Powdery mildew effectors AVRA1 and BEC1016 target the ER J-domain protein HvERdj3B required for immunity in barley

The barley powdery mildew fungus, Blumeria hordei (Bh), secretes hundreds of candidate secreted effector proteins (CSEPs) to facilitate pathogen infection and colonization. One of these, CSEP0008, is directly recognized by the barley nucleotide-binding leucine-rich-repeat (NLR) receptor MLA1 and therefore is designated AVRA1. Here, we show that AVRA1 and the sequence-unrelated Bh effector BEC1016 (CSEP0491) suppress immunity in barley. We used yeast two-hybrid next-generation interaction screens (Y2H-NGIS), followed by binary Y2H and in planta protein-protein interactions studies, and identified a common barley target of AVRA1 and BEC1016, the endoplasmic reticulum (ER)-localized J-domain protein HvERdj3B. Silencing of this ER quality control (ERQC) protein increased Bh penetration. HvERdj3B is ER luminal, and we showed using split GFP that AVRA1 and BEC1016 translocate into the ER signal peptide-independently. Overexpression of the two effectors impeded trafficking of a vacuolar marker through the ER; silencing of HvERdj3B also exhibited this same cellular phenotype, coinciding with the effectors targeting this ERQC component. Together, these results suggest that the barley innate immunity, preventing Bh entry into epidermal cells, requires ERQC. Here, the J-domain protein HvERdj3B appears to be essential and can be regulated by AVRA1 and BEC1016. Plant disease resistance often occurs upon direct or indirect recognition of pathogen effectors by host NLR receptors. Previous work has shown that AVRA1 is directly recognized in the cytosol by the immune receptor MLA1. We speculate that the AVRA1 J-domain target being inside the ER, where it is inapproachable by NLRs, has forced the plant to evolve this challenging direct recognition.

The Fungal Endophyte Penicillium olsonii ML37 Reduces Fusarium Head Blight by Local Induced Resistance in Wheat Spikes

The fungal endophyte Penicillium olsonii ML37 is a biocontrol agent of Fusarium head blight in wheat (caused by Fusarium graminearum), which has shown a limited direct inhibition of fungal growth in vitro. We used RNA-seq and LC-MS/MS analyses to elucidate metabolic interactions of the three-way system Penicillium–wheat–Fusarium in greenhouse experiments. We demonstrated that P. olsonii ML37 colonises wheat spikes and transiently activates plant defence mechanisms, as pretreated spikes show a faster and stronger expression of the defence metabolism during the first 24 h after pathogen inoculation. This effect was transient and the expression of the same genes was lower in the pathogen-infected spikes than in those infected by P. olsonii alone. This response to the endophyte includes the transcriptional activation of several WRKY transcription factors. This early activation is associated with a reduction in FHB symptoms and significantly lower levels of the F. graminearum metabolites 15-acetyl-DON and culmorin. An increase in the Penicillium-associated metabolite asperphanamate confirms colonisation by the endophyte. Our results suggest that the mode of action used by P. olsonii ML37 is via a local defence activation in wheat spikes, and that this fungus has potential as a novel biological alternative in wheat disease control.

Regulation of Tomato Specialised Metabolism after Establishment of Symbiosis with the Endophytic Fungus Serendipita indica

Specialised metabolites produced during plant-fungal associations often define how symbiosis between the plant and the fungus proceeds. They also play a role in the establishment of additional interactions between the symbionts and other organisms present in the niche. However, specialised metabolism and its products are sometimes overlooked when studying plant-microbe interactions. This limits our understanding of the specific symbiotic associations and potentially future perspectives of their application in agriculture. In this study, we used the interaction between the root endophyte Serendipita indica and tomato (Solanum lycopersicum) plants to explore how specialised metabolism of the host plant is regulated upon a mutualistic symbiotic association. To do so, tomato seedlings were inoculated with S. indica chlamydospores and subjected to RNAseq analysis. Gene expression of the main tomato specialised metabolism pathways was compared between roots and leaves of endophyte-colonised plants and tissues of endophyte-free plants. S. indica colonisation resulted in a strong transcriptional response in the leaves of colonised plants. Furthermore, the presence of the fungus in plant roots appears to induce expression of genes involved in the biosynthesis of lignin-derived compounds, polyacetylenes, and specific terpenes in both roots and leaves, whereas pathways producing glycoalkaloids and flavonoids were expressed in lower or basal levels.

