Comparative transcriptome analysis of two contrasting resistant and susceptible Aegilops tauschii accessions to wheat leaf rust (Puccinia triticina) using RNA-sequencing

Elucidating the mechanism of resistance to anthracnose in litchi leaves through transcriptome analysis

BACKGROUND

Litchi, an important tropical fruit, is severely affected by anthracnose disease. However, the mechanism of its disease resistance response remains unknown, and resistant accession genetic resources and resistance-related genes have not yet been identified.

RESULTS

In this study, 82 accessions of litchi were evaluated for resistance to Colletotrichum gloeosporioides, and the accessions 'Haiken 5' and 'Nongmei 5 hao' were identified as resistant and susceptible, respectively. Leaves from these two accessions were inoculated with C. gloeosporioides and collected at 6 and 24 h for use as materials for transcriptome analysis. Analyses of the differentially expressed genes (DEGs) between the accessions and their controls, which were inoculated with potato dextrose agar medium, revealed that the resistant accession presented more DEGs with smaller changes in magnitude, whereas the susceptible accession presented fewer DEGs with greater changes in magnitude. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were performed, and phenylpropanoid biosynthesis, amino sugar and nucleotide sugar metabolism, and plant-pathogen interactions were identified as common pathways. Chitinase activity, oxidoreductase activity, aminoglycan and glucosamine-containing compounds, and cell wall metabolic processes also participated in the defence reaction. Salicylic acid signalling in litchi leaves contributed to resistance to C. gloeosporioides. Short Time-series Expression Miner (STEM) and weighted correlation network analysis (WGCNA) were also employed to evaluate the gene expression trends and identify highly correlated genes.

CONCLUSION

Litchi accessions presented different resistance responses to anthracnose disease. Small changes in the expression levels of critical resistance-related genes were sufficient to produce the defence reaction. Calcium ion regulatory mechanisms and transcription factors have been preliminarily identified as contributors to disease resistance. Multiple pathways and molecular processes participate in the defence response. These results identify candidate genes and pathways involved in litchi plant defence against anthracnose.

Synergistic interaction between wheat streak mosaic virus and Triticum mosaic virus modulates wheat transcriptome to favor disease severity

Wheat streak mosaic virus (WSMV; Tritimovirus tritici) and Triticum mosaic virus (TriMV; Poacevirus tritici), the type members of the genera Tritimovirus and Poacevirus, respectively, in the family Potyviridae, are economically important wheat viruses in the Great Plains region of the USA. Co-infection of wheat by WSMV and TriMV results in disease synergism. Wheat transcriptome from singly (WSMV or TriMV) and doubly (WSMV+TriMV) infected upper uninoculated leaves were analyzed by RNA-Seq at 9, 12, and 21 days postinoculation. A total of 31,754 differentially expressed wheat genes were identified among all comparisons. Weighted gene co-expression network analysis resulted in 11 co-expression modules that broadly indicated gene expression profiles attributable to control, single, and double infections. Gene ontology, protein domain and KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway enrichment analysis revealed that genes specifically related to photosynthesis, growth, stress, senescence, and defense were differentially enriched. Analyses of transcription factor families indicated that genes encoding MADS-Box and ARFs were strongly enriched in control plants, moderately repressed in TriMV-infected plants, and more strongly repressed in WSMV- and doubly-infected plants, whereas genes encoding WRKYs and NACs were more enriched in WSMV or doubly infected plants. Synergistic interactions between WSMV and TriMV drastically enhanced disease phenotype compared to individual virus infections. The progression of disease phenotype was correlated to transcriptomic changes, indicating the strong disruption to plant metabolism and likely channeling of energy and metabolites for viral replication. There also appeared to be a connection between viral replication and plastid health, with stronger downregulation of genes needed for chloroplast functions and integrity and increased synergism between TriMV and WSMV. This study provides an overview of transcriptomic changes distinctly influenced by TriMV and WSMV either singly or in combination and provides a good correlation between specific transcription factors and genes associated with metabolism to observed phenotypic changes in plant growth and disease synergism.

