G-protein beta subunit of Cochliobolus heterostrophus involved in virulence, asexual and sexual reproductive ability, and morphogenesis

Emergence of Lasiodiplodia theobromae induced leaf necrosis in tea (Camellia sinensis [L.] O. Kuntze) from India

The tea plant, Camellia sinensis [L.] O. Kuntze, is a vital global agricultural commodity, yet faces challenges from fungal infections, which affects its production. To reduce the loss in the tea production, the fungal infections must be removed which is managed with fungicides, which are harmful to the environment. Leaf necrosis, which decreases tea quality and quantity, was investigated across Assam, revealing Lasiodiplodia theobromae as the causative agent. Pathogenicity tests, alongside morphological and molecular analyses, confirmed its role in leaf necrosis. Genome and gene analysis of L. theobromae showed multiple genes related to its pathogenicity. The study also assessed the impact of chemical pesticides on this pathogen. Additionally, the findings in this study highlight the significance of re-assessing management approaches in considering the fungal infection in tea.

Botrytis cinerea G Protein β Subunit Bcgb1 Controls Growth, Development and Virulence by Regulating cAMP Signaling and MAPK Signaling

Botrytis cinerea is a necrotrophic phytopathogenic fungus that causes gray mold disease in many crops. To better understand the role of G protein signaling in the development and virulence of this fungus, the G protein β subunit gene Bcgb1 was knocked out in this study. The ΔBcgb1 mutants showed reduced mycelial growth rate, but increased aerial hyphae and mycelial biomass, lack of conidiation, failed to form sclerotia, increased resistance to cell wall and oxidative stresses, delayed formation of infection cushions, and decreased virulence. Deletion of Bcgb1 resulted in a significant reduction in the expression of several genes involved in cAMP signaling, and caused a notable increase in intracellular cAMP levels, suggesting that G protein β subunit Bcgb1 plays an important role in cAMP signaling. Furthermore, phosphorylation levels of MAP kinases (Bmp1 and Bmp3) were increased in the ΔBcgb1 mutants. Yeast two-hybrid assays showed that Bcgb1 interacts with MAPK (Bmp1 and Bmp3) cascade proteins (BcSte11, BcBck1, BcMkk1, and BcSte50), and the Bmp1-regulated gene Bcgas2 was up-regulated in the ΔBcgb1 mutant. These results indicated that Gβ protein Bcgb1 is involved in the MAPK signaling pathway in B. cinerea. In summary, our results revealed that Gβ protein Bcgb1 controls development and virulence through both the cAMP and MAPK (Bmp1 and Bmp3) signaling pathways in B. cinerea.

The heterotrimeric G-protein beta subunit Gpb1 controls hyphal growth under low oxygen conditions through the protein kinase A pathway and is essential for virulence in the fungus Mucor circinelloides

Mucor circinelloides, a dimorphic opportunistic pathogen, expresses three heterotrimeric G-protein beta subunits (Gpb1, Gpb2 and Gpb3). The Gpb1-encoding gene is up-regulated during mycelial growth compared with that in the spore or yeast stage. gpb1 deletion mutation analysis revealed its relevance for an adequate development during the dimorphic transition and for hyphal growth under low oxygen concentrations. Infection assays in mice indicated a phenotype with considerably reduced virulence and tissue invasiveness in the deletion mutants (Δgpb1) and decreased host inflammatory response. This finding could be attributed to the reduced filamentous growth in animal tissues compared with that of the wild-type strain. Mutation in a regulatory subunit of cAMP-dependent protein kinase A (PKA) subunit (PkaR1) resulted in similar phenotypes to Δgpb1. The defects exhibited by the Δgpb1 strain were genetically suppressed by pkaR1 overexpression, indicating that the PKA pathway is controlled by Gpb1 in M. circinelloides. Moreover, during growth under low oxygen levels, cAMP levels were much higher in the Δgpb1 than in the wild-type strain, but similar to those in the ΔpkaR1 strain. These findings reveal that M. circinelloides possesses a signal transduction pathway through which the Gpb1 heterotrimeric G subunit and PkaR1 control mycelial growth in response to low oxygen levels.