Identification of a bio-signature for barley resistance against Pyrenophora teres infection based on physiological, molecular and sensor-based phenotyping

Necrotic and chlorotic symptoms induced during Pyrenophora teres infection in barley leaves indicate a compatible interaction that allows the hemi-biotrophic fungus Pyrenophora teres to colonise the host. However, it is unexplored how this fungus affects the physiological responses of resistant and susceptible cultivars during infection. To assess the degree of resistance in four different cultivars, we quantified visible symptoms and fungal DNA and performed expression analyses of genes involved in plant defence and ROS scavenging. To obtain insight into the interaction between fungus and host, we determined the activity of 19 key enzymes of carbohydrate and antioxidant metabolism. The pathogen impact was also phenotyped non-invasively by sensor-based multireflectance and -fluorescence imaging. Symptoms, regulation of stress-related genes and pathogen DNA content distinguished the cultivar Guld as being resistant. Severity of net blotch symptoms was also strongly correlated with the dynamics of enzyme activities already within the first day of infection. In contrast to the resistant cultivar, the three susceptible cultivars showed a higher reflectance over seven spectral bands and higher fluorescence intensities at specific excitation wavelengths. The combination of semi high-throughput physiological and molecular analyses with non-invasive phenotyping enabled the identification of bio-signatures that discriminates the resistant from susceptible cultivars.

A Sesquiterpene Synthase from the Endophytic Fungus Serendipita indica Catalyzes Formation of Viridiflorol

Interactions between plant-associated fungi and their hosts are characterized by a continuous crosstalk of chemical molecules. Specialized metabolites are often produced during these associations and play important roles in the symbiosis between the plant and the fungus, as well as in the establishment of additional interactions between the symbionts and other organisms present in the niche. Serendipita indica, a root endophytic fungus from the phylum Basidiomycota, is able to colonize a wide range of plant species, conferring many benefits to its hosts. The genome of S. indica possesses only few genes predicted to be involved in specialized metabolite biosynthesis, including a putative terpenoid synthase gene (SiTPS). In our experimental setup, SiTPS expression was upregulated when the fungus colonized tomato roots compared to its expression in fungal biomass growing on synthetic medium. Heterologous expression of SiTPS in Escherichia coli showed that the produced protein catalyzes the synthesis of a few sesquiterpenoids, with the alcohol viridiflorol being the main product. To investigate the role of SiTPS in the plant-endophyte interaction, an SiTPS-over-expressing mutant line was created and assessed for its ability to colonize tomato roots. Although overexpression of SiTPS did not lead to improved fungal colonization ability, an in vitro growth-inhibition assay showed that viridiflorol has antifungal properties. Addition of viridiflorol to the culture medium inhibited the germination of spores from a phytopathogenic fungus, indicating that SiTPS and its products could provide S. indica with a competitive advantage over other plant-associated fungi during root colonization.

Insights into the community structure and lifestyle of the fungal root endophytes of tomato by combining amplicon sequencing and isolation approaches with phytohormone profiling

Little is known about the influence of host genotype and phytohormones on the composition of fungal endophytic communities. We investigated the influence of host genotype and phytohormones on the structure of the fungal endophytic communities of tomato roots by amplicon sequencing of the ITS1 region and combined this approach with isolation and functional characterization of the isolates. A significant effect of the host genotype on the dominant fungal species was found by comparing the cultivars Castlemart and UC82B and, surprisingly, root pathogens were among the most abundant taxa. In contrast, smaller changes in the relative abundance of the dominant species were found in mutants impaired in jasmonic acid biosynthesis (def1) and ethylene biosynthesis (8338) compared to the respective wild types. However, def1 showed significantly higher species richness compared to the wild type. Analysis of the phytohormone profiles of these genotypes indicates that changes in the phytohormone balance may contribute to this difference in species richness. Assessing the lifestyle of isolated fungi on tomato seedlings revealed the presence of both beneficial endophytes and latent pathogens in roots of asymptomatic plants, suggesting that the interactions between members of the microbiome maintain the equilibrium in the community preventing pathogens from causing disease.

