Closing the ranks to attack by powdery mildew

Early and specific gene expression triggered by rice resistance gene Pi33 in response to infection by ACE1 avirulent blast fungus

* Our view of genes involved in rice disease resistance is far from complete. Here we used a gene-for-gene relationship corresponding to the interaction between atypical avirulence gene ACE1 from Magnaporthe grisea and rice resistance gene Pi33 to better characterize early rice defence responses induced during such interaction. * Rice genes differentially expressed during early stages of Pi33/ACE1 interaction were identified using DNA chip-based differential hybridization and QRT-PCR survey of the expression of known and putative regulators of disease resistance. * One hundred genes were identified as induced or repressed during rice defence response, 80% of which are novel, including resistance gene analogues. Pi33/ACE1 interaction also triggered the up-regulation of classical PR defence genes and a massive down-regulation of chlorophyll a/b binding genes. Most of these differentially expressed genes were induced or repressed earlier in Pi33/ACE1 interaction than in the gene-for-gene interaction involving Nipponbare resistant cultivar. * Besides demonstrating that an ACE1/Pi33 interaction induced classical and specific expression patterns, this work provides a list of new genes likely to be involved in rice disease resistance.

The protective role of silicon in the Arabidopsis-powdery mildew pathosystem

The role and essentiality of silicon (Si) in plant biology have been debated for >150 years despite numerous reports describing its beneficial properties. To obtain unique insights regarding the effect of Si on plants, we performed a complete transcriptome analysis of both control and powdery mildew-stressed Arabidopsis plants, with or without Si application, using a 44K microarray. Surprisingly, the expression of all but two genes was unaffected by Si in control plants, a result contradicting reports of a possible direct effect of Si as a fertilizer. In contrast, inoculation of plants, treated or not with Si, altered the expression of a set of nearly 4,000 genes. After functional categorization, many of the up-regulated genes were defense-related, whereas a large proportion of down-regulated genes were involved in primary metabolism. Regulated defense genes included R genes, stress-related transcription factors, genes involved in signal transduction, the biosynthesis of stress hormones (SA, JA, ethylene), and the metabolism of reactive oxygen species. In inoculated plants treated with Si, the magnitude of down-regulation was attenuated by >25%, an indication of stress alleviation. Our results demonstrate that Si treatment had no effect on the metabolism of unstressed plants, suggesting a nonessential role for the element but that it has beneficial properties attributable to modulation of a more efficient response to pathogen stress.

Stage-specific suppression of basal defense discriminates barley plants containing fast- and delayed-acting Mla powdery mildew resistance alleles

Nonspecific recognition of pathogen-derived general elicitors triggers the first line of plant basal defense, which in turn, preconditions the host towards resistance or susceptibility. To elucidate how basal defense responses influence the onset of Mla (mildew resistance locus a)-specified resistance, we performed a meta-analysis of GeneChip mRNA expression for 155 basal defense-related genes of barley (Hordeum vulgare) challenged with Blumeria graminis f. sp. hordei, the causal agent of powdery mildew disease. In plants containing the fast-acting Mla1, Mla6, or Mla13 alleles, transcripts hyper-accumulated from 0 to 16 h after inoculation (hai) in both compatible and incompatible interactions. Suppression of basal defense-related transcripts was observed after 16 hai only in compatible interactions, whereas these transcripts were sustained or increased in incompatible interactions. By contrast, in plants containing wild-type and mutants of the delayed-acting Mla12 allele, an early hyper-induction of transcripts from 0 to 8 hai was observed, but the expression of many of these genes is markedly suppressed from 8 to 16 hai. These results suggest that the inhibition of basal defense facilitates the development of haustoria by the pathogen, consequently delaying the onset of host resistance responses. Thus, we hypothesize that the regulation of basal defense influences host-cell accessibility to the fungal pathogen and drives allelic diversification of gene-specific resistance phenotypes.

Tomato defense to the powdery mildew fungus: differences in expression of genes in susceptible, monogenic- and polygenic resistance responses are mainly in timing

Oidium neolycopersici is a causal agent of tomato powdery mildew. In this paper, gene expression profiles were investigated of susceptible, monogenic- and polygenic resistant tomato genotypes in response to O. neolycopersici infection by using cDNA-AFLP. Around 30,000 TDFs (Transcript Derived Fragments), representing approximately 22% of the transcriptome based on in silico estimation, were identified and 887 TDFs were differentially expressed (DE-TDFs) upon inoculation with O. neolycopersici spores. Forty-two percent of the identified DE-TDFs were detected in both the compatible and incompatible interactions, a subset of these were studied for their temporal patterns. All of these common induced DE-TDFs displayed an expression peak at 7 days post incoluation in monogenic resistant response but sustained up-regulation in the susceptible and the polygenic resistant response. While more than half of these common DE-TDFs showed earlier timing in incompatible interactions compared to compatible interaction. Only 2% of the identified DE-TDFs were specific to either the monogenic or the polygenic resistant response. By annotation of the 230 sequenced DE-TDFs we found that 34% of the corresponding transcripts were known to be involved in plant defense, whereas the other transcripts played general roles in signal transduction (11%), regulation (24%), protein synthesis and degradation (11%), energy metabolism (12%) including photosynthesis, photorespiration and respiration.

