Barley Host Research Articles

Site-specific analysis reveals candidate cross-kingdom small RNAs, tRNA and rRNA fragments, and signs of fungal RNA phasing in the barley-powdery mildew interaction.

The establishment of host-microbe interactions requires molecular communication between both partners, which may involve the mutual transfer of noncoding small RNAs. Previous evidence suggests that this is also true for powdery mildew disease in barley, which is caused by the fungal pathogen Blumeria hordei. However, previous studies lacked spatial resolution regarding the accumulation of small RNAs upon host infection by B. hordei. Here, we analysed site-specific small RNA repertoires in the context of the barley-B. hordei interaction. To this end, we dissected infected leaves into separate fractions representing different sites that are key to the pathogenic process: epiphytic fungal mycelium, infected plant epidermis, isolated haustoria, a vesicle-enriched fraction from infected epidermis, and extracellular vesicles. Unexpectedly, we discovered enrichment of specific 31-33-base 5'-terminal fragments of barley 5.8S ribosomal RNA in extracellular vesicles and infected epidermis, as well as particular B. hordei transfer RNA fragments in haustoria. We describe canonical small RNAs from both the plant host and the fungal pathogen that may confer cross-kingdom RNA interference activity. Interestingly, we found first evidence of phased small interfering RNAs in B. hordei, a feature usually attributed to plants, which may be associated with the posttranscriptional control of fungal coding genes, pseudogenes, and transposable elements. Our data suggest a key and possibly site-specific role for cross-kingdom RNA interference and noncoding RNA fragments in the host-pathogen communication between B. hordei and its host barley.

Fungal Endophytes Isolated from Elymus repens, a Wild Relative of Barley, Have Potential for Biological Control of Fusarium culmorum and Pyrenophora teres in Barley.

Twenty-four fungal endophytes, isolated from a wild relative of barley, Elymus repens, were screened in barley against an isolate of Fusarium culmorum and an isolate of Pyrenophora teres under controlled conditions. In all experiments, the endophytes were applied individually as seed dressings. Five endophytes could significantly reduce symptoms of Fusarium culmorum (Periconia macrospinosa E1 and E2, Epicoccum nigrum E4, Leptodontidium sp. E7 and Slopeiomyces cylindrosporus E18). In particular, treatment with Periconia macrospinosa E1 significantly reduced Fusarium symptoms on roots by 29–63% in two out of four experiments. Using, a gfp transformed isolate of P. macrospinosa E1, it was possible to show that the fungus was present on roots 14 days after sowing, coinciding with the disease scoring. To test for a potential systemic effect of the seed treatment, eight endophyte isolates were tested against the leaf pathogen Pyrenophora teres. Three isolates could significantly reduce symptoms of P. teres (Lasiosphaeriaceae sp. E10, Lindgomycetaceae sp. E13 and Leptodontidium sp. E16). Seed treatment with Lasiosphaeriaceae sp. E10 reduced net blotch leaf lesion coverage by 89%, in one out of three experiments. In conclusion, specific endophyte isolates exerted varying degrees of protection in the different experiments. Nevertheless, data suggest that endophytic strains from E. repens in a few cases are antagonistic against F. culmorum and P. teres, but otherwise remain neutral when introduced to a barley host in a controlled environment.

Pathotype Identification and Virulence Variation in Cochliobolus sativus in China

Spot blotch caused by Cochliobolus sativus has become an important disease in the wheat-growing regions in China that has resulted from changes in the regional climate, agricultural cultivation patterns, and the susceptible wheat varieties that are widely grown. Little information is available about virulence variability and pathogenic specialization of the C. sativus isolates from major wheat-growing regions in China. Here, 12 representative wheat varieties and foundation breeding stocks were selected to characterize the pathotypes of C. sativus isolates from infected wheat plants. Based on the infection phenotypes in the 12 differential genotypes at the seedling stage, 70 Chinese pathotypes were identified from 110 isolates and clustered into three virulence groups. The high virulence isolates were collected from wheat leaves, crowns, and roots, with most (10 of 14) from the Henan province in the Huang-Huai plain. No relationship was evident between virulence variability of C. sativus isolates and their geographic origins or types of diseased wheat tissues. C. sativus showed a significant pathogenic specialization in hosts of wheat and barley. Most of the wheat isolates (50 of 65) were avirulent to all the differential barley genotypes, and a few were virulent only to highly susceptible barley genotypes. These results indicated that C. sativus isolates from the wheat-growing regions in China varied considerably for their virulence in wheat varieties, and showed significant pathogenic specialization to the wheat and barley hosts.

