Puccinia striiformis f. sp. tritici microRNA-like RNA 1 (Pst-milR1), an important pathogenicity factor of Pst, impairs wheat resistance to Pst by suppressing the wheat pathogenesis-related 2 gene

Fol-milR1, a pathogenicity factor of Fusarium oxysporum, confers tomato wilt disease resistance by impairing host immune responses

Although it is well known that miRNAs play crucial roles in multiple biological processes, there is currently no evidence indicating that milRNAs from Fusarium oxysporum f. sp. lycopersici (Fol) interfere with tomato resistance during infection. Here, using sRNA-seq, we demonstrate that Fol-milR1, a trans-kingdom small RNA, is exported into tomato cells after infection. The knockout strain ∆Fol-milR1 displays attenuated pathogenicity to the susceptible tomato cultivar 'Moneymaker'. On the other hand, Fol-milR1 overexpression strains exhibit enhanced virulence against the resistant cultivar 'Motelle'. Several tomato mRNAs are predicted targets of Fol-milR1. Among these genes, Solyc06g007430 (encoding the CBL-interacting protein kinase, SlyFRG4) is regulated at the posttranscriptional level by Fol-milR1. Furthermore, SlyFRG4 loss-of-function alleles created using CRISPR/Cas9 in tomato ('Motelle') exhibit enhanced disease susceptibility to Fol, further supporting the idea that SlyFRG4 is essential for tomato wilt disease resistance. Notably, our results using immunoprecipitation with specific antiserum suggest that Fol-milR1 interferes with the host immunity machinery by binding to tomato ARGONAUTE 4a (SlyAGO4a). Furthermore, virus-induced gene silenced (VIGS) knock-down SlyAGO4a plants exhibit reduced susceptibility to Fol. Together, our findings support a model in which Fol-milR1 is an sRNA fungal effector that suppresses host immunity by silencing a disease resistance gene, thus providing a novel virulence strategy to achieve infection.

Extracellular Vesicles: Emerging Players in Plant Defense Against Pathogens

Communication between plants and interacting microorganisms requires functional molecule trafficking, which is essential for host defense and pathogen virulence. Extracellular vesicles (EVs) are single membrane-bound spheres that carry complex cargos, including lipids, proteins, and nucleic acids. They mediate cell-to-cell communication via the transfer of molecules between cells. Plant EVs have been isolated from many plant species and play a prominent role in immune system modulation and plant defense response. Recent studies have shown that plant EVs are emerging players in cross-kingdom regulation and contribute to plant immunity by mediating the trafficking of regulatory small RNA into pathogens, leading to the silencing of pathogen virulence-related genes. This review summarizes the current understanding of plant EV isolation technologies, the role of plant EVs in plant immunity, and the mechanism of plant EV biogenesis, as well as approaches for how these findings can be developed into innovative strategies for crop protection.

The stem rust fungus Puccinia graminis f. sp. tritici induces centromeric small RNAs during late infection that are associated with genome-wide DNA methylation

Background

Silencing of transposable elements (TEs) is essential for maintaining genome stability. Plants use small RNAs (sRNAs) to direct DNA methylation to TEs (RNA-directed DNA methylation; RdDM). Similar mechanisms of epigenetic silencing in the fungal kingdom have remained elusive.

Results

We use sRNA sequencing and methylation data to gain insight into epigenetics in the dikaryotic fungus Puccinia graminis f. sp. tritici (Pgt), which causes the devastating stem rust disease on wheat. We use Hi-C data to define the Pgt centromeres and show that they are repeat-rich regions (~250 kb) that are highly diverse in sequence between haplotypes and, like in plants, are enriched for young TEs. DNA cytosine methylation is particularly active at centromeres but also associated with genome-wide control of young TE insertions. Strikingly, over 90% of Pgt sRNAs and several RNAi genes are differentially expressed during infection. Pgt induces waves of functionally diversified sRNAs during infection. The early wave sRNAs are predominantly 21 nts with a 5′ uracil derived from genes. In contrast, the late wave sRNAs are mainly 22-nt sRNAs with a 5′ adenine and are strongly induced from centromeric regions. TEs that overlap with late wave sRNAs are more likely to be methylated, both inside and outside the centromeres, and methylated TEs exhibit a silencing effect on nearby genes.

Conclusions

We conclude that rust fungi use an epigenetic silencing pathway that might have similarity with RdDM in plants. The Pgt RNAi machinery and sRNAs are under tight temporal control throughout infection and might ensure genome stability during sporulation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01123-z.

Functional Characterization of the Wheat Macrophage Migration Inhibitory Factor TaMIF1 in Wheat-Stripe Rust (Puccinia striiformis) Interaction

Simple Summary

There have been many breakthroughs in MIF function and mechanism investigation in vertebrates, but it has rarely been studied in plants. Here, we aimed to characterize the function of MIF in wheat and its potential role in Wheat-Stripe rust interaction. We showed that wheat MIF has some similarities with that MIF in vertebrates, such as subcellular localization in both the cytosol and nuclei, as well as significant tautomerase activity, and both can inhibit Bax-induced programmed cell death. In the wheat–Pst interaction, TaMIF1 is upregulated during Pst infection. Silencing TaMIF1 decreased Pst infection of wheat tissues, and the accumulation of ROS was increased in TaMIF1-silenced wheat leaves, which hinted that TaMIF1 mainly modulates the ROS signaling and then alters the subsequent immune responses. The function characterization of TaMIF1 provides significant insight into the role of MIFs across kingdoms and helpful in-depth functional mechanism analysis on these proteins.

Abstract

Macrophage migration inhibitory factor (MIF), named for its role in inhibiting macrophage/monocyte migration, has multiple functions in modulation of inflammation, cell proliferation, angiogenesis, and tumorigenesis in vertebrates. Although homologs of this gene can be found in plants, the function of MIF in plants remains obscure. Here, we characterized TaMIF1 in Triticum aestivum resembling the MIF secreted from Homo sapiens. Transcript analysis revealed that TaMIF1 responded to stripe rust infection of wheat and was upregulated during the infection stage. TaMIF1 was localized to both the cytosol and nuclei in wheat mesophyll protoplast. Additionally, TaMIF1 possessed significant tautomerase activity, indicating conservation of MIFs across kingdoms. Agrobacterium tumefaciens infiltration assay demonstrated that TaMIF1 was capable of suppressing programmed cell death hinting its role in plant immunity. Heterologous expression of TaMIF1 increased fission yeast sensitivity to oxidative stress. Silencing TaMIF1 decreased the susceptibility of wheat to Pst seemingly through increasing reactive oxygen species accumulation. In conclusion, functions of the TaMIF1 were investigated in this study, which provides significant insight into understanding the role of MIFs across kingdoms.

Small RNAs in Plant Immunity and Virulence of Filamentous Pathogens

Gene silencing guided by small RNAs governs a broad range of cellular processes in eukaryotes. Small RNAs are important components of plant immunity because they contribute to pathogen-triggered transcription reprogramming and directly target pathogen RNAs. Recent research suggests that silencing of pathogen genes by plant small RNAs occurs not only during viral infection but also in nonviral pathogens through a process termed host-induced gene silencing, which involves trans-species small RNA trafficking. Similarly, small RNAs are also produced by eukaryotic pathogens and regulate virulence. This review summarizes the small RNA pathways in both plants and filamentous pathogens, including fungi and oomycetes, and discusses their role in host-pathogen interactions. We highlight secondarysmall interfering RNAs of plants as regulators of immune receptor gene expression and executors of host-induced gene silencing in invading pathogens. The current status and prospects of trans-species gene silencing at the host-pathogen interface are discussed.

