Inhibitory phosphorylation of a mitotic cyclin-dependent kinase regulates the morphogenesis, cell size and virulence of the smut fungusUstilago maydis

Bacillus subtilis KLBMPGC81 suppresses appressorium-mediated plant infection by altering the cell wall integrity signaling pathway and multiple cell biological processes in Magnaporthe oryzae

Magnaporthe oryzae is one of the most destructive crop pathogens in the world, causing huge losses in rice harvest every year. Bacillus subtilis is a potential biocontrol agent that has been explored in many crop systems because it is a potent producer of bioactive compounds. However, the mechanisms by which these agents control rice blasts are not fully understood. We show that B. subtilis KLBMPGC81 (KC81) and its supernatant (SUP) have high antimicrobial activity against M. oryzae strain Guy11. To better exploit KC81 as a biocontrol agent, the mechanism by which KC81 suppresses rice blast pathogens was investigated. This study shows that KC81 SUP is effective in controlling rice blast disease. The SUP has a significant effect on suppressing the growth of M. oryzae and appressorium-mediated plant infection. KC81 SUP compromises cell wall integrity, microtubules and actin cytoskeleton, mitosis, and autophagy, all of which are required for M. oryzae growth, appressorium development, and host infection. We further show that the SUP reduces the activity of the cyclin-dependent kinase Cdc2 by enhancing the phosphorylation of Cdc2 Tyr 15, thereby impairing mitosis in M. oryzae cells. SUP induces the cell wall sensor MoWsc1 to activate the cell wall integrity pathway and Mps1 and Pmk1 mitogen-activated protein kinases. Taken together, our findings reveal that KC81 is an effective fungicide that suppresses M. oryzae growth, appressorium formation, and host infection by abnormally activating the cell wall integrity pathway, disrupting the cytoskeleton, mitosis, and autophagy.

Biocontrol ability and action mechanism of dihydromaltophilin against Colletotrichum fructicola causing anthracnose of pear fruit

BACKGROUND

Anthracnose caused by Colletotrichum fructicola is one of the most important diseases in pear fruit, resulting in huge economic losses. Public awareness of protecting the environment and food safety, together with pathogen resistance to many key fungicides have led to an urgent need to develop alternative strategies for controlling fruit diseases. Here, the antifungal activity of a natural product, dihydromaltophilin [heat-stable antifungal factor (HSAF)], against C. fructicola in vitro and in vivo was investigated to determine its efficacy for anthracnose management.

RESULTS

HSAF exhibited pronounced antifungal activity against in vitro mycelial growth of C. fructicola, with a half-inhibition concentration of 0.43 mg L^-1 . Hyphae treated with HSAF showed defects such as hyperbranching, swelling and depolarized growth. Conidia germination in the pathogen was inhibited by HSAF in a dose-dependent manner. In the presence of 4 mg L^-1 HSAF, conidia germination was significantly delayed, and germ tube growth was inhibited. HSAF at 8 mg L^-1 completely blocked conidia germination in C. fructicola. In addition, HSAF disrupted coordination of cytokinesis with growth and nuclear division, induced reactive oxygen species production in conidia, and damaged the integrity of the conidia cell wall. Moreover, an in vivo test confirmed that 50 mg L^-1 HSAF significantly reduced the development of anthracnose decay in pear fruit caused by C. fructicola.

CONCLUSION

HSAF was highly effective in reducing pear anthracnose caused by C. fructicola and has great potential to become a new type of fruit preservative.

Incompatibility between proliferation and plant invasion is mediated by a regulator of appressorium formation in the corn smut fungus Ustilago maydis

