Comparative transcriptome analysis and identification of candidate effectors in two related rust species (Gymnosporangium yamadae and Gymnosporangium asiaticum)

Effects of Temperature, Moisture, and Ultraviolet Light on Germination, Infection, and Survival of Gymnosporangium yamadae Teliospores and Basidiospores

Apple rust caused by Gymnosporangium yamadae is a significant disease in China's main apple production areas. We evaluated the effects of temperature, moisture, and ultraviolet (UV) light on the germination, infection, and survival of teliospore horns and basidiospores under artificially controlled environmental conditions. The temperature required for the germination and infection of teliospores and basidiospores of G. yamadae ranged from 5 to 25°C, with an optimum temperature of approximately 17°C. The teliospore horns germinated after soaking in distilled water for 5 min and required at least 2.3 h of development to produce basidiospores under the most favorable conditions. The basidiospores germinated only in free water and produced germ tubes 0.8 h after being placed in the water. The half-life of the basidiospore was 72.5 h in the dark and only 9.5 h when exposed to intense UV light. The basidiospores inoculated on the host leaves required at least 2.3 h of water exposure to cause rust lesions. A revised Weibull model could describe the relationships between the germination and infection of teliospore horns and basidiospores with temperature and wetness duration. Collectively, these results can serve as a valuable guide for developing a model to predict future apple rust epidemics and establish a method for effective control strategies.

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.

A decade after the first Pucciniales genomes: A bibliometric snapshot of (post) genomics studies in three model rust fungi

Pucciniales (rust fungi) are one of the largest fungal order of plant pathogens. They collectively infect key crops such as wheat and soybean, and threaten global food security. In the early 2010s, the genome sequences of three rust fungi were released: Melampsora larici-populina (the poplar leaf rust fungus), Puccinia graminis f. sp. tritici (the wheat stem rust fungus), and Puccinia striiformis f. sp. triciti (the wheat stripe rust or wheat yellow rust fungus). The availability of those genomes has forwarded rust biology into the post-genomic era, sparking a series of genomics, transcriptomics, in silico, and functional studies. Here, we snapshot the last 10 years of post-genomics studies addressing M. larici-populina, P. graminis f. sp. tritici, and/or P. striiformis f. sp. tritici. This mini-review notably reveals the model species-centered structure of the research community, and highlights the drastic increase of the number of functional studies focused on effectors since 2014, which notably revealed chloroplasts as a central host compartment targeted by rust fungi. This mini-review also discusses genomics-facilitated studies in other rust species, and emerging post-genomic research trends related to fully-phased rust genomes.

Evolution of pathogenicity in obligate fungal pathogens and allied genera

Obligate fungal pathogens (ascomycetes and basidiomycetes) and oomycetes are known to cause diseases in cereal crop plants. They feed on living cells and most of them have learned to bypass the host immune machinery. This paper discusses some of the factors that are associated with pathogenicity drawing examples from ascomycetes, basidiomycetes and oomycetes, with respect to their manifestation in crop plants. The comparisons have revealed a striking similarity in the three groups suggesting convergent pathways that have arisen from three lineages independently leading to an obligate lifestyle. This review has been written with the intent, that new information on adaptation strategies of biotrophs, modifications in pathogenicity strategies and population dynamics will improve current strategies for breeding with stable resistance.

Reference Genes Selection of Gymnosporangium yamadae during the Interaction with Apple Leaves