Unveiling the wheat-rust battleground: A transcriptomic journey

The global wheat production faces significant challenges due to major rust-causing fungi, namely Puccinia striiformis f. sp. tritici, P. triticina, and P. graminis f. sp. tritici, responsible for stripe, leaf, and stem rust diseases, respectively. The evolutionary relationship between wheat (host) and Puccinia (pathogen) renders existing wheat resistance ineffective over time. The most viable solution to this issue lies in the development of new resistant wheat varieties. However, achieving this requires a comprehensive understanding of wheat's defense mechanisms against ever-evolving pathogens. Transcriptomics emerges as a powerful tool for analyzing gene activity at the molecular level. Over the last decade, this technique has transformed our comprehension of the wheat-rust interaction. Transcriptomics has unveiled a compelling "biphasic model" of gene expression in wheat infected with rust fungi, delineating two distinct phases of defense activation. Moreover, it has illuminated the intricate signaling pathways, hormonal interactions, and diverse defense mechanisms employed by wheat. These mechanisms encompass the oxidative burst, reinforcement of cell walls, and controlled cessation of photosynthesis, all aimed at combatting the invading pathogen. However, the utility of transcriptomics extends beyond elucidating defense strategies; it enables the identification of novel genes linked to resistance or susceptibility. By unraveling the functions of these genes, researchers can uncover new avenues for breeding resistant wheat varieties, arming wheat with the molecular arsenal necessary to prevail in the ongoing battle against rust fungi. This review represents a pioneering effort in exploring transcriptomic techniques and accumulated data to present a comprehensive overview of the wheat-Puccinia interaction at the system-wide level.

Metabolome and Mycobiome of Aegilops tauschii Subspecies Differing in Susceptibility to Brown Rust and Powdery Mildew Are Diverse

The present study demonstrated the differences in the seed metabolome and mycobiome of two Aegilops tauschii Coss accessions with different resistance to brown rust and powdery mildew. We hypothesized that the seeds of resistant accession k-1958 Ae. tauschii ssp. strangulata can contain a larger number of metabolites with antifungal activity compared with the seeds of susceptible Ae. tauschii ssp meyeri k-340, which will determine differences in the seed fungal community. Our study emphasizes the differences in the seed metabolome of the studied Ae. tauschii accessions. The resistant accession k-1958 had a higher content of glucose and organic acids, including pyruvic, salicylic and azelaic acid, as well as pipecolic acids, galactinol, glycerol and sitosterol. The seeds of Ae. tauschii-resistant accession k-1958 were found to contain more active substances with antifungal activity. The genera Cladosporium and Alternaria were dominant in the seed mycobiome of the resistant accession. The genera Alternaria, Blumeria and Cladosporium dominated in seed mycobiome of susceptible accession k-340. In the seed mycobiome of the resistant k-1958, a higher occurrence of saprotrophic micromycetes was found, and many of the micromycetes were biocontrol agents. It was concluded that differences in the seed metabolome of Ae. tauschii contributed to the determination of the differences in mycobiomes.

Molecular Mechanisms of the Stripe Rust Interaction with Resistant and Susceptible Wheat Genotypes

Rust fungi cause significant damage to wheat production worldwide. In order to mitigate disease impact and improve food security via durable resistance, it is important to understand the molecular basis of host-pathogen interactions. Despite a long history of research and high agricultural importance, still little is known about the interactions between the stripe rust fungus and wheat host on the gene expression level. Here, we present analysis of the molecular interactions between a major wheat pathogen-Puccinia striiformis f. sp. tritici (Pst)-in resistant and susceptible host backgrounds. Using plants with durable nonrace-specific resistance along with fully susceptible ones allowed us to show how gene expression patterns shift in compatible versus incompatible interactions. The pathogen showed significantly greater number and fold changes of overexpressed genes on the resistant host than the susceptible host. Stress-related pathways including MAPK, oxidation-reduction, osmotic stress, and stress granule formation were, almost exclusively, upregulated in the resistant host background, suggesting the requirement of the resistance-countermeasure mechanism facilitated by Pst. In contrast, the susceptible host background allowed for broad overrepresentation of the nutrient uptake pathways. This is the first study focused on the stripe rust pathogen-wheat interactions, on the whole transcriptome level, from the pathogen side. It lays a foundation for the better understanding of the resistant/susceptible hosts versus pathogenic fungus interaction in a broader sense.