A mutation in the GTP hydrolysis site of Arabidopsis dynamin-related protein 1E confers enhanced cell death in response to powdery mildew infection

We screened for mutants of Arabidopsis thaliana that displayed enhanced disease resistance to the powdery mildew pathogen Erysiphe cichoracearum and identified the edr3 mutant, which formed large gray lesions upon infection with E. cichoracearum and supported very little sporulation. The edr3-mediated disease resistance and cell death phenotypes were dependent on salicylic acid signaling, but independent of ethylene and jasmonic acid signaling. In addition, edr3 plants displayed enhanced susceptibility to the necrotrophic fungal pathogen Botrytis cinerea, but showed normal responses to virulent and avirulent strains of Pseudomonas syringae pv. tomato. The EDR3 gene was isolated by positional cloning and found to encode Arabidopsis dynamin-related protein 1E (DRP1E). The edr3 mutation caused an amino acid substitution in the GTPase domain of DRP1E (proline 77 to leucine) that is predicted to block GTP hydrolysis, but not GTP binding. A T-DNA insertion allele in DRP1E did not cause powdery mildew-induced lesions, suggesting that this phenotype is caused by DRP1E being locked in the GTP-bound state, rather than by a loss of DRP1E activity. Analysis of DRP1E-green fluorescent protein fusion proteins revealed that DRP1E is at least partially localized to mitochondria. These observations suggest a mechanistic link between salicylic acid signaling, mitochondria and programmed cell death in plants.

Reconstitution of cyanogenesis in barley (Hordeum vulgare L.) and its implications for resistance against the barley powdery mildew fungus

Barley (Hordeum vulgare L.) produces a leucine-derived cyanogenic beta-D-glucoside, epiheterodendrin that accumulates specifically in leaf epidermis. Barley leaves are not cyanogenic, i.e. they do not possess the ability to release hydrogen cyanide, because they lack a cyanide releasing beta-D-glucosidase. Cyanogenesis was reconstituted in barley leaf epidermal cells through single cell expression of a cDNA encoding dhurrinase-2, a cyanogenic beta-D-glucosidase from sorghum. This resulted in a 35-60% reduction in colonization rate by an obligate parasite Blumeria graminis f. sp. hordei, the causal agent of barley powdery mildew. A database search for barley homologues of dhurrinase-2 identified a (1,4)-beta-D-glucan exohydrolase isozyme betaII that is located in the starchy endosperm of barley grain. The purified barley (1,4)-beta-D-glucan exohydrolase isozyme betaII was found to hydrolyze the cyanogenic beta-D-glucosides, epiheterodendrin and dhurrin. Molecular modelling of its active site based on the crystal structure of linamarase from white clover, demonstrated that the disposition of the catalytic active amino acid residues was structurally conserved. Epiheterodendrin stimulated appressoria and appressorial hook formation of B. graminis in vitro, suggesting that loss of cyanogenesis in barley leaves has enabled the fungus to utilize the presence of epiheterodendrin to facilitate host recognition and to establish infection.

A proteomic approach to study pea (Pisum sativum) responses to powdery mildew (Erysiphe pisi)

As a global approach to gain a better understanding of the mechanisms involved in pea resistance to Erysiphe pisi, changes in the leaf proteome of two pea genotypes differing in their resistance phenotype were analyzed by a combination of 2-DE and MALDI-TOF/TOF MS. Leaf proteins from control non-inoculated and inoculated susceptible (Messire) and resistant (JI2480) plants were resolved by 2-DE, with IEF in the 5-8 pH range and SDS-PAGE on 12% gels. CBB-stained gels revealed the existence of quantitative and qualitative differences between extracts from: (i) non-inoculated leaves of both genotypes (77 spots); (ii) inoculated and non-inoculated Messire leaves (19 spots); and (iii) inoculated and non-inoculated JI2480 leaves (12 spots). Some of the differential spots have been identified, after MALDI-TOF/TOF analysis and database searching, as proteins belonging to several functional categories, including photosynthesis and carbon metabolism, energy production, stress and defense, protein synthesis and degradation and signal transduction. Results are discussed in terms of constitutive and induced elements involved in pea resistance against Erysiphe pisi.

Regulation of plant defense responses in Arabidopsis by EDR2, a PH and START domain-containing protein

We have identified an Arabidopsis mutant that displays enhanced disease resistance (edr2) to the biotrophic powdery mildew pathogen Erysiphe cichoracearum. Inhibition of fungal growth on edr2 mutant leaves occurred at a late stage of the infection process and coincided with formation of necrotic lesions approximately 5 days after inoculation. Double-mutant analysis revealed that edr2-mediated resistance is suppressed by mutations that inhibit salicylic acid (SA)-induced defense signaling, including npr1, pad4 and sid2, demonstrating that edr2-mediated disease resistance is dependent on SA. However, edr2 showed normal responses to the bacterial pathogen Pseudomonas syringae pv. tomato strain DC3000. EDR2 appears to be constitutively transcribed in all tissues and organs and encodes a novel protein, consisting of a putative pleckstrin homology (PH) domain and a steroidogenic acute regulatory protein-related lipid-transfer (START) domain, and contains an N-terminal mitochondrial targeting sequence. The PH and START domains are implicated in lipid binding, suggesting that EDR2 may provide a link between lipid signaling and activation of programmed cell death mediated by mitochondria.