Defence responses triggered by Blumeria graminis f.sp. hordei in non‐host wheat genotypes results in a decrease in Puccinia triticina infection

AbstractNon‐host resistance is the resistance of plants against all so‐called non‐adapted pathogens, and is considered the most durable and efficient immune system of plants. We investigated barley powdery mildew (Blumeria graminis f.sp. hordei, Bgh)‐triggered defence responses in seven wheat genotypes, and the induction of resistance by this non‐host pathogen to challenge infection with leaf rust, Puccinia triticina Eriks (formerly known as Puccinia recondita f.sp. tritici). Bright‐field and ultraviolet microscopy revealed that although the germination of conidia on non‐host wheat leaves was generally lower than on host barley leaves, the 100% germination on Mv Mambó and only 50% germination on Mv 1 wheat genotypes are the two extremes. If we consider only the germinated conidia, papilla formation was the most frequently observed interaction on Mv 2 (90%) and Mv 4 (88%) genotypes. Epidermal cell death was pronounced on Mv Mambó (31%), but rare on Mv 2 (10%) and Mv 4 (11%), and a mesophyll hypersensitive response could even be detected. At sites of penetration attempts accumulation of H2O2 was shown by 3,3′‐diaminobenzidine staining. Ascorbate peroxidase, glutathione S‐transferase and catalase enzyme activities were augmented by 24.5%, 40.4%, and 21.5%, respectively, in wheat leaves 2 days after Bgh infection. Primary inoculation with Bgh, although with high variability, reduced the number of pustules in all wheat genotypes at first and third leaf stages upon inoculation with leaf rust races 12 and 61 by 10%–70% depending on the genotype, leaf stage and rust race.

Cross-infectivity of Helminthosporium isolates occurring on barley and other graminaceous hosts

Foliar blights of graminaceous host are one of the important fungal diseases causing considerable losses to the crops. Under Punjab conditions, foliar blight/spot blotch of wheat and barley (Bipolaris sorokiniana) and leaf spot of oats (Drechslera avenae) are one of the devastating fungal diseases causing considerable reduction in the grain yield. Fifty leaf samples showing typical disease symptoms of foliar blight were collected from different barley, wheat and oats growing areas of Punjab. Samples were also collected from the commonly found weed in the wheat fields i.e. Phalaris minor. Considering the pathogenic behaviour of different isolates on their respective susceptible checks viz., PL426 (barley), HD2329 (wheat), OL-9 (oats) and P. minor, the most virulent isolate from each host i.e. Bsb1, Bsb32, Bsp46 and Da46 from barley, wheat, P. minor and oats, respectively were selected to check their cross- infectivity on different graminaceous hosts including weed hosts under pot house conditions. It was observed that isolate Bsb1 and Bsw32 from barley and wheat, respectively were more aggressive exhibiting moderately resistant to susceptible reaction on different hosts, followed by Bsp34 from P. minor. Isolate Da46 from oats was least aggressive thus exhibiting resistant to moderate reaction on the different tested hosts. Hence, for B. sorokiniana, wheat, barley and oats can serve as collateral hosts. The test isolates developed moderately resistant and resistant reaction on rice also, hence, it can also help in the perpetuation of the pathogen.

The inconspicuous gatekeeper: endophytic Serendipita vermifera acts as extended plant protection barrier in the rhizosphere.

In nature, beneficial and pathogenic fungi often simultaneously colonise plants. Despite substantial efforts to understand the composition of natural plant-microbe communities, the mechanisms driving such multipartite interactions remain largely unknown. Here we address how the interaction between the beneficial root endophyte Serendipita vermifera and the pathogen Bipolaris sorokiniana affects fungal behaviour and determines barley host responses using a gnotobiotic soil-based split-root system. Fungal confrontation in soil resulted in induction of B.sorokiniana genes involved in secondary metabolism and a significant repression of genes encoding putative effectors. In S.vermifera, genes encoding hydrolytic enzymes were strongly induced. This antagonistic response was not activated during the tripartite interaction in barley roots. Instead, we observed a specific induction of S.vermifera genes involved in detoxification and redox homeostasis. Pathogen infection but not endophyte colonisation resulted in substantial host transcriptional reprogramming and activation of defence. In the presence of S.vermifera, pathogen infection and disease symptoms were significantly reduced despite no marked alterations of the plant transcriptional response. The activation of stress response genes and concomitant repression of putative effector gene expression in B.sorokiniana during confrontation with the endophyte suggest a reduction of the pathogen's virulence potential before host plant infection.