Apple rust disease caused by Gymnosporangium yamadae is the one of the major threats to the development of the apple industry in China, but the pathogenic molecular mechanism of the disease remains unclear. It is imperative to screen out appropriate reference genes during the interaction between G. yamadae and apple leaves to analyze the gene expression patterns during the pathogenesis of G. yamadae. ACT, EF1, EF2, GAPDH, 40S, 60S, α-TUB, β-TUB and UBCE3 were selected as candidate reference genes based on the transcriptomic dataset of G. yamadae. The expression levels were tested by real-time quantitative PCR during time-course infection of apple leaves and the expression stabilities were evaluated by △Ct method as well as by three software (NormFinder, geNorm and BestKeeper) and one web-based analysis software (RefFinder). The expression stability of the candidate reference genes was further validated by using the effector candidate gene Cluster-3395.48660 as the target gene in RT-qPCR. According to the results by △Ct and BestKeeper, 40S, EF2 and EF1 were the most stable reference genes, while EF1, EF2 and GAPDH were the most stable reference genes based on the NormFinder analysis result. The geNorm recommended the most stable genes EF1, EF2 and α-TUB as reference genes. Comprehensive analysis results of the RefFinder indicated EF1, EF2 and α-TUB were the most suitable genes. Based on these results, EF1, EF2 and α-TUB were considered as reference genes for analyzing the gene expression profiles of Cluster-3395.48660 in different infection stages, and the results were consistent with the transcriptome data. All the results suggest that the combination of EF1, EF2 and α-TUB proved to be acceptable reference genes during the interaction between G. yamadae and apple leaves.

A Bursaphelenchus xylophilus Effector, BxSCD3, Suppresses Plant Defense and Contributes to Virulence

Bursaphelenchus xylophilus is the most economically important species of migratory plant-parasitic nematodes (PPNs) and causes severe damage to forestry in China. The successful infection of B. xylophilus relies on the secretion of a repertoire of effector proteins. The effectors, which suppress the host pine immune response, are key to the facilitation of B. xylophilus parasitism. An exhaustive list of candidate effectors of B. xylophilus was predicted, but not all have been identified and characterized. Here, an effector, named BxSCD3, has been implicated in the suppression of host immunity. BxSCD3 could suppress pathogen-associated molecular patterns (PAMPs) PsXEG1- and INF1-triggered cell death when it was secreted into the intracellular space in Nicotiana benthamiana. BxSCD3 was highly up-regulated in the early infection stages of B. xylophilus. BxSCD3 does not affect B. xylophilus reproduction, either at the mycophagous stage or the phytophagous stage, but it contributes to the virulence of B. xylophilus. Moreover, BxSCD3 significantly influenced the relative expression levels of defense-related (PR) genes PtPR-3 and PtPR-6 in Pinus thunbergii in the early infection stage. These results suggest that BxSCD3 is an important toxic factor and plays a key role in the interaction between B. xylophilus and host pine.

The haustorium: The root of biotrophic fungal pathogens

Biotrophic plant pathogenic fungi are among the dreadful pathogens that continuously threaten the production of economically important crops. The interaction of biotrophic fungal pathogens with their hosts necessitates the development of unique infection mechanisms and involvement of various virulence-associated components. Biotrophic plant pathogenic fungi have an exceptional lifestyle that supports nutrient acquisition from cells of a living host and are fully dependent on the host for successful completion of their life cycle. The haustorium, a specialized infection structure, is the key organ for biotrophic fungal pathogens. The haustorium is not only essential in the uptake of nutrients without killing the host, but also in the secretion and delivery of effectors into the host cells to manipulate host immune system and defense responses and reprogram the metabolic flow of the host. Although there is a number of unanswered questions in this area yet, results from various studies indicate that the haustorium is the root of biotrophic fungal pathogens. This review provides an overview of current knowledge of the haustorium, its structure, composition, and functions, which includes the most recent haustorial transcriptome studies.

Host Adaptation and Virulence in Heteroecious Rust Fungi

Rust fungi (Pucciniales, Basidiomycota) are obligate biotrophic pathogens that cause rust diseases in plants, inflicting severe damage to agricultural crops. Pucciniales possess the most complex life cycles known in fungi. These include an alternation of generations, the development of up to five different sporulating stages, and, for many species, the requirement of infecting two unrelated host plants during different parts of their life cycle, termed heteroecism. These fungi have been extensively studied in the past century through microscopy and inoculation studies, providing precise descriptions of their infection processes, although the molecular mechanisms underlying their unique biology are poorly understood. In this review, we cover recent genomic and life cycle transcriptomic studies in several heteroecious rust species, which provide insights into the genetic tool kits associated with host adaptation and virulence, opening new avenues for unraveling their unique evolution.