A pattern recognition tool for quantitative analysis of in planta hyphal growth of powdery mildew fungi

The development of fungal pathogens can be quantified easily at the level of spore germination or penetration. However, the exact quantification of hyphal growth rates after initial, successful host invasion is much more difficult. Here, we report on the development of a new pattern recognition software (HyphArea) for automated quantitative analysis of hyphal growth rates of powdery mildew fungi on plant surfaces that usually represent highly irregular and noisy image backgrounds. By using HyphArea, we measured growth rates of colonies of the barley powdery mildew, Blumeria graminis f. sp. hordei, on susceptible and induced-resistant host plants. Hyphal growth was not influenced by the resistance state of the plants up to 48 h postinoculation. At later time points, growth rate increased on susceptible plants, whereas it remained restricted on induced-resistant plants. This difference in hyphal growth rate was accompanied by lack of secondary haustoria formation on induced-resistant plants, suggesting that induced resistance in barley against Blumeria graminis is caused mainly by reduced penetration rates of primary as well as secondary appressoria leading, finally, to fewer and less-developed fungal colonies. No evidence was found for reduced nutrient-uptake efficiency of the primary haustoria in induced-resistant leaves, which would be expected to have resulted in reduced hyphal growth rates during the first 48 h of the interaction.

Virus-induced gene silencing-based functional characterization of genes associated with powdery mildew resistance in barley

We successfully implemented virus-induced gene silencing (VIGS) in barley (Hordeum vulgare) for the functional characterization of genes required for Mla13-mediated resistance toward the biotrophic barley pathogen Blumeria graminis f. sp. hordei. Initially, barley cultivars were screened for their ability to host the barley stripe mosaic virus (BSMV)-VIGS vector by allowing its replication and systemic movement without causing excessive symptoms. Phytoene desaturase silencing leading to photobleaching was used as a phenotypic marker alongside reverse transcription-PCR data to characterize the silencing response at the molecular level. Barley cultivar Clansman, harboring the Mla13 resistance gene, was chosen as the most suitable host for BSMV-VIGS-based functional characterization of Rar1, Sgt1, and Hsp90 in the Mla-mediated resistance toward powdery mildew. BSMV-induced gene silencing of these candidate genes, which are associated in many but not all race-specific pathways, proved to be robust and could be detected at both mRNA and protein levels for up to 21 d postinoculation. Systemic silencing was observed not only in the newly developed leaves from the main stem but also in axillary shoots. By examining fungal development from an incompatible mildew strain carrying the cognate Avr13 gene on plants BSMV silenced for Rar1, Sgt1, and Hsp90, a resistance-breaking phenotype was observed, while plants infected with BSMV control constructs remained resistant. We demonstrate that Hsp90 is a required component for Mla13-mediated race-specific resistance and that BSMV-induced VIGS is a powerful tool to characterize genes involved in pathogen resistance in barley.

Contrasting mechanisms of defense against biotrophic and necrotrophic pathogens

It has been suggested that effective defense against biotrophic pathogens is largely due to programmed cell death in the host, and to associated activation of defense responses regulated by the salicylic acid-dependent pathway. In contrast, necrotrophic pathogens benefit from host cell death, so they are not limited by cell death and salicylic acid-dependent defenses, but rather by a different set of defense responses activated by jasmonic acid and ethylene signaling. This review summarizes results from Arabidopsis-pathogen systems regarding the contributions of various defense responses to resistance to several biotrophic and necrotrophic pathogens. While the model above seems generally correct, there are exceptions and additional complexities.

Stable expression of a defense-related gene in wheat epidermis under transcriptional control of a novel promoter confers pathogen resistance

Tissue-specific or regulated expression of transgenes is desirable in order to prevent pleiotropic side effects of putatively harmful transgene products as well as loss of energy resources due to unnecessary accumulation of transgene products. Epidermis-specific expression would be useful for many defense-related genes directed against attack by fungal pathogens that enter the plant body by direct penetration through the epidermis. In an approach to enhance resistance of wheat to the powdery mildew fungus Blumeria graminis f.sp. tritici, a novel epidermis-specific promoter was developed and used for expression of two defense-related genes. A 2.3 kb fragment of the wheat GstA1 promoter in combination with an intron-containing part of the wheat WIR1a gene was found to drive strong and constitutive transient expression in wheat epidermis. This promoter-intron combination was used for overexpression of oxalate oxidase 9f-2.8 and TaPERO peroxidase, two defense-related wheat genes expressed in inner leaf tissues. Expression studies of several transgenic lines by in situ oxalate-oxidase staining, RNA and protein blot analyses, as well as real-time PCR, demonstrated strong and constitutive transgene expression in the shoot epidermis. Transient as well as stable over-expression of the TaPERO peroxidase gene in wheat epidermis under the control of the GstA1i promoter resulted in enhanced resistance against Blumeria graminis f.sp. tritici, whereas oxalate-oxidase overexpression had no effect in either system. The data suggest that the wheat GstA1 promoter in combination with the WIR1a intron is useful for transgenic approaches to fungal disease resistance in cereals.

