Sequence of the Erysiphe graminis f. sp. hordei gene encoding beta-tubulin

Mechanisms and significance of fungicide resistance

In this review article, we show that occurrence of fungicide resistance is one of the most important issues in modern agriculture. Fungicide resistance may be due to mutations of genes encoding fungicide targets (qualitative fungicide resistance) or to different mechanisms that are induced by sub-lethal fungicide stress. These mechanisms result in different and varying levels of resistance (quantitative fungicide resistance). We discuss whether or not extensive use of fungicides in agricultural environments is related to the occurrence of fungicide resistance in clinical environments. Furthermore, we provide recommendations of how development of fungicide resistant pathogen populations may be prevented or delayed.

Multilocus phylogenetic analyses within Blumeria graminis, a powdery mildew fungus of cereals

Blumeria graminis, a powdery mildew fungus, is an important plant pathogen that causes serious damage to a variety of cereal crops. In spite of the importance of the pathogen, information on phylogenetic structure within B. graminis is scarce. In this study we conducted phylogenetic analyses of B. graminis based on the DNA sequences of four different DNA regions (ITS, 28S rDNA, chitin synthase 1, and beta-tubulin). The analyses revealed that the protein-coding regions have higher amounts of phylogenetic signals than rDNA regions and are useful for phylogenetic analyses of B. graminis. The present phylogenetic analyses revealed nine distinct groups in the B. graminis isolates used in this study, a result which was commonly supported by all trees constructed from the four DNA regions. Isolates from a single host genus belonged to a single group except for isolates from Lolium and Bromus, in which the isolates were split into two and three groups, respectively. Isolates from Agropyron, Secale and Triticum formed a distinct clade (Triticum clade) with identical or similar DNA sequences. The Hordeum clade was a sister of the Triticum clade, and Poa and Avena clades were distantly related to the Triticum and Hordeum clades. This phylogenetic relationship of B. graminis is well concordant with the level of reproductive isolation between formae speciales and also with phylogeny inferred from a cytological study. Shimodaira-Hasegawa and Templeton tests using sequences of four different DNA regions significantly rejected the tree topology of plants. Therefore, possibility of co-speciation between B. graminis and its host plants was obscure in this study.

Blumeria graminis secretes an extracellular catalase during infection of barley: potential role in suppression of host defence

SUMMARY The obligate biotrophic fungal pathogen of barley, Blumeria graminis f.sp. hordei (Bgh), elicits a burst of H(2)O(2) in its host barley at sites of germ tube invasion. To evaluate whether this specialized pathogen has any antioxidant response to this oxidative burst, the Bgh catB gene was characterized and transcript-profiled together with other genes implicated in the management of oxidative stress (catalase-peroxidase, cpx; glutathione peroxidase, gpx; superoxide dismutase, sod1) and in comparison with the constitutively expressed Bghbeta-tubulin and elongation factor1 (ef1) genes. Gel-based and real-time RT-PCR revealed enhanced numbers of catB transcripts at mature primary germ tube and appressorium germ tube (AGT) stages in a susceptible host. Moreover, an anti-CATB polyclonal antibody, from Aspergillus fumigatus, which recognizes both native and recombinant Bgh CATB, revealed an intense circle of immunofluorescence at the host-pathogen interface at the AGT tip and within the halo area surrounding the host papilla. A new diaminobenzidine-based 'scavenger' assay revealed areas of H(2)O(2) clearing at sites of fungal invasion, provoking speculation that Bgh catalase activity may contribute to pathogenicity in Bgh.

Quinoxyfen perturbs signal transduction in barley powdery mildew (Blumeria graminis f.sp. hordei)

SUMMARY Quinoxyfen is a protectant fungicide which controls powdery mildew diseases by interfering with germination and/or appressorium formation. Mutants of barley powdery mildew, Blumeria graminis f.sp. hordei, which are resistant to quinoxyfen produce fewer conidia, which germinate and form appressoria more promiscuously than do the prolific numbers of wild-type spores. This suggests that resistance bypasses host recognition signals. RT-PCR profiles of signal transduction genes, recorded during wild-type germling morphogenesis, reveals that quinoxyfen alters the accumulation of Protein Kinase C (pkc), pkc-like and catalytic subunit of Protein Kinase A (cpka) transcripts. Differential display-reverse transcription PCR identified a gene transcript in wild-type conidia that was absent, or much less abundant, in conidia from quinoxyfen-resistant mutants. This mRNA was not detectable 24 h after wild-type conidia were inoculated on to barley. It encodes a GTPase activating protein (GAP), which may interact with a small molecular weight Ras-type GTP binding protein. In the presence of quinoxyfen, the gap mRNA remains throughout germling morphogenesis. The involvement of GAP in resistance suggests that quinoxyfen inhibits mildew infection by disrupting early cell signalling events.

