Calcineurin phosphatase and phospholipase C are required for developmental and pathological functions in the citrus fungal pathogen Alternaria alternata

The cell functions of phospholipase C-1, Ca2+/H+ exchanger-1, and secretory phospholipase A2 in tolerance to stress conditions and cellulose degradation in Neurospora crassa

We investigated the cell functions of the Ca2+ signaling genes phospholipase C-1 (plc-1), Ca2+/H+ exchanger (cpe-1), and secretory phospholipase A2 (splA2) for stress responses and cellulose utilization in Neurospora crassa. The Δplc-1, Δcpe-1, and ΔsplA2 mutants displayed increased sensitivity to the alkaline pH and reduced survival during induced thermotolerance. The ΔsplA2 mutant also exhibited hypersensitivity to the DTT-induced endoplasmic reticulum (ER) stress, increased microcrystalline cellulose utilization, increased protein secretion, and glucose accumulation in the culture supernatants. Moreover, the ΔsplA2 mutant could not grow on microcrystalline cellulose during ER stress. Furthermore, plc-1, cpe-1, and splA2 synthetically regulate the acquisition of thermotolerance induced by heat shock, responses to alkaline pH and ER stress, and utilization of cellulose and other alternate carbon sources in N. crassa. In addition, expression of the alkaline pH regulator, pac-3, and heat shock proteins, hsp60, and hsp80 was reduced in the Δplc-1, Δcpe-1, and ΔsplA2 single and double mutants. The expression of the unfolded protein response (UPR) markers grp-78 and pdi-1 was also significantly reduced in the mutants showing growth defect during ER stress. The increased cellulolytic activities of the ΔsplA2 and Δcpe-1; ΔsplA2 mutants were due to increased cbh-1, cbh-2, and endo-2 expression in N. crassa. Therefore, plc-1, cpe-1, and splA2 are involved in stress responses and cellulose utilization in N. crassa.

Phospholipase C: Diverse functions in plant biotic stress resistance and fungal pathogenicity

Phospholipase C (PLC) generates various second messenger molecules and mediates phospholipid hydrolysis. In recent years, the important roles of plant and fungal PLC in disease resistance and pathogenicity, respectively, have been determined. However, the roles of PLC in plants and fungi are unintegrated and relevant literature is disorganized. This makes it difficult for researchers to implement PLC-based strategies to improve disease resistance in plants. In this comprehensive review, we summarize the structure, classification, and phylogeny of the PLCs involved in plant biotic stress resistance and fungal pathogenicity. PLCs can be divided into two groups, nonspecific PLC (NPC) and phosphatidylinositol-specific PLC (PI-PLC), which present marked differences in phylogenetic evolution. The products of PLC genes in fungi play significant roles in physiological activity and pathogenesis, whereas those encoded by plant PLC genes mediate the immune response to fungi. This review provides a perspective for the future control of plant fungal diseases.

Interaction of calcium responsive proteins and transcriptional factors with the PHO regulon in yeasts and fungi

Phosphate and calcium ions are nutrients that play key roles in growth, differentiation and the production of bioactive secondary metabolites in filamentous fungi. Phosphate concentration regulates the biosynthesis of hundreds of fungal metabolites. The central mechanisms of phosphate transport and regulation, mediated by the master Pho4 transcriptional factor are known, but many aspects of the control of gene expression need further research. High ATP concentration in the cells leads to inositol pyrophosphate molecules formation, such as IP3 and IP7, that act as phosphorylation status reporters. Calcium ions are intracellular messengers in eukaryotic organisms and calcium homeostasis follows elaborated patterns in response to different nutritional and environmental factors, including cross-talking with phosphate concentrations. A large part of the intracellular calcium is stored in vacuoles and other organelles forming complexes with polyphosphate. The free cytosolic calcium concentration is maintained by transport from the external medium or by release from the store organelles through calcium permeable transient receptor potential (TRP) ion channels. Calcium ions, particularly the free cytosolic calcium levels, control the biosynthesis of fungal metabolites by two mechanisms, 1) direct interaction of calcium-bound calmodulin with antibiotic synthesizing enzymes, and 2) by the calmodulin-calcineurin signaling cascade. Control of very different secondary metabolites, including pathogenicity determinants, are mediated by calcium through the Crz1 factor. Several interactions between calcium homeostasis and phosphate have been demonstrated in the last decade: 1) The inositol pyrophosphate IP3 triggers the release of calcium ions from internal stores into the cytosol, 2) Expression of the high affinity phosphate transporter Pho89, a Na+/phosphate symporter, is controlled by Crz1. Also, mutants defective in the calcium permeable TRPCa7-like of Saccharomyces cerevisiae shown impaired expression of Pho89. This information suggests that CrzA and Pho89 play key roles in the interaction of phosphate and calcium regulatory pathways, 3) Finally, acidocalcisomes organelles have been found in mycorrhiza and in some melanin producing fungi that show similar characteristics as protozoa calcisomes. In these organelles there is a close interaction between orthophosphate, pyrophosphate and polyphosphate and calcium ions that are absorbed in the polyanionic polyphosphate matrix. These advances open new perspectives for the control of fungal metabolism.