Host and non-host pathogens elicit different jasmonate/ethylene responses in Arabidopsis

Arabidopsis does not support the growth and asexual reproduction of the barley pathogen, Blumeria graminis f. sp. hordei Bgh). A majority of germlings fail to penetrate the epidermal cell wall and papillae. To gain additional insight into this interaction, we determined whether the salicylic acid (SA) or jasmonate (JA)/ethylene (ET) defence pathways played a role in blocking barley powdery mildew infections. Only the eds1 mutant and NahG transgenics supported a modest increase in penetration success by the barley powdery mildew. We also compared the global gene expression patterns of Arabidopsis inoculated with the non-host barley powdery mildew to those inoculated with a virulent, host powdery mildew, Erysiphe cichoracearum. Genes repressed by inoculations with non-host and host powdery mildews relative to non-inoculated control plants accounted for two-thirds of the differentially expressed genes. A majority of these genes encoded components of photosynthesis and general metabolism. Consistent with this observation, Arabidopsis growth was inhibited following inoculation with Bgh, suggesting a shift in resource allocation from growth to defence. A number of defence-associated genes were induced during both interactions. These genes likely are components of basal defence responses, which do not effectively block host powdery mildew infections. In addition, genes encoding defensins, anti-microbial peptides whose expression is under the control of the JA/ET signalling pathway, were induced exclusively by non-host pathogens. Ectopic activation of JA/ET signalling protected Arabidopsis against two biotrophic host pathogens. Taken together, these data suggest that biotrophic host pathogens must either suppress or fail to elicit the JA/ET signal transduction pathway.

Mutations in PMR5 result in powdery mildew resistance and altered cell wall composition

Powdery mildews and other obligate biotrophic pathogens are highly adapted to their hosts and often show limited host ranges. One facet of such host specialization is likely to be penetration of the host cell wall, a major barrier to infection. A mutation in the pmr5 gene rendered Arabidopsis resistant to the powdery mildew species Erysiphe cichoracearum and Erysiphe orontii, but not to the unrelated pathogens Pseudomonas syringae or Peronospora parasitica. PMR5 belongs to a large family of plant-specific genes of unknown function. pmr5-mediated resistance did not require signaling through either the salicylic acid or jasmonic acid/ethylene defense pathways, suggesting resistance in this mutant may be due either to the loss of a susceptibility factor or to the activation of a novel form of defense. Based on Fourier transform infrared analysis, the pmr5 cell walls were enriched in pectin and exhibited a reduced degree of pectin modification relative to wild-type cell walls. In addition, the mutant had smaller cells, suggesting a defect in cell expansion. A double mutant with pmr6 (defective in a glycosylphosphatidylinositol-anchored pectate lyase-like gene) exhibited a strong increase in total uronic acid content and a more severe reduction in size, relative to the single mutants, suggesting that the two genes affect pectin composition, either directly or indirectly, via different mechanisms. These two mutants highlight the importance of the host cell wall in plant-microbe interactions.

Interaction-dependent gene expression in Mla-specified response to barley powdery mildew

Plant recognition of pathogen-derived molecules influences attack and counterattack strategies that affect the outcome of host-microbe interactions. To ascertain the global framework of host gene expression during biotrophic pathogen invasion, we analyzed in parallel the mRNA abundance of 22,792 host genes throughout 36 (genotype x pathogen x time) interactions between barley (Hordeum vulgare) and Blumeria graminis f. sp hordei (Bgh), the causal agent of powdery mildew disease. A split-split-plot design was used to investigate near-isogenic barley lines with introgressed Mla6, Mla13, and Mla1 coiled-coil, nucleotide binding site, Leu-rich repeat resistance alleles challenged with Bgh isolates 5874 (AvrMla6 and AvrMla1) and K1 (AvrMla13 and AvrMla1). A linear mixed model analysis was employed to identify genes with significant differential expression (P value < 0.0001) in incompatible and compatible barley-Bgh interactions across six time points after pathogen challenge. Twenty-two host genes, of which five were of unknown function, exhibited highly similar patterns of upregulation among all incompatible and compatible interactions up to 16 h after inoculation (hai), coinciding with germination of Bgh conidiospores and formation of appressoria. By contrast, significant divergent expression was observed from 16 to 32 hai, during membrane-to-membrane contact between fungal haustoria and host epidermal cells, with notable suppression of most transcripts identified as differentially expressed in compatible interactions. These findings provide a link between the recognition of general and specific pathogen-associated molecules in gene-for-gene specified resistance and support the hypothesis that host-specific resistance evolved from the recognition and prevention of the pathogen's suppression of plant basal defense.