Bulked segregant RNA-sequencing (BSR-seq) identified a novel rare allele of eIF4E effective against multiple isolates of BaYMV/BaMMV.

A novel rare allele of the barley host factor gene eIF4E for BaMMV/BaYMV infection was identified in an Iranian landrace that showed broad resistance to barley yellow mosaic virus disease, and molecular markers facilitating efficient selection were developed. The soil-borne yellow mosaic virus disease caused by different strains of barley yellow mosaic virus (BaYMV) and barley mild mosaic virus (BaMMV) is a major threat to winter barley (Hordeum vulgare) production in Europe and East Asia. However, the exploration of resistant germplasm or casual genes for barley breeding is rather limited in relation to the rapid diversification of viral strains. Here, we identified an Iranian barley landrace 'HOR3298,' which represented complete resistance to BaYMV and BaMMV. In contrastto rym4 and rym5, which act as the predominant source in Europe and East Asia for breeding resistant cultivars over decades and which have been overcome by several virulent isolates, this landrace showed broad-spectrum resistance to multiple isolates of BaYMV/BaMMV in the fields of Germany and China. By employment of bulked segregant RNA sequencing, test for allelism, and haplotype analysis, a recessive resistance gene in 'HOR3298' was genetically mapped coincident with the host factor eukaryotic translation initiation factor 4E (eIF4E, causal gene of rym4 and rym5). The eIF4EHOR3298 allele encoded for a novel haplotype that contained an exclusive nucleotide mutation (G565A) in the coding sequence. The easily handled markers were developed based on the exclusively rare variation, providing precise selection of this allele. Thus, this work provided a novel reliable resistance source and the feasible marker-assisted selection assays that can be used in breeding for barley yellow mosaic virus disease resistance in cultivated barley.

Investigating hybridisation between the forms of Pyrenophora teres based on Australian barley field experiments and cultural collections

Pyrenophora teres f. teres (Ptt) and P. teres f. maculata (Ptm) cause net and spot form of net blotch of barley (Hordeum vulgare L.), respectively. Both pathogens co-exist in barley fields and each can reproduce sexually, resulting in hybridisation and potential generation of novel virulences that could overcome barley host resistances. In this study, three field experiments were conducted during three successive years to investigate the occurrence of hybridisation. Susceptible barley was sown and inoculated with Ptt and Ptm. Form-specific PCR markers were used to analyse 822 conidia and 223 ascospores sampled from infected leaf tissue and 317 P. teres isolates collected across Australia during 1976–2015. None of the isolates were hybrids. Investigation of ascospores indicated that hybridisation had taken place within the forms, demonstrating preference for recombination within forms. Possible contributions of reproductive barriers have been appraised but further investigation is required to explore the rare hybridisation between the forms.

Could vectors\u2019 fear of predators reduce the spread of plant diseases?

Predators influence the behaviour of prey and by doing so they potentially reduce pathogen transmission by a vector. Arthropod predators have been shown to reduce the consumption of plant biomass by pest herbivores, but their cascading non-consumptive effect on vector insects’ feeding behaviour and subsequent pathogen transmission has not been investigated experimentally before. Here we experimentally examined predator-mediated pathogen transmission mechanisms using the plant pathogen Wheat Dwarf Virus that is transmitted by the leafhopper, Psammotettix alienus. We applied in situ hybridization to localize which leaf tissues were infected with transmitted virus DNA in barley host plants, proving that virus occurrence is restricted to phloem tissues. In the presence of the spider predator, Tibellus oblongus, we recorded the within leaf feeding behaviour of the herbivore using electrical penetration graph. The leafhopper altered its feeding behaviour in response to predation risk. Phloem ingestion, the feeding phase when virus acquisition occurs, was delayed and was less frequent. The phase when pathogen inoculation takes place, via the secretion of virus infected vector saliva, was shorter when predator was present. Our study thus provides experimental evidence that predators can potentially limit the spread of plant pathogens solely through influencing the feeding behaviour of vector organisms.