Plant pathogenic fungi often developed specialized infection structures to breach the outer surface of a host plant. These structures, called appressoria, lead the invasion of the plant by the fungal hyphae. Studies in different phytopathogenic fungi showed that appressorium formation seems to be subordinated to the cell cycle. This subordination ensures the loading in the invading hypha of the correct genetic information to proceed with plant infection. However, how the cell cycle transmits its condition to the genetic program controlling appressorium formation and promoting the plant's invasion is unknown. Our results have uncovered how this process occurs for the appressorium of Ustilago maydis, the agent responsible for corn smut disease. Here, we described that the complex Clb2-cyclin-dependent kinase (Cdk)1, one of the master regulators of G2/M cell cycle progression in U. maydis, interacts and controls the subcellular localization of Biz1, a transcriptional factor required for the activation of the appressorium formation. Besides, Biz1 can arrest the cell cycle by down-regulation of the gene encoding a second b-cyclin Clb1 also required for the G2/M transition. These results revealed a negative feedback loop between appressorium formation and cell cycle progression in U. maydis, which serves as a "toggle switch" to control the fungal decision between infecting the plant or proliferating out of the plant.

Sexual Differentiation Is Coordinately Regulated by Cryptococcus neoformans CRK1 and GAT1

The heterothallic basidiomycetous fungus Cryptococcus neoformans has two mating types, MATa and MATα. Morphological progression of bisexual reproduction in C. neoformans is as follows: yeast to hyphal transition, filament extension, basidium formation, meiosis, and sporulation. C. neoformans Cdk-related kinase 1 (CRK1) is a negative regulator of bisexual mating. In this study, we characterized the morphological features of mating structures in the crk1 mutant and determined the genetic interaction of CRK1 in the regulatory networks of sexual differentiation. In the bilateral crk1 mutant cross, despite shorter length of filaments than in the wild-type cross, dikaryotic filaments and other structures still remained intact during bisexual mating, but the timing of basidium formation was approximately 18 h earlier than in the cross between wild type strains. Furthermore, gene expression analyses revealed that CRK1 modulated the expression of genes involved in the progression of hyphal elongation, basidium formation, karyogamy and meiosis. Phenotypic results showed that, although deletion of C. neoformans CRK1 gene increased the efficiency of bisexual mating, filamentation in the crk1 mutant was blocked by MAT2 or ZNF2 mutation. A bioinformatics survey predicted the C. neoformans GATA transcriptional factor Gat1 as a potential substrate of Crk1 kinase. Our genetic and phenotypic findings revealed that C. neoformansGAT1 and CRK1 formed a regulatory circuit to negatively regulate MAT2 to control filamentation progression and transition during bisexual mating.

Cytoplasmic retention and degradation of a mitotic inducer enable plant infection by a pathogenic fungus

In the fungus Ustilago maydis, sexual pheromones elicit mating resulting in an infective filament able to infect corn plants. Along this process a G2 cell cycle arrest is mandatory. Such as cell cycle arrest is initiated upon the pheromone recognition in each mating partner, and sustained once cell fusion occurred until the fungus enter the plant tissue. We describe that the initial cell cycle arrest resulted from inhibition of the nuclear transport of the mitotic inducer Cdc25 by targeting its importin, Kap123. Near cell fusion to take place, the increase on pheromone signaling promotes Cdc25 degradation, which seems to be important to ensure the maintenance of the G2 cell cycle arrest to lead the formation of the infective filament. This way, premating cell cycle arrest is linked to the subsequent steps required for establishment of the infection. Disabling this connection resulted in the inability of fungal cells to infect plants.

MoCDC14 is important for septation during conidiation and appressorium formation in Magnaporthe oryzae

As a typical foliar pathogen, appressorium formation and penetration are critical steps in the infection cycle of Magnaporthe oryzae. Because appressorium formation and penetration are closely co-regulated with the cell cycle, and Cdc14 phosphatases have an antagonistic relationship with cyclin-dependent kinases (CDKs) on proteins related to mitotic exit and cytokinesis, in this study, we functionally characterized the MoCDC14 gene in M. oryzae. The Mocdc14 deletion mutant showed significantly reduced growth rate and conidiation. It was also defective in septum formation and nuclear distribution. Septation was irregular in Mocdc14 hyphae and hyphal compartments became multi-nucleate. Mutant conidia often showed incomplete septa or lacked any septum. During appressorium formation, the septum delimiting appressoria from the rest of the germ tubes was often formed far away from the neck of the appressoria or not formed at all. Unlike the wild-type, some mutant appressoria had more than one nucleus at 24 h. In addition to appressoria, melanization occurred on parts of the germ tubes and conidia, depending on the irregular position of the appressorium-delimiting septum. The Mocdc14 mutant was also defective in glycogen degradation during appressorium formation and appressorial penetration of intact plant cells. Similar defects in septum formation, melanization and penetration were observed with appressorium-like structures formed at hyphal tips in the Mocdc14 mutant. Often a long fragment of mutant hyphae was melanized, together with the apical appressorium-like structures. These results indicate that MoCDC14 plays a critical role in septation, nuclear distribution and pathogenesis in M. oryzae, and correct septum formation during conidiogenesis and appressorium formation requires the MoCdc14 phosphatase.