Limited DNA Repair Gene Repertoire in Ascomycete Yeast Revealed by Comparative Genomics

Ascomycota is the largest phylogenetic group of fungi that includes species important to human health and wellbeing. DNA repair is important for fungal survival and genome evolution. Here, we describe a detailed comparative genomic analysis of DNA repair genes in Ascomycota. We determined the DNA repair gene repertoire in Taphrinomycotina, Saccharomycotina, Leotiomycetes, Sordariomycetes, Dothideomycetes, and Eurotiomycetes. The subphyla of yeasts, Saccharomycotina and Taphrinomycotina, have a smaller DNA repair gene repertoire comparing to Pezizomycotina. Some genes were absent from most, if not all, yeast species. To study the conservation of these genes in Pezizomycotina, we used the Gain Loss Mapping Engine algorithm that provides the expectations of gain or loss of genes given the tree topology. Genes that were absent from most of the species of Taphrinomycotina or Saccharomycotina showed lower conservation in Pezizomycotina. This suggests that the absence of some DNA repair in yeasts is not random; genes with a tendency to be lost in other classes are missing. We ranked the conservation of DNA repair genes in Ascomycota. We found that Rad51 and its paralogs were less conserved than other recombinational proteins, suggesting that there is a redundancy between Rad51 and its paralogs, at least in some species. Finally, based on the repertoire of UV repair genes, we found conditions that differentially kill the wine pathogen Brettanomyces bruxellensis and not Saccharomyces cerevisiae. In summary, our analysis provides testable hypotheses to the role of DNA repair proteins in the genome evolution of Ascomycota.

Copper Ions are Required for Cochliobolus heterostrophus in Appressorium Formation and Virulence on Maize

Cochliobolus heterostrophus is the causal agent of southern corn leaf blight, a destructive disease on maize worldwide. However, how it regulates virulence on maize is still largely unknown. Here, we report that two copper transporter genes, ChCTR1 and ChCTR4, are required for its virulence. chctr1 and chctr4 mutants showed attenuated virulence on maize compared with the wild-type strain TM17 but development phenotypes of those mutants on media with or without infection-related stress agents were the same as the wild-type strain. Moreover, ChCTR1 and ChCTR4 play critical roles in appressorium formation and mutation of ChCTR1 or ChCTR4 suppresses the appressorium formation. Furthermore, copper-chelating agent ammonium tetrathiomolybdate suppressed the appressorium formation and virulence of C. heterostrophus on maize, whereas copper ions enhanced the appressorium formation and virulence on maize. The results indicate that copper ions are required for appressorium formation and virulence of C. heterostrophus on maize and are acquired from the environment by two copper transporters: ChCTR1 and ChCTR4.

Genome Assembly of the Fungus Cochliobolus miyabeanus, and Transcriptome Analysis during Early Stages of Infection on American Wildrice (Zizania palustris L.)

The fungus Cochliobolus miyabeanus causes severe leaf spot disease on rice (Oryza sativa) and two North American specialty crops, American wildrice (Zizania palustris) and switchgrass (Panicum virgatum). Despite the importance of C. miyabeanus as a disease-causing agent in wildrice, little is known about either the mechanisms of pathogenicity or host defense responses. To start bridging these gaps, the genome of C. miyabeanus strain TG12bL2 was shotgun sequenced using Illumina technology. The genome assembly consists of 31.79 Mbp in 2,378 scaffolds with an N50 = 74,921. It contains 11,000 predicted genes of which 94.5% were annotated. Approximately 10% of total gene number is expected to be secreted. The C. miyabeanus genome is rich in carbohydrate active enzymes, and harbors 187 small secreted peptides (SSPs) and some fungal effector homologs. Detoxification systems were represented by a variety of enzymes that could offer protection against plant defense compounds. The non-ribosomal peptide synthetases and polyketide synthases (PKS) present were common to other Cochliobolus species. Additionally, the fungal transcriptome was analyzed at 48 hours after inoculation in planta. A total of 10,674 genes were found to be expressed, some of which are known to be involved in pathogenicity or response to host defenses including hydrophobins, cutinase, cell wall degrading enzymes, enzymes related to reactive oxygen species scavenging, PKS, detoxification systems, SSPs, and a known fungal effector. This work will facilitate future research on C. miyabeanus pathogen-associated molecular patterns and effectors, and in the identification of their corresponding wildrice defense mechanisms.