Fusarium graminearum DICER-like-dependent sRNAs are required for the suppression of host immune genes and full virulence

In filamentous fungi, gene silencing by RNA interference (RNAi) shapes many biological processes, including pathogenicity. Recently, fungal small RNAs (sRNAs) have been shown to act as effectors that disrupt gene activity in interacting plant hosts, thereby undermining their defence responses. We show here that the devastating mycotoxin-producing ascomycete Fusarium graminearum (Fg) utilizes DICER-like (DCL)-dependent sRNAs to target defence genes in two Poaceae hosts, barley (Hordeum vulgare, Hv) and Brachypodium distachyon (Bd). We identified 104 Fg-sRNAs with sequence homology to host genes that were repressed during interactions of Fg and Hv, while they accumulated in plants infected by the DCL double knock-out (dKO) mutant PH1-dcl1/2. The strength of target gene expression correlated with the abundance of the corresponding Fg-sRNA. Specifically, the abundance of three tRNA-derived fragments (tRFs) targeting immunity-related Ethylene overproducer 1-like 1 (HvEOL1) and three Poaceae orthologues of Arabidopsis thaliana BRI1-associated receptor kinase 1 (HvBAK1, HvSERK2 and BdSERK2) was dependent on fungal DCL. Additionally, RNA-ligase-mediated Rapid Amplification of cDNA Ends (RLM-RACE) identified infection-specific degradation products for the three barley gene transcripts, consistent with the possibility that tRFs contribute to fungal virulence via targeted gene silencing.

How to resist parasitic plants: pre- and post-attachment strategies

The lifecycle of parasitic plants can be divided into pre-attachment and post-attachment phases that equate to free living and parasitic stages. Similarly, plant resistance to parasitic plants can be defined as pre-attachment and post-attachment resistance. Parasitic plants rely on host cues for successful host invasion. During pre-attachment resistance, changes in the composition of host signals can disrupt parasitic plant development and ultimately host invasion. Recent studies have only now begun to elucidate the genetic elements in the host that promote pre-attachment resistance. In comparison, new research points to post-attachment resistance using the common molecular mechanisms utilized by the plant immune system during plant-pathogen interactions. In kind, parasitic plants secrete proteinaceous and RNA-based effectors post-attachment to subvert the host immune system.

Identification of microRNA-like RNAs from Trichoderma asperellum DQ-1 during its interaction with tomato roots using bioinformatic analysis and high-throughput sequencing

MicroRNA-like small RNAs (milRNAs) and their regulatory roles in the interaction between plant and fungus have recently aroused keen interest of plant pathologists. Trichoderma spp., one of the widespread biocontrol fungi, can promote plant growth and induce plant disease resistance. To investigate milRNAs potentially involved in the interaction between Trichoderma and tomato roots, a small RNA (sRNA) library expressed during the interaction of T. asperellum DQ-1 and tomato roots was constructed and sequenced using the Illumina HiSeqTM 2500 sequencing platform. From 13,464,142 sRNA reads, we identified 21 milRNA candidates that were similar to other known microRNAs in the miRBase database and 22 novel milRNA candidates that possessed a stable microRNA precursor hairpin structure. Among them, three milRNA candidates showed different expression level in the interaction according to the result of stem-loop RT-PCR indicating that these milRNAs may play a distinct regulatory role in the interaction between Trichoderma and tomato roots. The potential transboundary milRNAs from T. asperellum and their target genes in tomato were predicted by bioinformatics analysis. The results revealed that several interesting proteins involved in plant growth and development, disease resistance, seed maturation, and osmotic stress signal transduction might be regulated by the transboundary milRNAs. To our knowledge, this is the first report of milRNAs taking part in the process of interaction of T. asperellum and tomato roots and associated with plant promotion and disease resistance. The results might be useful to unravel the mechanism of interaction between Trichoderma and tomato.

Genome-Wide Expression Profiling of Small RNAs in Indian Strain of Rhizoctonia solani AG1-1A Reveals Differential Regulation of milRNAs during Pathogenesis and Crosstalk of Gene Regulation

Rhizoctonia solani AG1-1A is a necrotrophic fungus that causes sheath blight disease in rice. The reliable resistant source against this phytopathogenic fungus is not available in the gene pool of rice. Better understanding of pathogen genomics and gene regulatory networks are critical to devise alternate strategies for developing resistance against this noxious pathogen. In this study, miRNA-like RNAs (milRNAs) of an Indian strain of R. solani were identified by deep sequencing of small RNAs. We identified 128 known and 22 novel milRNAs from 20,963,123 sequence reads. These milRNAs showed 1725 target genes in the fungal genome which include genes associated with growth, development, pathogenesis and virulence of R. solani. Notably, these fungal milRNAs showed their target genes in host (rice) genome also which were later verified by qRT-PCR. The host target genes are associated with auxin metabolism, hypersensitive response, defense genes, and genes related to growth and development of rice. Osa-vacuolar-sorting receptor precursor: Rhi-milR-13, Osa-KANADI1:Rhi-milR-124, Osa-isoflavone reductase: Rhi-milR-135, Osa-nuclear transcription factor Y:Rhi-milR-131, Osa-NB-ARC domain containing protein: Rhi-milR-18, and Osa-OsFBX438: Rhi-milR-142 are notable potential regulons of host target genes: fungal milRNAs that need to be investigated for better understanding of the crosstalk of RNAi pathways between R. solani and rice. The detailed expression analysis of 17 milRNAs by qRT-PCR was analysed during infection at different time points of inoculation, at different growth stages of the host, in four different genotypes of the host, and also in four different strains of fungi which revealed differential regulation of milRNAs associated with pathogenesis and virulence. This study highlights several important findings on fungal milRNAs which need to be further studied and characterized to decipher the gene expression and regulation of this economically important phytopathogen.

mRNA Inventory of Extracellular Vesicles from Ustilago maydis

Extracellular vesicles (EVs) can transfer diverse RNA cargo for intercellular communication. EV-associated RNAs have been found in diverse fungi and were proposed to be relevant for pathogenesis in animal hosts. In plant-pathogen interactions, small RNAs are exchanged in a cross-kingdom RNAi warfare and EVs were considered to be a delivery mechanism. To extend the search for EV-associated molecules involved in plant-pathogen communication, we have characterised the repertoire of EV-associated mRNAs secreted by the maize smut pathogen, Ustilago maydis. For this initial survey, we examined EV-enriched fractions from axenic filamentous cultures that mimic infectious hyphae. EV-associated RNAs were resistant to degradation by RNases and the presence of intact mRNAs was evident. The set of mRNAs enriched inside EVs relative to the fungal cells are functionally distinct from those that are depleted from EVs. mRNAs encoding metabolic enzymes are particularly enriched. Intriguingly, mRNAs of some known effectors and other proteins linked to virulence were also found in EVs. Furthermore, several mRNAs enriched in EVs are also upregulated during infection, suggesting that EV-associated mRNAs may participate in plant-pathogen interactions.