An LRR/Malectin Receptor-Like Kinase Mediates Resistance to Non-adapted and Adapted Powdery Mildew Fungi in Barley and Wheat.

Pattern recognition receptors (PRRs) belonging to the multigene family of receptor-like kinases (RLKs) are the sensing devices of plants for microbe- or pathogen-associated molecular patterns released from microbial organisms. Here we describe Rnr8 (for Required for non-host resistance 8) encoding HvLEMK1, a LRR-malectin domain-containing transmembrane RLK that mediates non-host resistance of barley to the non-adapted wheat powdery mildew fungus Blumeria graminis f.sp. tritici. Transgenic barley lines with silenced HvLEMK1 allow entry and colony growth of the non-adapted pathogen, although sporulation was reduced and final colony size did not reach that of the adapted barley powdery mildew fungus B. graminis f.sp. hordei. Transient expression of the barley or wheat LEMK1 genes enhanced resistance in wheat to the adapted wheat powdery mildew fungus while expression of the same genes did not protect barley from attack by the barley powdery mildew fungus. The results suggest that HvLEMK1 is a factor mediating non-host resistance in barley and quantitative host resistance in wheat to the wheat powdery mildew fungus.

The genome of the emerging barley pathogen Ramularia collo-cygni.

BackgroundRamularia collo-cygni is a newly important, foliar fungal pathogen of barley that causes the disease Ramularia leaf spot. The fungus exhibits a prolonged endophytic growth stage before switching life habit to become an aggressive, necrotrophic pathogen that causes significant losses to green leaf area and hence grain yield and quality.ResultsThe R. collo-cygni genome was sequenced using a combination of Illumina and Roche 454 technologies. The draft assembly of 30.3 Mb contained 11,617 predicted gene models. Our phylogenomic analysis confirmed the classification of this ascomycete fungus within the family Mycosphaerellaceae, order Capnodiales of the class Dothideomycetes. A predicted secretome comprising 1053 proteins included redox-related enzymes and carbohydrate-modifying enzymes and proteases. The relative paucity of plant cell wall degrading enzyme genes may be associated with the stealth pathogenesis characteristic of plant pathogens from the Mycosphaerellaceae. A large number of genes associated with secondary metabolite production, including homologs of toxin biosynthesis genes found in other Dothideomycete plant pathogens, were identified.ConclusionsThe genome sequence of R. collo-cygni provides a framework for understanding the genetic basis of pathogenesis in this important emerging pathogen. The reduced complement of carbohydrate-degrading enzyme genes is likely to reflect a strategy to avoid detection by host defences during its prolonged asymptomatic growth. Of particular interest will be the analysis of R. collo-cygni gene expression during interactions with the host barley, to understand what triggers this fungus to switch from being a benign endophyte to an aggressive necrotroph.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-2928-3) contains supplementary material, which is available to authorized users.

Infection strategy of Ramularia collo‐cygni and development of ramularia leaf spot on barley and alternative graminaceous hosts

Ramularia leaf spot (RLS) is a newly important disease of barley across temperate regions worldwide. Despite this recent change in importance, the infection biology of the causal agent Ramularia collo‐cygni (Rcc) remains poorly understood. Confocal microscopy of the infection process of two transgenic Rcc isolates, expressing either GFP or DsRed reporter markers, was combined with light microscopy during field infection to track the progression of Rcc in planta. Infection of stomata, including the development of a previously unreported stomatopodium structure, results in symptomless development and intercellular colonization of the mesophyll tissue. Transition to necrotrophy is associated with breakdown of host chloroplasts and the formation of aggregates of conidiophores. In addition to barley, Rcc forms a compatible interaction with winter wheat and a number of perennial grass species. An incompatible reaction was observed with two dicotyledonous species. These results provide further insights into the host interactions of this fungus and suggest that RLS could be a potential threat to other agriculturally important crops.