Cell Biology of Hyphal Growth

Filamentous fungi are a large and ancient clade of microorganisms that occupy a broad range of ecological niches. The success of filamentous fungi is largely due to their elongate hypha, a chain of cells, separated from each other by septa. Hyphae grow by polarized exocytosis at the apex, which allows the fungus to overcome long distances and invade many substrates, including soils and host tissues. Hyphal tip growth is initiated by establishment of a growth site and the subsequent maintenance of the growth axis, with transport of growth supplies, including membranes and proteins, delivered by motors along the cytoskeleton to the hyphal apex. Among the enzymes delivered are cell wall synthases that are exocytosed for local synthesis of the extracellular cell wall. Exocytosis is opposed by endocytic uptake of soluble and membrane-bound material into the cell. The first intracellular compartment in the endocytic pathway is the early endosomes, which emerge to perform essential additional functions as spatial organizers of the hyphal cell. Individual compartments within septated hyphae can communicate with each other via septal pores, which allow passage of cytoplasm or organelles to help differentiation within the mycelium. This article introduces the reader to more detailed aspects of hyphal growth in fungi.

The Botrytis cinerea PAK kinase BcCla4 mediates morphogenesis, growth and cell cycle regulating processes downstream of BcRac

Rac proteins are involved in a variety of cellular processes. Effector proteins that interact with active Rac convey the GTPase-generated signal to downstream developmental cascades and processes. Here we report on the analysis of the main effector and signal cascade downstream of BcRac, the Rac homolog of the grey mold fungus Botrytis cinerea. Several lines of evidence highlighted the p21-activated kinase Cla4 as an important effector of Rac in fungi. Analysis of Δbccla4 strains revealed that the BcCla4 protein was sufficient to mediate all of the examined BcRac-driven processes, including hyphal growth and morphogenesis, conidia production and pathogenicity. In addition, the Δbccla4 strains had altered nuclei content, a phenomenon that was previously observed in Δbcrac isolates, thus connecting the BcRac/BcCla4 module with cell cycle control. Further analyses revealed that BcRac/BcCla4 control mitotic entry through changes in phosphorylation status of the cyclin dependent kinase BcCdk1. The complete cascade includes the kinase BcWee1, which is downstream of BcCla4 and upstream of BcCdk1. These results provide a mechanistic insight on the connection of cell cycle, morphogenesis and pathogenicity in fungi, and position BcCla4 as the most essential effector and central regulator of all of these processes downstream of BcRac.

Virulence-specific cell cycle and morphogenesis connections in pathogenic fungi

To initiate pathogenic development, pathogenic fungi respond to a set of inductive cues. Some of them are of an extracellular nature (environmental signals), while others are intracellular (developmental signals). These signals must be integrated into a single response whose major outcome is changes in the morphogenesis of the fungus. The regulation of the cell cycle is pivotal during these cellular differentiation steps; therefore, cell cycle regulation would likely provide control points for infectious development by fungal pathogens. Here, we provide clues to understanding how the control of the cell cycle is integrated with the morphogenesis program in pathogenic fungi, and we review current examples that support these connections.