Mechanistic and genetic overlap of barley host and non-host resistance to Blumeria graminis

SUMMARY Non-host resistance of barley to Blumeria graminis f.sp. tritici (Bgt), an inappropriate forma specialis of the grass powdery mildew fungus, is associated with formation of cell wall appositions (papillae) at sites of attempted fungal penetration and a hypersensitive cell death reaction (HR) of single attacked cells. Penetration resistance and HR are also typical features of race-non-specific and race-specific resistance of barley to the appropriate Blumeria graminis f.sp. hordei (Bgh), raising the question of whether genotypic differences in the cellular response of barley to Bgt are detectable. First, we analysed fungal penetration frequencies and HR in different barley accessions known to show altered non-host resistance. In genotypes with limited resistance to inappropriate cereal rust fungi, we concomitantly detected low penetration resistance to Bgt and significant differences of HR rates during attack from Bgt. Second, we tested barley mutants known to show altered host responses to Bgh. The rar1-mutation that suppresses many types of race-cultivar-specific resistances did not influence the non-host response of the Bgt-isolate used in this study. However, mutants of Ror1 and Ror2, two genes required for full race non-specific penetration resistance of mlo-barley to barley powdery mildew fungus, exhibited altered defence response to Bgt, including higher frequencies of fungal penetration. On these mutants, growth of the inappropriate fungus was arrested subsequent to penetration by HR. Together, the data show that barley defence response to the wheat powdery mildew fungus is determined by similar factors as race-specific and race-non-specific resistance to appropriate Bgh.

The barley apoptosis suppressor homologue BAX inhibitor-1 compromises nonhost penetration resistance of barley to the inappropriate pathogen Blumeria graminis f. sp. tritici

BAX inhibitor-1 (BI-1) proteins have been characterized as suppressors of programmed cell death in mammals and plants. The barley BI-1 is a suppressor of nonspecific background resistance and mlo-mediated penetration resistance to the biotrophic fungal pathogen Blumeria graminis f. sp. hordei when overexpressed in epidermal cells of barley. We report here that BI-1 expression is also slightly up-regulated during interaction with the inappropriate wheat pathogen Blumeria graminis f. sp. tritici. Significantly, overexpression of BI-1 in single epidermal cells of barley by microprojectile-mediated transformation rendered cells susceptible to penetration by inappropriate B. graminis f. sp. tritici. The degree of transgene-induced accessibility to B. graminis f. sp. tritici was thereby similar to the effect achieved by overexpression of the defense suppressor gene Mlo and could not be further enhanced by double expression of both BI-1 and Mlo. Confocal laser scanning microscopy was used to locate a functional green fluorescing GFP:BI-1 fusion protein in endomembranes and the nuclear envelope of barley epidermal cells. Together, enhanced expression of barley BI-1 suppresses penetration resistance to B. graminis f. sp. tritici, linking barley nonhost resistance with cell death regulation.

A single-amino acid substitution in the sixth leucine-rich repeat of barley MLA6 and MLA13 alleviates dependence on RAR1 for disease resistance signaling

Interactions between barley and the powdery mildew pathogen, Blumeria graminis f. sp. hordei, (Bgh) are determined by unique combinations of host resistance genes, designated Mildew-resistance locus (Ml), and cognate pathogen avirulence genes. These interactions occur both dependent and independent of Rar1 (required for Mla12 resistance) and Sgt1 (Suppressor of G-two allele of skp1), which are differentially required for diverse plant disease-resistance pathways. We have isolated two new functional Mla alleles, Rar1-independent Mla7 and Rar1-dependent Mla10, as well as the Mla paralogs, Mla6-2 and Mla13-2. Utilizing the inherent diversity amongst Mla-encoded proteins, we identified the only two amino acids exclusively conserved in RAR1-dependent MLA6, MLA10, MLA12, and MLA13 that differ at the corresponding position in RAR1-independent MLA1 and MLA7. Two- and three-dimensional modeling places these residues on a predicted surface of the sixth leucine-rich repeat (LRR) domain at positions distinct from those within the beta-sheets hypothesized to determine resistance specificity. Site-directed mutagenesis of these residues indicates that RAR1 independence requires the presence of an aspartate at position 721, as mutation of this residue to a structurally similar, but uncharged, asparagine did not alter RAR1 dependence. These results demonstrate that a single-amino acid substitution in the sixth MLA LRR can alter host signaling but not resistance specificity to B. graminis.

The germinlike protein GLP4 exhibits superoxide dismutase activity and is an important component of quantitative resistance in wheat and barley

Germinlike proteins (GLP) are encoded in plants by a gene family with proposed functions in plant development and defense. Genes of GLP subfamily 4 of barley (HvGLP4, formerly referred to as HvOxOLP) and the wheat orthologue TaGLP4 (formerly referred to as TaGLP2a) were previously found to be expressed in pathogen-attacked epidermal tissue of barley and wheat leaves, and the corresponding proteins are proposed to accumulate in the apoplast. Here, the role of HvGLP4 and TaGLP4 in the defense of barley and wheat against Blumeria graminis (DC.) E. O. Speer, the cereal powdery mildew fungus, was examined in an epidermal transient expression system and in transgenic Arabidopsis thaliana plants overexpressing His-tagged HvGLP4. Leaf extracts of transgenic Arabidopsis overexpressing HvGLP4 contained a novel His-tagged protein with superoxide dismutase activity and HvGLP4 epitopes. Transient overexpression of TaGLP4 and HvGLP4 enhanced resistance against B. graminis in wheat and barley, whereas transient silencing by RNA interference reduced basal resistance in both cereals. The effect of GLP4 overexpression or silencing was strongly influenced by the genotype of the plant. The data suggest that members of GLP subfamily 4 are components of quantitative resistance in both barley and wheat, acting together with other, as yet unknown, plant components.

