Soybean-Phakopsora pachyrhizi interactions: towards the development of next-generation disease-resistant plants

Soybean rust (SBR), caused by the obligate biotrophic fungus Phakopsora pachyrhizi, is a devastating foliar disease threatening soybean production. To date, no commercial cultivars conferring durable resistance to SBR are available. The development of long-lasting SBR resistance has been hindered by the lack of understanding of this complex pathosystem, encompassing challenges posed by intricate genetic structures in both the host and pathogen, leading to a gap in the knowledge of gene-for-gene interactions between soybean and P. pachyrhizi. In this review, we focus on recent advancements and emerging technologies that can be used to improve our understanding of the P. pachyrhizi-soybean molecular interactions. We further explore approaches used to combat SBR, including conventional breeding, transgenic approaches and RNA interference, and how advances in our understanding of plant immune networks, the availability of new molecular tools, and the recent sequencing of the P. pachyrhizi genome could be used to aid in the development of better genetic resistance against SBR. Lastly, we discuss the research gaps of this pathosystem and how new technologies can be used to shed light on these questions and to develop durable next-generation SBR-resistant soybean plants.

RNA-Seq and Comparative Transcriptomic Analyses of Asian Soybean Rust Resistant and Susceptible Soybean Genotypes Provide Insights into Identifying Disease Resistance Genes

Asian soybean rust (ASR), caused by Phakopsora pachyrhizi, is one of the most destructive foliar diseases that affect soybeans. Developing resistant cultivars is the most cost-effective, environmentally friendly, and easy strategy for controlling the disease. However, the current understanding of the mechanisms underlying soybean resistance to P. pachyrhizi remains limited, which poses a significant challenge in devising effective control strategies. In this study, comparative transcriptomic profiling using one resistant genotype and one susceptible genotype was performed under infected and control conditions to understand the regulatory network operating between soybean and P. pachyrhizi. RNA-Seq analysis identified a total of 6540 differentially expressed genes (DEGs), which were shared by all four genotypes. The DEGs are involved in defense responses, stress responses, stimulus responses, flavonoid metabolism, and biosynthesis after infection with P. pachyrhizi. A total of 25,377 genes were divided into 33 modules using weighted gene co-expression network analysis (WGCNA). Two modules were significantly associated with pathogen defense. The DEGs were mainly enriched in RNA processing, plant-type hypersensitive response, negative regulation of cell growth, and a programmed cell death process. In conclusion, these results will provide an important resource for mining resistant genes to P. pachyrhizi infection and valuable resources to potentially pyramid quantitative resistance loci for improving soybean germplasm.

An NBS-LRR protein in the Rpp1 locus negates the dominance of Rpp1-mediated resistance against Phakopsora pachyrhizi in soybean

The soybean Rpp1 locus confers resistance to Phakopsora pachyrhizi, causal agent of rust, and resistance is usually dominant over susceptibility. However, dominance of Rpp1-mediated resistance is lost when a resistant genotype (Rpp1 or Rpp1b) is crossed with susceptible line TMG06_0011, and the mechanism of this dominant susceptibility (DS) is unknown. Sequencing the Rpp1 region reveals that the TMG06_0011 Rpp1 locus has a single nucleotide-binding site leucine-rich repeat (NBS-LRR) gene (DS-R), whereas resistant PI 594760B (Rpp1b) is similar to PI 200492 (Rpp1) and has three NBS-LRR resistance gene candidates. Evidence that DS-R is the cause of DS was reflected in virus-induced gene silencing of DS-R in Rpp1b/DS-R or Rpp1/DS-R heterozygous plants with resistance partially restored. In heterozygous Rpp1b/DS-R plants, expression of Rpp1b candidate genes was not significantly altered, indicating no effect of DS-R on transcription. Physical interaction of the DS-R protein with candidate Rpp1b resistance proteins was supported by yeast two-hybrid studies and in silico modeling. Thus, we conclude that suppression of resistance most likely does not occur at the transcript level, but instead probably at the protein level, possibly with Rpp1 function inhibited by binding to the DS-R protein. The DS-R gene was found in other soybean lines, with an estimated allele frequency of 6% in a diverse population, and also found in wild soybean (Glycine soja). The identification of a dominant susceptible NBS-LRR gene provides insight into the behavior of NBS-LRR proteins and serves as a reminder to breeders that the dominance of an R gene can be influenced by a susceptibility allele.

