitative trait loci and defense-related genes identified in Brassicaceae and unrelated species which, upon introgression or overexpression, confer resistance. Finally, the studies conducted on developing resistant transgenics in Brassicaceae have been covered in which chitinase and glucanase genes are mostly used. The knowledge gained from this review can further be used for imparting resistance against major fungal pathogens.

Comparative Transcriptome and Targeted Metabolome Profiling Unravel the Key Role of Phenylpropanoid and Glucosinolate Pathways in Defense against Alternaria brassicicola in Broccoli

Alternaria brassicicola (Ab) can cause a major yield and quality-limiting disease of Brassica oleracea called black spot, and the genetic resources conferring complete resistance against Ab have not been identified to date. Here, comparative transcriptome and targeted metabolome analysis were performed utilizing a newly identified resistant (R) line and a broccoli susceptible (S) line at 6, 24, and 72 h post-inoculation (hpi). Kyoto encyclopedia of genes and genomes pathway enrichment and the weighted gene co-expression network analyses showed that the phenylpropanoid pathway regulates the resistance to Ab in broccoli. One metabolite, cinnamic acid, was significantly upregulated in the Ab_inoculated R line compared with the mock treatment but no significant difference in the S line, indicating that the cinnamic acid may cause the resistance difference between R and S lines. Our results also revealed that three indolic glucosinolates of I3G, 4MI3G, and 1MI3G were significantly increased in the Ab_inoculated R line compared with the mock treatment, and some related genes were differentially expressed between the R and S lines. These results provided new insights into the mechanism of Ab defense in B. oleracea and have laid a theoretical foundation for effectively utilizing resistant germplasm resources in broccoli breeding.

Transcriptome Analysis of the Necrotrophic Pathogen Alternaria brassicae Reveals Insights into Its Pathogenesis in Brassica juncea

Alternaria blight or leaf spot caused by Alternaria brassicae has an enormous economic impact on the Brassica crops grown worldwide. Although the genome of A. brassicae has been sequenced, little is known about the genes that play a role during the infection of the host species. In this study, the transcriptome expression profile of A. brassicae during growth and infection was determined. Differential expression analysis revealed that 4,430 genes were differentially expressed during infection. Weighted gene coexpression network analysis helped identify 10 modules, which were highly correlated with growth and infection. Subsequent gene ontology (GO) enrichment analysis of the modules highlighted the involvement of biological processes such as toxin metabolism, ribosome biogenesis, polysaccharide catabolism, copper ion transport, and vesicular trafficking during infection. Additionally, 200 carbohydrate-active enzymes (CAZymes) and 80 potential effectors were significantly upregulated during infection. Furthermore, 18 secondary metabolite gene clusters were also differentially expressed during infection. The clusters responsible for the production of destruxin B, brassicicene C, and HC-toxin were significantly upregulated during infection. Collectively, these results provide an overview of the critical pathways underlying the pathogenesis of A. brassicae and highlight the distinct gene networks that are temporally regulated. The study thus provides novel insights into the transcriptional plasticity of a necrotrophic pathogen during infection of its host. Additionally, the in planta expression evidence for many potential effectors provides a theoretical basis for further investigations into the effector biology of necrotrophic pathogens such as A. brassicae. IMPORTANCE Alternaria brassicae is a necrotrophic pathogen that can infect almost all members of the Brassicaceae family. A. brassicae causes extensive yield losses in oilseed mustard and has practically restricted the cultivation of oilseed brassicas in regions with cool and foggy climatic conditions (foothills and mountainous terrains) where the severity of the pathogen is the highest. In this study, I identified the differentially expressed genes associated with the pathogenicity of A. brassicae through transcriptome sequencing. Also, I have been able to delineate pathways that are active during the early and late stages of infection. Consequently, this study has provided crucial insights into the molecular mechanisms underlying the pathogenesis of A. brassicae, an important necrotrophic pathogen.

Development of genome-specific SSR markers for the identification of introgressed segments of Sinapis alba in the Brassica juncea background