RNAs - a new frontier in crop protection

Small RNA (sRNA)-mediated RNA interference (RNAi) is a regulatory mechanism conserved in almost all eukaryotes. sRNAs play a critical role in host pathogen interactions either endogenously or by traveling between the interacting organisms and inducing 'cross-Kingdom RNAi' in the counterparty. Cross-kingdom RNAi is the mechanistic basis of host-induced gene silencing (HIGS), which relies on genetically expressing pathogen-gene targeting RNAs in crops, and has been successfully utilized against both microbial pathogens and pests. HIGS is limited by the need to produce genetically engineered crops. Recent studies have demonstrated that double-stranded RNAs and sRNAs can be efficiently taken up by many fungal pathogens, and induce gene silencing in fungal cells. This mechanism, termed 'environmental RNAi', allows direct application of pathogen-gene targeting RNAs onto crops to silence fungal virulence-related genes for plant protection. In this review, we will focus on how we can leverage cross-kingdom RNAi and environmental RNAi for crop disease control.

Message in a Bubble: Shuttling Small RNAs and Proteins Between Cells and Interacting Organisms Using Extracellular Vesicles

Communication between plant cells and interacting microorganisms requires the secretion and uptake of functional molecules to and from the extracellular environment and is essential for the survival of both plants and their pathogens. Extracellular vesicles (EVs) are lipid bilayer-enclosed spheres that deliver RNA, protein, and metabolite cargos from donor to recipient cells and participate in many cellular processes. Emerging evidencehas shown that both plant and microbial EVs play important roles in cross-kingdom molecular exchange between hosts and interacting microbes to modulate host immunity and pathogen virulence. Recent studies revealed that plant EVs function as a defense system by encasing and delivering small RNAs (sRNAs) into pathogens, thereby mediating cross-species and cross-kingdom RNA interference to silence virulence-related genes. This review focuses on the latest advances in our understanding of plant and microbial EVs and their roles in transporting regulatory molecules, especially sRNAs, between hosts and pathogens. EV biogenesis and secretion are also discussed, as EV function relies on these important processes.

Trichoderma and the Plant Heritable Priming Responses

There is no doubt that Trichoderma is an inhabitant of the rhizosphere that plays an important role in how plants interact with the environment. Beyond the production of cell wall degrading enzymes and metabolites, Trichoderma spp. can protect plants by inducing faster and stronger immune responses, a mechanism known as priming, which involves enhanced accumulation of dormant cellular proteins that function in intracellular signal amplification. One example of these proteins is the mitogen-activated protein kinases (MAPK) that are triggered by the rise of cytosolic calcium levels and cellular redox changes following a stressful challenge. Transcription factors such as WRKYs, MYBs, and MYCs, play important roles in priming as they act as regulatory nodes in the transcriptional network of systemic defence after stress recognition. In terms of long-lasting priming, Trichoderma spp. may be involved in plants epigenetic regulation through histone modifications and replacements, DNA (hypo)methylation, and RNA-directed DNA methylation (RdDM). Inheritance of these epigenetic marks for enhanced resistance and growth promotion, without compromising the level of resistance of the plant’s offspring to abiotic or biotic stresses, seems to be an interesting path to be fully explored.

miRNA Mediated Regulation and Interaction between Plants and Pathogens

Plants have evolved diverse molecular mechanisms that enable them to respond to a wide range of pathogens. It has become clear that microRNAs, a class of short single-stranded RNA molecules that regulate gene expression at the transcriptional or post-translational level, play a crucial role in coordinating plant-pathogen interactions. Specifically, miRNAs have been shown to be involved in the regulation of phytohormone signals, reactive oxygen species, and NBS-LRR gene expression, thereby modulating the arms race between hosts and pathogens. Adding another level of complexity, it has recently been shown that specific lncRNAs (ceRNAs) can act as decoys that interact with and modulate the activity of miRNAs. Here we review recent findings regarding the roles of miRNA in plant defense, with a focus on the regulatory modes of miRNAs and their possible applications in breeding pathogen-resistance plants including crops and trees. Special emphasis is placed on discussing the role of miRNA in the arms race between hosts and pathogens, and the interaction between disease-related miRNAs and lncRNAs.

Vm-milR37 contributes to pathogenicity by regulating glutathione peroxidase gene VmGP in Valsa mali

MicroRNAs play important roles in various biological processes by regulating their corresponding target genes. However, the function and regulatory mechanism of fungal microRNA-like RNAs (milRNAs) are still largely unknown. In this study, a milRNA (Vm-milR37) was isolated and identified from Valsa mali, which causes the most serious disease on the trunk of apple trees in China. Based on the results of deep sequencing and quantitative reverse transcription PCR, Vm-milR37 was found to be expressed in the mycelium, while it was not expressed during the V. mali infection process. Overexpression of Vm-milR37 did not affect vegetative growth, but significantly decreased pathogenicity. Based on degradome sequencing, the target of Vm-milR37 was identified as VmGP, a glutathione peroxidase. The expression of Vm-milR37 and VmGP showed a divergent trend in V. mali-apple interaction samples and Vm-milR37 overexpression transformants. The expression of VmGP could be suppressed significantly by Vm-milR37 when coexpressed in tobacco leaves. Deletion of VmGP showed significantly reduced pathogenicity compared with the wild type. VmGP deletion mutants showed more sensitivity to hydrogen peroxide. Apple leaves inoculated with Vm-milR37 overexpression transformants and VmGP deletion mutant displayed increased accumulation of reactive oxygen species compared with the wild type. Thus, Vm-milR37 plays a critical role in pathogenicity by regulating VmGP, which contributes to the oxidative stress response during V. mali infection. These results provide important evidence to define the roles of milRNAs and their corresponding target genes in pathogenicity.

Rhizospheric Plant-Microbe Interactions: miRNAs as a Key Mediator

The importance of microorganisms in plant development, nutrition, and stress resistance is unquestioned and has led to a more holistic approach of plant-microbe interactions, under the holobiont concept. The structure of the plant microbiota is often described as host driven, especially in the rhizosphere, where microbial communities are shaped by diverse rhizodeposits. Gradually, this anthropogenic vision is fading and being replaced by the idea that plants and microorganisms co-shape the plant microbiota. Through coevolution, plants and microbes have developed cross-kingdom communication channels. Here, we propose that miRNAs are crucial mediators of plant-microbe interactions and microbiota shaping in the rhizosphere. Moreover, we suggest, as an alternative to generally unsuccessful strategies based on microbial inoculants, miRNAs as a promising tool for novel holobiont engineering.

In Silico Identification of the Full Complement of Subtilase-Encoding Genes and Characterization of the Role of TaSBT1.7 in Resistance Against Stripe Rust in Wheat

Plant subtilases (SBTs) or subtilisin-like proteases comprise a very diverse family of serine peptidases that participates in a broad spectrum of biological functions. Despite increasing evidence for roles of SBTs in plant immunity in recent years, little is known about wheat (Triticum aestivum) SBTs (TaSBTs). Here, we identified 255 TaSBT genes from bread wheat using the latest version 2.0 of the reference genome sequence. The SBT family can be grouped into five clades, from TaSBT1 to TaSBT5, based on a phylogenetic tree constructed with deduced protein sequences. In silico protein-domain analysis revealed the existence of considerable sequence diversification of the TaSBT family which, together with the local clustered gene distribution, suggests that TaSBT genes have undergone extensive functional diversification. Among those TaSBT genes whose expression was altered by biotic factors, TaSBT1.7 was found to be induced in wheat leaves by chitin and flg22 elicitors, as well as six examined pathogens, implying a role for TaSBT1.7 in plant defense. Transient overexpression of TaSBT1.7 in Nicotiana benthamiana leaves resulted in necrotic cell death. Moreover, knocking down TaSBT1.7 in wheat using barley stripe mosaic virus-induced gene silencing compromised the hypersensitive response and resistance against Puccinia striiformis f. sp. tritici, the causal agent of wheat stripe rust. Taken together, this study defined the full complement of wheat SBT genes and provided evidence for a positive role of one particular member, TaSBT1.7, in the incompatible interaction between wheat and a stripe rust pathogen.