Overview of the functional virulent genome of the coffee leaf rust pathogen Hemileia vastatrix with an emphasis on early stages of infection

Hemileia vastatrix is the causal agent of coffee leaf rust, the most important disease of coffee Arabica. In this work, a 454-pyrosequencing transcriptome analysis of H. vastatrix germinating urediniospores (gU) and appressoria (Ap) was performed and compared to previously published in planta haustoria-rich (H) data. A total of 9234 transcripts were identified and annotated. Ca. 50% of these transcripts showed no significant homology to international databases. Only 784 sequences were shared by the three conditions, and 75% were exclusive of either gU (2146), Ap (1479) or H (3270). Relative transcript abundance and RT-qPCR analyses for a selection of genes indicated a particularly active metabolism, translational activity and production of new structures in the appressoria and intense signaling, transport, secretory activity and cellular multiplication in the germinating urediniospores, suggesting the onset of a plant-fungus dialogue as early as at the germ tube stage. Gene expression related to the production of carbohydrate-active enzymes and accumulation of glycerol in germinating urediniospores and appressoria suggests that combined lytic and physical mechanisms are involved in appressoria-mediated penetration. Besides contributing to the characterization of molecular processes leading to appressoria-mediated infection by rust fungi, these results point toward the identification of new H. vastatrix candidate virulence factors, with 516 genes predicted to encode secreted proteins.

Crossover fungal pathogens: the biology and pathogenesis of fungi capable of crossing kingdoms to infect plants and humans

The outbreak of fungal meningitis associated with contaminated methylprednisolone acetate has thrust the importance of fungal infections into the public consciousness. The predominant pathogen isolated from clinical specimens, Exserohilum rostratum (teleomorph: Setosphaeria rostrata), is a dematiaceous fungus that infects grasses and rarely humans. This outbreak highlights the potential for fungal pathogens to infect both plants and humans. Most crossover or trans-kingdom pathogens are soil saprophytes and include fungi in Ascomycota and Mucormycotina phyla. To establish infection, crossover fungi must overcome disparate, host-specific barriers, including protective surfaces (e.g. cuticle, skin), elevated temperature, and immune defenses. This review illuminates the underlying mechanisms used by crossover fungi to cause infection in plants and mammals, and highlights critical events that lead to human infection by these pathogens. Several genes including veA, laeA, and hapX are important in regulating biological processes in fungi important for both invasive plant and animal infections.

Structure-activity relationships delineate how the maize pathogen Cochliobolus heterostrophus uses aromatic compounds as signals and metabolites

The necrotrophic maize pathogen Cochliobolus heterostrophus senses plant-derived phenolic compounds, which promote nuclear retention of the redox-sensitive transcription factor ChAP1 and alter gene expression. The intradiol dioxygenase gene CCHD1 is strongly upregulated by coumaric and caffeic acids. Plant phenolics are potential nutrients but some of them are damaging compounds that need to be detoxified. Using coumaric acid as an inducer (16 to 160 μM), we demonstrated the rapid and simultaneous upregulation of most of the β-ketoadipate pathway genes in C. heterostrophus. A cchd1 deletion mutant provided genetic evidence that protocatechuic acid is an intermediate in catabolism of a wide range of aromatic acids. Aromatics catabolism was slowed for compounds showing toxicity, and this was strongly correlated with nuclear retention of GFP-ChAP1. The activity of a structure series of compounds showed complementary requirements for upregulation of CCHD1 and for ChAP1 nuclear retention. Thus, there is an inverse correlation between the ability to metabolize a compound and the stress response (ChAP1 nuclear retention) that it causes. The ability to metabolize phenolics and to respond to them as signals should be an advantage to plant pathogens and may explain the presence of at least two response pathways detecting these compounds.

