Strain Specific Factors Control Effector Gene Silencing in Phytophthora sojae

GmARF15 Enhances the Resistance of Soybean to Phytophthora sojae by Promoting GmPT10d Expression in Response to Salicylic Acid Signalling

Phytophthora root and stem rot caused by Phytophthora sojae (P. sojae) is a globally prevalent oomycete disease. The use of resistant cultivars is an effective and environmentally friendly strategy to manage this disease. It is important to understand the molecular mechanisms underlying the response of Glycine max (soybean) to P. sojae infection. In this study, we demonstrated that an isoflavonoid-specific prenyltransferase gene (GmPT10d, Glyma.10G070300) was significantly upregulated in the soybean cultivar Williams 82 with high resistance to P. sojae infection. Transgenic soybean seedlings overexpressing GmPT10d exhibited enhanced resistance to P. sojae, and those subjected to RNA interference showed increased susceptibility to the pathogen. Yeast-one-hybrid and electrophoretic mobility shift assays revealed that GmARF15 could directly bind to the promoter of GmPT10d. Further analysis of the GmARF15 function showed that transgenic soybean seedlings overexpressing GmARF15 also exhibited enhanced resistance to P. sojae. Transactivation assay, luciferase assay, and qPCR analysis showed that GmARF15 could promote the expression of GmPT10d. Further analysis indicated that elevated salicylic acid levels were associated with increased expression of GmARF15 and GmPT10d. Taken together, these findings reveal a regulatory mechanism by which GmARF15 enhances soybean resistance to P. sojae, potentially by promoting the expression of GmPT10d through the salicylic acid signaling pathway.

Specific Transcriptional Regulation Controls Plant Organ-Specific Infection by the Oomycete Pathogen Phytophthora sojae

The organs of a plant species vary in cell structure, metabolism and defence responses. However, the mechanisms that enable a single pathogen to colonise different plant organs remain unclear. Here we compared the transcriptome of the oomycete pathogen Phytophthora sojae during infection of roots versus leaves of soybeans. We found differences in the transcript levels of hundreds of pathogenicity-related genes, particularly genes encoding carbohydrate-active enzymes, secreted (effector) proteins, oxidoreductase-related proteins and transporters. To identify the key regulator for root-specific infection, we knocked out root-specific transcription factors (TFs) and found the mutants of PsBZPc29, which encodes a member of an oomycete-specific class of basic leucine zipper (bZIP) TFs, displayed reduced virulence on soybean roots but not on leaves. More than 60% of the root-specific genes showed reduced expression in the mutants during root infection. The results suggest that transcriptional regulation underlies the organ-specific infection by P. sojae, and that a bZIP TF plays a key role in root-specific transcriptional regulation.

Molecular mechanisms of Phytophthora sojae avirulence effectors escaping host recognition

Phytophthora sojae is a well-known destructive oomycete pathogen, which causes soybean stem and root rot and poses a serious threat to global food security. Growing soybean cultivars with the appropriate resistance to P. sojae (Rps) genes are the primary management strategy to reduce losses. In most Phytophthora pathosystems, host resistance protein encoded by a specific R gene in the plant recognizes corresponding RxLR effector protein, encoded by an avirulence gene. This gene-for-gene relationship has been exploited to help breeders and agronomists deploy soybean cultivars. To date, 6 Rps genes have been incorporated into commercial soybean germplasm and trigger plant immunity in response to 8 P. sojae avirulence effectors. The incorporation of Rps genes in the soybean population creates selection pressure in favor of novel pathotypes of P. sojae. The 8 avirulence genes evolved to evade the host immune system, driven by genetic selection pressures. Understanding the evading strategies has important reference value for the prevention and control of Phytophthora stem and root rot. This investigation primarily highlights the research on the strategies of P. sojae avirulence effector evasion of host recognition, looking forward to creating durable resistance genes and thereby enabling successful disease management.