Sinapis alba L. (white mustard) is recognized for carrying host resistance against several biotic stresses including, Alternaria brassicae, which is responsible for blight disease in cultivated Brassica. However, another cultivated Brassica has a dearth for genetic resistance for these stresses due to its narrow genetic base. Therefore, we performed introgression of the genomic regions of S. alba into backcross progenies of B. juncea + S. alba somatic hybrids. These advanced generations with S. alba chromosomal segments are named B. juncea-S. alba introgression lines (ILs). In the present study, we developed the S. alba genome-specific microsatellites from the draft genome to track the S. alba genome introgressions and responsible regions for resistance to A. brassicae. For developing these SSR markers, the unique contigs of S. alba draft genome were identified through BLASTN with B. juncea, B. rapa, B. nigra, and B. oleracea reference genome assemblies, including mitochondrial and chloroplast genomes, and further used for marker development. Out of 403,423 contigs, we have identified 65,343 non-hit contigs of S. alba that yielded a total of 1231 genome-specific microsatellites, out of which 1107 were expected to produce a single allele upon amplification. Out of the total SSRs, 234 primer pairs were randomly picked from whole-genome and validated between B. juncea and S. alba genomes for their specificity. In the validation experiment, these markers gave a single amplicon into S. alba, while they did not amplify in B. juncea genome. Of these, 59 microsatellites were used to track S. alba introgressions in 80 BC2F3 lines. To the best of our knowledge, this is the first time that these two genetic resources are developed in the form of B. juncea-S. alba ILs and S. alba-specific markers. Therefore, both the resources unlock a new avenue of Brassica breeding for biotic and abiotic stresses along with quality traits.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03402-0.

Introgression and QTL mapping conferring resistance for Alternaria brassicae in the backcross progeny of Sinapis alba + Brassica juncea somatic hybrids

A total of three QTLs, responsible for A. brassicae resistance were introgressed into S. alba - B. juncea introgression lines from S. alba and mapped through donor genome-specific SSR markers. Alternaria brassicae is a key pathogen of the Brassicaceae family causing severe blight disease to oilseed crops that leads to heavy yield losses due to lack of resistance source within cultivated Brassicas. However, the host resistance present in the Sinapis alba, an allied member of the Brassicaceae family is still unexplored precisely due to the unavailability of segregating population for Alternaria blight resistance and scarcity of donor genome-specific genetic markers. The present study was undertaken to identify quantitative trait loci governing resistance to Alternaria blight which was introgressed from S. alba to the backcross population of stable S. alba + B. juncea somatic hybrids (2n = 60; AABBSS). The second backcross population showed significant phenotypic variations for Alternaria blight ranging from immune to highly susceptible phenotype, thus suggesting quantitative nature of resistance for the disease. A subset of 154 BC₂F_3-4 lines was used for disease screening and genotyping with 234 S. alba genome-specific microsatellite markers. As a result of the study, twelve linkage groups were developed corresponding to 12 chromosomes of S. alba (n = 12) covering a length of 1694.02 cM. The chromosomes 5 and 11 harbored a total of 1 (Abr-01), and 2 (Abr-02, and Abr-03) QTLs detected by ICIM-ADD mapping method at LOD score values 3.7, 5.12, and 6.74, respectively. The QTLs identified during the study have a range of 5.51-10.87 percent phenotypic variations for disease resistance. To the best of our knowledge, this is the first report of QTLs introgression for A. brassicae resistance in cultivated Brassica from an allied member of Brassicaceae.

Functional Characterization of the Nep1-Like Protein Effectors of the Necrotrophic Pathogen - Alternaria brassicae

Alternaria brassicae is an important necrotrophic pathogen that infects the Brassicaceae family. A. brassicae, like other necrotrophs, also secretes various proteinaceous effectors and metabolites that cause cell death to establish itself in the host. However, there has been no systematic study of A. brassicae effectors and their roles in pathogenesis. The availability of the genome sequence of A. brassicae in public domain has enabled the search for effectors and their functional characterization. Nep1-like proteins (NLPs) are a superfamily of proteins that induce necrosis and ethylene biosynthesis. They have been reported from a variety of microbes including bacteria, fungi, and oomycetes. In this study, we identified two NLPs from A. brassicae viz. AbrNLP1 and AbrNLP2 and functionally characterized them. Although both AbrNLPs were found to be secretory in nature, they localized differentially inside the plant. AbrNLP2 was found to induce necrosis in both host and non-host species, while AbrNLP1 could not induce necrosis in both species. Additionally, AbrNLP2 was shown to induce pathogen-associated molecular pattern (PAMP)-triggered immunity in both host and non-host species. Overall, our study indicates that AbrNLPs are functionally and spatially (subcellular location) distinct and may play different but important roles during the pathogenesis of A. brassicae.