Identification and partial characterization of Rhizopus nigricans Gβ proteins and their expression in the presence of progesterone

The mammalian steroid hormone progesterone actuates a signalling pathway in the zygomycete Rhizopus nigricans which includes heterotrimeric G proteins. To investigate the possibility that the Gβ subunit of these proteins is involved in the signalling, a cDNA library from R. nigricans exposed to progesterone was prepared and a sequence coding for a Gβ subunit was searched for. Using degenerate primers, two sequences, RnGPB1 and RnGPB2, were identified that exhibited a high degree of identity with those for Gβ from other filamentous fungi, but not from yeast. The presence of more than one Gβ subunit is very rare among the fungi, and it has been to date reported only for Rhizopus oryzae. We have shown that progesterone increases the expression of RnGPB1, but has no influence on the expression of RnGPB2. Therefore, our studies imply the involvement of Gβ subunit 1 in the response of R. nigricans to progesterone. Moreover, the Gβ subunit is subjected to endogenous ADP-ribosylation in the presence of NAD, which could be important in some, as yet unknown, cell process. Article from a special issue on steroids and microorganisms.

The G protein β subunit controls virulence and multiple growth- and development-related traits in Verticillium dahliae

To gain insight into the role of G protein-mediated signaling in virulence and development of the soilborne, wilt causing fungus Verticillium dahliae, the G protein β subunit gene (named as VGB) was disrupted in tomato race 1 strain of V. dahliae. A resulting mutant strain, 70ΔGb15, displayed drastic reduction in virulence, increased microsclerotia formation and conidiation, and decreased ethylene production compared to the corresponding wild type (wt) strain 70wt-r1. Moreover, 70ΔGb15 exhibited an elongated rather than radial growth pattern on agar media. A transformant of 70ΔGb15 (named as 70ΔGbPKAC1) that carries an extra copy of VdPKAC1, a V. dahliae gene encoding the catalytic subunit of the cAMP-dependent protein kinase A, exhibited wt growth pattern and conidiation, was unable to form microsclerotia, produced high amounts of ethylene, and exhibited virulence between that of 70ΔGb15 and 70wt-r1 on tomato plants. Phenotypical changes observed in 70ΔGb15 and 70ΔGbPKAC1 correlated with transcriptional changes in several genes involved in signaling (MAP kinase VMK1) and development (hydrophobin VDH1 and ACC synthase ACS1) of V. dahliae. Results from the present work suggest a linkage between VGB and VdPKAC1 signaling pathways in regulating virulence, hormone production and development in V. dahliae.

A homeobox gene is essential for conidiogenesis of the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae starts its infection by the attachment of pyriform conidia on rice tissues, and severity of the disease epidemic is proportional to the quantity of conidia produced in the rice blast lesions. However, the mechanism of conidial production is not well understood. Homeodomain proteins play critical roles in regulating various growth and developmental processes in fungi and other eukaryotes. Through targeted gene replacement, we find that deletion of HTF1, one of seven homeobox genes in the fungal genome, does not affect mycelial growth but causes total defect of conidial production. Further observation revealed that the Deltahtf1 mutant produces significantly more conidiophores, which curve slightly near the tip but could not develop sterigmata-like structures. Although the Deltahtf1 mutant fails to form conidia, it could still develop melanized appressoria from hyphal tips and infect plants. The expression level of HTF1 is significantly reduced in the Deltamgb1 G-beta and DeltacpkA deletion mutant, and the ACR1 but not CON7 gene that encodes transcription factor required for normal conidiogenesis is significantly downregulated in the Deltahtf1 mutant. These data suggest that the HTF1 gene is essential for conidiogenesis, and may be functionally related to the trimeric G-protein signaling and other transcriptional regulators that are known to be important for conidiation in M. oryzae.