Ubiquitin-specific protease 22 is a deubiquitinase of CCNB1

The elevated level of CCNB1 indicates more aggressive cancer and poor prognosis. However, the factors that cause CCNB1 upregulation remain enigmatic. Herein, we identify USP22 as a CCNB1 interactor and discover that both USP22 and CCNB1 are dramatically elevated with a strong positive correlation in colon cancer tissues. USP22 stabilizes CCNB1 by antagonizing proteasome-mediated degradation in a cell cycle-specific manner. Phosphorylation of USP22 by CDK1 enhances its activity in deubiquitinating CCNB1. The ubiquitin ligase anaphase-promoting complex (APC/C) targets USP22 for degradation by using the substrate adapter CDC20 during cell exit from M phase, presumably allowing CCNB1 degradation. Finally, we discover that USP22 knockdown leads to slower cell growth and reduced tumor size. Our study demonstrates that USP22 is a CCNB1 deubiquitinase, suggesting that targeting USP22 might be an effective approach to treat cancers with elevated CCNB1 expression.

MRN- and 9-1-1-Independent Activation of the ATR-Chk1 Pathway during the Induction of the Virulence Program in the Phytopathogen Ustilago maydis

DNA damage response (DDR) leads to DNA repair, and depending on the extent of the damage, to further events, including cell death. Evidence suggests that cell differentiation may also be a consequence of the DDR. During the formation of the infective hypha in the phytopathogenic fungus Ustilago maydis, two DDR kinases, Atr1 and Chk1, are required to induce a G2 cell cycle arrest, which in turn is essential to display the virulence program. However, the triggering factor of DDR in this process has remained elusive. In this report we provide data suggesting that no DNA damage is associated with the activation of the DDR during the formation of the infective filament in U. maydis. We have analyzed bulk DNA replication during the formation of the infective filament, and we found no signs of impaired DNA replication. Furthermore, using RPA-GFP fusion as a surrogate marker of the presence of DNA damage, we were unable to detect any sign of DNA damage at the cellular level. In addition, neither MRN nor 9-1-1 complexes, both instrumental to transmit the DNA damage signal, are required for the induction of the above mentioned cell cycle arrest, as well as for virulence. In contrast, we have found that the claspin-like protein Mrc1, which in other systems serves as scaffold for Atr1 and Chk1, was required for both processes. We discuss possible alternative ways to trigger the DDR, independent of DNA damage, in U. maydis during virulence program activation.

Distinct cell cycle regulation during saprophytic and pathogenic growth in fungal pathogens

In a number of dimorphic and hemibiotrophic pathogens, cell cycle regulation has been shown to be important for morphological changes related to infectious growth or infection-related morphogenesis. However, the role of mitotic CDK kinase Cdc2, the key regulator of cell cycle, in pathogenic growth is not clear, because most fungal pathogens have a single CDC2 gene that is essential for cell cycle progression and viability. Interestingly, the wheat scab fungus Fusarium graminearum has two CDC2 genes. Although CDC2A and CDC2B have redundant functions in vegetative growth and asexual production, only CDC2A is required for invasive growth and plant infection. In this study, we showed that Cdc2A and Cdc2B interacted with each other and may form homo- and heterodimers in vegetative hyphae. We also identified sequence and structural differences between Cdc2A and Cdc2B that may be related to their functional divergence. These results, together with earlier studies with cyclins, important for differentiation and infection in Candida albicans and Ustilago maydis, indicated that dimorphic and hemibiotrophic fungal pathogens may have stage-specific cyclin-CDK combinations or CDK targets during saprophytic and pathogenic growth.

Two Cdc2 Kinase Genes with Distinct Functions in Vegetative and Infectious Hyphae in Fusarium graminearum