Genomic regions associated with resistance to soybean rust (Phakopsora pachyrhizi) under field conditions in soybean germplasm accessions from Japan, Indonesia and Vietnam

Eight soybean genomic regions, including six never before reported, were found to be associated with resistance to soybean rust (Phakopsora pachyrhizi) in the southeastern USA. Soybean rust caused by Phakopsora pachyrhizi is one of the most important foliar diseases of soybean [Glycine max (L.) Merr.]. Although seven Rpp resistance gene loci have been reported, extensive pathotype variation in and among fungal populations increases the importance of identifying additional genes and loci associated with rust resistance. One hundred and ninety-one soybean plant introductions from Japan, Indonesia and Vietnam, and 65 plant introductions from other countries were screened for resistance to P. pachyrhizi under field conditions in the southeastern USA between 2008 and 2015. The results indicated that 84, 69, and 49% of the accessions from southern Japan, Vietnam or central Indonesia, respectively, had negative BLUP values, indicating less disease than the panel mean. A genome-wide association analysis using SoySNP50K Infinium BeadChip data identified eight genomic regions on seven chromosomes associated with SBR resistance, including previously unreported regions of Chromosomes 1, 4, 6, 9, 13, and 15, in addition to the locations of the Rpp3 and Rpp6 loci. The six unreported genomic regions might contain novel Rpp loci. The identification of additional sources of rust resistance and associated genomic regions will further efforts to develop soybean cultivars with broad and durable resistance to soybean rust in the southern USA.

SdhC-I86F Mutation in Phakopsora pachyrhizi Is Stable and Can Be Related to Fitness Penalties

Succinate dehydrogenase inhibitors (SDHIs) fungicides are used to control Asian soybean rust (Phakopsora pachyrhizi), and the SdhC-I86F mutation is related to pathogen resistance. The objective of this study was to determine whether fitness penalties are associated with SDHI resistance (SdhC-I86F mutation) in P. pachyrhizi populations. Moreover, the study investigated whether the SdhC-I86F mutation remained stable after the fungus propagation both in the absence and presence of fungicide. The populations used in this study presented mutations for all genes analyzed (Cyp51, Cytb, and SdhC), except for a wild-type population (WT_SdhC) found with no SdhC-I86F mutation. The frequencies of the SdhC-I86F mutant populations were stable after 36 generations in the absence of fungicide. However, in the case of the WT_SdhC population, the SdhC-I86F mutation was further detected after one generation of the fungus in the presence of the SDHI fungicide, according to the results of a detached leaf assay. Three tests were performed to evaluate fitness components and sensitivity to fungicides (half maximal effective concentration). SdhC-I86F mutant populations were more sensitive to osmotic and oxidative stress than the WT_SdhC population; however, the sensitivity to ultraviolet radiation was similar for both populations. All mutated populations were less sensitive than the WT_SdhC when using SDHI (azoxystrobin + benzovindiflupyr), but more sensitive to mancozeb. The presence of fitness penalties, the mutation stability, and the sensitivity to mancozeb presented by the SdhC-I86F mutant populations can be relevant to the management of the disease in the field.

Host-Induced Gene Silencing Using BPMV on Soybean to Study Genes in the Soybean Rust Fungus Phakopsora pachyrhizi

To obtain direct evidence for the influence of an effector on the virulence or pathogenicity of a pathogen, it is necessary to knock out, knock down, or silence the respective gene. Since genetic transformation is not yet possible for rust fungi, silencing the gene is the only option. Posttranscriptional gene silencing uses RNAi. RNAi in plant pathogens can be accomplished by introducing dsRNA either by direct application of in vitro synthesized dsRNA or through positive-strand or double-strand RNA plant viruses. For studying effectors in Phakopsora pachyrhizi, we have implemented a host-induced silencing procedure based on virus-induced gene silencing using the bean pod mottle virus system. Here, procedures and interpretations of results are described and limitations of the system are discussed.