Protoplast transformation of filamentous fungi

The protoplast method for the transformation of filamentous fungi is described in detail, as is the Restriction Enzyme-Mediated Integration (REMI) procedure for introducing tagged mutations into the fungal genome. A split marker method for generating PCR fragments for targeted integration and deletion of genes of interest is also detailed.

G(alpha) and Gbeta proteins regulate the cyclic AMP pathway that is required for development and pathogenicity of the phytopathogen Mycosphaerella graminicola

We identified and functionally characterized genes encoding three Galpha proteins and one Gbeta protein in the dimorphic fungal wheat pathogen Mycosphaerella graminicola, which we designated MgGpa1, MgGpa2, MgGpa3, and MgGpb1, respectively. Sequence comparisons and phylogenetic analyses showed that MgGPA1 and MgGPA3 are most related to the mammalian Galpha(i) and Galpha(s) families, respectively, whereas MgGPA2 is not related to either of these families. On potato dextrose agar (PDA) and in yeast glucose broth (YGB), MgGpa1 mutants produced significantly longer spores than those of the wild type (WT), and these developed into unique fluffy mycelia in the latter medium, indicating that this gene negatively controls filamentation. MgGpa3 mutants showed more pronounced yeast-like growth accompanied with hampered filamentation and secreted a dark-brown pigment into YGB. Germ tubes emerging from spores of MgGpb1 mutants were wavy on water agar and showed a nested type of growth on PDA that was due to hampered filamentation, numerous cell fusions, and increased anastomosis. Intracellular cyclic AMP (cAMP) levels of MgGpb1 and MgGpa3 mutants were decreased, indicating that both genes positively regulate the cAMP pathway, which was confirmed because the WT phenotype was restored by adding cAMP to these mutant cultures. The cAMP levels in MgGpa1 mutants and the WT were not significantly different, suggesting that this gene might be dispensable for cAMP regulation. In planta assays showed that mutants of MgGpa1, MgGpa3, and MgGpb1 are strongly reduced in pathogenicity. We concluded that the heterotrimeric G proteins encoded by MgGpa3 and MgGpb1 regulate the cAMP pathway that is required for development and pathogenicity in M. graminicola.

The coiled-coil protein-binding motif in Fusarium verticillioides Fsr1 is essential for maize stalk rot virulence

Fusarium verticillioides (Sacc.) Nirenberg (teleomorph Gibberella moniliformis Wineland) is one of the key pathogens of maize stalk rot disease. However, a clear understanding of stalk rot pathogenesis is still lacking. Previously, we identified the F. verticillioides FSR1 gene, which plays a key role in fungal virulence and sexual mating. The predicted Fsr1 protein contains multiple protein-binding domains, namely a caveolin-binding domain, a coiled-coil structure, and a calmodulin-binding motif at the N terminus and a WD40 repeat domain at the C terminus. Fsr1 shares significant similarity to a family of striatin proteins that play a critical role in cellular mechanisms that regulate a variety of developmental processes. Significantly, FSR1 function is conserved in Fusarium graminearum, where it also plays a direct role in pathogenesis. In this study, our goal was to determine the motif(s) in Fsr1 that are directly associated with fungal virulence. We complemented the FSR1 knockout (Deltafsr1) strain with mutated versions of the FSR1 gene, and determined that the Fsr1 C-terminal WD40 repeat domain is dispensable for vegetative growth and maize stalk rot virulence. We also examined the potential link between FSR1-mediated virulence and cell wall-degrading enzyme (alpha-amylase, pectinase and cellulase) activities. Further characterization of the N-terminal region revealed that the coiled-coil structure is essential for virulence in F. verticillioides. The coiled-coil domain is involved in a variety of protein-protein interactions in eukaryotic systems, and thus we hypothesize that the interaction between Fsr1 and the putative Fsr1-binding protein triggers downstream gene signalling that is associated with F. verticillioides virulence.