Distinct Transcriptomic Reprogramming in the Wheat Stripe Rust Fungus During the Initial Infection of Wheat and Barberry

Puccinia striiformis f. sp. tritici is the causal agent of wheat stripe rust that causes severe yield losses all over the world. As a macrocyclic heteroecious rust fungus, it is able to infect two unrelated host plants, wheat and barberry. Its urediniospores infect wheat and cause disease epidemic, while its basidiospores parasitize barberry to fulfill the sexual reproduction. This complex life cycle poses interesting questions on the different mechanisms of pathogenesis underlying the infection of the two different hosts. In the present study, transcriptomes of P. striiformis f. sp. tritici during the initial infection of wheat and barberry leaves were qualitatively and quantitatively compared. As a result, 142 wheat-specifically expressed genes (WEGs) were identified, which was far less than the 2,677 barberry-specifically expressed genes (BEGs). A larger proportion of evolutionarily conserved genes were observed in BEGs than that in WEGs, implying a longer history of the interaction between P. striiformis f. sp. tritici and barberry. Additionally, P. striiformis f. sp. tritici differentially expressed genes (DEGs) between wheat at 1 and 2 days postinoculation (dpi) and barberry at 3 and 4 dpi were identified by quantitative analysis. Gene Ontology analysis of these DEGs and expression patterns of P. striiformis f. sp. tritici pathogenic genes, including those encoding candidate secreted effectors, cell wall-degrading enzymes, and nutrient transporters, demonstrated that urediniospores and basidiospores exploited distinct strategies to overcome host defense systems. These results represent the first analysis of the P. striiformis f. sp. tritici transcriptome in barberry and contribute to a better understanding of the evolutionary processes and strategies of different types of rust spores during the infection process on different hosts.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Plants under the Attack of Allies: Moving towards the Plant Pathobiome Paradigm

Plants are functional macrobes living in a close association with diverse communities of microbes and viruses as complex systems that continuously interact with the surrounding environment. The microbiota within the plant holobiont serves various essential and beneficial roles, such as in plant growth at different stages, starting from seed germination. Meanwhile, pathogenic microbes—differentiated from the rest of the plant microbiome based on their ability to damage the plant tissues through transient blooming under specific conditions—are also a part of the plant microbiome. Recent advances in multi-omics have furthered our understanding of the structure and functions of plant-associated microbes, and a pathobiome paradigm has emerged as a set of organisms (i.e., complex eukaryotic, microbial, and viral communities) within the plant’s biotic environment which interact with the host to deteriorate its health status. Recent studies have demonstrated that the one pathogen–one disease hypothesis is insufficient to describe the disease process in many cases, particularly when complex organismic communities are involved. The present review discusses the plant holobiont and covers the steady transition of plant pathology from the one pathogen–one disease hypothesis to the pathobiome paradigm. Moreover, previous reports on model plant diseases, in which more than one pathogen or co-operative interaction amongst pathogenic microbes is implicated, are reviewed and discussed.

Recognition and defence of plant-infecting fungal pathogens

Attempted infections of plants with fungi result in diverse outcomes ranging from symptom-less resistance to severe disease and even death of infected plants. The deleterious effect on crop yield have led to intense focus on the cellular and molecular mechanisms that explain the difference between resistance and susceptibility. This research has uncovered plant resistance or susceptibility genes that explain either dominant or recessive inheritance of plant resistance with many of them coding for receptors that recognize pathogen invasion. Approaches based on cell biology and phytochemistry have contributed to identifying factors that halt an invading fungal pathogen from further invasion into or between plant cells. Plant chemical defence compounds, antifungal proteins and structural reinforcement of cell walls appear to slow down fungal growth or even prevent fungal penetration in resistant plants. Additionally, the hypersensitive response, in which a few cells undergo a strong local immune reaction, including programmed cell death at the site of infection, stops in particular biotrophic fungi from spreading into surrounding tissue. In this review, we give a general overview of plant recognition and defence of fungal parasites tracing back to the early 20th century with a special focus on Triticeae and on the progress that was made in the last 30 years.

Integrated Analysis of MicroRNA and Target Genes in Brachypodium distachyon Infected by Magnaporthe oryzae by Small RNA and Degradome Sequencing

Rice blast caused by Magnaporthe oryzae is one of the most important diseases that seriously threaten rice production. Brachypodium distachyon is a grass species closely related to grain crops, such as rice, barley, and wheat, and has become a new model plant of Gramineae. In this study, 15 small RNA samples were sequenced to examine the dynamic changes in microRNA (miRNA) expression in B. distachyon infected by M. oryzae at 0, 24, and 48 h after inoculation. We identified 432 conserved miRNAs and 288 predicted candidate miRNAs in B. distachyon. Additionally, there were 7 and 19 differentially expressed miRNAs at 24 and 48 h post-inoculation, respectively. Furthermore, using degradome sequencing, we identified 2,126 genes as targets for 308 miRNAs; using quantitative real-time PCR (qRT-PCR), we validated five miRNA/target regulatory units involved in B. distachyon-M. oryzae interactions. Moreover, using co-transformation technology, we demonstrated that BdNAC21 was negatively regulated by miR164c. This study provides a new approach for identifying resistance genes in B. distachyon by mining the miRNA regulatory network of host-pathogen interactions.

The Evolutionary Significance of RNAi in the Fungal Kingdom

RNA interference (RNAi) was discovered at the end of last millennium, changing the way scientists understood regulation of gene expression. Within the following two decades, a variety of different RNAi mechanisms were found in eukaryotes, reflecting the evolutive diversity that RNAi entails. The essential silencing mechanism consists of an RNase III enzyme called Dicer that cleaves double-stranded RNA (dsRNA) generating small interfering RNAs (siRNAs), a hallmark of RNAi. These siRNAs are loaded into the RNA-induced silencing complex (RISC) triggering the cleavage of complementary messenger RNAs by the Argonaute protein, the main component of the complex. Consequently, the expression of target genes is silenced. This mechanism has been thoroughly studied in fungi due to their proximity to the animal phylum and the conservation of the RNAi mechanism from lower to higher eukaryotes. However, the role and even the presence of RNAi differ across the fungal kingdom, as it has evolved adapting to the particularities and needs of each species. Fungi have exploited RNAi to regulate a variety of cell activities as different as defense against exogenous and potentially harmful DNA, genome integrity, development, drug tolerance, or virulence. This pathway has offered versatility to fungi through evolution, favoring the enormous diversity this kingdom comprises.

