Mutagenesis of putative acylation sites alters function, localization, and accumulation of a Gi alpha subunit of the chestnut blight fungus Cryphonectria parasitica

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.

Evidence for a role of the regulator of G-protein signaling protein CPRGS-1 in Galpha subunit CPG-1-mediated regulation of fungal virulence, conidiation, and hydrophobin synthesis in the chestnut blight fungus Cryphonectria parasitica

We previously reported that the chestnut blight fungus Cryphonectria parasitica expresses at least three G-protein alpha subunits and that Galpha subunit CPG-1 is essential for regulated growth, pigmentation, sporulation, and virulence. We now report the cloning and characterization of a C. parasitica regulator of G-protein signaling (RGS) protein, CPRGS-1. The phylogenetic relationship of CPRGS-1 to orthologs from other fungi was inferred and found to be generally concordant with species relationships based on 18S ribosomal sequences and on morphology. However, Hemiascomycotine RGS branch lengths in particular were longer than for their 18S sequence counterparts, which correlates with functional diversification in the signaling pathway. Deletion of cprgs-1 resulted in reduced growth, sparse aerial mycelium, and loss of pigmentation, sporulation, and virulence. Disruption of cprgs-1 was also accompanied by a severe posttranscriptional reduction in accumulation of CPG-1 and Gbeta subunit CPGB-1 and severely reduced expression of the hydrophobin-encoding gene cryparin. The changes in phenotype, cryparin expression, and CPGB-1 accumulation resulting from cprgs-1 gene deletion were also observed in a strain containing a mutationally activated copy of CPG-1 but not in strains containing constitutively activated mutant alleles of the other two identified Galpha subunits, CPG-2 and CPG-3. Furthermore, cprgs-1 transcript levels were increased in the activated CPG-1 strain but were unaltered in activated CPG-2 and CPG-3 strains. The results strongly suggest that CPRGS-1 is involved in regulation of Galpha subunit CPG-1-mediated signaling and establish a role for a RGS protein in the modulation of virulence, conidiation, and hydrophobin synthesis in a plant pathogenic fungus.

Heterologous diploid nuclei in the violet root rot fungus, Helicobasidium mompa

Allelic genes hga1-1 and hga1-2, which encode G protein alpha subunit in the violet root rot fungus, Helicobasidium mompa, were sequenced and characterized. Restriction fragment polymorphism (RFLP) analysis determined that the gene is present as a single locus in the single basidiospore isolates, while strain V169 possessed both alleles of this gene. Therefore, although basidiospore isolates are dikaryon, they are homokaryotic. Field-isolated strain V169, the putative parent strain, is a dikaryotic heterokaryon. Allelic genes hga1-1 and hga1-2 segregated in almost a 1:3 ratio among single basidiospore isolates from the same fruiting body. Moreover, the copy number of hga1-1 was found to be less than that of hga1-2 in the V169 strain. These results suggest that one of the nuclei in the V169 parent strain is homozygous diploid and the other heterozygous diploid. This parent strain produced four homokaryotic and dikaryon basidiospores on each basidium.

Analysis of altered G-protein subunit accumulation in Cryphonectria parasitica reveals a third Galpha homologue

Heterotrimeric G-proteins mediate many responses of eukaryotic cells to external stimuli and have been shown to be important for fungal pathogenicity. In this study, we explored the accumulation of G-protein subunits of the chestnut blight fungus, Cryphonectria parasitica, in mutant strains deleted for one or more putative partner subunits. Using a series of extraction buffers and immunoblot end-point dilution analysis, we established a convenient method to assess the relative abundance of these membrane-associated proteins. Disruption of either cpg-1, which encodes the Galpha subunit CPG-1, or cpgb-1, the Gbeta subunit CPGB-1, consistently reduced the level of its presumptive partner protein. This was not observed in the case of a second Galpha subunit, CPG-2, suggesting that CPG-1 and CPGB-1 regulate each other's stability. Further, analysis of transcript levels indicated that the Galpha and Gbeta protein turnover rates were increased in the mutant strains. Additionally, a previously unidentified protein that was cross-reactive with anti-CPG-1 antiserum was found to be enhanced in liquid culture. We describe the sequence of a new Galpha subunit, CPG-3, that is most similar to three other filamentous fungal Galpha proteins that form a phylogenetically distinct grouping.