Eukaryotic cell cycle involves a number of protein kinases important for the onset and progression through mitosis, most of which are well characterized in the budding and fission yeasts and conserved in other fungi. However, unlike the model yeast and filamentous fungi that have a single Cdc2 essential for cell cycle progression, the wheat scab fungus Fusarium graminearum contains two CDC2 orthologs. The cdc2A and cdc2B mutants had no obvious defects in growth rate and conidiation but deletion of both of them is lethal, indicating that these two CDC2 orthologs have redundant functions during vegetative growth and asexual reproduction. However, whereas the cdc2B mutant was normal, the cdc2A mutant was significantly reduced in virulence and rarely produced ascospores. Although deletion of CDC2A had no obvious effect on the formation of penetration branches or hyphopodia, the cdc2A mutant was limited in the differentiation and growth of infectious growth in wheat tissues. Therefore, CDC2A plays stage-specific roles in cell cycle regulation during infectious growth and sexual reproduction. Both CDC2A and CDC2B are constitutively expressed but only CDC2A was up-regulated during plant infection and ascosporogenesis. Localization of Cdc2A- GFP to the nucleus but not Cdc2B-GFP was observed in vegetative hyphae, ascospores, and infectious hyphae. Complementation assays with chimeric fusion constructs showed that both the N- and C-terminal regions of Cdc2A are important for its functions in pathogenesis and ascosporogenesis but only the N-terminal region is important for its subcellular localization. Among the Sordariomycetes, only three Fusarium species closely related to F. graminearum have two CDC2 genes. Furthermore, F. graminearum uniquely has two Aurora kinase genes and one additional putative cyclin gene, and its orthologs of CAK1 and other four essential mitotic kinases in the budding yeast are dispensable for viability. Overall, our data indicate that cell cycle regulation is different between vegetative and infectious hyphae in F. graminearum and Cdc2A, possibly by interacting with a stage-specific cyclin, plays a more important role than Cdc2B during ascosporogenesis and plant infection.

In the model yeasts and filamentous fungi, CDC2 is an essential gene that encodes the only CDK essential for mitotic cell cycle progression. However, the wheat scab fungus F. graminearum contains two CDC2 orthologs. The cdc2A and cdc2B deletion mutants had no defects in vegetative growth but deletion of both is lethal. Whereas the cdc2B mutant was normal, the cdc2A mutant was almost non-pathogenic, indicating that only Cdc2A is essential in infectious hyphae. Cdc2A and Cdc2B differ in subcellular localization and only localization of Cdc2A to the nucleus was increased in cells active in mitosis. Furthermore, F. graminearum uniquely has two orthologs of Ipl1 Aurora kinase and mutants deleted of orthologs of five essential yeast mitotic kinase genes were viable. However, most of these mutants were significantly reduced in virulence. Overall, our data indicate that F. graminearum differs from the model fungi in CDK and other key mitotic kinase genes, and cell cycle regulation is different between vegetative and infectious hyphae. This is the first report on two Cdc2 kinases in fungi and they differ in subcellular localization and functions during sexual reproduction and plant infection.

Fungal Ku prevents permanent cell cycle arrest by suppressing DNA damage signaling at telomeres

The Ku heterodimer serves in the initial step in repairing DNA double-strand breaks by the non-homologous end-joining pathway. Besides this key function, Ku also plays a role in other cellular processes including telomere maintenance. Inactivation of Ku can lead to DNA repair defects and telomere aberrations. In model organisms where Ku has been studied, inactivation can lead to DNA repair defects and telomere aberrations. In general Ku deficient mutants are viable, but a notable exception to this is human where Ku has been found to be essential. Here we report that similar to the situation in human Ku is required for cell proliferation in the fungus Ustilago maydis. Using conditional strains for Ku expression, we found that cells arrest permanently in G2 phase when Ku expression is turned off. Arrest results from cell cycle checkpoint activation due to persistent signaling via the DNA damage response (DDR). Our results point to the telomeres as the most likely source of the DNA damage signal. Inactivation of the DDR makes the Ku complex dispensable for proliferation in this organism. Our findings suggest that in U. maydis, unprotected telomeres arising from Ku depletion are the source of the signal that activates the DDR leading to cell cycle arrest.

Programmed cell cycle arrest is required for infection of corn plants by the fungus Ustilago maydis

Ustilago maydis is a plant pathogen that requires a specific structure called infective filament to penetrate the plant tissue. Although able to grow, this filament is cell cycle arrested on the plant surface. This cell cycle arrest is released once the filament penetrates the plant tissue. The reasons and mechanisms for this cell cycle arrest are unknown. Here, we have tried to address these questions. We reached three conclusions from our studies. First, the observed cell cycle arrest is the result of the cooperation of at least two distinct mechanisms: one involving the activation of the DNA damage response (DDR) cascade; and the other relying on the transcriptional downregulation of Hsl1, a kinase that modulates the G2/M transition. Second, a sustained cell cycle arrest during the infective filament step is necessary for the virulence in U. maydis, as a strain unable to arrest the cell cycle was severely impaired in its ability to infect corn plants. Third, production of the appressorium, a structure required for plant penetration, is incompatible with an active cell cycle. The inability to infect plants by strains defective in cell cycle arrest seems to be caused by their failure to induce the appressorium formation process. In summary, our findings uncover genetic circuits to arrest the cell cycle during the growth of this fungus on the plant surface, thus allowing the penetration into plant tissue.