Transcriptome analysis in Aegilops tauschii unravels further insights into genetic control of stripe rust resistance

MAIN CONCLUSION

The Aegilops tauschii resistant accession prevented the pathogen colonization by controlling the sugar flow and triggering the hypersensitive reaction. This study suggested that NBS-LRRs probably induce resistance through bHLH by controlling JA- and SA-dependent pathways. Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst) is one of wheat's most destructive fungal diseases that causes a severe yield reduction worldwide. The most effective and economically-friendly strategy to manage this disease is genetic resistance which can be achieved through deploying new and effective resistance genes. Aegilops tauschii, due to its small genome and co-evolution with Pst, can provide detailed information about underlying resistance mechanisms. Hence, we used RNA-sequencing approach to identify the transcriptome variations of two contrasting resistant and susceptible Ae. tauschii accessions in interaction with Pst and differentially expressed genes (DEGs) for resistance to stripe rust. Gene ontology, pathway analysis, and search for functional domains, transcription regulators, resistance genes, and protein-protein interactions were used to interpret the results. The genes encoding NBS-LRR, CC-NBS-kinase, and phenylalanine ammonia-lyase, basic helix-loop-helix (bHLH)-, basic-leucine zipper (bZIP)-, APETALA2 (AP2)-, auxin response factor (ARF)-, GATA-, and LSD-like transcription factors were up-regulated exclusively in the resistant accession. The key genes involved in response to salicylic acid, amino sugar and nucleotide sugar metabolism, and hypersensitive response contributed to plant defense against stripe rust. The activation of jasmonic acid biosynthesis and starch and sucrose metabolism pathways under Pst infection in the susceptible accession explained the colonization of the host. Overall, this study can fill the gaps in the literature on host-pathogen interaction and enrich the Ae. tauschii transcriptome sequence information. It also suggests candidate genes that could guide future breeding programs attempting to develop rust-resistant cultivars.

Identification of drought-tolerant hub genes in Iranian KC-2226 genotype of Aegilops tauschii using transcriptomic analysis

Aegilops tauschii, as a donor of D genome to the bread wheat with a valuable source of resistance to different biotic and abiotic stresses, is used to improve the quality of wheat cultivars. Every genotype has a specific genetic content, the investigation of which can lead to the identification of useful genes such as stress tolerance genes, including drought. Therefore, 23 genotypes of Ae. tauschii were selected to evaluate their morphological and physiological traits under greenhouse conditions. Among them, a superior tolerant genotype (KC-2226) was chosen for transcriptomic analysis. Our result showed that 5007 and 3489 genes were deferentially up- and downregulated, respectively. Upregulated genes were involved in photosynthesis, glycolysis/gluconeogenesis, and amino acid biosynthesis whereas downregulated genes were often engaged in DNA synthesis, replication, repair and topological changes. The result of protein-protein interaction network analysis showed that AT1G76550 (1.46), AT1G20950 (1.42), IAR4 (1.19), and PYD2 (1.16) among upregulated genes and THY-1 (44), PCNA1 (41) and TOPII (22) among down-regulated genes had the highest interactions with other genes. In conclusion, Ae. tauschii employs elevated transcription of specific genes involved in photosynthesis, glycolysis and gluconeogenesis and amino acid biosynthesis pathways rather than genes active in DNA synthesis and repair to provide the energy needed for the plant to survive under stress conditions.