Genetic and forma specialis diversity in Blumeria graminis of cereals and its implications for host-pathogen co-evolution

SUMMARY The grass powdery mildew fungus, Blumeria graminis is classified into eight formae speciales (ff.spp.) based on strict host specialization. However, evidence suggests that host ranges extend to more than one genus and are particularly diverse among samples from the Middle East, the proposed centre of origin and diversification of crop plants. This study investigated whether geographical origin, host species or both determine the genetic variation in B. graminis that is found in cereals, sampled from Europe, Asia and North America, and whether there is any evidence for co-evolution between pathogen and host. Phylogenetic analysis of nucleotide sequence variation within the ribosomal DNA Internal Transcribed Spacer (ITS) regions and the beta-tubulin (tub2) gene gives rise to two dendrograms with different topologies. In both trees, isolates of B. graminis from cultivated cereals are grouped according to their principal host genus. This grouping was supported by amplified fragment length polymorphism (AFLP) analysis and cross-infectivity tests. However, there was no evidence of co-evolution. There was far greater divergence between ff.spp. in tub2 sequences than ITS regions and a faster rate of mutation of tub2, especially in the third base position of exons. It is proposed that variation in the rDNA-ITS regions is constrained either by their functional role in the processing of rDNA precursor molecules or by concerted evolution, hence limiting their use in phylogenetic studies. AFLP data suggests an overall lack of correlation between geographical and genetic distances. This may be related to the long distance dispersal exhibited by B. graminis.

The barley powdery mildew protein kinase C gene, pkc1 and pkc-like gene, are differentially expressed during morphogenesis

summary Protein kinase C agonist assays revealed the phorbol ester, phorbol 12-myristate 13-acetate, invoked germling morphogenesis and enhanced PKC activity in Blumeria graminis. No antagonist of mildew PKC activity was found but the data fuelled a hunt for powdery mildew pkc genes. Oligonucleotides, designed on the basis of conserved ATP-binding and kinase domains within the catalytic core of eukaryotic protein kinase proteins, were used as primers to amplify chromosomal and cDNA fragments from the barley powdery mildew fungus graminis. Three kinase gene fragments were isolated (pkc1, pkc-like and cpka) and the full length genomic sequences of the mildew pkc and pkc-like genes were determined by 'step down' PCR. RT-PCR transcript profiles showed the three genes to be differentially regulated during germling morphogenesis.

Gene identification in the obligate fungal pathogen Blumeria graminis by expressed sequence tag analysis

Powdery mildew of barley is caused by the obligate fungal pathogen Blumeria graminis f. sp. hordei. Haploid conidia of B. graminis, landing on the barley leaf, germinate to form first a primary germ tube and then an appressorial germ tube. The appressorial germ tube differentiates into a mature appressorium from which direct penetration of host epidermis occurs. Here we present data on 4908 expressed sequence tags obtained from B. graminis conidia. The combined sequences represent 2676 clones describing 1669 individual genes. Comparison with sequences from other pathogenic and nonpathogenic fungi defines hypotheses on the genes required for pathogenicity and growth on the host. The putative roles of some of the identified genes are discussed.

Expression and sequence analysis of the Blumeria graminis mitogen-activated protein kinase genes, mpk1 and mpk2

Mitogen-activated protein (MAP) kinases represent a group of serine/threonine kinases which play a pivotal role in signal transduction processes in eukaryotic cells. Using degenerate PCR primer design based on published and aligned MAP kinase sequences we have cloned and characterised two MAP kinase genes from the barley powdery mildew fungus, Blumeria graminis. We have utilised 'step down' PCR to attain the full length mildew genomic clones. The single-copy genes, named mpk1 and mpk2, encode putative proteins of 356 and 410 amino acids and carry three and four introns, respectively. Expression studies, using RT-PCR, reveal a differing pattern of tissue gene expression with mpk1 and mpk2 during germling morphogenesis and this is compared with the constitutive expression of the 'control' beta-tubulin gene.