A Detailed Spatial Expression Analysis of Wing Phenotypes Reveals Novel Patterns of Odorant Binding Proteins in the Soybean Aphid, Aphis glycines

The wide range of insect niches has led to a rapid expansion of chemosensory gene families as well as their relatively independent evolution and a high variation. Previous studies have revealed some functions for odorant-binding proteins (OBPs) in processes beyond olfaction, such as gustation and reproduction. In this study, a comparative transcriptomic analysis strategy was applied for the soybean aphid, Aphis glycines, focusing on various functional tissues and organs of winged aphids, including the antenna, head, leg, wing, thorax, cauda, and cornicle. Detailed spatial OBP expression patterns in winged and wingless parthenogenetic aphids were detected by RT-qPCR. Twelve OBPs were identified, and three new OBPs in A. glycines are first reported. All OBPs showed comparatively higher expression in sensory organs and tissues, such as the antenna, head, or leg. Additionally, we found some novel expression patterns for aphid OBPs (Beckendorf et al., 2008). Five OBPs exhibited high-expression levels in the cauda and four in the cornicle (Biasio et al., 2015). Three genes (OBP2/3/15) were highly expressed in the wing (Calvello et al., 2003). Two (OBP3/15) were significantly more highly expressed in the wingless thorax than in the winged thorax with the wings removed, and these transcripts were significantly enriched in the removed wings. More details regarding OBP spatial expression were revealed under our strategy. These findings supported the existence of carrier transport functions other than for foreign chemicals and therefore broader ligand ranges of aphid OBPs. It is important for understanding how insect OBPs function in chemical perception as well as their other potential physiological functions.

Comparative transcriptome analysis and identification of candidate adaptive evolution genes of Miscanthus lutarioriparius and Miscanthus sacchariflorus

Miscanthus species are perennial C4 grasses that are considered promising energy crops because of their high biomass yields, excellent adaptability and low management costs. Miscanthus lutarioriparius and Miscanthus sacchariflorus are closely related subspecies that are distributed in different habitats. However, there are only a few reports on the mechanisms by which Miscanthus adapts to different environments. Here, comparative transcriptomic and morphological analyses were used to study the evolutionary adaptation of M. lutarioriparius and M. sacchariflorus to different habitats. In total, among 7586 identified orthologs, 2060 orthologs involved in phenylpropanoid biosynthesis and plant hormones were differentially expressed between the two species. Through an analysis of the Ka/Ks ratios of the orthologs, we estimated that the divergence time between the two species was approximately 4.37 Mya. In addition, 37 candidate positively selected orthologs (PSGs) that played important roles in the adaptation of these species to different habitats were identified. Then, the expression levels of 20 PSGs in response to flooding and drought stress were analyzed, and the analysis revealed significant changes in their expression levels. These results facilitate our understanding of the evolutionary adaptation to habitats and the speciation of M. lutarioriparius and M. sacchariflorus. We hypothesise that lignin synthesis genes are the main cause of the morphological differences between the two species. In summary, the plant nonspecific phospholipase C gene family and the receptor-like protein kinase gene family played important roles in the evolution of these two species.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01030-1.

Comparative transcriptomic analysis of candidate effectors to explore the infection and survival strategy of Bursaphelenchus xylophilus during different interaction stages with pine trees

BACKGROUND

The pine wood nematode (PWN), Bursaphelenchus xylophilus, is a devastating pathogen of many Pinus species in China. The aim of this study was to understand the interactive molecular mechanism of PWN and its host by comparing differentially expressed genes and candidate effectors from three transcriptomes of B. xylophilus at different infection stages.

RESULTS

In total, 62, 69 and 46 candidate effectors were identified in three transcriptomes (2.5 h postinfection, 6, 12 and 24 h postinoculation and 6 and 15 d postinfection, respectively). In addition to uncharacterized pioneers, other candidate effectors were involved in the degradation of host tissues, suppression of host defenses, targeting plant signaling pathways, feeding and detoxification, which helped B. xylophilus survive successfully in the host. Seven candidate effectors were identified in both our study and the B. xylophilus transcriptome at 2.5 h postinfection, and one candidate effector was identified in all three transcriptomes. These common candidate effectors were upregulated at infection stages, and one of them suppressed pathogen-associated molecular pattern (PAMP) PsXEG1-triggered cell death in Nicotiana benthamiana.