The hawthorn CpLRR-RLK1 gene targeted by ACLSV-derived vsiRNA positively regulate resistance to bacteria disease

Virus-derived small interfering RNAs (vsiRNAs) can target not only viruses but also plant genes. Apple chlorotic leaf spot virus (ACLSV) is an RNA virus that infects Rosaceae plants extensively, including apple, pear and hawthorn. Here, we report an ACLSV-derived vsiRNA [vsiR1360(-)] that targets and down-regulates the leucine-rich repeat receptor-like kinase 1 (LRR-RLK1) gene of hawthorn (Crataegus pinnatifida). The targeting and cleavage of the CpLRR-RLK1 gene by vsiR1360(-) were validated by RNA ligase-mediated 5' rapid amplification of cDNA ends and tobacco transient transformation assays. And the CpLRR-RLK1 protein fused to green fluorescent protein localized to the cell membrane. Conserved domain and phylogenetic tree analyses showed that CpLRR-RLK1 is closely related to the proteins of the LRRII-RLK subfamily. The biological function of CpLRR-RLK1 was explored by heterologous overexpression of CpLRR-RLK1 gene in Arabidopsis. The results of inoculation of Pst DC3000 in Arabidopsis leaves showed that the symptoms of CpLRR-RLK1 overexpression plants infected with Pst DC3000 were significantly reduced compared with the wild type. In addition, the detection of reactive oxygen species and callose deposition and the expression analysis of defense-related genes showed that the CpLRR-RLK1 gene can indeed enhance the resistance of Arabidopsis to bacteria disease.

Role of Genetics, Genomics, and Breeding Approaches to Combat Stripe Rust of Wheat

Puccinia striiformis (Pst) is a devastating biotrophic fungal pathogen that causes wheat stripe rust. It usually loves cool and moist places and can cause 100% crop yield losses in a single field when ideal conditions for disease incidence prevails. Billions of dollars are lost due to fungicide application to reduce stripe rust damage worldwide. Pst is a macrocyclic, heteroecious fungus that requires primary (wheat or grasses) as well as secondary host (Berberis or Mahonia spp.) for completion of life cycle. In this review, we have summarized the knowledge about pathogen life cycle, genes responsible for stripe rust resistance, and susceptibility in wheat. In the end, we discussed the importance of conventional and modern breeding tools for the development of Pst-resistant wheat varieties. According to our findings, genetic engineering and genome editing are less explored tools for the development of Pst-resistant wheat varieties; hence, we highlighted the putative use of advanced genome-modifying tools, i.e., base editing and prime editing, for the development of Pst-resistant wheat.

Characterization of microRNA-like RNAs associated with sclerotial development in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a model necrotrophic pathogen causing great economic losses worldwide. Sclerotia are dormant structures that play significant biological and ecological roles in the life and disease cycles of S. sclerotiorum and other species of sclerotia-forming fungi. microRNA-like RNAs (milRNAs) as non-coding small RNAs play regulatory roles in fungal development and pathogenicity. Therefore, milRNAs associated with sclerotial development in S. sclerotiorum were investigated in this study. A total of 275 milRNAs with induced expression during sclerotia development were identified, in which 51 were differentially expressed. The target genes of all milRNAs were predicted. The putative functions of the targets regulated by milRNAs were annotated by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses. The expression levels of six selected milRNAs that coordinated with their corresponding targets were validated by qRT-PCR. Among these six milRNAs, Ssc-milR-240 was potentially associated with sclerotial development by epigenetic regulation of its target histone acetyltransferase. This study will facilitate the better understanding of the milRNA regulation associated with sclerotial development in S. sclerotiorum and even other sclerotia-forming fungi. This work will provide novel insights into the molecular regulations of fungal morphogenesis and the candidate targets of milRNAs used for the sustainable management of plant diseases caused by S. sclerotiorum.

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.

Adaptive regulation of virulence genes by microRNA-like RNAs in Valsa mali

MicroRNAs play important roles in the regulation of gene expression in plants and animals. However, little information is known about the action mechanism and function of fungal microRNA-like RNAs (milRNAs). In this study, combining deep sequencing, molecular and histological assays, milRNAs and their targets in the phytopathogenic fungus Valsa mali were isolated and identified. A critical milRNA, Vm-milR16, was identified to adaptively regulate the expression of virulence genes. Fourteen isolated milRNAs showed high expression abundance. Based on the assessment of a pathogenicity function of these milRNAs, Vm-milR16 was found to be a critical milRNA in V. mali by regulating sucrose non-fermenting 1 (VmSNF1), 4,5-DOPA dioxygenase extradiol (VmDODA), and a hypothetical protein (VmHy1). During V. mali infection, Vm-milR16 is downregulated, while its targets are upregulated. Overexpression of Vm-milR16, but not mutated Vm-milR16, significantly reduces the expression of targets and virulence of V. mali. Furthermore, deletion of VmSNF1, VmDODA and VmHy1 significantly reduce virulence of V. mali. All three targets seem to be essential for oxidative stress response and VmSNF1 is required for expression of pectinase genes during V. mali-host interaction. Our results demonstrate Vm-milRNAs contributing to the infection of V. mali on apple trees by adaptively regulating virulence genes.

Identification and characterization of Dicer-like genes in leaf rust pathogen (Puccinia triticina) of wheat

Puccinia triticina (P. triticina) is one of the most devastating fungal pathogens of wheat which causes significant annual yield loss to the crop. Understanding the gene regulatory mechanism of the biotrophic pathogen is one of the important aspects of host-pathogen interaction studies. Dicer-like genes are considered as important mediators of RNAi-based gene regulation. In this study, we report the presence of three Dicer-like genes (Pt-DCL1, Pt-DCL2, Pt-DCL3) in P. triticina genome identified through computational and biological analyses. Quantitative real-time PCR studies revealed an increase in the expression of these genes in germinating spore stages. Heterologous expression combined with mass spectrometry analysis of Pt-DCL2 confirmed the presence of a canonical Dicer-like gene in P. triticina. Phylogenetic analysis of the Pt-DCLs with the Dicer-like proteins from other organisms showed a distinct cluster of rust pathogens from the order Pucciniales. The results indicated a species-specific duplication of Dicer-like genes within the wheat rust pathogens. This study, for the first time, reports the presence of Dicer-dependent RNAi pathway in P. triticina that may play a role in gene regulatory mechanism of the pathogen during its development. Our study serves as a vital source of information for further RNAi-based molecular studies for better understanding and management of the wheat leaf rust disease.

Role of Fungicide Applications on the Integrated Management of Wheat Stripe Rust

First described in Europe in 1777, stripe rust (SR) caused by Puccinia striiformis Westend. f. sp. tritici Erikss (Pst) is one of the most important and destructive diseases of wheat worldwide. Until 2000, SR was mainly endemic to cooler regions, but since then, new aggressive strains have emerged, spread intercontinentally, and caused severe epidemics in warmer regions across the world. This has put SR as a disease that poses a threat to the world food security. At present, the preferred strategy for control of SR is the access to wheat cultivars with adequate levels of SR resistance. However, wheat breeding programs are not sufficiently advanced to cope with the recently emerged Pst strains. Under this scenario, foliar fungicide applications have become an important component of SR management, but information on the effects of fungicide applications on SR control and wheat cultivar yield response is scarce. This review seeks to provide an overview of the impact and role of fungicides on SR management. With focus on wheat management in the major wheat-growing regions of the world, the review addresses: (a) the efficacy of different fungicide active ingredients, optimal fungicide timing and number of applications in controlling SR, and (b) the impact of fungicide on wheat grain yield response. Inclusion of fungicides in an integrated crop management approach is discussed.