Functional analysis of barley RAC/ROP G-protein family members in susceptibility to the powdery mildew fungus

Small monomeric G-proteins of the plant ras (rat sarcome oncogene product) related C3 botulinum toxin substrate (RAC)/Rho of plants (ROP) family are molecular switches in signal transduction of many cellular processes. RAC/ROPs regulate hormone effects, subcellular gradients of Ca2+, the organisation of the actin cytoskeleton and the production of reactive oxygen intermediates. Therefore, we followed a genetic bottom-up strategy to study the role of these proteins during the interaction of barley (Hordeum vulgare L.) with the fungal biotrophic pathogen Blumeria graminis f.sp. hordei (Bgh). We identified six barley RAC/ROP proteins and studied their gene expression. Five out of six Rac/Rop genes were expressed constitutively in the leaf epidermis, which is the site of interaction with Bgh. None of the genes showed enhancement of mRNA abundance after inoculation with Bgh. After microprojectile mediated transformation of single barley epidermal cells with constitutively activated mutant RAC/ROP proteins, we found an RAC/ROP-specific enhancement of pathogen accessibility, tagging HvRACB, HvRAC3 and HvROP6 as host proteins potentially involved in the establishment of susceptibility to Bgh. Confocal laser scanning microscopy (CLSM) of green fluorescent protein (GFP):HvRAC/ROP-transformed cells revealed varying strengths of plasma membrane association of barley RAC/ROPs. The C-terminal CAAX motif for presumable prenylation or the C-terminal hypervariable region (HVR), respectively, were required for membrane association of the RAC/ROPs. Proper intracellular localisation was essential for HvRACB and HvRAC3 function. Together, our data support the view that different paths of host signal transduction via RAC/ROP G-proteins are involved in processes supporting parasitic entry into epidermal host cells.

Apoplastic ascorbate metabolism and its role in the regulation of cell signalling

The apoplast has crucial functions in plant biology. It comprises all the compartments beyond the plasmalemma, including the cell wall. As the reservoir of information on the biotic and abiotic environment surrounding the cell and a major conduit of information between cells, the apoplast has functions in stress perception and the subsequent appropriate control of growth and defence. The oxidative burst phenomenon, caused by environmental challenges and pathogen attack in particular, oxidises the apoplast. Ascorbic acid (AA), the major and probably the only antioxidant buffer in the apoplast, becomes oxidised in these conditions. The apoplastic enzyme ascorbate oxidase (AO) also regulates the reduction/oxidation (redox) state of the apoplastic ascorbate pool. We propose that a key function of the oxidative burst and of AO is to modify the apoplastic redox state in such a way as to modify receptor activity and signal transduction to regulate defence and growth.

The monosaccharide transporter gene, AtSTP4, and the cell-wall invertase, Atbetafruct1, are induced in Arabidopsis during infection with the fungal biotroph Erysiphe cichoracearum

Powdery mildew fungi are biotrophic pathogens that form a complex interface, the haustorium, between the host plant and the parasite. The pathogen acts as an additional sink, competing with host sinks, resulting in considerable modification of photoassimilate production and partitioning within the host tissue. Here, we examine the factors that may contribute to these changes. We show for the first time in one biotrophic interaction (Arabidopsis/Erysiphe cichoracearum) all of the following responses: Glc uptake in host tissues is enhanced after fungal infection; this coincides with the induction of expression of the monosaccharide transporter gene, Arabidopsis sugar transport protein 4 (AtSTP4), in infected leaves; invertase activity and transcript levels for a cell wall invertase, Atbetafruct1, increase substantially in Arabidopsis during attack by this pathogen. Before infection, Arabidopsis plants transformed with an AtSTP4 promoter-beta-glucuronidase construct show expression mainly in sink tissues such as roots; after infection, AtSTP4 expression is induced in the mature leaves and increases over the 6-d time period. Sections of infected leaves stained for beta-glucuronidase show that AtSTP4 expression is not confined to infected epidermal cells but is also evident in a wider range of cells, including those of the vascular tissue. The results are discussed in relation to the possible coordinated expression of hexose transporters and cell wall invertase in the host response to powdery mildew infection.

Overexpression of barley BAX inhibitor 1 induces breakdown of mlo-mediated penetration resistance to Blumeria graminis

Cell death regulation is linked to pathogen defense in plants and animals. Execution of apoptosis as one type of programmed cell death in animals is irreversibly triggered by cytochrome c release from mitochondria via pores formed by BAX proteins. This type of programmed cell death can be prevented by expression of BAX inhibitor 1 (BI-1), a membrane protein that protects cells from the effects of BAX by an unknown mechanism. In barley, a homologue of the mammalian BI-1 is expressed in response to inoculation with the barley powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh). We found differential expression of BI-1 in response to Bgh in susceptible and resistant plants. Chemical induction of resistance to Bgh by soil drench treatment with 2,6-dichloroisonicotinic acid led to down-regulation of the expression level of BI-1. Importantly, single-cell transient overexpression of BI-1 in epidermal leaf tissue of susceptible barley cultivar Ingrid led to enhanced accessibility, resulting in a higher penetration efficiency of Bgh on BI-1-transformed cells. In Bgh-resistant mlo5 genotypes, which do not express the negative regulator of defense and cell death MLO, overexpression of BI-1 almost completely reconstituted susceptibility to fungal penetration. We suggest that BI-1 is a regulator of cellular defense in barley sufficient to substitute for MLO function in accessibility to fungal parasites.