Functional analysis of the Aspergillus nidulans kinome

The filamentous fungi are an ecologically important group of organisms which also have important industrial applications but devastating effects as pathogens and agents of food spoilage. Protein kinases have been implicated in the regulation of virtually all biological processes but how they regulate filamentous fungal specific processes is not understood. The filamentous fungus Aspergillus nidulans has long been utilized as a powerful molecular genetic system and recent technical advances have made systematic approaches to study large gene sets possible. To enhance A. nidulans functional genomics we have created gene deletion constructs for 9851 genes representing 93.3% of the encoding genome. To illustrate the utility of these constructs, and advance the understanding of fungal kinases, we have systematically generated deletion strains for 128 A. nidulans kinases including expanded groups of 15 histidine kinases, 7 SRPK (serine-arginine protein kinases) kinases and an interesting group of 11 filamentous fungal specific kinases. We defined the terminal phenotype of 23 of the 25 essential kinases by heterokaryon rescue and identified phenotypes for 43 of the 103 non-essential kinases. Uncovered phenotypes ranged from almost no growth for a small number of essential kinases implicated in processes such as ribosomal biosynthesis, to conditional defects in response to cellular stresses. The data provide experimental evidence that previously uncharacterized kinases function in the septation initiation network, the cell wall integrity and the morphogenesis Orb6 kinase signaling pathways, as well as in pathways regulating vesicular trafficking, sexual development and secondary metabolism. Finally, we identify ChkC as a third effector kinase functioning in the cellular response to genotoxic stress. The identification of many previously unknown functions for kinases through the functional analysis of the A. nidulans kinome illustrates the utility of the A. nidulans gene deletion constructs.

Cla4, but not Rac1, regulates the filamentous response of Ustilago maydis to low ammonium conditions

Ustilago maydis, the fungal pathogen of maize, undergoes a dimorphic transition from budding yeast-like growth to filamentous growth, both as part of its program for pathogenesis and distinctly, in response to environmental cues, such as acid pH or low nitrogen availability. Smu1 is a p21-activated protein kinase (PAK) with roles in both the mating response required for the former function, as well as for the nutrient response. Hsl7 may be a negative regulator of Smu1 and appears to play a role in cell length and cell cycle.  Additional proteins that participate in cell polarity and filamentation pathways include the small G protein, Rac1, and its effector PAK kinase, Cla4. Here we describe further experiments that explore the roles of Cla4 and Rac1 in the response to nitrogen availability. While deletion of rac1severely delays filamentous growth on solid media low in ammonium (SLAD), we found that deletion of cla4 does not abolish filamentous cell morphology on solid SLAD. Unexpectedly, however, the Dcla4 mutants also filament in liquid SLAD. The filamentous cell morphology of the cla4 mutant in liquid SLAD has only been seen previously for one other mutant, a strain deleted for hsl7 that simultaneously over-expresses smu1. 

The distinct interaction between cell cycle regulation and the widely conserved morphogenesis-related (MOR) pathway in the fungus Ustilago maydis determines morphology