AaCaMKs Positively Regulate Development, Infection Structure Differentiation and Pathogenicity in Alternaria alternata, Causal Agent of Pear Black Spot

Calcium/calmodulin-dependent protein kinase (CaMK), a key downstream target protein in the Ca²⁺ signaling pathway of eukaryotes, plays an important regulatory role in the growth, development and pathogenicity of plant fungi. Three AaCaMKs (AaCaMK1, AaCaMK2 and AaCaMK3) with conserved PKC_like superfamily domains, ATP binding sites and ACT sites have been cloned from Alternaria alternata, However, their regulatory mechanism in A. alternata remains unclear. In this study, the function of the AaCaMKs in the development, infection structure differentiation and pathogenicity of A. alternata was elucidated through targeted gene disruption. The single disruption of AaCaMKs had no impact on the vegetative growth and spore morphology but significantly influenced hyphae growth, sporulation, biomass accumulation and melanin biosynthesis. Further expression analysis revealed that the AaCaMKs were up-regulated during the infection structure differentiation of A. alternata on hydrophobic and pear wax substrates. In vitro and in vivo analysis further revealed that the deletion of a single AaCaMKs gene significantly reduced the A. alternata conidial germination, appressorium formation and infection hyphae formation. In addition, pharmacological analysis confirmed that the CaMK specific inhibitor, KN93, inhibited conidial germination and appressorium formation in A. alternata. Meanwhile, the AaCaMKs genes deficiency significantly reduced the A. alternata pathogenicity. These results demonstrate that AaCaMKs regulate the development, infection structure differentiation and pathogenicity of A. alternata and provide potential targets for new effective fungicides.

A zinc finger suppressor involved in stress resistance, cell wall integrity, conidiogenesis, and autophagy in the necrotrophic fungal pathogen Alternaria alternata

The tangerine pathotype of Alternaria alternata can withstand high-level reactive oxygen species (ROS). By analyzing loss- and gain-of-function mutants, this study demonstrated that a Cys2His2 zinc finger-containing transcription regulator, A. alternata Stress Response Regulator 1 (AaSRR1), plays a negative role in resistance to peroxides and singlet-oxygen-generating compounds. AaSRR1 plays no role in cellular susceptibility or resistance to superoxide-producing compounds. AaSRR1 also negatively regulates conidiogenesis, maintenance of cell wall and membrane integrities, and chitin biosynthesis. Some wild-type hyphae displayed necrosis after exposure to 30 mM H₂O₂, whereas AaSRR1 deficient mutant (ΔAaSRR1) hyphae had visible granules and vacuoles. sGFP-AaATG8 proteolysis assays revealed that H₂O₂ and starvation could trigger autophagy formation in both wild type and ΔAaSRR1. Autophagy occurred at higher rates in ΔAaSRR1 than wild type under both conditions, particularly after H₂O₂ treatments, indicating that autophagy might contribute to ROS resistance. Upon exposure to H₂O₂ or under starvation, AaSRR1 was translocated into the nucleus, even though the expression of AaSRR1 was decreased. AaSRR1 is required for vegetative growth but is dispensable for fungal virulence as assayed on detached calamondin leaves. AaSRR1 suppressed the expression of the gene encoding a HOG1 mitogen-activated protein (MAP) kinase implicated in ROS resistance. Mutation of AaSRR1 increased catalase activity but decreased superoxide dismutase activity, leading to fewer ROS accumulation in the cytosol. Nevertheless, our results indicated that AaSRR1 is a transcription suppressor for ROS resistance. This study also revealed tradeoffs between stress responses and hyphal growth in A. alternata.