Positive-strand RNA viruses stimulate host phosphatidylcholine synthesis at viral replication sites

All positive-strand RNA viruses reorganize host intracellular membranes to assemble their viral replication complexes (VRCs); however, how these viruses modulate host lipid metabolism to accommodate such membrane proliferation and rearrangements is not well defined. We show that a significantly increased phosphatidylcholine (PC) content is associated with brome mosaic virus (BMV) replication in both natural host barley and alternate host yeast based on a lipidomic analysis. Enhanced PC levels are primarily associated with the perinuclear ER membrane, where BMV replication takes place. More specifically, BMV replication protein 1a interacts with and recruits Cho2p (choline requiring 2), a host enzyme involved in PC synthesis, to the site of viral replication. These results suggest that PC synthesized at the site of VRC assembly, not the transport of existing PC, is responsible for the enhanced accumulation. Blocking PC synthesis by deleting the CHO2 gene resulted in VRCs with wider diameters than those in wild-type cells; however, BMV replication was significantly inhibited, highlighting the critical role of PC in VRC formation and viral replication. We further show that enhanced PC levels also accumulate at the replication sites of hepatitis C virus and poliovirus, revealing a conserved feature among a group of positive-strand RNA viruses. Our work also highlights a potential broad-spectrum antiviral strategy that would disrupt PC synthesis at the sites of viral replication but would not alter cellular processes.

The knottin-like Blufensin family regulates genes involved in nuclear import and the secretory pathway in barley-powdery mildew interactions.

Plants have evolved complex regulatory mechanisms to control a multi-layered defense response to microbial attack. Both temporal and spatial gene expression are tightly regulated in response to pathogen ingress, modulating both positive and negative control of defense. BLUFENSINs, small knottin-like peptides in barley, wheat, and rice, are highly induced by attack from fungal pathogens, in particular, the obligate biotrophic fungus, Blumeria graminis f. sp. hordei (Bgh), causal agent of barley powdery mildew. Previous research indicated that Blufensin1 (Bln1) functions as a negative regulator of basal defense mechanisms. In the current report, we show that BLN1 and BLN2 can both be secreted to the apoplast and Barley stripe mosaic virus (BSMV)-mediated overexpression of Bln2 increases susceptibility of barley to Bgh. Bimolecular fluorescence complementation (BiFC) assays signify that BLN1 and BLN2 can interact with each other, and with calmodulin. We then used BSMV-induced gene silencing to knock down Bln1, followed by Barley1 GeneChip transcriptome analysis, to identify additional host genes influenced by Bln1. Analysis of differential expression revealed a gene set enriched for those encoding proteins annotated to nuclear import and the secretory pathway, particularly Importin α1-b and Sec61 γ subunits. Further functional analysis of these two affected genes showed that when silenced, they also reduced susceptibility to Bgh. Taken together, we postulate that Bln1 is co-opted by Bgh to facilitate transport of disease-related host proteins or effectors, influencing the establishment of Bgh compatibility on its barley host.

Association mapping of seedling resistance to spot form net blotch in a worldwide collection of barley.

Spot form net blotch (SFNB), caused by the necrotrophic fungal pathogen Pyrenophora teres f. maculata, is an important foliar disease of barley in major production regions around the world. Deployment of adequate host resistance is challenging because the virulence of P. teres f. maculata is highly variable and characterized minor-effect resistances are typically ineffective against the diverse pathogen populations. A world barley core collection consisting of 2,062 barley accessions of diverse origin and genotype were phenotyped at the seedling stage with four P. teres f. maculata isolates collected from the United States (FGO), New Zealand (NZKF2), Australia (SG1), and Denmark (DEN 2.6). Of the 2,062 barley accessions phenotyped, 1,480 were genotyped with the Illumina barley iSelect chip and passed the quality controls with 5,954 polymorphic markers used for further association mapping analysis. Genome-wide association mapping was utilized to identify and map resistance loci from the seedling disease response data and the single nucleotide polymorphism (SNP) marker data. The best among six different regression models was identified for each isolate and association analysis was performed separately for each. A total of 138 significant (-log10P value>3.0) marker-trait associations (MTA) were detected. Using a 5 cM cutoff, a total of 10, 8, 13, and 10 quantitative trait loci (QTL) associated with SFNB resistance were identified for the FGO, SG1, NZKF2, and DEN 2.6 isolates, respectively. Loci containing from 1 to 34 MTA were identified on all seven barley chromosomes with one locus at 66 to 69 cM on chromosome 2H common to all four isolates. Six distinct loci were identified by the association mapping (AM) analysis that corresponded to previously characterized SFNB resistance QTL identified by biparental population analysis (QRpt4, QRpt6, Rpt4, Rpt6, Rpt7, and a QTL on 4H that was not given a provisional gene or QTL nomenclature). The 21 putative novel loci identified may represent a broad spectrum of resistance and or susceptibility loci. This is the first comprehensive AM study to characterize SFNB resistance loci underlying broad populations of the barley host and P. teres f. maculata pathogen.

