The antagonistic role of an E3 ligase pair in regulating plant NLR-mediated autoimmunity and fungal pathogen resistance.

Plant immune homeostasis is achieved through a balanced immune activation and suppression, enabling effective defense while averting autoimmunity. In Arabidopsis, disrupting a mitogen-activated protein (MAP) kinase cascade triggers nucleotide-binding leucine-rich-repeat (NLR) SUPPRESSOR OF mkk1/2 2 (SUMM2)-mediated autoimmunity. Through an RNAi screen, we identify PUB5, a putative plant U-box E3 ligase, as a critical regulator of SUMM2-mediated autoimmunity. In contrast to typical E3 ligases, PUB5 stabilizes CRCK3, a calmodulin-binding receptor-like cytoplasmic kinase involved in SUMM2 activation. A closely related E3 ligase, PUB44, functions oppositely with PUB5 to degrade CRCK3 through monoubiquitylation and internalization. Furthermore, CRCK3, highly expressed in roots and conserved across plant species, confers resistance to Fusarium oxysporum, a devastating soil-borne fungal pathogen, in both Arabidopsis and cotton. These findings demonstrate the antagonistic role of an E3 ligase pair in fine-tuning kinase proteostasis for the regulation of NLR-mediated autoimmunity and highlight the function of autoimmune activators in governing plant root immunity against fungal pathogens.

Loop-Mediated Isothermal Amplification for Rapid Detection of Mating Types of Villosiclava virens.

Rice false smut (RFS), caused by Villosiclava virens, is an important fungal disease in panicles of rice. V. virens is a heterothallic ascomycete controlled by two opposite idiomorphs, MAT1-1 and MAT1-2. Previous study showed that sexual reproduction of V. virens plays an important role in the epidemic of RFS. In this study, we developed a loop-mediated isothermal amplification (LAMP) assay to detect the mating type of V. virens easily and rapidly by using specific primers based on the mating type genes MAT1-1-2 and MAT1-2-1, respectively. The LAMP assay used only a water/dry bath and could recognize the mating type of V. virens in just 45 min. The LAMP assay was so sensitive that it could detect small amounts of V. virens genomic DNA (as low as 2.0 pg of MAT1-1 and 200.0 pg of MAT1-2) and was 10 times more sensitive than PCR. In addition, we demonstrated the application of mating type via LAMP assay by assessing the genomic DNA of V. virens isolated from rice fields. The high efficiency and specificity of this LAMP assay suggest that it can be used as a rapid testing tool in mating type recognition of V. virens isolates in the field.

Pyricularia sp. jiangsuensis, a new cryptic rice panicle blast pathogen from rice fields in Jiangsu Province, China.

Pyricularia oryzae is a multi-host pathogen causing cereal disease, including the devastating rice blast. Panicle blast is a serious stage, leading to severe yield loss. Thirty-one isolates (average 4.1%) were collected from the rice panicle lesions at nine locations covering Jiangsu province from 2010 to 2017. These isolates were characterized as Pyricularia sp. jiangsuensis distinct from known Pyricularia species. The representative strain 18-2 can infect rice panicle, root and five kinds of grasses. Intriguingly, strain 18-2 can co-infect rice leaf with P. oryzae Guy11. The whole genome of P. sp. jiangsuensis 18-2 was sequenced. Nine effectors were distributed in translocation or inversion region, which may link to the rapid evolution of effectors. Twenty-one homologues of known blast-effectors were identified in strain 18-2, seven effectors including the homologues of SLP1, BAS2, BAS113, CDIP2/3, MoHEG16 and Avr-Pi54, were upregulated in the sample of inoculated panicle with strain 18-2 at 24 hpi compared with inoculation at 8 hpi. Our results provide evidences that P. sp. jiangsuensis represents an addition to the mycobiota of blast disease. This study advances our understanding of the pathogenicity of P. sp. jiangsuensis to hosts, which sheds new light on the adaptability in the co-evolution of pathogen and host.

The N-terminus of an Ustilaginoidea virens Ser-Thr-rich glycosylphosphatidylinositol-anchored protein elicits plant immunity as a MAMP.

Many pathogens infect hosts through specific organs, such as Ustilaginoidea virens, which infects rice panicles. Here, we show that a microbe-associated molecular pattern (MAMP), Ser-Thr-rich Glycosyl-phosphatidyl-inositol-anchored protein (SGP1) from U. virens, induces immune responses in rice leaves but not panicles. SGP1 is widely distributed among fungi and acts as a proteinaceous, thermostable elicitor of BAK1-dependent defense responses in N. benthamiana. Plants specifically recognize a 22 amino acid peptide (SGP1 N terminus peptide 22, SNP22) in its N-terminus that induces cell death, oxidative burst, and defense-related gene expression. Exposure to SNP22 enhances rice immunity signaling and resistance to infection by multiple fungal and bacterial pathogens. Interestingly, while SGP1 can activate immune responses in leaves, SGP1 is required for U. virens infection of rice panicles in vivo, showing it contributes to the virulence of a panicle adapted pathogen.