Sensitivity assessment and SDHC-I86F mutation frequency of Phakopsora pachyrhizi populations to benzovindiflupyr and fluxapyroxad fungicides from 2015 to 2019 in Brazil

BACKGROUND

Fungicides of the succinate dehydrogenase inhibitors (SDHIs) group have been used in soybean to control Asian soybean rust (ASR) caused by Phakopsora pachyrhizi. Fungal populations with less sensitivity to SDHI fungicides have been reported since 2015.

RESULTS

In this study, fungal sensitivity to benzovindiflupyr (BZV) and fluxapyroxad (FXD) was assessed using a total of 770 P. pachyrhizi populations sampled over four soybean growing seasons. Cross-resistance, intrinsic activity, and frequency of SDHC-I86F mutation were also analyzed. The average effective concentration to inhibit 50% (EC₅₀ ) and SDHC-I86F frequency increased over the 2015/2016, 2016/2017, 2017/2018 and 2018/2019 soybean-seasons. Fourteen P. pachyrhizi populations had the EC₅₀ value above 10 mg L^-1 for both carboxamides. No difference was found in intrinsic active to BZV and FXD fungicides for sensitive P. pachyrhizi populations. For P. pachyrhizi classified as less sensitive BZV showed the highest fungitoxicity effect. High frequency of the C-I86F mutation was observed in samples collected in volunteer soybean plants. The maximum frequency of SDHC-I86F mutation in the population was 50% and resulting in ASR populations with low sensitivity to SDHIs. A low correlation between bioassay and SDHC-I86F mutation was observed possible due to the dikaryotic nature of rust fungi or other mutations in the other succinate dehydrogenase subunits.

CONCLUSION

The present work provides an overview of a large sampling size of P. pachyrhizi populations and their performance over the four crop seasons. The high frequency of SDHC-I86F mutation and low sensitivity to SDHIs are widely distributed in the main soybean growing regions in Brazil and present in volunteer plants in the soybean-free period. Further detailed studies are needed to identify novel point mutations affecting the effectiveness of SDHIs. © 2021 Society of Chemical Industry.

Identification and characterization of a new soybean promoter induced by Phakopsora pachyrhizi, the causal agent of Asian soybean rust

BACKGROUND

Phakopsora pachyrhizi is a biotrophic fungal pathogen responsible for the Asian soybean rust disease causing important yield losses in tropical and subtropical soybean-producing countries. P. pachyrhizi triggers important transcriptional changes in soybean plants during infection, with several hundreds of genes being either up- or downregulated.

RESULTS

Based on published transcriptomic data, we identified a predicted chitinase gene, referred to as GmCHIT1, that was upregulated in the first hours of infection. We first confirmed this early induction and showed that this gene was expressed as early as 8 h after P. pachyrhizi inoculation. To investigate the promoter of GmCHIT1, transgenic soybean plants expressing the green fluorescence protein (GFP) under the control of the GmCHIT1 promoter were generated. Following inoculation of these transgenic plants with P. pachyrhizi, GFP fluorescence was detected in a limited area located around appressoria, the fungal penetration structures. Fluorescence was also observed after mechanical wounding whereas no variation in fluorescence of pGmCHIT1:GFP transgenic plants was detected after a treatment with an ethylene precursor or a methyl jasmonate analogue.

CONCLUSION

We identified a soybean chitinase promoter exhibiting an early induction by P. pachyrhizi located in the first infected soybean leaf cells. Our results on the induction of GmCHIT1 promoter by P. pachyrhizi contribute to the identification of a new pathogen inducible promoter in soybean and beyond to the development of a strategy for the Asian soybean rust disease control using biotechnological approaches.

Variation in the Internal Transcribed Spacer Region of Phakopsora pachyrhizi and Implications for Molecular Diagnostic Assays

Phakopsora pachyrhizi, the causal agent of soybean rust (SBR), is a global threat to soybean production. Since the discovery of SBR in the continental United States, quantitative polymerase chain reaction assays based on the internal transcribed spacer (ITS) ribosomal DNA locus were established for its rapid detection. However, insufficient data were initially available to test assays against factors that could give rise to misidentification. This study aimed to reevaluate current assays for (i) the potential for false-positive detection caused by nontarget Phakopsora species and (ii) the potential for false-negative detection caused by intraspecific variation within the ITS locus of P. pachyrhizi. A large amount of intraspecific and intragenomic variation in ITS was detected, including the presence of polymorphic ITS copies within single leaf samples and within single rust sori. The diagnostic assays were not affected by polymorphisms in the ITS region; however, current assays are at risk of false positives when screened against other species of Phakopsora. This study raises caveats to the use of multicopy genes (e.g., ITS) in single-gene detection assays and discusses the pitfalls of inferences concerning the aerobiological pathways of disease spread made in the absence of an evaluation of intragenomic ITS heterogeneity.