Constitutively activated Galpha negatively regulates virulence, reproduction and hydrophobin gene expression in the chestnut blight fungus Cryphonectria parasitica

Disruption of the gene encoding Galpha subunit cpg-1 in the chestnut blight fungus Cryphonectria parasitica reduces growth and pigmentation and abolishes reproduction and virulence. We now report the consequences of mutations designed to constitutively activate (Q204-L and R178-C) CPG-1-mediated signaling. Introduction of cpg-1-QL or cpg-1-RC into wild type, Deltacpg-1 and Deltacpgb-1 (Gbeta) mutant strains resulted in a dominant phenotype characterized by a complete absence of aerial hyphae, pigmentation, conidia production and virulence. Opposing responses of Deltacpg-1 and activated mutant strains to chronic heat, hyperosmolarity and oxidative stress suggested that CPG-1 plays a role in mediating stress response. Growth of the cpg-1 mutant strains proceeded at wild-type level in rich liquid medium, but was severely curtailed on solid medium and absent in chestnut tissue, indicating the importance of CPG-1 mediated signaling under these harsher conditions. Both cpg-1 deletion and activating CPG-1 mutations resulted in post-transcriptional alterations in the accumulation of CPG-1 and/or CPGB-1, providing evidence for extensive post-transcriptional regulation of G-protein subunit accumulation in C. parasitica. Finally, the absence of aerial hyphae and the easily wettable phenotype exhibited by the QL and RC mutants correlated with reduced expression of the gene encoding cryparin, suggesting G-protein-mediated regulation of a fungal hydrophobin.

Differential modulation of cellular signaling pathways by mild and severe hypovirus strains

Hypoviruses persistently alter multiple phenotypic traits, stably modify gene expression, and attenuate virulence (hypovirulence) of their pathogenic fungal host, the chestnut blight fungus Cryphonectria parasitica. The pleiotropic nature of these changes is consistent with hypovirus-mediated perturbation of one or more cellular signal transduction pathways. We now report that two hypoviruses that differ in the severity of symptom expression differentially perturb specific cellular signaling pathways. The C. parasitica 13-1 gene, originally identified as a hypovirus-inducible and cyclic AMP (cAMP)-regulated gene, was used to design a promoter-GFP reporter construct with which to monitor perturbation of cAMP-mediated signaling. Virus-mediated modulation of calcium/calmodulin/inositol trisphosphate-dependent signaling was monitored by measuring transcript accumulation from the C. parasitica laccase gene, lac-1. Infection by the severe hypovirus strain CHV1-EP713 caused a substantial induction of 13-1 promoter activity and a reduction of total extracellular laccase enzymatic activity (LAC-1 and LAC-3). In contrast, 13-1 promoter activity and total laccase activity were only marginally altered upon infection with the mild hypovirus strain CHV1-Euro7. However, examination of lac-1-specific transcript accumulation under previously defined culture conditions revealed that both CHV1-EP713 and CHV1-Euro7 perturbed calcium/calmodulin/inositol trisphosphate-dependent signaling. CHV1-EP713/CHV1-Euro7 chimeric viruses were used to map viral determinants responsible for modulation of cAMP-dependent signaling to domains within the central portion of the second open reading frame.

Hypoviruses and chestnut blight: exploiting viruses to understand and modulate fungal pathogenesis

Fungal viruses are considered unconventional because they lack an extracellular route of infection and persistently infect their hosts, often in the absence of apparent symptoms. Because mycoviruses are limited to intracellular modes of transmission, they can be considered as intrinsic fungal genetic elements. Such long-term genetic interactions, even involving apparently asymptomatic mycoviruses, are likely to have an impact on fungal ecology and evolution. One of the clearest examples supporting this view is the phenomenon of hypovirulence (virulence attenuation) observed for strains of the chestnut blight fungus, Cryphonectria parasitica, harboring members of the virus family Hypoviridae. The goal of this chapter is to document recent advances in hypovirus molecular genetics and to provide examples of how that progress is leading to the identification of virus-encoded determinants responsible for altering fungal host phenotype, insights into essential and dispensable elements of hypovirus replication, revelations concerning the role of G-protein signaling in fungal pathogenesis, and new avenues for enhancing biological control potential.

Molecular cloning of three genes encoding G protein alpha subunits in the white root rot fungus, Rosellinia necatrix

Three genes encoding G protein alpha subunits were cloned from the white root rot fungus, Rosellinia necatrix, and characterized. Only one copy of each gene was present in the genome. The protein sequences of Rga1, Rga2, and Rga3 are very similar to those of MagA, MagB and MagC of Magnaporthe grisea, respectively. Moreover, Rga1 is similar to Mod-D which is closely related to vegetative incompatibility in Podospora anserina, which suggests that Rga1 is important in the vegetative incompatibility reaction in R. necatrix. Reverse transcription PCR (RT-PCR) analysis of Rga1, Rga2, and Rga3 mRNA expression showed that the three genes were all transcribed in R. necatrix cells.