Differential expression of two Blumeria graminis chitin synthase genes

Abstract Two Blumeria graminis chitin synthase genes, designated BgChs1 and BgChs2 were cloned and characterized following the synthesis and use of degenerate PCR primers designed to the conserved regions of fungal chitin synthase (Chs) genes. Their sequences revealed high similarity with the Chs genes previously cloned from other fungi and placed BgChs1 and BgChs2 with the classes I and V, respectively. Each gene was present as a single copy within the barley powdery mildew genome. Semi-quantitative RT-PCR assays revealed BgChs1 to be up-regulated at both the primary germ tube (PGT) and appressorial germ tube (AGT) stages of differentiation whilst the BgChs2 transcript was up-regulated at the PGT stage. The B. graminisbeta-tubulin gene was used as a control for all RT-PCR reactions. The BgChs1 transcript was some 30 fold less abundant than the beta-tubulin transcript and BgChs2 was some 30 fold rarer than the BgChs1 transcript. The effects of the chitin substrate analogues nikkomycin Z and polyoxin D on conidial morphogenesis were assessed. These nucleoside peptide inhibitors did not affect germination but both polyoxin D and nikkomycin Z treatment led to a large population of abnormally swollen 'balloon-shaped' AGTs, whilst by 12 h after inoculation polyoxin treatment caused the swollen germ tubes to burst.

Stable transformation of erysiphe graminis an obligate biotrophic pathogen of barley

Barley powdery mildew, Erysiphe graminis f.sp. hordei, is an obligate biotrophic pathogen and as such cannot complete its life cycle without a living host. The inability to transform this fungus and manipulate its genome has constrained research towards understanding its life cycle and pathogenicity. Here we describe an in planta transformation system based on delivery of DNA using a gold-particle gun and selection using benomyl or bialaphos. Using this method, we consistently obtained stable transformants with efficiencies comparable to other filamentous fungi. Stable expression of the beta-glucuronidase in E. graminis was demonstrated by co-transforming the uidA gene with the selectable markers.

Molecular analysis and characterization of the Cochliobolus heterostrophus beta-tubulin gene and its possible role in conferring resistance to benomyl

Cochliobolus heterostrophus Tub1 described here is the first beta-tubulin gene characterized from a naturally occurring benomyl-resistant ascomycete plant pathogen. The gene encodes a protein of 447 amino acids. The coding region of Tub1 is interrupted by three introns, of 116, 55, and 56 nt, situated after codons 4, 12, and 53, respectively. As a result of the preference for pyrimidines in the third position of the codons when a choice exists between purines and pyrimidines, codon usage in the Tub1 gene is biased. Tub1 shows high homology with beta-tubulin genes of other ascomycete species. However, Tub1 is exceptional in having Tyr(167), compared with Phe(167), possessed by beta-tubulin genes of other ascomycetes sequenced thus far. The Tyr(167) residue has been associated with benomyl resistance in other organisms. In contrast, all other benomyl-implicated residues of Tub1 correspond to sensitivity. Based on these results, we suggest that benomyl resistance in the fungus probably is attributed to Tyr(167).

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Characterization of the transcript of a new class of retroposon-type repetitive element cloned from the powdery mildew fungus, Erysiphe graminis

The putative master transcript of a novel class of repetitive element has been cloned from the fungus erysiphe graminis f.sp. hordei. Sequence analysis of the cDNA revealed that the element, designated Eg-R1, is a member of the retroposon superfamily with properties in common with SINEs and LINEs (short or long interspersed elements). SINE-like properties include the transcript size (approximately 700 bp), and the lack of major open reading frames. In contrast, the fact that the transcript is polyadenylated and is most probably transcribed by RNA polymerase II, suggests a functional relationship to LINEs. Except for a short, but striking, sequence identity to a published SINE from the same fungus, no similar sequence was found in database searches. A constitutively high transcript level is found throughout the asexual life cycle of the fungus. Small differences in band patterns of Southern blots were observed between two isolates of E. graminis f.sp. hordei, while the band patterns in an isolate of the very close relative E. graminis f.sp. tritici in general appear dissimilar. This may imply that the element is currently active. Recent dispersal is confirmed by the observation that an approximately 550 bp internal hinfI fragment is conserved in the majority of the copies in all three isolates. Approximately 50 copies are present in E. graminis f.sp. hordei.