Phospholipase C (AoPLC2) regulates mycelial development, trap morphogenesis, and pathogenicity of the nematode-trapping fungus Arthrobotrys oligospora

AIMS

Phospholipase C (PLC) is a hydrolase involved in signal transduction in eukaryotic cells. This study aimed to understand the function of PLC in the nematode-trapping fungus Arthrobotrys oligospora.

METHODS AND RESULTS

Orthologous PLC (AoPLC2) of A. oligospora was functionally analysed using gene disruption and multi-phenotypic analysis. Disrupting Aoplc2 caused a deformation of partial hyphal cells (about 10%) and conidia (about 50%), decreased the number of nuclei in both conidia and hyphal cells, and increased the accumulation of lipid droplets. Meanwhile, the sporulation-related genes fluG and abaA were downregulated in ΔAoplc2 mutants than in the wild-type strain. Moreover, ΔAoplc2 mutants were more sensitive to osmotic stressors. Importantly, the number of traps, electron-dense bodies in traps, and nematicidal activity of ΔAoplc2 mutants were reduced, and the shape of the traps was deformed. In addition, AoPLC2 was involved in the biosynthesis of secondary metabolites in A. oligospora.

CONCLUSIONS

AoPLC2 plays an important role in the development of hyphae, spores, and cell nuclei, responses to stress, formation of traps, and predation of nematodes in A. oligospora.

SIGNIFICANCE AND IMPACT OF STUDY

This study reveals the various functions of phospholipase C and elucidates the regulation of trap morphogenesis in nematode-trapping fungi.

The basal transcription factor II H subunit Tfb5 is required for stress response and pathogenicity in the tangerine pathotype of Alternaria alternata

The basal transcription factor II H (TFIIH) is a multicomponent complex. In the present study, we characterized a TFIIH subunit Tfb5 by analysing loss- and gain-of-function mutants to gain a better understanding of the molecular mechanisms underlying stress resistance and pathogenicity in the citrus fungal pathogen Alternaria alternata. Tfb5 deficiency mutants (ΔAatfb5) decreased sporulation and pigmentation, and were impaired in the maintenance of colony surface hydrophobicity and cell wall integrity. ΔAatfb5 increased sensitivity to ultraviolet light, DNA-damaging agents, and oxidants. The expression of Aatfb5 was up-regulated in the wild type upon infection in citrus leaves, implicating the requirement of Aatfb5 in fungal pathogenesis. Biochemical and virulence assays revealed that ΔAatfb5 was defective in toxin production and cellwall-degrading enzymes, and failed to induce necrotic lesions on detached citrus leaves. Aatfb5 fused with green fluorescent protein (GFP) was localized in the cytoplasm and nucleus and physically interacted with another subunit, Tfb2, based on yeast two-hybrid and co-immunoprecipitation analyses. Transcriptome and Antibiotics & Secondary Metabolite Analysis Shell (antiSMASH) analyses revealed the positive and negative roles of Aatfb5 in the production of various secondary metabolites and in the regulation of many metabolic and biosynthetic processes in A. alternata. Aatfb5 may play a negative role in oxidative phosphorylation and a positive role in peroxisome biosynthesis. Two cutinase-coding genes (AaCut2 and AaCut15) required for full virulence were down-regulated in ΔAatfb5. Overall, this study expands our understanding of how A. alternata uses the basal transcription factor to deal with stress and achieve successful infection in the plant host.