Fusarium Head Blight Modifies Fungal Endophytic Communities During Infection of Wheat Spikes

Fusarium head blight (FHB) is a devastating disease of wheat heads. It is caused by several species from the genus Fusarium. Several endophytic fungi also colonize wheat spikes asymptomatically. Pathogenic and commensal fungi share and compete for the same niche and thereby influence plant performance. Understanding the natural dynamics of the fungal community and how the pre-established species react to pathogen attack can provide useful information on the disease biology and the potential use of some of these endophytic organisms in disease control strategies. Fungal community composition was assessed during anthesis as well as during FHB attack in wheat spikes during 2016 and 2017 in two locations. Community metabarcoding revealed that endophyte communities are dominated by basidiomycete yeasts before anthesis and shift towards a more opportunistic ascomycete-rich community during kernel development. These dynamics are interrupted when Fusarium spp. colonize wheat spikes. The Fusarium pathogens appear to exclude other fungi from floral tissues as they are associated with a reduction in community diversity, especially in the kernel which they colonize rapidly. Similarly, the presence of several endophytes was negatively correlated with Fusarium spp. and linked with spikes that stayed healthy despite exposure to the pathogen. These endophytes belonged to the genera Cladosporium, Itersonillia and Holtermanniella. These findings support the hypothesis that some naturally occurring endophytes could outcompete or prevent FHB and represent a source of potential biological control agents in wheat.

Transcriptional reprogramming of wheat and the hemibiotrophic pathogen Septoria tritici during two phases of the compatible interaction

The disease septoria leaf blotch of wheat, caused by fungal pathogen Septoria tritici, is of worldwide concern. The fungus exhibits a hemibiotrophic lifestyle, with a long symptomless, biotrophic phase followed by a sudden transition to necrotrophy associated with host necrosis. Little is known about the systematic interaction between fungal pathogenicity and host responses at specific growth stages and the factors triggering the transition. In order to gain some insights into global transcriptome alterations in both host and pathogen during the two phases of the compatible interaction, disease transition was monitored using pathogenesis-related gene markers and H₂O₂ signature prior to RNA-Seq. Transcriptome analysis revealed that the slow symptomless growth was accompanied by minor metabolic responses and slightly suppressed defences in the host, whereas necrotrophic growth was associated with enhanced host responses involving energy metabolism, transport, signalling, defence and oxidative stress as well as a decrease in photosynthesis. The fungus expresses distinct classes of stage-specific genes encoding potential effectors, probably first suppressing plant defence responses/facilitating the symptomless growth and later triggering life style transition and inducing host necrosis/facilitating the necrotrophic growth. Transport, signalling, anti-oxidative stress mechanisms and apoplastic nutrient acquisition play important roles in the entire infection process of S. tritici. Our findings uncover systematic S. tritici-induced expression profiles of wheat related to specific fungal infection strategies and provide a transcriptome resource for studying both hosts and pathogens in plant-Dothideomycete interactions.

Battle through signaling between wheat and the fungal pathogen Septoria tritici revealed by proteomics and phosphoproteomics