Biolistic transformation of the obligate plant pathogenic fungus, Erysiphe graminis f.sp. hordei

Particle gun acceleration appears to be a possible way to transform mycelium cells of obligate plant parasites growing on host surfaces. GUS expression was obtained in E. graminis f.sp. hordei cells after bombardment with the GUS gene under the control of the E. graminis f.sp. hordei β-tubulin promoter. Three heterologous promoters, onefrom Aspergillus nidulans and two from Cochliobolus heterostrophus, gave very low or no expression of GUS.

A putative O-methyltransferase from barley is induced by fungal pathogens and UV light

A cDNA clone, pBH72-F1 (F1), was isolated from a cDNA library prepared from barley leaves 72 h after inoculation with Erysiphe graminis f.sp. hordei. The 1388 bp nucleotide sequence of pBH72-F1 contains an open reading frame encoding a 42.3 kDa polypeptide of 390 amino acids which shows sequence similarity to O-methyltransferases (OMTs) from different plant species; the highest identity (41%) was observed with a putative OMT expressed in roots of maize. A phylogenetic analysis shows that the barley and maize sequences are distinctly different from the ortho-diphenol-OMTs involved in lignin formation. A putative S-adenosyl-L-methionine-binding motif (KELVDDSITN) determined for a rabbit protein-carboxyl OMT is partially conserved in the encoded amino acid sequence. Genomic Southern blot hybridization shows that pBH72-F1 probably represents a single copy gene. The F1 clone corresponds to a gene transcript exhibiting a relatively late accumulation in mildew-infected barley leaves compared to other pathogen-induced transcripts, such as transcripts encoding PR proteins, a peroxidase, and transcripts homologous to a maize caffeic acid OMT. No transcript was detected in plants exhibiting papilla resistance at time points when resistance is thought to be manifested. The atypical transcript accumulation pattern for F1 was also observed after infection by other pathogens and after UV-light treatment.

A nucleotide substitution in one of the beta-tubulin genes of Trichoderma viride confers resistance to the antimitotic drug methyl benzimidazole-2-yl-carbamate

We characterized a Trichoderma viride strain that is resistant to the antimitotic drug methyl benzimidazole-2-yl-carbamate (MBC). This species has two beta-tubulin genes (tub1 and tub2) and by reverse genetics we showed that a mutation in the tub2 gene confers MBC resistance in this strain. Comparison of the tub2 sequence of the mutant strain with that of the wild type revealed that a single amino acid substitution of tyrosine for histidine at a position 6 is responsible for the MBC tolerance. Furthermore, we showed that this gene can be used as a homologous dominant selectable marker in T. viride transformation. Both tubulin genes were completely sequenced. They differ by 48 residues and the degree of identity between their deduced amino acid sequences is 86.3%.

SINE-like properties of a highly repetitive element in the genome of the obligate parasitic fungus Erysiphe graminis f.sp. hordei

The genomic organization of repetitive DNA in the obligate parasitic fungus Erysiphe graminis DC ex Mérat f.sp. hordei Em. Marchal was investigated using a cosmid library of the fungal genome. Three repetitive sequences were shown to be dispersed throughout the genome, and in a few cases they were found closely associated with long poly(dA) tracts. The most prevalent sequence is 903 bp long and accounts for at least 5% of the genome. Sequence analysis revealed features resembling mammalian Short INterspersed Elements (SINEs), namely the presence of a poly(dA) tail (33 bp), flanking direct repeats (13 bp), putative "A" and "B" blocks for RNA polymerase III binding; the corresponding transcript would be capable of forming a complex secondary structure.

Molecular analysis of the Septoria nodorum beta-tubulin gene and characterization of a benomyl-resistance mutation

The complete nucleotide sequence of a benomyl-resistant allele of the Septoria nodorum beta-tubulin gene (tubAR) has been determined including 745 and 1024 nucleotides 5' and 3' to the tubAR coding region, respectively. tubAR encodes a 447 amino acid polypeptide which shows a high degree of homology with other fungal beta-tubulins. The gene contains three introns at codons 4, 12 and 53, uses 48 of the possible 61 sense codons and has a GC content of 59.1% in its coding region. S1 nuclease mapping has identified two transcriptional start sites 80 bp and 83 bp upstream of the translation start, and a transcriptional termination site 192 bp downstream of the stop codon. The two transcriptional start sites lie just 8 bp and 5 bp downstream of a CT motif consisting of 18 pyrimidine nucleotides interrupted by a single adenine. The wild-type allele tubA+ has been cloned using the polymerase chain reaction and the mutation producing the benomyl-resistant phenotype of tubAR mapped to a C to T transition at the first position of codon 6, resulting in a histidine to tyrosine amino acid substitution.