Phospholipase C From Alternaria alternata Is Induced by Physiochemical Cues on the Pear Fruit Surface That Dictate Infection Structure Differentiation and Pathogenicity

To investigate the mechanisms of phospholipase C (PLC)-mediated calcium (Ca²⁺) signaling in Alternaria alternata, the regulatory roles of PLC were elucidated using neomycin, a specific inhibitor of PLC activity. Three isotypes of PLC designated AaPLC1, AaPLC2, and AaPLC3 were identified in A. alternata through genome sequencing. qRT-PCR analysis showed that fruit wax extracts significantly upregulated the expression of all three PLC genes in vitro. Pharmacological experiments showed that neomycin treatment led to a dose-dependent reduction in spore germination and appressorium formation in A. alternata. Appressorium formation was stimulated on hydrophobic and pear wax-coated surfaces but was significantly inhibited by neomycin treatment. The appressorium formation rates of neomycin treated A. alternata on hydrophobic and wax-coated surfaces decreased by 86.6 and 47.4%, respectively. After 4 h of treatment, exogenous CaCl₂ could partially reverse the effects of neomycin treatment. Neomycin also affected mycotoxin production in alternariol (AOH), alternariol monomethyl ether (AME), altenuene (ALT), and tentoxin (TEN), with exogenous Ca²⁺ partially reversing these effects. These results suggest that PLC is required for the growth, infection structure differentiation, and secondary metabolism of A. alternata in response to physiochemical signals on the pear fruit surface.

Calcium signaling is involved in diverse cellular processes in fungi

Calcium (Ca²⁺) is a universal signalling molecule of life. The Ca²⁺ signalling is an evolutionarily conserved process from prokaryotes to eukaryotes. Ca²⁺ at high concentration is deleterious to the cell; therefore, cell maintains a low resting level of intracellular free Ca²⁺ concentration ([Ca²⁺]_c). The resting [Ca²⁺]_c is tightly regulated, and a transient increase of the [Ca²⁺]_c initiates a signalling cascade in the cell. Ca²⁺ signalling plays an essential role in various processes, including growth, development, reproduction, tolerance to stress conditions, and virulence in fungi. In this review, we describe the evolutionary aspects of Ca²⁺ signalling and cell functions of major Ca²⁺ signalling proteins in different fungi.

The Role of a Nascent Polypeptide-Associated Complex Subunit Alpha in Siderophore Biosynthesis, Oxidative Stress Response, and Virulence in Alternaria alternata

The present study demonstrates that a nascent polypeptide-associated complex α subunit (Nac1) functions as a transcriptional regulator and plays both positive and negative roles in a vast array of functions in Alternaria alternata. Gain- and loss-of-function studies reveal that Nac1 is required for the formation and germination of conidia, likely via the regulation of Fus3 and Slt2 mitogen-activated protein kinase (MAPK)-coding genes, both implicated in conidiation. Nac1 negatively regulates hyphal branching and the production of cell wall-degrading enzymes. Importantly, Nac1 is required for the biosynthesis of siderophores, a novel phenotype that has not been reported to be associated with a Nac in fungi. The expression of Nac1 is positively regulated by iron, as well as by the Hog1 MAPK and the NADPH-dependent oxidase (Nox) complex. Nac1 confers cellular susceptibility to reactive oxygen species (ROS) likely via negatively regulating the expression of the genes encoding Yap1, Skn7, Hog1, and Nox, all involved in ROS resistance. The involvement of Nac1 in sensitivity to glucose-, mannitol-, or sorbitol-induced osmotic stress could be due to its ability to suppress the expression of Skn7. The requirement of Nac1 in resistance to salts is unlikely mediated through the transcriptional activation of Hog1. Although Nac1 plays no role in toxin production, Nac1 is required for fungal full virulence. All observed deficiencies can be restored by re-expressing a functional copy of Nac1, confirming that Nac1 contributes to the phenotypes. Thus, a dynamic regulation of gene expression via Nac1 is critical for developmental, physiological, and pathological processes of A. alternata.

Calcium-Calmodulin-Calcineurin Signaling: A Globally Conserved Virulence Cascade in Eukaryotic Microbial Pathogens

Calcium is an abundant intracellular ion, and calcium homeostasis plays crucial roles in several cellular processes. The calcineurin signaling cascade is one of the major pathways governed by intracellular calcium. Calcineurin, a conserved protein from yeast to humans, is a calcium-calmodulin-dependent serine-threonine-specific phosphatase that orchestrates cellular stress responses. In eukaryotic microbial pathogens, calcineurin controls essential virulence pathways, such as the ability to grow at host temperature, morphogenesis to enable invasive hyphal growth, drug tolerance and resistance, cell wall integrity, and sexual development. Therefore, the calcineurin cascade is an attractive target in drug development against eukaryotic pathogens. In the present review, we summarize and discuss the current knowledge on the roles of calcineurin in eukaryotic microbial pathogens, focusing on fungi and parasitic protists.