Oomycete small RNAs bind to the plant RNA-induced silencing complex for virulence

The exchange of small RNAs (sRNAs) between hosts and pathogens can lead to gene silencing in the recipient organism, a mechanism termed cross-kingdom RNAi (ck-RNAi). While fungal sRNAs promoting virulence are established, the significance of ck-RNAi in distinct plant pathogens is not clear. Here, we describe that sRNAs of the pathogen Hyaloperonospora arabidopsidis, which represents the kingdom of oomycetes and is phylogenetically distant from fungi, employ the host plant’s Argonaute (AGO)/RNA-induced silencing complex for virulence. To demonstrate H. arabidopsidis sRNA (HpasRNA) functionality in ck-RNAi, we designed a novel CRISPR endoribonuclease Csy4/GUS reporter that enabled in situ visualization of HpasRNA-induced target suppression in Arabidopsis. The significant role of HpasRNAs together with AtAGO1 in virulence was revealed in plant atago1 mutants and by transgenic Arabidopsis expressing a short-tandem-target-mimic to block HpasRNAs, that both exhibited enhanced resistance. HpasRNA-targeted plant genes contributed to host immunity, as Arabidopsis gene knockout mutants displayed quantitatively enhanced susceptibility.

RNA-Spray-Mediated Silencing of Fusarium graminearum AGO and DCL Genes Improve Barley Disease Resistance

Over the last decade, several studies have revealed the enormous potential of RNA-silencing strategies as a potential alternative to conventional pesticides for plant protection. We have previously shown that targeted gene silencing mediated by an in planta expression of non-coding inhibitory double-stranded RNAs (dsRNAs) can protect host plants against various diseases with unprecedented efficiency. In addition to the generation of RNA-silencing (RNAi) signals in planta, plants can be protected from pathogens, and pests by spray-applied RNA-based biopesticides. Despite the striking efficiency of RNA-silencing-based technologies holds for agriculture, the molecular mechanisms underlying spray-induced gene silencing (SIGS) strategies are virtually unresolved, a requirement for successful future application in the field. Based on our previous work, we predict that the molecular mechanism of SIGS is controlled by the fungal-silencing machinery. In this study, we used SIGS to compare the silencing efficiencies of computationally-designed vs. manually-designed dsRNA constructs targeting ARGONAUTE and DICER genes of Fusarium graminearum (Fg). We found that targeting key components of the fungal RNAi machinery via SIGS could protect barley leaves from Fg infection and that the manual design of dsRNAs resulted in higher gene-silencing efficiencies than the tool-based design. Moreover, our results indicate the possibility of cross-kingdom RNA silencing in the Fg-barley interaction, a phenomenon in which sRNAs operate as effector molecules to induce gene silencing between species from different kingdoms, such as a plant host and their interacting pathogens.

Progress on Molecular Genetics and Manipulation of Rust Fungi

Among the thousands of rust species described, many are known for their devastating effects on their hosts, which include major agriculture crops and trees. Hence, for over a century, these basidiomycete pathogenic fungi have been researched and experimented with. However, due to their biotrophic nature, they are challenging organisms to work with and, needing their hosts for propagation, represent pathosystems that are not easily experimentally accessible. Indeed, efforts to perform genetics have been few and far apart for the rust fungi, though one study performed in the 1940s was famously instrumental in formulating the gene-for-gene hypothesis describing pathogen-host interactions. By taking full advantage of the molecular genetic tools developed in the 1980s, research on many plant pathogenic microbes thrived, yet similar work on the rusts remained very challenging though not without some successes. However, the genomics era brought real breakthrough research for the biotrophic fungi and with innovative experimentation and the use of heterologous systems, molecular genetic analyses over the last 2 decades have significantly advanced our insight into the function of many rust fungus genes and their role in the interaction with their hosts. This has allowed optimizing efforts for resistance breeding and the design and testing of various novel strategies to reduce the devastating diseases they cause.

Silencing Dicer-Like Genes Reduces Virulence and sRNA Generation in Penicillium italicum, the Cause of Citrus Blue Mold

The Dicer protein is one of the most important components of RNAi machinery because it regulates the production of small RNAs (sRNAs) in eukaryotes. Here, Dicer1-like gene (Pit-DCL1) and Dicer2-like gene (Pit-DCL2) RNAi transformants were generated via pSilent-1 in Penicillium italicum (Pit), which is the causal agent of citrus blue mold. Neither transformant showed a change in mycelial growth or sporulation ability, but the pathogenicity of the Pit-DCL2 RNAi transformant to citrus fruits was severely impaired, compared to that of the Pit-DCL1 RNAi transformant and the wild type. We further developed a citrus wound-mediated RNAi approach with a double-stranded fragment of Pit-DCL2 generated in vitro, which achieved an efficiency in reducing Pi-Dcl2 expression and virulence that was similar to that of protoplast-mediated RNAi in P. italicum, suggesting that this approach is promising in the exogenous application of dsRNA to control pathogens on the surface of citrus fruits. In addition, sRNA sequencing revealed a total of 69.88 million potential sRNAs and 12 novel microRNA-like small RNAs (milRNAs), four of which have been predicated on target innate immunity or biotic stress-related genes in Valencia orange. These data suggest that both the Pit-DCL1 and Pit-DCL2 RNAi transformants severely disrupted the biogenesis of the potential milRNAs, which was further confirmed for some milRNAs by qRT-PCR or Northern blot analysis. These data suggest the sRNAs in P. italicum that may be involved in a molecular virulence mechanism termed cross-kingdom RNAi (ck-RNAi) by trafficking sRNA from P. italicum to citrus fruits.

Small RNA Bidirectional Crosstalk During the Interaction Between Wheat and Zymoseptoria tritici

Cross-kingdom RNA interference (RNAi) has been shown to play important roles during plant-pathogen interactions, and both plants and pathogens can use small RNAs (sRNAs) to silence genes in each other. This bidirectional cross-kingdom RNAi was still unexplored in the wheat-Zymoseptoria tritici pathosystem. Here, we performed a detailed analysis of the sRNA bidirectional crosstalk between wheat and Z. tritici. Using a combination of small RNA sequencing (sRNA-seq) and microRNA sequencing (mRNA-seq), we were able to identify known and novel sRNAs and study their expression and their action on putative targets in both wheat and Z. tritici. We predicted the target genes of all the sRNAs in either wheat or Z. tritici transcriptome and used degradome analysis to validate the cleavage of these gene transcripts. We could not find any clear evidence of a cross-kingdom RNAi acting by mRNA cleavage in this pathosystem. We also found that the fungal sRNA enrichment was lower in planta than during in vitro growth, probably due to the lower expression of the only Dicer gene of the fungus during plant infection. Our results support the recent finding that Z. tritici sRNAs cannot play important roles during wheat infection. However, we also found that the fungal infection induced wheat sRNAs regulating the expression of specific wheat genes, including auxin-related genes, as an immune response. These results indicate a role of sRNAs in the regulation of wheat defenses during Z. tritici infection. Our findings contribute to improve our understanding of the interactions between wheat and Z. tritici.

Plant miRNA Cross-Kingdom Transfer Targeting Parasitic and Mutualistic Organisms as a Tool to Advance Modern Agriculture

MicroRNAs (miRNAs), defined as small non-coding RNA molecules, are fine regulators of gene expression. In plants, miRNAs are well-known for regulating processes spanning from cell development to biotic and abiotic stress responses. Recently, miRNAs have been investigated for their potential transfer to distantly related organisms where they may exert regulatory functions in a cross-kingdom fashion. Cross-kingdom miRNA transfer has been observed in host-pathogen relations as well as symbiotic or mutualistic relations. All these can have important implications as plant miRNAs can be exploited to inhibit pathogen development or aid mutualistic relations. Similarly, miRNAs from eukaryotic organisms can be transferred to plants, thus suppressing host immunity. This two-way lane could have a significant impact on understanding inter-species relations and, more importantly, could leverage miRNA-based technologies for agricultural practices. Additionally, artificial miRNAs (amiRNAs) produced by engineered plants can be transferred to plant-feeding organisms in order to specifically regulate their cross-kingdom target genes. This minireview provides a brief overview of cross-kingdom plant miRNA transfer, focusing on parasitic and mutualistic relations that can have an impact on agricultural practices and discusses some opportunities related to miRNA-based technologies. Although promising, miRNA cross-kingdom transfer remains a debated argument. Several mechanistic aspects, such as the availability, transfer, and uptake of miRNAs, as well as their potential to alter gene expression in a cross-kingdom manner, remain to be addressed.