MoTup1 is required for growth, conidiogenesis and pathogenicity of Magnaporthe oryzae.

The general transcriptional repressor Tup1 proteins play important regulatory roles in the growth and development of fungi. In this report, we characterized MoTup1, a protein homologous to Tup1 of Saccharomyces cerevisiae, from M. oryzae. Disruption of MoTUP1 resulted in severe mycelial growth reduction and a defect in conidiogenesis. We found that MoTup1 is required for the maintenance of cell wall integrity by regulating the expression of the genes involved in cell wall biosynthesis. Pathogenicity assays indicated that the ΔMotup1 mutants lost the ability to invade both rice and barley hosts. Moreover, observation of rice epidermis penetration showed that the hyphal tips of the mutants could still form appressorium-like structures, but were unable to invade host cells. Taken together, our results demonstrate that M. oryzae MoTup1 is an important regulatory factor in fungal growth, development and pathogenesis on hosts.

An Immunity-Triggering Effector from the Barley Smut Fungus Ustilago hordei Resides in an Ustilaginaceae-Specific Cluster Bearing Signs of Transposable Element-Assisted Evolution

The basidiomycete smut fungus Ustilago hordei was previously shown to comprise isolates that are avirulent on various barley host cultivars. Through genetic crosses we had revealed that a dominant avirulence locus UhAvr1 which triggers immunity in barley cultivar Hannchen harboring resistance gene Ruh1, resided within an 80-kb region. DNA sequence analysis of this genetically delimited region uncovered the presence of 7 candidate secreted effector proteins. Sequence comparison of their coding sequences among virulent and avirulent parental and field isolates could not distinguish UhAvr1 candidates. Systematic deletion and complementation analyses revealed that UhAvr1 is UHOR_10022 which codes for a small effector protein of 171 amino acids with a predicted 19 amino acid signal peptide. Virulence in the parental isolate is caused by the insertion of a fragment of 5.5 kb with similarity to a common U. hordei transposable element (TE), interrupting the promoter of UhAvr1 and thereby changing expression and hence recognition of UhAVR1p. This rearrangement is likely caused by activities of TEs and variation is seen among isolates. Using GFP-chimeric constructs we show that UhAvr1 is induced only in mated dikaryotic hyphae upon sensing and infecting barley coleoptile cells. When infecting Hannchen, UhAVR1p causes local callose deposition and the production of reactive oxygen species and necrosis indicative of the immune response. UhAvr1 does not contribute significantly to overall virulence. UhAvr1 is located in a cluster of ten effectors with several paralogs and over 50% of TEs. This cluster is syntenous with clusters in closely-related U. maydis and Sporisorium reilianum. In these corn-infecting species, these clusters harbor however more and further diversified homologous effector families but very few TEs. This increased variability may have resulted from past selection pressure by resistance genes since U. maydis is not known to trigger immunity in its corn host.

Direct and indirect influences of arbuscular mycorrhizal fungi on phosphorus uptake by two root hemiparasitic Pedicularis species: do the fungal partners matter at low colonization levels?