Role of phospholipase C in banana in response to anthracnose infection

Phospholipase C (PLC) plays an important role in plant immunity, and anthracnose caused by the Colletotrichum species is a common postharvest disease of the banana fruit. This study aims to evaluate the role of PLC in anthrax resistance in banana. The experimental group of banana samples was treated with a banana anthracnose conidia suspension, and the control group was treated with distilled water. After inoculation, the groups were sprayed with ethephon, and indicators, such as hardness and conductivity changes; PLC activity, 1,2-diacylglycerol (DAG) and phosphatidic acid (PA)content; and MaPLC-1and MaPLC-2 expression levels, were assessed at 0, 3, 6, 9, 12, and 15 days. Moreover, the expression levels of MaPLC-1 and MaPLC-2 were detected in various tissues. The hardness of banana fruits in the experimental group decreased faster than that in the control group. Furthermore, the conductivity was higher in the experimental group than in the control group. Regarding PLC activity, DAG, and PA content, bananas in the experimental group showed higher activities than those in the control group. Moreover, relatively higher expression of PLC mRNA was detected in anthracnose-inoculated tissues. The evaluation of MaPLC-1 and MaPLC-2 expression levels showed that the mature peel had the highest MaPLC-1 expression level. However, the MaPLC-2 gene was expressed at relatively low levels in the fruit and at relatively high levels in the flower organs. PLC might play a role in fruit ripening in response to anthracnose resistance.

SEM and qRT-PCR revealed quercetin inhibits morphogenesis of Aspergillus flavus conidia via modulating calcineurin-Crz1 signalling pathway

ASPERGILLUS FLAVUS

exploits diverse mechanisms to survive during exposure to antifungal agents including morphogenesis. Germination of dormant conidia involves cascades of reactions integrated into the signalling pathway. This study documents the effect of phytochemical-quercetin on A. flavus during germination of conidia using scanning electron microscopy (SEM). Significant inhibition of conidial swelling of A. flavus in comparison to control was observed at 4 and 7 h Quantitative real-time PCR for genes from calcium signalling pathway and heat-shock proteins family showed up-regulation of heat shock (Hsp70 and Hsp90) and calcium signalling pathway genes (calcium-transporting ATPase and calmodulin) in response to quercetin at initial 4 h in comparison to control sample whereas up-regulation of Hsp70, calcineurin and transcription factor Crz1, were observed in both the treated samples. Gene encoding for calcium-kinase, cAMP, Rho-gdp, Plc and Pkc showed a constitutively higher level of expression in quercetin-treated sample in comparison to control at both time points. These data showed a clear response from genes encoding calcineurin-Crz1 signalling pathways and may find its application in the screening of antifungal agents.

ABBREVIATIONS

Hsp: Hear shock protein; MIC: Minimum Inhibitory Concentration; SEM: Scanning Electron Microscopy; qRT-PCR: Quantitative Real-Time Polymerase Chain Reaction.

OsPLB gene expressed during seed germination encodes a phospholipase in rice

Hydrolysis of phospholipid monolayer by phospholipases is an important event in the mobilization of stored lipids for seed germination. However, the identification and functional characterization of cereal phospholipases, especially during rice germination, are limited. In the present study, we have identified and characterized a phospholipase OsPLB gene expressed during germination. The full-length coding region of OsPLB was cloned into pRSETA as well as pYES2/NTC vector. The recombinant protein was successfully expressed in both E. coli and Saccharomyces cerevisiae. The recombinant protein was purified to homogeneity by affinity chromatography, and it was further confirmed by MS/MS analysis. In vitro lipase assay and lipidome analysis using high-resolution mass spectrometry showed phosphatidylcholine (PC) specific phospholipase B activity. The results revealed that protein encoded by OsPLB gene prefers to hydrolyze PCs with C28, C32, and C34 containing unsaturated fatty acids. Collectively, the present study describes the identification and characterization of a phospholipase B, which hydrolyze PC, a major component of phospholipid monolayer covering storage lipid, as an initial event during rice seed germination.