Essential and dispensable virus-encoded replication elements revealed by efforts To develop hypoviruses as gene expression vectors

We have investigated whether hypoviruses, viral agents responsible for virulence attenuation (hypovirulence) of the chestnut blight fungus Cryphonectria parasitica, could serve as gene expression vectors. The infectious cDNA clone of the prototypic hypovirus CHV1-EP713 was modified to generate 20 different vector candidates. Although transient expression was achieved for a subset of vectors that contained the green fluorescent protein gene from Aequorea victoria, long-term expression (past day 8) was not observed for any vector construct. Analysis of viral RNAs recovered from transfected fungal colonies revealed that the foreign genes were readily deleted from the replicating virus, although small portions of foreign sequences were retained by some vectors after months of replication. However, the results of vector viability and progeny characterization provided unexpected new insights into essential and dispensable elements of hypovirus replication. The N-terminal portion (codons 1 to 24) of the 5'-proximal open reading frame (ORF), ORF A, was found to be required for virus replication, while the remaining 598 codons of this ORF were completely dispensable. Substantial alterations were tolerated in the pentanucleotide UAAUG that contains the ORF A termination codon and the overlapping putative initiation codon of the second of the two hypovirus ORFs, ORF B. Replication competence was maintained following either a frameshift mutation that caused a two-codon extension of ORF A or a modification that produced a single-ORF genomic organization. These results are discussed in terms of determinants of hypovirus replication, the potential utility of hypoviruses as gene expression vectors, and possible mechanisms by which hypoviruses recognize and delete foreign sequences.

Identification of bdm-1, a gene involved in G protein beta-subunit function and alpha-subunit accumulation

Targeted disruption of Galpha and Gbeta genes has established the requirement of an intact G protein signaling pathway for optimal execution of several important physiological processes, including pathogenesis, in the chestnut blight fungus Cryphonectria parasitica. We now report the identification of a G protein signal transduction component, beta disruption mimic factor-1, BDM-1. Disruption of the corresponding gene, bdm-1, resulted in a phenotype indistinguishable from that previously observed after disruption of the Gbeta subunit gene, cpgb-1. The BDM-1 deduced amino acid sequence contained several significant clusters of identity with mammalian phosducin, including a domain corresponding to a highly conserved 11-amino acid stretch that has been implicated in binding to the Gbetagamma dimer and two regions of defined Gbeta/phosducin contact points. Unlike the negative regulatory function proposed for mammalian phosducin, the genetic data presented in this report suggest that BDM-1 is required for or facilitates Gbeta function. Moreover, disruption of either bdm-1 or cpgb-1 resulted in a significant, posttranscriptional reduction in the accumulation of CPG-1, a key Galpha subunit required for a range of vital physiological processes.

A mitogen-activated protein kinase of the corn leaf pathogen Cochliobolus heterostrophus is involved in conidiation, appressorium formation, and pathogenicity: diverse roles for mitogen-activated protein kinase homologs in foliar pathogens

Fungal pathogens perceive and respond to molecules from the plant, triggering pathogenic development. Transduction of these signals may use heterotrimeric G proteins, and it is thought that protein phosphorylation cascades are also important. We have isolated a mitogen-activated protein kinase homolog from the corn pathogen Cochliobolus heterostrophus to test its role as a component of the transduction pathways. The new gene, CHK1, has a deduced amino acid sequence 90% identical to Pmk1 of the rice blast fungus Magnaporthe grisea and 59% identical to Fus3 of Saccharomyces cerevisiae. A series of chk1 deletion mutants has poorly developed aerial hyphae, autolysis, and no conidia. No pseudothecia are formed when a cross between two Deltachk1 mutants is attempted. The ability of Deltachk1 mutants to infect corn plants is reduced severely. The growth pattern of hyphae on a glass surface is strikingly altered from that of the wild type, forming coils or loops, but no appressoria. This set of phenotypes overlaps only partially with that of pmk1 mutants, the homologous gene of the rice blast fungus. In particular, sexual and asexual sporulation both require Chk1 function in Cochliobolus heterostrophus, in contrast to Pmk1, but perhaps more similar to yeast, where Fus3 transmits the mating signal. Chk1 is required for efficient colonization of leaf tissue, which can be compared with filamentous invasive growth of yeast, modulated through another closely related mitogen-activated protein kinase, Kss1. Ubiquitous signaling elements thus are used in diverse ways in different plant pathogens, perhaps the result of coevolution of the transducers and their targets.