CONCLUSIONS

The results indicated that B. xylophilus secreted various candidate effectors, and some of them continued to function throughout all infection stages. These various candidate effectors were important to B. xylophilus infection and survival, and they functioned in different ways (such as breaking down host cell walls, suppressing host defenses, promoting feeding efficiency, promoting detoxification and playing virulence functions). The present results provide valuable resources for in-depth research on the pathogenesis of B. xylophilus from the perspective of effectors.

Effector Biology of Biotrophic Plant Fungal Pathogens: Current Advances and Future Prospects

The interaction of fungal pathogens with their host requires a novel invading mechanism and the presence of various virulence-associated components responsible for promoting the infection. The small secretory proteins, explicitly known as effector proteins, are one of the prime mechanisms of host manipulation utilized by the pathogen to disarm the host. Several effector proteins are known to translocate from fungus to the plant cell for host manipulation. Many fungal effectors have been identified using genomic, transcriptomic, and bioinformatics approaches. Most of the effector proteins are devoid of any conserved signatures, and their prediction based on sequence homology is very challenging, therefore by combining the sequence consensus based upon machine learning features, multiple tools have also been developed for predicting apoplastic and cytoplasmic effectors. Various post-genomics approaches like transcriptomics of virulent isolates have also been utilized for identifying active consortia of effectors. Significant progress has been made in understanding biotrophic effectors; however, most of it is underway due to their complex interaction with host and complicated recognition and signaling networks. This review discusses advances, and challenges in effector identification and highlighted various features of the potential effector proteins and approaches for understanding their genetics and strategies for regulation.

Transcriptome Analysis of Apple Leaves Infected by the Rust Fungus Gymnosporangium yamadae at Two Sporulation Stages

Apple rust disease caused by Gymnosporangium yamadae is one of the major threats to apple orchards. In this study, dual RNA-seq analysis was conducted to simultaneously monitor gene expression profiles of G. yamadae and infected apple leaves during the formation of rust spermogonia and aecia. The molecular mechanisms underlying this compatible interaction at 10 and 30 days postinoculation (dpi) indicate a significant reaction from the host plant and comprise detoxication pathways at the earliest stage and the induction of secondary metabolism pathways at 30 dpi. Such host reactions have been previously reported in other rust pathosystems and may represent a general reaction to rust infection. G. yamadae transcript profiling indicates a conserved genetic program in spermogonia and aecia that is shared with other rust fungi, whereas secretome prediction reveals the presence of specific secreted candidate effector proteins expressed during apple infection. Unexpectedly, the survey of fungal unigenes in the transcriptome assemblies of inoculated and mock-inoculated apple leaves reveals that G. yamadae infection may modify the fungal community composition in the apple phyllosphere at 30 dpi. Collectively, our results provide novel insights into the compatible apple-G. yamadae interaction and advance the knowledge of this heteroecious demicyclic rust fungus.

Comparative transcriptomics of Gymnosporangium spp. teliospores reveals a conserved genetic program at this specific stage of the rust fungal life cycle

BACKGROUND

Gymnosporangium spp. are fungal plant pathogens causing rust disease and most of them are known to infect two different host plants (heteroecious) with four spore stages (demicyclic). In the present study, we sequenced the transcriptome of G. japonicum teliospores on its host plant Juniperus chinensis and we performed comparison to the transcriptomes of G. yamadae and G. asiaticum at the same life stage, that happens in the same host but on different organs.

RESULTS

Functional annotation for the three Gymnosporangium species showed the expression of a conserved genetic program with the top abundant cellular categories corresponding to energy, translation and signal transduction processes, indicating that this life stage is particularly active. Moreover, the survey of predicted secretomes in the three Gymnosporangium transcriptomes revealed shared and specific genes encoding carbohydrate active enzymes and secreted proteins of unknown function that could represent candidate pathogenesis effectors. A transcript encoding a hemicellulase of the glycoside hydrolase 26 family, previously identified in other rust fungi, was particularly highly expressed suggesting a general role in rust fungi. The comparison between the transcriptomes of the three Gymnosporangium spp. and selected Pucciniales species in different taxonomical families allowed to identify lineage-specific protein families that may relate to the biology of teliospores in rust fungi. Among clustered gene families, 205, 200 and 152 proteins were specifically identified in G. japonicum, G. yamadae and G. asiaticum, respectively, including candidate effectors expressed in teliospores.