Cell-Wall-Degrading Enzymes Required for Virulence in the Host Selective Toxin-Producing Necrotroph Alternaria alternata of Citrus

The necrotrophic fungal pathogen Alternaria alternata attacks many citrus species, causing brown spot disease. Its pathogenic capability depends primarily on the production of host-selective ACT toxin. In the current study a Ste12 transcription factor was characterized to be required for conidial formation and the production of cell-wall-degrading enzymes (CWDEs) in the tangerine pathotype of A. alternata. The Ste12 deficiency strain (ΔSte12) retained wild-type growth, ACT toxin production, and sensitivity to oxidative and osmotic stress. However, pathogenicity tests assayed on detached Dancy leaves revealed a marked reduction in virulence of ΔSte12. Transcriptome and quantitative RT-PCR analyses revealed that many genes associated with Carbohydrate-Active Enzymes (CAZymes) were downregulated in ΔSte12. Two cutinase-coding genes (AaCut3 and AaCut7) regulated by Ste12 were individually and simultaneously inactivated. The AaCut3 or AaCut7 deficiency strain unchanged in cutinase activities and incited wild-type lesions on Dancy leaves. However, the strain carrying an AaCut3 AaCut7 double mutation produced and secreted significantly fewer cutinases and incited smaller necrotic lesions than wild type. Not only is the host-selective toxin (HST) produced by A. alternata required for fungal penetration and lesion formation, but so too are CWDEs required for full virulence. Overall, this study expands our understanding of how A. alternata overcomes citrus physical barriers to carry out successful penetration and colonization.

The putative phospholipase Lip2 counteracts oxidative damage and influences the virulence of Ustilago maydis

Ustilago maydis is an obligate biotrophic fungal pathogen which causes common smut disease of corn. To proliferate in host tissue, U. maydis must gain access to nutrients and overcome plant defence responses, such as the production of reactive oxygen species. The elucidation of the mechanisms by which U. maydis meets these challenges is critical for the development of strategies to combat smut disease. In this study, we focused on the contributions of phospholipases (PLs) to the pathogenesis of corn smut disease. We identified 11 genes encoding putative PLs and characterized the transcript levels for these genes in the fungus grown in culture and during infection of corn tissue. To assess the contributions of specific PLs, we focused on two genes, lip1 and lip2, which encode putative phospholipase A₂ (PLA₂ ) enzymes with similarity to platelet-activating factor acetylhydrolases. PLA₂ enzymes are known to counteract oxidative damage to lipids in other organisms. Consistent with a role in the mitigation of oxidative damage, lip2 mutants were sensitive to oxidative stress provoked by hydrogen peroxide and by increased production of reactive oxygen species caused by inhibitors of mitochondrial functions. Importantly, mutants defective in lip2, but not lip1, were attenuated for virulence in corn seedlings. Finally, a comparative analysis of fatty acid and cardiolipin profiles in the wild-type strain and a lip2 mutant revealed differences consistent with a protective role for Lip2 in maintaining lipid homeostasis and mitochondrial health during proliferation in the hostile host environment.

The phospholipase C (FgPLC1) is involved in regulation of development, pathogenicity, and stress responses in Fusarium graminearum

Phospholipase C (PLC) is an important phospholipid hydrolase that plays critical roles in various biological processes in eukaryotic cells. To elucidate the functions of PLC in morphogenesis and pathogenesis in Fusarium graminearum, deletion mutants were constructed of all six FgPLC genes identified in this study. Deletion of FgPLC1, but not the other five FgPLC genes, affected hyphal growth and conidiation. The FgPLC1 deletion mutant (Δplc1) also was defective in conidium germination and germ tube growth. It was sterile in selfing crosses and had increased sensitivities to hyperosmotic and cell wall stresses. The Δplc1 mutant showed reduced DON production and virulence during infection in flowering wheat heads. Deletion of FgPLC1 decreased the phosphorylation levels of both Gpmk1 and Mgv1 MAP kinases. qRT-PCR analysis showed that several genes related to defective phenotypes were down-regulated in the Δplc1 mutant. Taken together, these results indicated that FgPLC1 is important for hyphal growth, plant infection, and sexual or asexual reproduction, and it may be functionally related to MAP kinases in F. graminearum.