MAT1-1-3, a Mating Type Gene in the Villosiclava virens, Is Required for Fruiting Bodies and Sclerotia Formation, Asexual Development and Pathogenicity.

Villosiclava virens is the prevalent causative pathogen of rice false smut, a destructive rice disease. Mating-type genes play a vital role in the evolution of mating systems in fungi. Some fungi have lost MAT1-1-3, one of the mating-type genes, during evolution, whereas others still retain MAT1-1-3. However, how MAT1-1-3 regulates the sexual development of heterothallic V. virens remains unknown. Here, we generated the MAT1-1-3 mutants, which exhibited defects in vegetative growth, stress response, pathogenicity, sclerotia formation and fruiting body maturation. An artificial outcrossing inoculation assay showed that the Δmat1-1-3 mutant was unable to produce sclerotia. Unexpectedly, the Δmat1-1-3 mutant could form immature fruiting bodies without mating on potato sucrose agar medium (PSA) compared with the wild-type strain, most likely by activating the truncated MAT1-2-1 transcription to regulate the sexual development. Moreover, RNA-seq data showed that knockout of MAT1-1-3 results in misregulation of a subset of genes involved in sexual development, MAPK signaling, cell wall integrity, autophagy, epigenetic modification, and transcriptional regulation. Collectively, this study reveals that MAT1-1-3 is required for asexual and sexual development, and pathogenicity of V. virens, thereby provides new insights into the function of mating-type genes in the fungi life cycle and infection process.

Two mating-type genes MAT1-1-1 and MAT1-1-2 with significant functions in conidiation, stress response, sexual development, and pathogenicity of rice false smut fungus Villosiclava virens.

Rice false smut caused by Villosiclava virens is one of the destructive diseases on panicles of rice. Sexual development of V. virens, controlled by mating-type locus, plays an important role in the prevalence of rice false smut and genetic diversity of the pathogen. However, how the mating-type genes mediate sexual development of the V. virens remains largely unknown. In this study, we characterized the two mating-type genes, MAT1-1-1 and MAT1-1-2, in V. virens. MAT1-1-1 knockout mutant showed defects in hyphal growth, conidia morphogenesis, sexual development, and increase in the tolerance to salt and osmotic stress. Targeted deletion of MAT1-1-2 not only impaired the sclerotia formation and pathogenicity of V. virens, but also reduced the production of conidia. The MAT1-1-2 mutant showed increases in tolerance to salt and hydrogen peroxide stress, but decreases in tolerance to osmotic stress. Yeast two-hybrid assay showed that MAT1-1-1 interacted with MAT1-1-2, indicating that those proteins might form a complex to regulate sexual development. In addition, MAT1-1-1 localized in the nucleus, and MAT1-1-2 localized in the cytoplasm. Collectively, our results demonstrate that MAT1-1-1 and MAT1-1-2 play important roles in the conidiation, stress response, sexual development, and pathogenicity of V. virens, thus providing new insights into the function of mating-type gene.

Genome-wide identification and analysis of the GATA transcription factor gene family in Ustilaginoidea virens.

In filamentous fungi, the conserved transcription factors play important roles in multiple cellular and developmental processes. The GATA proteins, a family of GATA-binding zinc finger transcription factors, play diverse functions in fungi. Ustilaginoidea virens is an economically important pathogen-causing rice false smut worldwide. To gain additional insight into the cellular and molecular mechanisms of this pathogen, in this study, we identified and functionally characterized seven GATA proteins from the U. virens genome (UvGATA). Sequences analysis indicated that these GATA proteins are divided into seven clades. The proteins in each clade contained conserved clade-specific sequences and structures, thus leading to the same motif serving different purposes in various contexts. The expression profiles of UvGATA genes at different infection stages and under H2O2 stress were detected. Results showed that the majority of UvGATA genes performed functions at both processes, thereby confirming the roles of these genes in pathogenicity and reactive oxygen species stress tolerance. This study provided an important starting point to further explore the biological functions of UvGATA genes and increased our understanding of their potential transcriptional regulatory mechanisms in U. virens.

A Homeobox Transcription Factor UvHOX2 Regulates Chlamydospore Formation, Conidiogenesis, and Pathogenicity in Ustilaginoidea virens.