Distinct and combined roles of the MAP kinases of Cochliobolus heterostrophus in virulence and stress responses

Pathogenicity mitogen-activated protein kinases (MAPKs), related to yeast FUS3/KSS1, are essential for virulence in fungi, including Cochliobolus heterostrophus, a necrotrophic pathogen causing Southern corn leaf blight. We compared the phenotypes of mutants in three MAPK genes: HOG1, MPS1, and CHK1. The chk1 and mps1 mutants show autolytic appearance, light pigmentation, and dramatic reduction in virulence and conidiation. Similarity of mps1 and chk1 mutants is reflected by coregulation by these two MAPKs of several genes. Unlike chk1, mps1 mutants are female-fertile and form normal-looking appressoria. HOG1 mediates resistance to hyperosmotic and, to a lesser extent, oxidative stress, and is required for stress upregulation of glycerol-3-phosphate phosphatase, transaldolase, and a monosaccharide transporter. Hog1, but not Mps1 or Chk1, was rapidly phosphorylated in response to increased osmolarity. The hog1 mutants have smaller appressoria and cause decreased disease symptoms on maize leaves. Surprisingly, loss of MPS1 in a wild-type or hog1 background improved resistance to some stresses. All three MAPKs contribute to the regulation of central developmental functions under normal and stress conditions, and full virulence cannot be achieved without appropriate input from all three pathways.

Functional analyses of heterotrimeric G protein G alpha and G beta subunits in Gibberella zeae

The homothallic ascomycete fungus Gibberella zeae (anamorph: Fusarium graminearum) is a major toxigenic plant pathogen that causes head blight disease on small-grain cereals. The fungus produces the mycotoxins deoxynivalenol (DON) and zearalenone (ZEA) in infected hosts, posing a threat to human and animal health. Despite its agricultural and toxicological importance, the molecular mechanisms underlying its growth, development and virulence remain largely unknown. To better understand such mechanisms, we studied the heterotrimeric G proteins of G. zeae, which are known to control crucial signalling pathways that regulate various cellular and developmental responses in fungi. Three putative Gα subunits, GzGPA1, GzGPA2 and GzGPA3, and one Gβ subunit, GzGPB1, were identified in the F. graminearum genome. Deletion of GzGPA1, a homologue of the Aspergillus nidulans Gα gene fadA, resulted in female sterility and enhanced DON and ZEA production, suggesting that GzGPA1 is required for normal sexual reproduction and repression of toxin biosynthesis. The production of DON and ZEA was also enhanced in the GzGPB1 mutant, suggesting that both GαGzGPA1 and GβGzGPB1 negatively control mycotoxin production. Deletion of GzGPA2, which encodes a Gα protein similar to A. nidulans GanB, caused reduced pathogenicity and increased chitin accumulation in the cell wall, implying that GzGPA2 has multiple functions. Our study shows that G. zeae heterotrimeric G protein subunits can regulate vegetative growth, sexual development, toxin production and pathogenicity.

Heterotrimeric G protein signaling in filamentous fungi

Filamentous fungi are multicellular eukaryotic organisms known for nutrient recycling as well as for antibiotic and food production. This group of organisms also contains the most devastating plant pathogens and several important human pathogens. Since the first report of heterotrimeric G proteins in filamentous fungi in 1993, it has been demonstrated that G proteins are essential for growth, asexual and sexual development, and virulence in both animal and plant pathogenic filamentous species. Numerous G protein subunit and G protein-coupled receptor genes have been identified, many from whole-genome sequences. Several regulatory pathways have now been delineated, including those for nutrient sensing, pheromone response and mating, and pathogenesis. This review provides a comparative analysis of G protein pathways in several filamentous species, with discussion of both unifying themes and important unique signaling paradigms.