The beta-tubulin gene from rat and human isolates of Pneumocystis carinii

The development of new drugs for treating Pneumocystis carinii infections in AIDS patients is hampered by the lack of long-term culture systems, and by our generally limited knowledge of this organism. Recently, however, we observed significant activity of various benzimidazoles against growth of this organism in short-term cultures. Benzimidazoles inhibit microtubule polymerization; there is strong evidence that the primary target is the beta-tubulin subunit. To understand the basis for benzimidazole activity against P. carinii, and to examine the apparent relatedness of this organism to fungi, we have cloned and sequenced the single beta-tubulin gene from a rat P. carinii isolate. There was 89-91% identity at the amino acid level to beta-tubulins from filamentous fungi, but only 79-82% identity to yeast and protozoal beta-tubulins. Also, eight introns were distributed throughout the P. carinii beta-tubulin gene in a pattern characteristic of filamentous fungi. Specific residues previously implicated in benzimidazole sensitivity were conserved in P. carinii beta-tubulin. The polymerase chain reaction was used to amplify a segment of P. carinii beta-tubulin DNA from bronchoalveolar lavages obtained from two patients with AIDS. There was considerable divergence at the DNA level between the human and rat sequences, but 100% identity at the amino-acid level.

Cloning, sequence and expression of a beta-tubulin-encoding gene in the homobasidiomycete Schizophyllum commune

The beta-tubulin (beta Tub)-encoding gene (tub-2) of Schizophyllum commune is the first tubulin gene isolated, cloned and sequenced from higher filamentous fungi (homobasidiomycetes). The S. commune tub-2 gene is organized into nine exons and eight introns. The introns vary from 48 to 107 nt in length, and are distributed throughout the gene. The tub-2 exons code for a protein of 445 amino acids (aa), which shows great homology with beta Tubs of filamentous ascomycetes, plants, and animals, but less homology with yeasts. The codon usage of tub-2 from S. commune is biased, as it is in most beta Tub-encoding genes of filamentous fungi. The S. commune beta Tub shows a conserved aa sequence in the C-terminal domain, which is suggested to interact with microtubule-associated proteins in animals. In contrast, the S. commune beta Tub deviates from most known beta Tubs by having a Cys165 residue, which might be significant for the insensitivity of S. commune haploid strains to the antimicrotubule drug, benomyl. In tub-2 of different haploid strains, sequence polymorphisms occur in the 5' and 3' flanking regions. The expression of tub-2 is high in young mycelium, which has a high number of extending apical cells, but decreases with the aging of the mycelium. No significant difference in the hybridization signal intensity for the tub-2 transcripts was recorded either during intercellular nuclear migration at early mating, or in mycelia with a mutation in the B mating-type gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of beta-tubulin sequences reveals highly conserved, coordinated amino acid substitutions. Evidence that these 'hot spots' are directly involved in the conformational change required for dynamic instability

Vertebrate beta-tubulins have been classified into six classes on the basis of their C-terminal sequences [(1987) J. Cell Biol. 105, 1707-1720]. In particular, the sequences starting at residue 430 differ between isotypes of the same animal but are conserved between species. We extend this analysis and show that there are three 'hot spots', at residues 35, 55-57 and 124 which exhibit intra-species heterogeneity but inter-species conservation. There is a remarkable correlation between the identity of these residues and the C-terminal sequences, and suggests that the vertebrate beta-tubulins fall into three broad types. This correlation extends to those non-vertebrate organisms which have the Type 1 C-terminal sequence. We propose that these three 'hot spots' and the C-terminal peptide interact in the tertiary structure. We have also noted that the C-terminal peptide almost always contains a single phenylalanine or tyrosine residue, and that there is a strong correlation between this residue and the amino acids at positions 217/218, in both the vertebrate and non-vertebrate sequences. We propose that the C-terminal aromatic amino acid interacts with residues 217/218 in the tertiary structure. Analysis of conditions which stabilise microtubules and/or lower the steady state critical concentration strongly suggests that these two sets of coordinated amino acid substitutions are directly involved in effecting the conformational change associated with GTP hydrolysis which results in dynamic instability. We propose that there is an interaction between the highly acidic sequence between residue 430 and the aromatic amino acid (termed peptide A) and conserved basic amino acids located close to the 'hot spots'. We suggest that this interaction is altered in response to the assembly-dependent GTP hydrolysis, with the consequential increase in the subunit dissociation rate constant.