Cytosolic free calcium dynamics as related to hyphal and colony growth in the filamentous fungal pathogen Colletotrichum graminicola

Tip growth of pollen tubes and root hairs of plants is oscillatory and orchestrated by tip-focussed variations of cytosolic free calcium ([Ca(2+)]cyt). Hyphae of filamentous fungi are also tubular tip-growing cells, and components of the Ca(2+) signalling machinery, such as Ca(2+) channels and Ca(2+) sensors, are known to be important for fungal growth. In this study, we addressed the questions if tip-focussed [Ca(2+)]cyt transients govern hyphal and whole-colony growth in the maize pathogen Colletotrichum graminicola, and whether colony-wide [Ca(2+)]cyt dynamics rely on external Ca(2+) or internal Ca(2+) stores. Ratiometric fluorescence microscopy of individual hyphae expressing the Ca(2+) reporter Yellow Cameleon 3.6 revealed that Ca(2+) spikes in hyphal tips precede the re-initiation of growth after wounding. Tip-focussed [Ca(2+)]cyt spikes were also observed in undisturbed growing hyphae. They occurred not regularly and at a higher rate in hyphae growing at a medium-glass interface than in those growing on an agar surface. Hyphal tip growth was non-pulsatile, and growth speed was not correlated with the rate of spike occurrence. A possible relationship of [Ca(2+)]cyt spike generation and growth of whole colonies was assessed by using a codon-optimized version of the luminescent Ca(2+) reporter Aequorin. Depletion of extracellular free Ca(2+) abolished [Ca(2+)]cyt spikes nearly completely, but had only a modest effect on colony growth. In a pharmacological survey, some inhibitors targeting Ca(2+) influx or release from internal stores repressed growth strongly. However, although some of those inhibitors also affected [Ca(2+)]cyt spike generation, the effects on both parameters were not correlated. Collectively, the results indicate that tip growth of C. graminicola is non-pulsatile and not mechanistically linked to tip-focused or global [Ca(2+)]cyt spikes, which are likely a response to micro-environmental parameters, such as the physical properties of the growth surface.

How the Pathogenic Fungus Alternaria alternata Copes with Stress via the Response Regulators SSK1 and SHO1

The tangerine pathotype of Alternaria alternata is a necrotrophic fungal pathogen causing brown spot disease on a number of citrus cultivars. To better understand the dynamics of signal regulation leading to oxidative and osmotic stress response and fungal infection on citrus, phenotypic characterization of the yeast SSK1 response regulator homolog was performed. It was determined that SSK1 responds to diverse environmental stimuli and plays a critical role in fungal pathogenesis. Experiments to determine the phenotypes resulting from the loss of SSK1 reveal that the SSK1 gene product may be fulfilling similar regulatory roles in signaling pathways involving a HOG1 MAP kinase during ROS resistance, osmotic resistance, fungicide sensitivity and fungal virulence. The SSK1 mutants display elevated sensitivity to oxidants, fail to detoxify H₂O₂ effectively, induce minor necrosis on susceptible citrus leaves, and displays resistance to dicarboximide and phenylpyrrole fungicides. Unlike the SKN7 response regulator, SSK1 and HOG1 confer resistance to salt-induced osmotic stress via an unknown kinase sensor rather than the “two component” histidine kinase HSK1. SSK1 and HOG1 play a moderate role in sugar-induced osmotic stress. We also show that SSK1 mutants are impaired in their ability to produce germ tubes from conidia, indicating a role for the gene product in cell differentiation. SSK1 also is involved in multi-drug resistance. However, deletion of the yeast SHO1 (synthetic high osmolarity) homolog resulted in no noticeable phenotypes. Nonetheless, our results show that A. alternata can sense and react to different types of stress via SSK1, HOG1 and SKN7 in a cooperative manner leading to proper physiological and pathological functions.