Novel genomic regions on chromosome 5B controlling wheat powdery mildew seedling resistance under Egyptian conditions

Wheat powdery mildew (PM) causes significant yield losses worldwide. None of the Egyptian wheat cultivars was detected to be highly resistant to such a severe disease. Therefore, a diverse spring wheat panel was evaluated for PM seedling resistance using different Bgt conidiospores collected from Egyptian fields in two growing seasons. The evaluation was done in two separate experiments. Highly significant differences were found between the two experiments suggesting the presence of different isolates populations. Highly significant differences were found among the tested genotypes confirming the ability to improve PM resistance using the recent panel. Genome-wide association study (GWAS) was done for each experiment separately and a total of 71 significant markers located within 36 gene models were identified. The majority of these markers are located on chromosome 5B. Haplotype block analysis identified seven blocks containing the significant markers on chromosome 5B. Five gene models were identified on the short arm of the chromosome. Gene enrichment analysis identified five and seven pathways based on the biological process and molecular functions respectively for the detected gene models. All these pathways are associated with disease resistance in wheat. The genomic regions on 5B seem to be novel regions that are associated with PM resistance under Egyptian conditions. Selection of superior genotypes was done and Grecian genotypes seem to be a good source for improving PM resistance under Egyptian conditions.

Comparative transcriptome analysis of juniper branches infected by Gymnosporangium spp. highlights their different infection strategies associated with cytokinins

BACKGROUND

Gymnosporangium asiaticum and G. yamadae can share Juniperus chinensis as the telial host, but the symptoms are completely different. The infection of G. yamadae causes the enlargement of the phloem and cortex of young branches as a gall, but not for G. asiaticum, suggesting that different molecular interaction mechanisms exist the two Gymnosporangium species with junipers.

RESULTS

Comparative transcriptome analysis was performed to investigate genes regulation of juniper in responses to the infections of G. asiaticum and G. yamadae at different stages. Functional enrichment analysis showed that genes related to transport, catabolism and transcription pathways were up-regulated, while genes related to energy metabolism and photosynthesis were down-regulated in juniper branch tissues after infection with G. asiaticum and G. yamadae. The transcript profiling of G. yamadae-induced gall tissues revealed that more genes involved in photosynthesis, sugar metabolism, plant hormones and defense-related pathways were up-regulated in the vigorous development stage of gall compared to the initial stage, and were eventually repressed overall. Furthermore, the concentration of cytokinins (CKs) in the galls tissue and the telia of G. yamadae was significantly higher than in healthy branch tissues of juniper. As well, tRNA-isopentenyltransferase (tRNA-IPT) was identified in G. yamadae with highly expression levels during the gall development stages.

CONCLUSIONS

In general, our study provided new insights into the host-specific mechanisms by which G. asiaticum and G. yamadae differentially utilize CKs and specific adaptations on juniper during their co-evolution.

The Keys to Controlling Wheat Rusts: Identification and Deployment of Genetic Resistance

Rust diseases are among the major constraints for wheat production worldwide due to the emergence and spread of highly destructive races of Puccinia. The most common approach to minimize yield losses due to rust is to use cultivars that are genetically resistant. Modern wheat cultivars, landraces, and wild relatives can contain undiscovered resistance genes, which typically encode kinase or nucleotide-binding site leucine rich repeat (NLR) domain containing receptor proteins. Recent research has shown that these genes can provide either resistance in all growth stages (all-stage resistance; ASR) or specially in later growth stages (adult-plant resistance; APR). ASR genes are pathogen and race-specific, meaning can function against selected races of the Puccinia fungus due to the necessity to recognize specific avirulence molecules in the pathogen. APR genes are either pathogen-specific or multipathogen resistant but often race-nonspecific. Prediction of resistance genes through rust infection screening alone remains complex when more than one resistance gene is present. However, breakthroughs during the past half century such as the single-nucleotide polymorphism-based genotyping techniques and resistance gene isolation strategies like mutagenesis, resistance gene enrichment, and sequencing (MutRenSeq), mutagenesis and chromosome sequencing (MutChromSeq), and association genetics combined with RenSeq (AgRenSeq) enables rapid transfer of resistance from source to modern cultivars. There is a strong need for combining multiple genes for better efficacy and longer-lasting resistance. Hence, techniques like gene cassette creation speeds up the gene combination process, but their widespread adoption and commercial use is limited due to their transgenic nature.