RXLR effector gene Avr3a from Phytophthora sojae is recognized by Rps8 in soybean

The use of resistance genes in elite soybean cultivars is one of the most widely used methods to manage Phytophthora sojae. This method relies on effector-triggered immunity, where a Resistant to P. sojae (Rps) gene product from the plant recognizes a specific effector from the pathogen, encoded by an avirulence (Avr) gene. Many Avr genes from P. sojae have been identified in the last decade, allowing a better exploitation of this type of resistance. The objective of the present study was to identify the Avr gene triggering immunity derived from the soybean resistance gene Rps8. The analysis of a segregating F2 progeny coupled with a genotyping-by-sequencing approach led to the identification of a putative Avr8 locus. The investigation of this locus using whole-genome sequencing data from 31 isolates of P. sojae identified Avr3a as the likely candidate for Avr8. Long-read sequencing also revealed that P. sojae isolates can carry up to five copies of the Avr3a gene, compared to the four previously reported. Haplotype and transcriptional analyses showed that amino acid changes and absence of Avr3a transcripts from P. sojae isolates caused changes in virulence towards Rps8. Functional analyses using CRISPR/Cas9 knockout and constitutive expression demonstrated that Rps8 interacted with Avr3a. We also showed that a specific allele of Avr3a is recognized by Rps3a but not Rps8. While Rps3a and Rps8 have been previously described as closely linked, this is the first report of a clear distinction hitherto undefined between these two resistance genes.

How to win a tug-of-war: the adaptive evolution of Phytophthora effectors

The 'zigzag' model formulates some of the fundamental principles underpinning the dynamic interactions between pathogen effectors and plant immunity. As key virulence factors, effectors often exhibit a pattern of rapid evolution, presumably as a result of the host-pathogen arms race. Here, we summarize the current knowledge of mechanisms that may accelerate effector evolution in the highly successful Phytophthora pathogens. Recent findings on epigenetic regulation of effector genes that allows evasion of host recognition and maintenance of cost/benefit balance, and a conserved structural unit in effector proteins that may promote the evolution of virulence activities are highlighted.

Silent control: microbial plant pathogens evade host immunity without coding sequence changes

Both animals and plants have evolved a robust immune system to surveil and defeat invading pathogenic microbes. Evasion of host immune surveillance is the key for pathogens to initiate successful infection. To evade the host immunity, plant pathogens evolved a variety of strategies such as masking themselves from host immune recognitions, blocking immune signaling transductions, reprogramming immune responses and adapting to immune microenvironmental changes. Gain of new virulence genes, sequence and structural variations enables plant pathogens to evade host immunity through changes in the genetic code. However, recent discoveries demonstrated that variations at the transcriptional, post-transcriptional, post-translational and glycome level enable pathogens to cope with the host immune system without coding sequence changes. The biochemical modification of pathogen associated molecular patterns and silencing of effector genes emerged as potent ways for pathogens to hide from host recognition. Altered processing in mRNA activities provide pathogens with resilience to microenvironment changes. Importantly, these hiding variants are directly or indirectly modulated by catalytic enzymes or enzymatic complexes and cannot be revealed by classical genomics alone. Unveiling these novel host evasion mechanisms in plant pathogens enables us to better understand the nature of plant disease and pinpoints strategies for rational diseases management in global food protection.

Effector gene silencing mediated by histone methylation underpins host adaptation in an oomycete plant pathogen

The relentless adaptability of pathogen populations is a major obstacle to effective disease control measures. Increasing evidence suggests that gene transcriptional polymorphisms are a strategy deployed by pathogens to evade host immunity. However, the underlying mechanisms of transcriptional plasticity remain largely elusive. Here we found that the soybean root rot pathogen Phytophthora sojae evades the soybean Resistance gene Rps1b through transcriptional polymorphisms in the effector gene Avr1b that occur in the absence of any sequence variation. Elevated levels of histone H3 Lysine27 tri-methylation (H3K27me3) were observed at the Avr1b locus in a naturally occurring Avr1b-silenced strain but not in an Avr1b-expressing strain, suggesting a correlation between this epigenetic modification and silencing of Avr1b. To genetically test this hypothesis, we edited the gene, PsSu(z)12, encoding a core subunit of the H3K27me3 methyltransferase complex by using CRISPR/Cas9, and obtained three deletion mutants. H3K27me3 depletion within the Avr1b genomic region correlated with impaired Avr1b gene silencing in these mutants. Importantly, these mutants lost the ability to evade immune recognition by soybeans carrying Rps1b. These data support a model in which pathogen effector transcriptional polymorphisms are associated with changes in chromatin epigenetic marks, highlighting epigenetic variation as a mechanism of pathogen adaptive plasticity.