Rice false smut fungus (teleomorph: Villosiclava virens; anamorph: Ustilaginoidea virens) can generate chlamydospores and survive winter under field conditions. The chlamydospore is considered as an important infection source of the disease. However, little is known about the regulatory mechanism of the chlamydospore production. In this study, we identified a defective homeobox transcription factor (designated as UvHOX2) gene in a U. virens random insertional mutant B-766 that could not form chlamydospores. To confirm the regulatory function of UvHOX2, an Agrobacterium tumefaciens mediated transformation- and CRISPR/Cas9- based targeted gene replacement method was developed. The UvHox2 deletion mutants completely failed to produce chlamydospores, showed reduced conidia production and decreased virulence, and was hyper-sensitive to oxidative, osmotic, and cell wall stresses. We confirmed that UvHOX2 is located in the nuclei of U. virens, and the expression of UvHox2 was the strongest during the early stage of chlamydospore and conidium formation. Global transcription pattern of UvHOX2 was also determined by RNA-seq in this study, and several genes that might be down-stream of UvHOX2 regulation were identified. The results will better our understanding of the molecular mechanism of chlamydospore formation in U. virens as a model fungus.

Cytological studies on the infection of rice root by Ustilaginoidea virens.

In recent years, false smut disease of rice has been one of the most important diseases of cultivated rice in China. Ustilaginoidea virens is an ascomycete fungal pathogen that causes false smut in rice. There is always controversy about whether the pathogen can infect the rice root and cause the occurrence of false smut, mainly due to lack direct cytological evidence. In our study, we observed the cytological structure of rice root invaded by U. virens. The results showed that U. virens could attach to the surface of young roots and penetrate into the intercellular space of the root epidermis. The cellulose microfibrils in root epidermal cell wall are very loose and soft, and their structural features are similar to filaments of rice. After the fungus infected the roots, a large number of fungal secretions were accumulated outside of the cell walls. At 40 days, the fungus began to degrade, but pathogens still had not infected the sclerenchyma, in which the cells are arranged densely and the cell walls are thicker. U. virens could not cross the sclerenchyma layer into the endodermis and phloem of the root. To some extent, the U. virens infection affected the leaf and root growth of the rice. After inoculation, there was no fungal mycelium found in transverse sections of the rice young stem. These results suggested that root colonization of U. virens does not lead to systemic invasion in rice.

Villosiclava virens infects specifically rice and barley stamen filaments due to the unique host cell walls.

Rice false smut, caused by the fungal pathogen Villosiclava virens, is one of the most important rice diseases in the world. Previous studies reported that the pathogen has less number of cell wall-degraded genes and attacks dominantly rice stamen filaments and extends intercellularly. To reveal why the fungus infects plant stamen filaments, inoculation test on barley was carried out with the similar protocol to rice. The experimental results showed that the fungus could penetrate quickly into barley stamen filaments and extends both intracellularly and intercellularly, usually resulting in severe damage of the stamen filament tissues. It also attacked young barley lodicules and grew intercellularly by chance. The light microscopic observations found that the epidermal and cortex cells in barley stamen filaments arranged loosely with very thick cell walls and large cell gaps. Cellulose microfibrils in barley stamen filament cell walls arranged very sparsely so that the cell walls looked like transparent. The cell walls were very soft and flexible, and often folded. However, V. virens extended dominantly in the noncellulose regions and seemed never to degrade microfibrils in barley and rice cell walls. This suggested that the unique structures of rice and barley stamen filaments should be fit for their function of elongation in anthesis, and also endow with the susceptibility to the fungus, V. virens.

Wheat gene TaS3 contributes to powdery mildew susceptibility.

KEY MESSAGE: Identification of TaS3 as a potential susceptibility gene encoding a protein homologous to ULP1 protease in wheat, which may regulate SUMO function facilitating powdery mildew attack. Some plant genes that are required for susceptibilities to certain pathogens are known as susceptibility genes or susceptibility factors, whose loss-of-function mutations can confer the plants resistances. To identify potential susceptibility genes to powdery mildew in wheat, differentially expressed genes in compatible and incompatible interactions between wheat and powdery mildew were examined by the cDNA chip assay. The genes exclusively expressed in the susceptible cultivar were interfered using biolistic transient transformation in wheat epidermal cells. The suppression of gene TaS3 (Triticum aestivum susceptibility) decreased the pathogen penetration by 19%, and its over-expression increased the disease susceptibility. The deduced protein from TaS3 belongs to the putative ubiquitin-like protease 1 peptidase domain family. Subcellular localization studies revealed that its protein was accumulated in the nucleus. Quantitative real-time polymerase chain reaction analysis revealed that TaS3 transcript was significantly induced in the compatible host. This suggests that TaS3 is a potential susceptible gene and its function may be related to regulate SUMO functions.