Transcriptional Analysis on Resistant and Susceptible Kiwifruit Genotypes Activating Different Plant-Immunity Processes against Pseudomonas syringae pv. actinidiae

Pseudomonas syringae pv. actinidiae (Psa), a bacterial pathogen, is a severe threat to kiwifruit production. To elucidate the species-specific interaction between Psa and kiwifruit, transcriptomic-profiles analyses were conducted, under Psa-infected treatment and mock-inoculated control, on shoots of resistant Maohua (MH) and susceptible Hongyang (HY) kiwifruit varieties. The plant hormone-signal transduction and plant–pathogen interaction were significantly enriched in HY compared with MH. However, the starch and sucrose metabolism, antigen processing and presentation, phagosome, and galactose metabolism were significantly enriched in MH compared with HY. Interestingly, the MAP2 in the pathogen/microbe-associated molecular patterns (PAMPs)-triggered immunity (PTI) was significantly up-regulated in MH. The genes RAR1, SUGT1, and HSP90A in the effector-triggered immunity (ETI), and the NPR1 and TGA genes involved in the salicylic acid signaling pathway as regulatory roles of ETI, were significantly up-regulated in HY. Other important genes, such as the CCRs involved in phenylpropanoid biosynthesis, were highly expressed in MH, but some genes in the Ca2+ internal flow or involved in the reactive oxygen metabolism were obviously expressed in HY. These results suggested that the PTI and cell walls involved in defense mechanisms were significant in MH against Psa infection, while the ETI was notable in HY against Psa infection. This study will help to understand kiwifruit bacterial canker disease and provide important theoretical support in kiwifruit breeding.

Comparative Transcriptome Analysis Reveals the Gene Expression and Regulatory Characteristics of Broad-Spectrum Immunity to Leaf Rust in a Wheat-Agropyron cristatum 2P Addition Line

Wheat leaf rust (caused by Puccinia triticina Erikss.) is among the major diseases of common wheat. The lack of resistance genes to leaf rust has limited the development of wheat cultivars. Wheat-Agropyron cristatum (A. cristatum) 2P addition line II-9-3 has been shown to provide broad-spectrum immunity to leaf rust. To identify the specific A. cristatum resistance genes and related regulatory pathways in II-9-3, we conducted a comparative transcriptome analysis of inoculated and uninoculated leaves of the resistant addition line II-9-3 and the susceptible cultivar Fukuhokomugi (Fukuho). The results showed that there were 66 A. cristatum differentially expressed genes (DEGs) and 1389 wheat DEGs in II-9-3 during P. triticina infection. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment and gene set enrichment analysis (GSEA) revealed that the DEGs of II-9-3 were associated with plant-pathogen interaction, MAPK signaling pathway-plant, plant hormone signal transduction, glutathione metabolism, and phenylpropanoid biosynthesis. Furthermore, many defense-related A. cristatum genes, such as two NLR genes, seven receptor kinase-encoding genes, and four transcription factor-encoding genes, were identified. Our results indicated that the key step of resistance to leaf rust involves, firstly, the gene expression of chromosome 2P upstream of the immune pathway and, secondly, the effect of chromosome 2P on the co-expression of wheat genes in II-9-3. The disease resistance regulatory pathways and related genes in the addition line II-9-3 thus could play a critical role in the effective utilization of innovative resources for leaf rust resistance in wheat breeding.