The cAMP pathway is important for controlling the morphological switch to the pathogenic yeast form of Paracoccidioides brasiliensis

Paracoccidioides brasiliensis is a human pathogenic fungus that switches from a saprobic mycelium to a pathogenic yeast. Consistent with the morphological transition being regulated by the cAMP-signalling pathway, there is an increase in cellular cAMP levels both transiently at the onset (< 24 h) and progressively in the later stages (> 120 h) of the transition to the yeast form, and this transition can be modulated by exogenous cAMP. We have cloned the cyr1 gene encoding adenylate cyclase (AC) and established that its transcript levels correlate with cAMP levels. In addition, we have cloned the genes encoding three Gα (Gpa1–3), Gβ (Gpb1) and Gγ (Gpg1) G proteins. Gpa1 and Gpb1 interact with one another and the N-terminus of AC, but neither Gpa2 nor Gpa3 interacted with Gpb1 or AC. The interaction of Gpa1 with Gpb1 was blocked by GTP, but its interaction with AC was independent of bound nucleotide. The transcript levels for gpa1, gpb1 and gpg1 were similar in mycelium, but there was a transient excess of gpb1 during the transition, and an excess of gpa1 in yeast. We have interpreted our findings in terms of a novel signalling mechanism in which the activity of AC is differentially modulated by Gpa1 and Gpb1 to maintain the signal over the 10 days needed for the morphological switch.

Fusarium verticillioides GBB1, a gene encoding heterotrimeric G protein beta subunit, is associated with fumonisin B biosynthesis and hyphal development but not with fungal virulence

Fusarium verticillioides (Sacc.) Nirenberg (teleomorph Gibberella moniliformis Wineland) is a maize pathogen that causes ear rots and stalk rots. The fungus also produces a group of mycotoxins, fumonisins, on infected ears, which cause considerable health and economic concerns for humans and animals worldwide. To date, our understanding of the molecular mechanisms associated with fungal virulence and fumonisin biosynthesis in F. verticillioides is limited. In this study, GBB1, a gene encoding a putative beta subunit of a heterotrimeric G protein, was disrupted and the effects on fumonisin biosynthesis and virulence were evaluated. A GBB1 deletion mutant (Deltagbb1) showed no significant differences in radial growth and mycelial mass but produced significantly less fumonisin B(1 )(FB(1)) than its wild-type progenitor. HPLC analysis showed that Deltagbb1 produced less than 10 p.p.m. FB(1) while the wild-type produced over 140 p.p.m. when strains were grown on cracked corn kernels. Reduced expression of the key FB(1 )biosynthetic genes, FUM1 and FUM8, in Deltagbb1 provides further evidence that GBB1 is involved in FB(1) regulation. Stalk rot virulence, as measured by mean lesion length and by area, was not significantly different in Deltagbb1 compared with the wild-type, suggesting that GBB1 does not regulate virulence in F. verticillioides. Developmentally, hyphae of Deltagbb1 do not deviate from the original axis of polarity established upon germ tube emergence in contrast to wild-type hyphae that meander on and off axis as they grow. Complementation of Deltagbb1 with GBB1 restored FB(1) production and hyphal growth to wild-type. The results of this study demonstrate that heterotrimeric G protein beta subunit plays an important role in regulation of FB(1) biosynthesis and hyphal growth, but not virulence in F. verticillioides.