Current Status of the Disease-Resistant Gene(s)/QTLs, and Strategies for Improvement in Brassica juncea

Brassica juncea is a major oilseed crop in tropical and subtropical countries, especially in south-east Asia like India, China, Bangladesh, and Pakistan. The widespread cultivation of genetically similar varieties tends to attract fungal pathogens which cause heavy yield losses in the absence of resistant sources. The conventional disease management techniques are often expensive, have limited efficacy, and cause additional harm to the environment. A substantial approach is to identify and use of resistance sources within the Brassica hosts and other non-hosts to ensure sustainable oilseed crop production. In the present review, we discuss six major fungal pathogens of B. juncea: Sclerotinia stem rot (Sclerotinia sclerotiorum), Alternaria blight (Alternaria brassicae), White rust (Albugo candida), Downy mildew (Hyaloperonospora parasitica), Powdery mildew (Erysiphe cruciferarum), and Blackleg (Leptoshaeria maculans). From discussing studies on pathogen prevalence in B. juncea, the review then focuses on highlighting the resistance sources and quantitative trait loci/gene identified so far from Brassicaceae and non-filial sources against these fungal pathogens. The problems in the identification of resistance sources for B. juncea concerning genome complexity in host subpopulation and pathotypes were addressed. Emphasis has been laid on more elaborate and coordinated research to identify and deploy R genes, robust techniques, and research materials. Examples of fully characterized genes conferring resistance have been discussed that can be transformed into B. juncea using advanced genomics tools. Lastly, effective strategies for B. juncea improvement through introgression of novel R genes, development of pre-breeding resistant lines, characterization of pathotypes, and defense-related secondary metabolites have been provided suggesting the plan for the development of resistant B. juncea.

Optimization of process parameters for the synthesis of silver nanoparticles from Piper betle leaf aqueous extract, and evaluation of their antiphytofungal activity

Biological methods offer eco-friendly and cost-effective alternatives for the synthesis of silver nanoparticles (AgNPs). The present study highlights a green process where AgNPs were synthesized and optimized by using silver nitrate (AgNO₃) and the aqueous extract of Piper betle (Pbet) leaf as the reducing and capping agent. The stable and optimized process for the synthesis of Pbet-AgNPs was exposure of reaction mixture into the sunlight for 40 min, pH 9.0, and 2 mM AgNO₃ using 1:4 diluted Pbet leaf aqueous extract. The optimized Pbet-AgNPs were characterized by UV-visible spectroscopy, high-resolution field emission scanning electron microscopy (FE-SEM), X-ray diffractometry (XRD), and Fourier-transform infrared spectroscopy (FTIR). The prepared Pbet-AgNPs were spherical in shape with size in the range of 6-14 nm. These nanoparticles were stable for 6 months in aqueous solution at room temperature under dark conditions. The biogenic synthesized Pbet-AgNPs are found to have significant antifungal activity against plant pathogenic fungi, Alternaria brassicae and Fusarium solani. Synthesized Pbet-AgNPs potentially reduced the fungal growth in a dose-dependent manner. Microscopic observation of treated mycelium showed that Pbet-AgNPs could disrupt the mycelium cell wall and induce cellular permeability. Protein leakage assay supports these findings. Overall, this study revealed the efficacy of green synthesized AgNPs to control the plant fungal pathogens. Pbet leaves are a rich source of phenolic biomolecule(s). It was hypothesized that these biomolecule(s) mediated metal reduction reactions. In this context, the present work investigates the phytobiomolecule(s) of the aqueous extract of Pbet leaves using high-resolution liquid chromatography-mass spectroscopy (HR-LCMS) method. The analysis revealed that eugenol, chavicol, and hydroxychavicol were present in the Pbet aqueous extract.

Perspectives on the utilization of resistance mechanisms from host and nonhost plants for durable protection of Brassica crops against Alternaria blight

BACKGROUND

Alternaria brassicae, the causal organism of Alternaria blight, is a necrotroph infecting crops of the Brassicaceae family at all growth stages. To circumvent this problem, several disease management strategies are being used in the field, and disease-resistant varieties have also been developed. However, no strategy has proven completely successful, owing to the high variability in virulence among A. brassicae isolates, which causes a diverse spectrum of symptoms. Nonhost resistance (NHR) is a robust and broad-spectrum defense mechanism available in plants, and the exploitation of gene pools from plant species that are nonhost to A. brassicae could serve as novel sources of resistance.

METHODOLOGY

We searched the literature using key words relevant to this study in various search engines, such as PubMed, Web of Science, and Google Scholar, as well as certain journal websites. The literature was retrieved, sorted, and mined to extract data pertinent to the present review.

RESULTS

In this review, we have comprehensively covered the recent progress made in developing Alternaria blight resistance in Brassica crops by exploiting host germplasm. We also enumerate the potential NHR sources available for A. brassicae and the NHR layers possibly operating against this pathogen. In addition, we propose different strategies for identifying NHR-related genes from nonhost plants and testing their relevance in imparting broad-spectrum resistance when transferred to host plants.

CONCLUSION

This review will help broaden the current knowledge base pertaining to the resistance sources available in host germplasm, the exploitation of NHR mechanisms, and their applications in protecting Brassica crops from Alternaria blight. The insights might also be applicable to a wider repertoire of plant pathogens.