The fungus Septoria tritici causes the disease septoria tritici blotch in wheat, one of the most economically devastating foliar diseases in this crop. To investigate signaling events and defense responses in the wheat-S. tritici interaction, we performed a time-course study of S. tritici infection in resistant and susceptible wheat using quantitative proteomics and phosphoproteomics, with special emphasis on the initial biotrophic phase of interactions. Our study revealed an accumulation of defense and stress-related proteins, suppression of photosynthesis, and changes in sugar metabolism during compatible and incompatible interactions. However, differential regulation of the phosphorylation status of signaling proteins, transcription and translation regulators, and membrane-associated proteins was observed between two interactions. The proteomic data were correlated with a more rapid or stronger accumulation of signal molecules, including calcium, H2O2, NO, and sugars, in the resistant than in the susceptible cultivar in response to the infection. Additionally, 31 proteins and 5 phosphoproteins from the pathogen were identified, including metabolic proteins and signaling proteins such as GTP-binding proteins, 14-3-3 proteins, and calcium-binding proteins. Quantitative PCR analysis showed the expression of fungal signaling genes and genes encoding a superoxide dismutase and cell-wall degrading enzymes. These results indicate roles of signaling, antioxidative stress mechanisms, and nutrient acquisition in facilitating the initial symptomless growth. Taken in its entirety, our dataset suggests interplay between the plant and S. tritici through complex signaling networks and downstream molecular events. Resistance is likely related to several rapidly and intensively triggered signal transduction cascades resulting in a multiple-level activation of transcription and translation processes of defense responses. Our sensitive approaches and model provide a comprehensive (phospho)proteomics resource for studying signaling from the point of view of both host and pathogen during a plant-pathogen interaction.

Fusarium graminearum and Its Interactions with Cereal Heads: Studies in the Proteomics Era

The ascomycete fungal pathogen Fusarium graminearum (teleomorph stage: Gibberella zeae) is the causal agent of Fusarium head blight in wheat and barley. This disease leads to significant losses of crop yield, and especially quality through the contamination by diverse fungal mycotoxins, which constitute a significant threat to the health of humans and animals. In recent years, high-throughput proteomics, aiming at identifying a broad spectrum of proteins with a potential role in the pathogenicity and host resistance, has become a very useful tool in plant-fungus interaction research. In this review, we describe the progress in proteomics applications toward a better understanding of F. graminearum pathogenesis, virulence, and host defense mechanisms. The contribution of proteomics to the development of crop protection strategies against this pathogen is also discussed briefly.

Secretomics identifies Fusarium graminearum proteins involved in the interaction with barley and wheat

Fusarium graminearum is a phytopathogenic fungus primarily infecting small grain cereals, including barley and wheat. Secreted enzymes play important roles in the pathogenicity of many fungi. In order to access the secretome of F. graminearum, the fungus was grown in liquid culture with barley or wheat flour as the sole nutrient source to mimic the host-pathogen interaction. A gel-based proteomics approach was employed to identify the proteins secreted into the culture medium. Sixty-nine unique fungal proteins were identified in 154 protein spots, including enzymes involved in the degradation of cell walls, starch and proteins. Of these proteins, 35% had not been identified in previous in planta or in vitro studies, 70% were predicted to contain signal peptides and a further 16% may be secreted in a nonclassical manner. Proteins identified in the 72 spots showing differential appearance between wheat and barley flour medium were mainly involved in fungal cell wall remodelling and the degradation of plant cell walls, starch and proteins. The in planta expression of corresponding F. graminearum genes was confirmed by quantitative reverse transcriptase-polymerase chain reaction in barley and wheat spikelets harvested at 2-6 days after inoculation. In addition, a clear difference in the accumulation of fungal biomass and the extent of fungal-induced proteolysis of plant β-amylase was observed in barley and wheat. The present study considerably expands the current database of F. graminearum secreted proteins which may be involved in Fusarium head blight.

Rhizobacterially induced protection of watermelon against Didymella bryoniae

AIMS

To identify rhizobacteria from the Mekong Delta of Vietnam, which can systemically protect watermelon against Didymella bryoniae and elucidate the mechanisms involved in the protection conferred by isolate Pseudomonas aeruginosa 23(1-1).

METHODS AND RESULTS

Bacteria were isolated from watermelon roots and their antagonistic ability tested in vitro. Of 190 strains, 68 were able to inhibit D. bryoniae by production of antibiotics. Four strains were able to reduce foliar infection by D. bryoniae when applied to watermelon seeds before sowing. Strain Ps. aeruginosa 23(1-1) was chosen for investigations of the mechanisms involved in protection and ability to control disease under field conditions. In the field, the bacterium was able to significantly reduce disease in two consecutive seasons and increase yield. Furthermore, it colonized watermelon plants endophytically, with higher numbers in plants infected by D. bryoniae than in noninoculated plants. To elucidate the mechanisms involved in protection, the infection biology of the pathogen was studied in bacterially treated and control plants. Pseudomonas aeruginosa 23(1-1) treatment inhibited pathogen penetration and this was associated with hydrogen peroxide accumulation, increased peroxidase activity and occurrence of new peroxidase isoforms, thus indicating that resistance was induced.