RNA-Spray-Mediated Silencing of Fusarium graminearum AGO and DCL Genes Improve Barley Disease Resistance

Over the last decade, several studies have revealed the enormous potential of RNA-silencing strategies as a potential alternative to conventional pesticides for plant protection. We have previously shown that targeted gene silencing mediated by an in planta expression of non-coding inhibitory double-stranded RNAs (dsRNAs) can protect host plants against various diseases with unprecedented efficiency. In addition to the generation of RNA-silencing (RNAi) signals in planta, plants can be protected from pathogens, and pests by spray-applied RNA-based biopesticides. Despite the striking efficiency of RNA-silencing-based technologies holds for agriculture, the molecular mechanisms underlying spray-induced gene silencing (SIGS) strategies are virtually unresolved, a requirement for successful future application in the field. Based on our previous work, we predict that the molecular mechanism of SIGS is controlled by the fungal-silencing machinery. In this study, we used SIGS to compare the silencing efficiencies of computationally-designed vs. manually-designed dsRNA constructs targeting ARGONAUTE and DICER genes of Fusarium graminearum (Fg). We found that targeting key components of the fungal RNAi machinery via SIGS could protect barley leaves from Fg infection and that the manual design of dsRNAs resulted in higher gene-silencing efficiencies than the tool-based design. Moreover, our results indicate the possibility of cross-kingdom RNA silencing in the Fg-barley interaction, a phenomenon in which sRNAs operate as effector molecules to induce gene silencing between species from different kingdoms, such as a plant host and their interacting pathogens.

Small RNAs - Big Players in Plant-Microbe Interactions

Eukaryotic small RNAs (sRNAs) are short non-coding regulatory molecules that induce RNA interference (RNAi). During microbial infection, host RNAi machinery is highly regulated and contributes to reprogramming gene expression and balancing plant immunity and growth. While most sRNAs function endogenously, some can travel across organismal boundaries between hosts and microbes and silence genes in trans in interacting organisms, a mechanism called "cross-kingdom RNAi." During the co-evolutionary arms race between fungi and plants, some fungi developed a novel virulence mechanism, sending sRNAs as effector molecules into plant cells to silence plant immunity genes, whereas plants also transport sRNAs, mainly using extracellular vesicles, into the pathogens to suppress virulence-related genes. In this Review, we highlight recent discoveries on these key roles of sRNAs and RNAi machinery. Understanding the molecular mechanisms of sRNA biogenesis, trafficking, and RNAi machinery will help us develop innovative strategies for crop protection.

A safe ride in extracellular vesicles - small RNA trafficking between plant hosts and pathogens

Communication between plants and pathogens requires the transport of regulatory molecules across cellular boundaries, which is essential for host defense and pathogen virulence. Previous research has largely focused on protein transport, but, which other molecules function in communication, and how they are transported remains under explored. Recent studies discovered that small RNAs (sRNAs) are transported between plants and pathogens, which can silence target genes in the interacting organisms and regulate host immunity and pathogen infection, a mechanism called 'cross-kingdom RNA interference (RNAi)'. Further studies indicate that plant extracellular vesicles (EVs) are essential for sRNA trafficking and host-pathogen communication. This review will focus on the latest advances in our understanding of plant EVs and their roles in transporting regulatory molecules, especially sRNAs, between hosts and pathogens.

Nonproteinaceous effectors: the terra incognita of plant-fungal interactions

Molecular plant-fungal interaction studies have mainly focused on small secreted protein effectors. However, accumulating evidence shows that numerous fungal secondary metabolites are produced at all stages of plant colonization, especially during early asymptomatic/biotrophic phases. The discovery of fungal small RNAs targeting plant transcripts has expanded the fungal repertoire of nonproteinaceous effectors even further. The challenge now is to develop specific functional methods to fully understand the biological roles of these effectors. Studies on fungal extracellular vesicles are also needed because they could be the universal carriers of all kinds of fungal effectors. With this review, we aim to stimulate the nonproteinaceous effector research field to move from descriptive to functional studies, which should bring a paradigm shift in plant-fungal interactions.

Small RNA Functions as a Trafficking Effector in Plant Immunity

Small RNAs represent a class of small but powerful agents that regulate development and abiotic and biotic stress responses during plant adaptation to a constantly challenging environment. Previous findings have revealed the important roles of small RNAs in diverse cellular processes. The recent discovery of bidirectional trafficking of small RNAs between different kingdoms has raised many interesting questions. The subsequent demonstration of exosome-mediated small RNA export provided a possible tool for further investigating how plants use small RNAs as a weapon during the arms race between plant hosts and pathogens. This review will focus on discussing the roles of small RNAs in plant immunity in terms of three aspects: the biogenesis of extracellular small RNAs and the transportation and trafficking small RNA-mediated gene silencing in pathogens.

Comprehensive analysis of full genome sequence and Bd-milRNA/target mRNAs to discover the mechanism of hypovirulence in Botryosphaeria dothidea strains on pear infection with BdCV1 and BdPV1

Pear ring rot disease, mainly caused by Botryosphaeria dothidea, is widespread in most pear and apple-growing regions. Mycoviruses are used for biocontrol, especially in fruit tree disease. BdCV1 (Botryosphaeria dothidea chrysovirus 1) and BdPV1 (Botryosphaeria dothidea partitivirus 1) influence the biological characteristics of B. dothidea strains. BdCV1 is a potential candidate for the control of fungal disease. Therefore, it is vital to explore interactions between B. dothidea and mycovirus to clarify the pathogenic mechanisms of B. dothidea and hypovirulence of B. dothidea in pear. A high-quality full-length genome sequence of the B. dothidea LW-Hubei isolate was obtained using Single Molecule Real-Time sequencing. It has high repeat sequence with 9.3% and DNA methylation existence in the genome. The 46.34 Mb genomes contained 14,091 predicted genes, which of 13,135 were annotated. B. dothidea was predicted to express 3833 secreted proteins. In bioinformatics analysis, 351 CAZy members, 552 transporters, 128 kinases, and 1096 proteins associated with plant-host interaction (PHI) were identified. RNA-silencing components including two endoribonuclease Dicer, four argonaute (Ago) and three RNA-dependent RNA polymerase (RdRp) molecules were identified and expressed in response to mycovirus infection. Horizontal transfer of the LW-C and LW-P strains indicated that BdCV1 induced host gene silencing in LW-C to suppress BdPV1 transmission. To investigate the role of RNA-silencing in B. dothidea defense, we constructed four small RNA libraries and sequenced B. dothidea micro-like RNAs (Bd-milRNAs) produced in response to BdCV1 and BdPV1 infection. Among these, 167 conserved and 68 candidate novel Bd-milRNAs were identified, of which 161 conserved and 20 novel Bd-milRNA were differentially expressed. WEGO analysis revealed involvement of the differentially expressed Bd-milRNA-targeted genes in metabolic process, catalytic activity, cell process and response to stress or stimulus. BdCV1 had a greater effect on the phenotype, virulence, conidiomata, vertical and horizontal transmission ability, and mycelia cellular structure biological characteristics of B. dothidea strains than BdPV1 and virus-free strains. The results obtained in this study indicate that mycovirus regulates biological processes in B. dothidea through the combined interaction of antiviral defense mediated by RNA-silencing and milRNA-mediated regulation of target gene mRNA expression.