Near-Complete Genome Assembly of Alternaria brassicae-A Necrotrophic Pathogen of Brassica Crops

Alternaria brassicae, a necrotrophic fungal pathogen, causes Alternaria blight, an important disease of brassica crops. Although many Alternaria spp. have been sequenced, no genome information is available for A. brassicae, a monotypic lineage within the Alternaria genus. A highly contiguous genome assembly of A. brassicae has been generated using Nanopore MinION sequencing with an N50 of 2.98 Mb, yielding nine full chromosome-level sequences. This study adds to the current genomic resources available for the genus Alternaria and will provide opportunities for further analyses to unravel the mechanisms underlying pathogenicity of this important necrotrophic pathogen.

Unravelling the Complex Genetics of Karnal Bunt (Tilletia indica) Resistance in Common Wheat (Triticum aestivum) by Genetic Linkage and Genome-Wide Association Analyses

Karnal bunt caused by Tilletia indica Mitra [syn. Neovossia indica (Mitra) Mundkur] is a significant biosecurity concern for wheat-exporting countries that are free of the disease. It is a seed-, soil-and air-borne disease with no effective chemical control measures. The current study used data from multi-year field experiments of two bi-parental populations and a genome-wide association (GWA) mapping panel to unravel the genetic basis for resistance in common wheat. Broad-sense heritability for Karnal bunt resistance in the populations varied from 0.52 in the WH542×HD29 population, to 0.61 in the WH542×W485 cross and 0.71 in a GWAS panel. Quantitative trait locus (QTL) analysis with seven years of phenotypic data identified a major locus on chromosome 3B (R² = 27.8%) and a minor locus on chromosome 1A (R² = 12.2%), in the WH542×HD29 population, with both parents contributing the high-value alleles. A major locus (R² = 27.8%) and seven minor loci (R² = 4.4–15.8%) were detected in the WH542×W485 population. GWA mapping validated QTL regions in the bi-parent populations, but also identified novel loci not previously associated with Karnal bunt resistance. Meta-QTL analysis aligned the results from this study with those reported in wheat over the last two decades. Two major clusters were detected, the first on chromosome 4B, which clustered with Qkb.ksu-4B, QKb.cimmyt-4BL, Qkb.cim-4BL, and the second on chromosome 3B, which clustered with Qkb.cnl-3B, QKb.cimmyt-3BS and Qkb.cim-3BS1. The results provide definitive chromosomal assignments for QTL/genes controlling Karnal bunt resistance in common wheat, and will be useful in pre-emptive breeding against the pathogen in wheat-producing areas that are free of the disease.

Genome-wide association mapping in Arabidopsis identifies novel genes underlying quantitative disease resistance to Alternaria brassicae

Quantitative disease resistance (QDR) is the predominant form of resistance against necrotrophic pathogens. The genes and mechanisms underlying QDR are not well known. In the current study, the Arabidopsis-Alternaria brassicae pathosystem was used to uncover the genetic architecture underlying resistance to A. brassicae in a set of geographically diverse Arabidopsis accessions. Arabidopsis accessions revealed a rich variation in the host responses to the pathogen, varying from complete resistance to high susceptibility. Genome-wide association (GWA) mapping revealed multiple regions to be associated with disease resistance. A subset of genes prioritized on the basis of gene annotations and evidence of transcriptional regulation in other biotic stresses was analysed using a reverse genetics approach employing T-DNA insertion mutants. The mutants of three genes, namely At1g06990 (GDSL-motif lipase), At3g25180 (CYP82G1) and At5g37500 (GORK), displayed an enhanced susceptibility relative to the wild-type. These genes are involved in the development of morphological phenotypes (stomatal aperture) and secondary metabolite synthesis, thus defining some of the diverse facets of quantitative resistance against A. brassicae.

Mapping X-Disease Phytoplasma Resistance in Prunus virginiana

Phytoplasmas such as "Candidatus Phytoplasma pruni," the causal agent of X-disease of stone fruits, lack detailed biological analysis. This has limited the understanding of plant resistance mechanisms. Chokecherry (Prunus virginiana L.) is a promising model to be used for the plant-phytoplasma interaction due to its documented ability to resist X-disease infection. A consensus chokecherry genetic map "Cho" was developed with JoinMap 4.0 by joining two parental maps. The new map contains a complete set of 16 linkage groups, spanning a genetic distance of 2,172 cM with an average marker density of 3.97 cM. Three significant quantitative trait loci (QTL) associated with X-disease resistance were identified contributing to a total of 45.9% of the phenotypic variation. This updated genetic linkage map and the identified QTL will provide the framework needed to facilitate molecular genetics, genomics, breeding, and biotechnology research concerning X-disease in chokecherry and other Prunus species.