CONCLUSIONS

This comprehensive comparative transcriptomics study of three Gymnosporangium spp. identified gene functions and metabolic pathways particularly expressed in teliospores, a stage of the life cycle that is mostly overlooked in rust fungi. Secreted protein encoding transcripts expressed in teliospores may reveal new candidate effectors related to pathogenesis. Although this spore stage is not involved in host plant infection but in the production of basidiospores infecting plants in the Amygdaloideae, we speculate that candidate effectors may be expressed as early as the teliospore stage for preparing further infection by basidiospores.

Evolutionary adaptation analysis of immune defense and hypoxia tolerance in two closely related Marsupenaeus species based on comparative transcriptomics

Kuruma shrimp, a major farmed shrimp species in the world, includes two cryptic or sibling species, Form I (Marsupenaeus japonicus) and Form II (Marsupenaeus pulchricaudatus). Due to the lack of genomic resources, little is known about the molecular mechanisms associated with immune defense and hypoxia tolerance. Here, we sequenced the transcriptomes of two closely related Marsupenaeus species and compared genomic divergence. This study obtained 77049 and 84561 unigenes with N50 values of 1281bp and 1244bp for M. japonicus and M. pulchricaudatus, respectively, and 5036 pairs of putative orthologs were identified between two Marsupenaeus species. Estimation of Ka/Ks ratios indicated that 165 orthologous genes may be under positive selection (Ka/Ks > 0.5), including 49 pairs with a Ka/Ks ratio >1. According to the peak of synonymous rates, the divergence time between M. japonicus and M. pulchricaudatus was about 0.26-0.69 Mya. These positively selected orthologous genes related to the immune process mainly comprised single VWC domain protein, legumain, ras-related C3 botulinum, caspase, C-type lectin and were enriched in functions related to immune (Toll-like receptor and PI3K-Akt signaling) and hypoxia signaling (HIF-1 signaling and VEGF signaling). In this study, dozens of caspase-like unigenes were screened from two Marsupenaeus transcriptomes. Among these, the PjCaspase orthologous gene was subjected to positive selection (Ka/Ks = 1.22), which had different secondary and three-dimensional structure prediction. Based on the single copy caspase gene, eight populations of Marsupenaeus species were divided into two phylogeographic lineages from the East and South China. We characterized the transcriptomes of the two Marsupenaeus species and obtained several key orthologs associated with immune defense and hypoxia tolerance, which provides new insights into the immunity and genetic divergence of the two varieties. Moreover, this study will facilitate further comparative genomic studies of the two varieties.

Advances in understanding obligate biotrophy in rust fungi

Contents Summary 1190 I. Introduction 1190 II. Rust fungi: a diverse and serious threat to agriculture 1191 III. The different facets of rust life cycles and unresolved questions about their evolution 1191 IV. The biology of rust infection 1192 V. Rusts in the genomics era: the ever-expanding list of candidate effector genes 1195 VI. Functional characterization of rust effectors 1197 VII. Putting rusts to sleep: Pucciniales research outlooks 1201 Acknowledgements 1202 References 1202 SUMMARY: Rust fungi (Pucciniales) are the largest group of plant pathogens and represent one of the most devastating threats to agricultural crops worldwide. Despite the economic importance of these highly specialized pathogens, many aspects of their biology remain obscure, largely because rust fungi are obligate biotrophs. The rise of genomics and advances in high-throughput sequencing technology have presented new options for identifying candidate effector genes involved in pathogenicity mechanisms of rust fungi. Transcriptome analysis and integrated bioinformatics tools have led to the identification of key genetic determinants of host susceptibility to infection by rusts. Thousands of genes encoding secreted proteins highly expressed during host infection have been reported for different rust species, which represents significant potential towards understanding rust effector function. Recent high-throughput in planta expression screen approaches (effectoromics) have pushed the field ahead even further towards predicting high-priority effectors and identifying avirulence genes. These new insights into rust effector biology promise to inform future research and spur the development of effective and sustainable strategies for managing rust diseases.