Trichoderma as a Model to Study Effector-Like Molecules

Plants are capable of perceiving microorganisms by coordinating processes to establish different forms of plant–microbe relationships. Plant colonization is governed in fungal and bacterial systems by secreted effector molecules, suppressing plant defense responses and modulating plant physiology to promote either virulence or compatibility. Proteins, secondary metabolites, and small RNAs have been described as effector molecules that use different mechanisms to establish the interaction. Effector molecules have been studied in more detail due to their involvement in harmful interactions, leading to a negative impact on agriculture. Recently, research groups have started to study the effectors in symbiotic interactions. Interestingly, most symbiotic effectors are members of the same families present in phytopathogens. Nevertheless, the quantity and ratio of secreted effectors depends on the microorganism and the host, suggesting a complex mechanism of recognition between the plant and their associated microorganisms. Fungi belonging to Trichoderma genus interact with plants by inducing their defense system and promoting plant growth. Research suggests that some of these effects are associated with effector molecules that Trichoderma delivers during the association with the plant. In this review, we will focus on the main findings concerning the effector molecules reported in Trichoderma spp. and their role during the interaction with plants, mainly in the molecular dialogue that takes place between them.

PsRPs26, a 40S Ribosomal Protein Subunit, Regulates the Growth and Pathogenicity of Puccinia striiformis f. sp. Tritici

Eukaryotic ribosomes are essential for proliferation, differentiation, and cell growth. RPs26 is a ribosomal subunit structural protein involved in the growth and development process. Little is known about the function of PsRPs26 in pathogenic fungi. In this study, we isolated the RPs26 gene, PsRPs26, from Puccinia striiformis f. sp. tritici (Pst). PsRPs26 contains a eukaryotic-specific Y62-K70 motif and is more than 90% identical with its ortholog gene in other fungi. PsRPs26 was found to be localized in both the nucleus and cytoplasm. Expression of PsRPs26 increased when wheat seedlings were inoculated with the Pst CYR31 isolate. Moreover, knockdown of PsRPs26 by a host-induced gene silencing system inhibited growth and limited urediospore production in Pst. Our discovery that PsRPs26 may contribute to the pathogenicity of Pst and open a new way in the pathogenic function of PsRPs26 in cereal rust fungi.

Cross-Kingdom Small RNAs Among Animals, Plants and Microbes

Small RNAs (sRNAs), a class of regulatory non-coding RNAs around 20~30-nt long, including small interfering RNAs (siRNAs) and microRNAs (miRNAs), are critical regulators of gene expression. Recently, accumulating evidence indicates that sRNAs can be transferred not only within cells and tissues of individual organisms, but also across different eukaryotic species, serving as a bond connecting the animal, plant, and microbial worlds. In this review, we summarize the results from recent studies on cross-kingdom sRNA communication. We not only review the horizontal transfer of sRNAs among animals, plants and microbes, but also discuss the mechanism of RNA interference (RNAi) signal transmission via cross-kingdom sRNAs. We also compare the advantages of host-induced gene silencing (HIGS) and spray-induced gene silencing (SIGS) technology and look forward to their applicable prospects in controlling fungal diseases.

Leaf rust (Puccinia triticina) mediated RNAi in wheat (Triticum aestivum L.) prompting host susceptibility

Significance of microRNAs in regulating gene expression in higher eukaryotes as well as in pathogens like fungi to suppress host defense is a well-established phenomenon. The present study focuses on leaf rust fungi Puccinia triticina (Pathotype 77-5) mediated RNAi to make wheat (Triticum aestivum L.) more susceptible. To reach such conclusions, we first confirmed the presence of argonaute (AGO) and dicer-like protein (DCL) family sequences in Puccinia. Bioinformatic tools were applied to retrieve the sequences from Puccinia genome followed by cloning and sequencing from P. triticina pathotype 77-5 cDNA to obtain the specific sequences. Their homologs were searched in other 14 Puccinia races to relate them with pathogenesis. Further, precursor sequences for three miRNA-like RNA molecules (milRs) were cloned from P. triticina cDNA. Their target genes like MAP kinase were successfully predicted and validated through degradome mapping and qRT-PCR. Gradual increase in milR2 (milR and milR*) expression over progressive time point of infection and positive expression for all the milRs within 77-5 urediniospores confirmed a complete host- independent RNAi activity by P. triticina.

The Catalase Gene Family in Cotton: Genome-Wide Characterization and Bioinformatics Analysis

Catalases (CATs), which were coded by the catalase gene family, were a type notably distinguished ROS-metabolizing proteins implicated to perform various physiological functions in plant growth, development and stress responses. However, no systematical study has been performed in cotton. In the present study, we identified 7 and 7 CAT genes in the genome of Gossypium hirsutum L. Additionally, G. barbadense L., respectively. The results of the phylogenetic and synteny analysis showed that the CAT genes were divided into two groups, and whole-genome duplication (WGD) or polyploidy events contributed to the expansion of the GossypiumCAT gene family. Expression patterns analysis showed that the CAT gene family possessed temporal and spatial specificity and was induced by the Verticillium dahliae infection. In addition, we predicted the putative molecular regulatory mechanisms of the CAT gene family. Based on the analysis and preliminary verification results, we hypothesized that the CAT gene family, which might be regulated by transcription factors (TFs), alternative splicing (AS) events and miRNAs at different levels, played roles in cotton development and stress tolerance through modulating the reactive oxygen species (ROS) metabolism. This is the first report on the genome-scale analysis of the cotton CAT gene family, and these data will help further study the roles of CAT genes during stress responses, leading to crop improvement.

Discovery and profiling of small RNAs from Puccinia triticina by deep sequencing and identification of their potential targets in wheat

Cross-kingdom RNAi is a well-documented phenomenon where sRNAs generated by host and pathogens may govern resistance or susceptible phenotypes during host-pathogen interaction. With the first example of the direct involvement of fungal generated sRNAs in virulence of plant pathogenic fungi Botrytis cinerea and recently from Puccinia striiformis f. sp. tritici, we attempted to identify sRNAs in Puccinia triticina (P. triticina). Four sRNA libraries were prepared and sequenced using Illumina sequencing technology and a total of ~ 1-1.28 million potential sRNAs and two microRNA-like small RNA (mil-RNAs) candidates were identified. Computational prediction of targets using a common set of sRNAs and P. triticina mil-RNAs (pt-mil-RNAs) within P. triticina and wheat revealed the majority of the targets as repetitive elements in P. triticina whereas in wheat, the target genes were identified to be involved in many biological processes including defense-related pathways. We found 9 receptor-like kinases (RLKs) and 14 target genes of each related to reactive oxygen species (ROS) pathway and transcription factors respectively, including significant numbers of target genes from various other categories. Expression analysis of twenty selected sRNAs, targeting host genes pertaining to ROS related, disease resistance, metabolic processes, transporter, apoptotic inhibitor, and transcription factors along with two pt-mil-RNAs by qRT-PCR showed distinct patterns of expression of the sRNAs in urediniospore-specific libraries. In this study, for the first time, we report identification of novel sRNAs identified in P. triticina including two pt-mil-RNAs that may play an important role in biotrophic growth and pathogenicity.