Transcriptome-based analyses of phosphite-mediated suppression of rust pathogens Puccinia emaculata and Phakopsora pachyrhizi and functional characterization of selected fungal target genes

Phosphite (Phi) is used commercially to manage diseases mainly caused by oomycetes, primarily due to its low cost compared with other fungicides and its persistent control of oomycetous pathogens. We explored the use of Phi in controlling the fungal pathogens Puccinia emaculata and Phakopsora pachyrhizi, the causal agents of switchgrass rust and Asian soybean rust, respectively. Phi primes host defenses and efficiently inhibits the growth of P. emaculata, P. pachyrhizi and several other fungal pathogens tested. To understand these Phi-mediated effects, a detailed molecular analysis was undertaken in both the host and the pathogen. Transcriptomic studies in switchgrass revealed that Phi activates plant defense signaling as early as 1 h after application by increasing the expression of several cytoplasmic and membrane receptor-like kinases and defense-related genes within 24 h of application. Unlike in oomycetes, RNA sequencing of P. emaculata and P. pachyrhizi did not exhibit Phi-mediated retardation of cell wall biosynthesis. The genes with reduced expression in either or both rust fungi belonged to functional categories such as ribosomal protein, actin, RNA-dependent RNA polymerase, and aldehyde dehydrogenase. A few P. emaculata genes that had reduced expression upon Phi treatment were further characterized. Application of double-stranded RNAs specific to P. emaculata genes encoding glutamate N-acetyltransferase and cystathionine gamma-synthase to switchgrass leaves resulted in reduced disease severity upon P. emaculata inoculation, suggesting their role in pathogen survival and/or pathogenesis.

Prediction of the in planta Phakopsora pachyrhizi secretome and potential effector families

Asian soybean rust (ASR), caused by the obligate biotrophic fungus Phakopsora pachyrhizi, can cause losses greater than 80%. Despite its economic importance, there is no soybean cultivar with durable ASR resistance. In addition, the P. pachyrhizi genome is not yet available. However, the availability of other rust genomes, as well as the development of sample enrichment strategies and bioinformatics tools, has improved our knowledge of the ASR secretome and its potential effectors. In this context, we used a combination of laser capture microdissection (LCM), RNAseq and a bioinformatics pipeline to identify a total of 36 350 P. pachyrhizi contigs expressed in planta and a predicted secretome of 851 proteins. Some of the predicted secreted proteins had characteristics of candidate effectors: small size, cysteine rich, do not contain PFAM domains (except those associated with pathogenicity) and strongly expressed in planta. A comparative analysis of the predicted secreted proteins present in Pucciniales species identified new members of soybean rust and new Pucciniales- or P. pachyrhizi-specific families (tribes). Members of some families were strongly up-regulated during early infection, starting with initial infection through haustorium formation. Effector candidates selected from two of these families were able to suppress immunity in transient assays, and were localized in the plant cytoplasm and nuclei. These experiments support our bioinformatics predictions and show that these families contain members that have functions consistent with P. pachyrhizi effectors.

Competitive Fitness of Phakopsora pachyrhizi Isolates with Mutations in the CYP51 and CYTB Genes

Soybean rust (Phakopsora pachyrhizi) in Brazil is mainly controlled with applications of fungicides, including demethylation inhibitors (DMI) and quinone outside inhibitors (QoI). Isolates with less sensitivity to DMI and QoI have been reported, and these have been found to have mutations in the CYP51 and CYTB genes, respectively. There have been no reports of fitness costs in isolates with mutations in CYP51 and CYTB, and the aim of this work was to compare the competitive ability of isolates with lower DMI or QoI sensitivities with that of sensitive (wild-type) isolates. Urediniospores of sensitive wild-type isolates and isolates with different CYP51 or CYTB alleles were mixed and inoculated on detached soybean leaves. After 3 weeks, urediniospores were harvested and used as inoculum for the next disease cycle. Frequencies of relevant target site mutations were monitored using the pyrosequencing method over four disease cycles. Isolates with lower DMI sensitivity and different CYP51 alleles had competitive disadvantages compared with a DMI-sensitive, wild-type CYP51 isolate. In contrast, the isolate with the F129L mutation in the CYTB gene competed equally well with a QoI-sensitive, wild-type CYTB isolate under the conditions of this experiment. The CYP51 and CYTB alleles were stable in all isolates over four disease cycles when cultivated alone.