CONCLUSIONS

The endophytic bacterium Ps. aeruginosa 23(1-1) can control D. bryoniae in watermelon by antibiosis and induced resistance under greenhouse and field conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

These findings suggest that rhizobacteria from native soils in Vietnam can be used to control gummy stem blight of watermelon through various mechanisms including induction of resistance.

Investigation of the effect of nitrogen on severity of Fusarium head blight in barley

The effect of nitrogen on Fusarium Head Blight (FHB) in a susceptible barley cultivar was investigated using gel-based proteomics. Barley grown with either 15 or 100kgha(-1)N fertilizer was inoculated with Fusarium graminearum (Fg). The storage protein fraction did not change significantly in response either to N level or Fg, whereas eighty protein spots in the water-soluble albumin fraction increased and 108 spots decreased more than two-fold in intensity in response to Fg. Spots with greater intensity in infected plants contained fungal proteins (9 spots) and proteolytic fragments of plant proteins (65 spots). Identified fungal proteins included two superoxide dismutases, L-xylulose reductase in two spots, peptidyl prolyl cis-trans isomerase and triosephosphate isomerase, and proteins of unknown function. Spots decreasing in intensity in response to Fg contained plant proteins possibly degraded by fungal proteases. Greater spot volume changes occurred in response to Fg in plants grown with low nitrogen, although proteomes of uninfected plants were similar for both treatments. Correlation of proteome changes with measurement of Fusarium-damaged kernels, fungal biomass and mycotoxin levels indicated that increased Fusarium infection occurred in barley with low N and suggests control of N fertilization as a possible way to minimise FHB in barley.

Engineering pathogen resistance in crop plants: current trends and future prospects

Transgenic crops are now grown commercially in 25 countries worldwide. Although pathogens represent major constraints for the growth of many crops, only a tiny proportion of these transgenic crops carry disease resistance traits. Nevertheless, transgenic disease-resistant plants represent approximately 10% of the total number of approved field trials in North America, a proportion that has remained constant for 15 years. In this review, we explore the socioeconomic and biological reasons for the paradox that although technically useful solutions now exist for providing transgenic disease resistance, very few new crops have been introduced to the global market. For bacteria and fungi, the majority of transgenic crops in trials express antimicrobial proteins. For viruses, three-quarters of the transgenics express coat protein (CP) genes. There is a notable trend toward more biologically sophisticated solutions involving components of signal transduction pathways regulating plant defenses. For viruses, RNA interference is increasingly being used.

Investigation of the effect of nitrogen on severity of Fusarium head blight in barley

The effect of nitrogen on Fusarium Head Blight (FHB) in a susceptible barley cultivar was investigated using gel-based proteomics. Barley grown with either 15 or 100kgha(-1)N fertilizer was inoculated with Fusarium graminearum (Fg). The storage protein fraction did not change significantly in response either to N level or Fg, whereas eighty protein spots in the water-soluble albumin fraction increased and 108 spots decreased more than two-fold in intensity in response to Fg. Spots with greater intensity in infected plants contained fungal proteins (9 spots) and proteolytic fragments of plant proteins (65 spots). Identified fungal proteins included two superoxide dismutases, L-xylulose reductase in two spots, peptidyl prolyl cis-trans isomerase and triosephosphate isomerase, and proteins of unknown function. Spots decreasing in intensity in response to Fg contained plant proteins possibly degraded by fungal proteases. Greater spot volume changes occurred in response to Fg in plants grown with low nitrogen, although proteomes of uninfected plants were similar for both treatments. Correlation of proteome changes with measurement of Fusarium-damaged kernels, fungal biomass and mycotoxin levels indicated that increased Fusarium infection occurred in barley with low N and suggests control of N fertilization as a possible way to minimise FHB in barley.