Calcium Restores Prepenetration Morphogenesis Abolished by Methylglyoxal-Bis-Guanyl Hydrazone in Cochliobolus miyabeanus Infecting Rice

ABSTRACT Cochliobolus miyabeanus forms a specialized infection structure, an appressorium, to infect its host rice plants. Curtailment of prepenetration development by spermidine and spermine was more evident in appressorium development and germination remained unaffected, whereas putrescine and methylglyoxal-bis-guanyl hydrazone (MGBG) impaired both morphogenetic events. Exogenous calcium nullified the inhibitory effect of MGBG on the prepenetration development in vitro and in vivo and the disease progression. High levels of polyamines were detected in freshly collected conidia, but the amounts were reduced during germination and appressorium formation. MGBG fortified the decrease of polyamines within conidia under development and calcium amendment did not affect the reduction. Hard-surface contact augmented messenger RNA synthesis of calmodulin gene (CmCaM) and protein kinase C (PKC) activity in germinating or appressorium-forming conidia. Calcium restored transcription of CmCaM and upregulation of PKC activity suppressed by MGBG. Taken together, fine-tuning of intracellular polyamine transition is indispensable for the conidial germination and appressorium formation in C. miyabeanus. Biochemical and molecular analyses revealed that the MGBG-acting site or sites are upstream of Ca(2+)-dependent signaling pathways regulating prepenetration morphogenesis of C. miyabeanus causing rice brown leaf spot.

The putative monomeric G-protein GBP1 is negatively associated with fumonisin B production in Fusarium verticillioides

SUMMARY Fumonisin B(1) (FB(1)) is a mycotoxin produced by Fusarium verticillioides that contaminates maize. FB(1) has been linked to a number of human and animal mycotoxicoses worldwide. Despite its significance, our understanding of the FB(1) biosynthesis regulatory mechanisms is limited. Here, we describe F. verticillioides GBP1, encoding a monomeric G-protein, and its role in FB(1) biosynthesis. GBP1 was discovered as an expressed sequence tag (EST) up-regulated in the F. verticillioides fcc1 mutant that showed reduced conidiation and no FB(1) biosynthesis when grown on maize kernels. Sequence analysis showed that GBP1 encodes a putative 368-amino-acid protein with similarity to DRG and Obg subclasses of G-proteins that are involved in development and stress responses. A GBP1 knockout mutant (Deltagbp1) exhibited normal growth, but increased FB(1) production (> 58%) compared with the wild-type when grown on corn kernels. Complementation of Deltagbp1 with the wild-type GBP1 gene restored FB(1) production levels to that of the wild-type. Our data indicate that GBP1 is negatively associated with FB(1) biosynthesis but not with conidiation in F. verticillioides. The deletion of GBP1 led to up-regulation of key FB(1) biosynthetic genes, FUM1 and FUM8, suggesting that the increased FB(1) production in Deltagbp1 is due to over-expression of FUM genes.

FSR1 is essential for virulence and female fertility in Fusarium verticillioides and F. graminearum

Fusarium verticillioides (teleomorph Gibberella moniliformis) and F. graminearum (teleomorph G. zeae) are well known to cause devastating diseases on cereal crops. Despite their importance, our understanding of the molecular mechanisms involved in these host-pathogen interactions is limited. The FSR1 locus in F. verticillioides was identified by screening REMI mutants for loss of virulence in maize stalk rot inoculation studies. FSR1 encodes an 823-codon open reading frame interrupted by two introns. The Fsr1 protein shares 60% sequence identity with the Sordaria macrospora Pro11, a multimodular protein with four putative protein-protein binding domains (caveolin-binding domain, coiled-coil structure, calmodulin-binding motif, and seven-WD40 repeats), which plays a regulatory role in cell differentiation and ascocarp development. Our data demonstrate that FSR1 is essential for female fertility and virulence in F. verticillioides. Significantly, targeted disruption of the FSR1 ortholog in F. graminearum (FgFSR1) reduced virulence on barley and deterred perithecia formation. Cross-complementation experiments demonstrated that the gene function is conserved in the two Fusarium species. FSR1 is expressed constitutively, and we hypothesize that Fsr1 regulates virulence by acting as a scaffold for a signal transduction pathway. A survey of available genome databases indicates Fsr1 homologs are present in a number of filamentous fungi and animal systems but not in budding yeast or plants. A maximum likelihood analysis of this gene family reveals well-supported monophyletic clades associated with fungi and animals.