Reactive oxygen intermediates in plant-microbe interactions: who is who in powdery mildew resistance?

Reactive oxygen intermediates (ROIs) such as hydrogen peroxide (H(2)O(2)) and the superoxide anion radical (O*(2)(-)) accumulate in many plants during attack by microbial pathogens. Despite a huge number of studies, the complete picture of the role of ROIs in the host-pathogen interaction is not yet fully understood. This situation is reflected by the controversially discussed question as to whether ROIs are key factors in the establishment and maintenance of either host cell inaccessibility or accessibility for fungal pathogens. On the one hand, ROIs have been implicated in signal transduction as well as in the execution of defence reactions such as cell wall strengthening and a rapid host cell death (hypersensitive reaction). On the other hand, ROIs accumulate in compatible interactions, and there are reports suggesting a function of ROIs in restricting the spread of leaf lesions and thus in suppressing cell death. Moreover, in situ analyses have demonstrated that different ROIs may trigger opposite effects in plants depending on their spatiotemporal distribution and subcellular concentrations. This demonstrates the need to determine the particular role of individual ROIs in distinct stages of pathogen development. The well-studied interaction of cereals with fungi from the genus Blumeria is an excellent model system in which signal transduction and defence reactions can be further elucidated in planta. This review article gives a synopsis of the role of ROI accumulation, with particular emphasis on the pathosystem Hordeum vulgare L.- Blumeria graminis.

Powdery mildew-induced Mla mRNAs are alternatively spliced and contain multiple upstream open reading frames

In barley (Hordeum vulgare), the Mla13 powdery mildew resistance gene confers Rar1-dependent, AvrMla13-specific resistance to Blumeria graminis f. sp. hordei (Bgh). We have identified cDNA and genomic copies of Mla13 and used this coiled-coil nucleotide-binding site leucine-rich repeat protein-encoding gene as a model for the regulation of host resistance to obligate biotrophic fungi in cereals. We demonstrate quantitatively that a rapid increase in the accumulation of Mla transcripts and transcripts of the Mla-signaling genes, Rar1 and Sgt1, is triggered between 16 and 20 h post inoculation, the same time frame that haustoria of avirulent Bgh make contact with the host cell plasma membrane. An abundance of Mla13 cDNAs revealed five classes of transcript leader regions containing two alternatively spliced introns and up to three upstream open reading frames (uORFs). Alternative splicing of introns in the transcript leader region results in a different number of uORFs and variability in the size of uORF2. These results indicate that regulation of Mla transcript accumulation is not constitutive and that induction is coordinately controlled by recognition-specific factors. The sudden increase in specific transcript levels could account for the rapid defense response phenotype conferred by Mla6 and Mla13.

Understanding the functions of plant disease resistance proteins

Many disease resistance (R) proteins of plants detect the presence of disease-causing bacteria, viruses, or fungi by recognizing specific pathogen effector molecules that are produced during the infection process. Effectors are often pathogen proteins that probably evolved to subvert various host processes for promotion of the pathogen life cycle. Five classes of effector-specific R proteins are known, and their sequences suggest roles in both effector recognition and signal transduction. Although some R proteins may act as primary receptors of pathogen effector proteins, most appear to play indirect roles in this process. The functions of various R proteins require phosphorylation, protein degradation, or specific localization within the host cell. Some signaling components are shared by many R gene pathways whereas others appear to be pathway specific. New technologies arising from the genomics and proteomics revolution will greatly expand our ability to investigate the role of R proteins in plant disease resistance.

Overexpression of a kinase-deficient form of the EDR1 gene enhances powdery mildew resistance and ethylene-induced senescence in Arabidopsis

The EDR1 gene of Arabidopsis has previously been reported to encode a Raf-like mitogen-activated protein kinase kinase (MAPKK) kinase, and to function as a negative regulator of disease resistance. A phylogenetic analysis of plant and animal protein kinases revealed, however, that plant Raf-like kinases are more closely related to animal mixed lineage kinases (MLKs) than Raf-like kinases, and are deeply divergent from both classes of animal kinases, making inferences of substrate specificity questionable. We, therefore, assayed the kinase activity of recombinant EDR1 protein in vitro. The EDR1 kinase domain displayed autophosphorylation activity and phosphorylated the common MAP kinase substrate myelin basic protein. The EDR1 kinase domain also phosphorylated a kinase-deficient EDR1 protein, indicating that EDR1 autophosphorylation can occur via an intermolecular mechanism. Overexpression of a kinase-deficient full-length EDR1 gene (35S::dnEDR1) in wild-type Arabidopsis plants caused a dominant negative phenotype, conferring resistance to powdery mildew (Erysiphe cichoracearum) and enhancing ethylene-induced senescence. RNA-gel blot analyses showed that the 35S::dnEDR1 transgene was highly transcribed in transgenic plants. Western blot analysis, however, revealed that neither the wild-type nor mutant EDR1 protein could be detected in these lines, indicating that the dominant negative phenotype may be caused by a translational inhibition mechanism rather than by a protein level effect. Overexpression of orthologous dnEDR1 constructs may provide a novel strategy for controlling powdery mildew disease in crops.