Detection of the F129L mutation in the cytochrome b gene in Phakopsora pachyrhizi

BACKGROUND

Asian soybean rust, caused by Phakopsora pachyrhizi, is mostly controlled by demethylation inhibitor (DMI) and quinone outside inhibitor (QoI) fungicides. Mutations in the cytochrome b (CYTB) gene can lead to pathogen resistance to QoIs. The occurrence of the mutations in codons 129, 137 and 143 in the CYTB gene was investigated, and a pyrosequencing assay was developed for rapid and quantitative detection of the F129L mutation.

RESULTS

Molecular analysis of the CYTB gene showed the presence of the F129L mutation in field samples and monouredinial isolates, while other mutations (G143A and G137R) were not found. The pyrosequencing was an effective method for quantitative detection of the F129L mutation, and many of the P. pachyrhizi samples showed high frequency of F129L.

CONCLUSION

This is the first report of the occurrence of the F129L mutation in P. pachyrhizi. The practical relevance of this mutation for field efficacy of QoIs needs further investigation. © 2015 Society of Chemical Industry.

Correlations between traits in soybean (Glycine max L.) naturally infected with Asian rust (Phakopsora pachyrhizi)

Soybean (Glycine max L.)-breeding programs aim to develop cultivars with high grain yields and high tolerance to Asian soybean rust (Phakopsora pachyrhizi). Considering that the traits targeted for breeding are mainly quantitative in nature, knowledge of associations between traits allows the breeder to formulate indirect selection strategies. In this study, we investigated phenotypic, genotypic, and environmental correlations between the agronomic traits of soybean plants naturally infected with P. pachyrhizi, and identified agronomic traits that would be useful in indirectly selecting soybean genotypes for high yields. The study was conducted on the Capim Branco Farm, Uberlândia, Brazil, with 15 soybean genotypes, which were cultivated in a completely randomized block design with four replications. Fourteen phenotypic traits were evaluated using the GENES software. The phenotypic and genotypic correlations were positive and of a high magnitude between the total number of pods and the number of pods with two or three grains, indicating that the total number of pods is a useful trait for the indirect selection of soybean genotypes for high grain yields. Strong environmental correlations were found between plant height at blooming and maturity and grain yield and yield components.

Reference Genes in the Pathosystem Phakopsora pachyrhizi/ Soybean Suitable for Normalization in Transcript Profiling

Phakopsora pachyrhizi is a devastating pathogen on soybean, endangering soybean production worldwide. Use of Host Induced Gene Silencing (HIGS) and the study of effector proteins could provide novel strategies for pathogen control. For both approaches quantification of transcript abundance by RT-qPCR is essential. Suitable stable reference genes for normalization are indispensable to obtain accurate RT-qPCR results. According to the Minimum Information for Publication of Quantitative Real-Time PCR Experiments (MIQE) guidelines and using algorithms geNorm and NormFinder we tested candidate reference genes from P. pachyrhizi and Glycine max for their suitability in normalization of transcript levels throughout the infection process. For P. pachyrhizi we recommend a combination of CytB and PDK or GAPDH for in planta experiments. Gene expression during in vitro stages and over the whole infection process was found to be highly unstable. Here, RPS14 and UbcE2 are ranked best by geNorm and NormFinder. Alternatively CytB that has the smallest Cq range (Cq: quantification cycle) could be used. We recommend specification of gene expression relative to the germ tube stage rather than to the resting urediospore stage. For studies omitting the resting spore and the appressorium stages a combination of Elf3 and RPS9, or PKD and GAPDH should be used. For normalization of soybean genes during rust infection Ukn2 and cons7 are recommended.