Phytophthora infestans Sporangia Produced in Artificial Media and Plant Lesions Have Subtly Divergent Transcription Profiles but Equivalent Infection Potential and Aggressiveness

Sporangia of the potato late blight agent Phytophthora infestans are often used in studies of pathogen biology and plant responses to infection. Investigations of spore biology can be challenging in oomycetes because their sporangia are physiologically active and change in response to environmental factors and aging. Whether sporangia from artificial media and plant lesions are functionally equivalent has been a topic of debate. To address these issues, we compared the transcriptomes and infection ability of sporangia from rye-sucrose media, potato and tomato leaflets, and potato tubers. Small differences were observed between the mRNA profiles of sporangia from all sources, including variation in genes encoding metabolic enzymes, cell-wall-degrading enzymes, and ABC transporters. Small differences in sporangia age also resulted in variation in the transcriptome. Taking care to use sporangia of similar maturity, we observed that those sourced from media or plant lesions had similar rates of zoospore release and cyst germination. There were also no differences in infection rates or aggressiveness on leaflets, based on single-spore inoculation assays. Such results are discordant with those of a recent publication in this journal. Nevertheless, we conclude that sporangia from plant and media cultures are functionally similar and emphasize the importance of using "best practices" in experiments with sporangia to obtain reliable results.

Phenotypic instability in fungi

Fungi are prone to phenotypic instability, that is, the vegetative phase of these organisms, be they yeasts or molds, undergoes frequent switching between two or more behaviors, often with different morphologies, but also sometime having different physiologies without any obvious morphological outcome. In the context of industrial utilization of fungi, this can have a negative impact on the maintenance of strains and/or on their productivity. Instabilities have been shown to result from various mechanisms, either genetic or epigenetic. This chapter will review different types of instabilities and discuss some lesser-known ones, mostly in filamentous fungi, while it will direct readers to additional literature in the case of well-known phenomena such as the amyloid prions or fungal senescence. It will present in depth the "white/opaque" switch of Candida albicans and the "crippled growth" degeneration of the model fungus Podospora anserina. These are two of the most thoroughly studied epigenetic phenotypic switches. I will also discuss the "sectors" presented by many filamentous ascomycetes, for which a prion-based model exists but is not demonstrated. Finally, I will also describe intriguing examples of phenotypic instability for which an explanation has yet to be provided.

Isoflavonoid-specific prenyltransferase gene family in soybean: GmPT01, a pterocarpan 2-dimethylallyltransferase involved in glyceollin biosynthesis

Phytoalexin glyceollins are soybean-specific antimicrobial compounds that are derived from the isoflavonoid pathway. They are synthesized by soybean in response to extrinsic stress such as pathogen attack or injury, thereby conferring partial resistance if synthesized rapidly at the site of infection and at the required concentration. Soybean produces multiple forms of glyceollins that result from the differential prenylation reaction catalyzed by prenyltransferases (PTs) on either the C-2 or C-4 carbon of a pterocarpan glycinol. The soybean genome contains 77 PT-encoding genes (GmPTs) where at least 11 are (iso)flavonoid-specific. Transcript accumulation of five candidates GmPTs was increased in response to Phytophthora sojae infection, suggesting their role in phytoalexin synthesis. The induced GmPTs localize to plastids and display tissue-specific expression. We have in this study identified two additional GmPTs: an isoflavone dimethylallyltransferase 3 (IDT3); and a glycinol 2-dimethylallyl transferase GmPT01. GmPT01 prenylates (-)-glycinol at the C-2 position, localizes in the plastid, and exhibits root-specific gene expression. Furthermore, its expression is induced rapidly in response to stress, and is associated with a quantitative trait loci linked with resistance to P. sojae. Based on these results, we conclude that GmPT01 are possibly one of the loci involved in conferring partial resistance against stem and root rot disease in soybean.

Host-Pathogen interactions modulated by small RNAs

Biological processes such as defense mechanisms and microbial offense strategies are regulated through RNA induced interference in eukaryotes. Genetic mutations are modulated through biogenesis of small RNAs which directly impacts upon host development. Plant defense mechanisms are regulated and supported by a diversified group of small RNAs which are involved in streamlining several RNA interference pathways leading toward the initiation of pathogen gene silencing mechanisms. In the similar context, pathogens also utilize the support of small RNAs to launch their offensive attacks. Also there are strong evidences about the active involvement of these RNAs in symbiotic associations. Interestingly, small RNAs are not limited to the individuals in whom they are produced; they also show cross kingdom influences through variable interactions with other species thus leading toward the inter-organismic gene silencing. The phenomenon is understandable in the microbes which utilize these mechanisms to overcome host defense line. Understanding the mechanism of triggering host defense strategies can be a valuable step toward the generation of disease resistant host plants. We think that the cross kingdom trafficking of small RNA is an interesting insight that is needed to be explored for its vitality.