Cell polarization, a crucial process in fungal defence

Plant cells responding to fungal attack undergo large morphological alterations, along with rapid and extensive metabolic reprogramming. Cytological analysis of single infected plant cells revealed a large complexity of interdependent, rapid and dynamic changes of a multitude of cellular components. Among these changes are major rearrangements of the cytoskeleton, translocation of cytoplasm and of the cell nucleus to the fungal penetration site, and local apposition of barrier material around this site, which results in massive cell-wall reinforcement. If this first line of defence is overcome by the pathogen, in many cases, it is followed by hypersensitive plant cell death, which stops growth of the penetrating fungus and finally leads to its death. The speed and magnitude of the initial defence response appear to be crucial to plant disease resistance.

Tobacco Rar1, EDS1 and NPR1/NIM1 like genes are required for N-mediated resistance to tobacco mosaic virus

The tobacco N gene confers resistance to tobacco mosaic virus (TMV) and encodes a Toll-interleukin-1 receptor/nucleotide binding site/leucine-rich repeat (TIR-NBS-LRR) class protein. We have developed and used a tobacco rattle virus (TRV) based virus induced gene silencing (VIGS) system to investigate the role of tobacco candidate genes in the N-mediated signalling pathway. To accomplish this we generated transgenic Nicotiana benthamiana containing the tobacco N gene. The transgenic lines exhibit hypersensitive response (HR) to TMV and restrict virus spread to the inoculated site. This demonstrates that the tobacco N gene can confer resistance to TMV in heterologous N. benthamiana. We have used this line to study the role of tobacco Rar1-, EDS1-, and NPR1/NIM1- like genes in N-mediated resistance to TMV using a TRV based VIGS approach. Our VIGS analysis suggests that these genes are required for N function. EDS1-like gene requirement for the N function suggests that EDS1 could be a common component of bacterial, fungal and viral resistance signalling mediated by the TIR-NBS-LRR class of resistance proteins. Requirement of Rar1- like gene for N-mediated resistance to TMV and some powdery mildew resistance genes in barley provide the first example of converging points in the disease resistance signalling pathways mediated by TIR-NBS-LRR and CC-NBS-LRR proteins. The TRV based VIGS approach as described here to study N-mediated resistance signalling will be useful for the analysis of not only disease resistance signalling pathways but also of other signalling pathways in genetically intractable plant systems.

A small GTP-binding host protein is required for entry of powdery mildew fungus into epidermal cells of barley

Small GTP-binding proteins such as those from the RAC family are cytosolic signal transduction proteins that often are involved in processing of extracellular stimuli. Plant RAC proteins are implicated in regulation of plant cell architecture, secondary wall formation, meristem signaling, and defense against pathogens. We isolated a RacB homolog from barley (Hordeum vulgare) to study its role in resistance to the barley powdery mildew fungus (Blumeria graminis f.sp. hordei). RacB was constitutively expressed in the barley epidermis and its expression level was not strongly influenced by inoculation with B. graminis. However, after biolistic bombardment of barley leaf segments with RacB-double-stranded RNA, sequence-specific RNA interference with RacB function inhibited fungal haustorium establishment in a cell-autonomous and genotype-specific manner. Mutants compromised in function of the Mlo wild-type gene and the Ror1 gene (genotype mlo5 ror1) that are moderately susceptible to B. graminis showed no alteration in powdery mildew resistance upon RacB-specific RNA interference. Thus, the phenotype, induced by RacB-specific RNA interference, was apparently dependent on the same processes as mlo5-mediated broad resistance, which is suppressed by ror1. We conclude that an RAC small GTP-binding protein is required for successful fungal haustorium establishment and that this function may be linked to MLO-associated functions.

Non-host resistance of barley is associated with a hydrogen peroxide burst at sites of attempted penetration by wheat powdery mildew fungus

Summary In barley, non-host resistance against the wheat powdery mildew fungus (Blumeria graminis f.sp. tritici, Bgt) is associated with the formation of cell wall appositions and a hypersensitive reaction in which epidermal cells die rapidly in response to fungal attack. In the interaction of barley with the pathogenic barley powdery mildew fungus (Blumeria graminis f.sp. hordei, Bgh), these defence reactions are also associated with accumulation of H(2)O(2). To elucidate the mechanism of non-host resistance, the accumulation of H(2)O(2) in response to Bgt was studied in situ by histochemical staining with diaminobenzidine. H(2)O(2) accumulation was found in cell wall appositions under appressoria from Bgt and in cells undergoing a hypersensitive reaction. A mutation (mlo5) at the barley Mlo locus, that confers broad spectrum resistance to Bgh, did not influence the barley defence phenotype to Bgt. Significantly, Bgt triggered cell death on mlo5-barley while Bgh did not.