Because most parasitic plants do not form mycorrhizal associations, the nutritional roles of arbuscular mycorrhizal (AM) fungi in them have hardly been tested. Some facultative root hemiparasitic Pedicularis species form AM associations and hence are ideal for testing both direct and indirect effects of AM fungi on their nutrient acquisition. The aim of this study was to test the influence of AM inoculation on phosphorus (P) uptake by Pedicularis rex and P. tricolor. (32)P labelling was used in compartmented pots to assess the contribution of the AM pathway and the influence of AM inoculation on P uptake from a host plant into the root hemiparasites. Laboratory isolates of fungal species (Glomus mosseae and G. intraradices) and the host species (Hordeum vulgare 'Fleet') to which the two Pedicularis species showed obvious responses in haustorium formation and growth in previous studies were used. The AM colonization of both Pedicularis spp. was low (<15 % root length) and only a very small proportion of total plant P (<1 %) was delivered from the soil via the AM fungus. In a separate experiment, inoculation with AM fungi strongly interfered with P acquisition by both Pedicularis species from their host barley, almost certainly because the numbers of haustoria formed by the parasite were significantly reduced in AM plants. Roles of AM fungi in nutrient acquisition by root parasitic plants were quantitatively demonstrated for the first time. Evidence was obtained for a novel mechanism of preventing root parasitic plants from overexploiting host resources through AM fungal-induced suppression of the absorptive structures in the parasites.

Mosaic genome structure of the barley powdery mildew pathogen and conservation of transcriptional programs in divergent hosts.

Barley powdery mildew, Blumeria graminis f. sp. hordei (Bgh), is an obligate biotrophic ascomycete fungal pathogen that can grow and reproduce only on living cells of wild or domesticated barley (Hordeum sp.). Domestication and deployment of resistant barley cultivars by humans selected for amplification of Bgh isolates with different virulence combinations. We sequenced the genomes of two European Bgh isolates, A6 and K1, for comparative analysis with the reference genome of isolate DH14. This revealed a mosaic genome structure consisting of large isolate-specific DNA blocks with either high or low SNP densities. Some of the highly polymorphic blocks likely accumulated SNPs for over 10,000 years, well before the domestication of barley. These isolate-specific blocks of alternating monomorphic and polymorphic regions imply an exceptionally large standing genetic variation in the Bgh population and might be generated and maintained by rare outbreeding and frequent clonal reproduction. RNA-sequencing experiments with isolates A6 and K1 during four early stages of compatible and incompatible interactions on leaves of partially immunocompromised Arabidopsis mutants revealed a conserved Bgh transcriptional program during pathogenesis compared with the natural host barley despite ~200 million years of reproductive isolation of these hosts. Transcripts encoding candidate-secreted effector proteins are massively induced in successive waves. A specific decrease in candidate-secreted effector protein transcript abundance in the incompatible interaction follows extensive transcriptional reprogramming of the host transcriptome and coincides with the onset of localized host cell death, suggesting a host-inducible defense mechanism that targets fungal effector secretion or production.

Increased feeding damage under elevated Co2: The case of the Russian wheat aphid

We report the results of a baseline study on the effects of Russian wheat aphid infestation on barley lines grown under ambient and elevated (450 and 550μmolmol−1) CO2 concentrations [CO2]. Elevated CO2 impacted on plant biomass, C:N ratios and leaf nitrogen concentrations. Visible manifestation of aphid feeding related damage was assessed by examining resultant chlorosis and leaf roll under ambient and two elevated [CO2] levels using a control and three resistant barley host combinations. Elevated [CO2] had a significant positive effect on the growth of the four barley lines that were not infested by the aphids. However under the same conditions aphid feeding under elevated CO2 conditions caused very high biomass loss, which was more noticeable in experiments involving non-resistant PUMA than in the resistant barley lines. The results of this study demonstrate that CO2 enrichment substantially increases aphid populations of RWASA1 and RWASA2 on the four barley lines investigated. Furthermore, aphid populations were higher on non-resistant PUMA than the three resistant lines and the RWASA2 biotype out-performed RWASA1 in each case. Under elevated [CO2], aphid feeding, resulted in a significant increase in the leaf C:N ratios (as a percentage change) in most treatments, compared to levels recorded on uninfested plants. The resistant lines also showed a significant reduction in leaf nitrogen (~40% for PUMA and not less than 30% for the resistant STARS lines tested). C:N ratio changes and N loss correlated to [CO2] and aphid biotype. By 28days of infestation, most of the non-resistant PUMA line in particular showed significant irrecoverable levels of leaf chlorosis. At level 9 rating on the chlorosis scale (i.e. plant death when recovery was not possible), experiments were terminated. As aphid success is unlikely to be the sole product of [CO2], but also of other limiting nutrients such as N, it may be worth further investigating the effect of plant quality and ultimately plant nutrition on the population growth of aphids.