The morphogenesis-related NDR kinase (MOR) pathway regulates morphogenesis in fungi. In spite of the high conservation of its components, impairing their functions results in highly divergent cellular responses depending on the fungal species. The reasons for such differences are unclear. Here we propose that the species-specific connections between cell cycle regulation and the MOR pathway could be partly responsible for these divergences. We based our conclusion on the characterization of the MOR pathway in the fungus Ustilago maydis. Each gene that encodes proteins of this pathway in U. maydis was deleted. All mutants exhibited a constitutive hyperpolarized growth, contrasting with the loss of polarity observed in other fungi. Using a conditional allele of the central NDR kinase Ukc1, we found that impairing MOR function resulted in a prolonged G2 phase. This cell cycle delay appears to be the consequence of an increase in Cdk1 inhibitory phosphorylation. Strikingly, prevention of the inhibitory Cdk1 phosphorylation abolished the hyperpolarized growth associated with MOR pathway depletion. We found that the prolonged G2 phase resulted in higher levels of expression of crk1, a conserved kinase that promotes polar growth in U. maydis. Deletion of crk1 also abolished the dramatic activation of polar growth in cells lacking the MOR pathway. Taken together, our results suggest that Cdk1 inhibitory phosphorylation may act as an integrator of signaling cascades regulating fungal morphogenesis and that the distinct morphological response observed in U. maydis upon impairment of the MOR pathway could be due to a cell cycle deregulation.

Functional analysis of the kinome of the wheat scab fungus Fusarium graminearum

As in other eukaryotes, protein kinases play major regulatory roles in filamentous fungi. Although the genomes of many plant pathogenic fungi have been sequenced, systematic characterization of their kinomes has not been reported. The wheat scab fungus Fusarium graminearum has 116 protein kinases (PK) genes. Although twenty of them appeared to be essential, we generated deletion mutants for the other 96 PK genes, including 12 orthologs of essential genes in yeast. All of the PK mutants were assayed for changes in 17 phenotypes, including growth, conidiation, pathogenesis, stress responses, and sexual reproduction. Overall, deletion of 64 PK genes resulted in at least one of the phenotypes examined, including three mutants blocked in conidiation and five mutants with increased tolerance to hyperosmotic stress. In total, 42 PK mutants were significantly reduced in virulence or non-pathogenic, including mutants deleted of key components of the cAMP signaling and three MAPK pathways. A number of these PK genes, including Fg03146 and Fg04770 that are unique to filamentous fungi, are dispensable for hyphal growth and likely encode novel fungal virulence factors. Ascospores play a critical role in the initiation of wheat scab. Twenty-six PK mutants were blocked in perithecia formation or aborted in ascosporogenesis. Additional 19 mutants were defective in ascospore release or morphology. Interestingly, F. graminearum contains two aurora kinase genes with distinct functions, which has not been reported in fungi. In addition, we used the interlog approach to predict the PK-PK and PK-protein interaction networks of F. graminearum. Several predicted interactions were verified with yeast two-hybrid or co-immunoprecipitation assays. To our knowledge, this is the first functional characterization of the kinome in plant pathogenic fungi. Protein kinase genes important for various aspects of growth, developmental, and infection processes in F. graminearum were identified in this study.

Fusarium head blight caused by Fusarium graminearum is one of the most important diseases on wheat and barley. Although protein kinases are known to play major regulatory roles in fungi, systematic characterization of fungal kinomes has not been reported in plant pathogens. In this study we generated deletion mutants for 96 protein kinase genes. All of the resulting knockout mutants were assayed for changes in 17 phenotypes, including growth, reproduction, stress responses, and plant infection. Overall, deletion of 64 kinase genes resulted in at least one of the phenotypes examined. In total, 42 kinase mutants were significantly reduced in virulence or non-pathogenic. A number of these protein kinase genes, including two that are unique to filamentous fungi, are dispensable for hyphal growth and likely encode novel fungal virulence factors. Ascospores are the primary inoculum for wheat scab. We identified 26 mutants blocked in ascospore. We also used the in silico approach to predict the kinase-kinase interactions and verified some of them by yeast two-hybrid or co-IP assays. Overall, in this study we functionally characterize the kinome of F. graminearum. Protein kinase genes that are important for various aspects of growth, developmental, and plant infection processes were identified.

The DNA damage response signaling cascade regulates proliferation of the phytopathogenic fungus Ustilago maydis in planta

In the phytopathogenic fungus Ustilago maydis, the dikaryotic state dominates the period of growth occurring during the infectious phase. Dikaryons are cells in which two nuclei, one from each parent cell, share a single cytoplasm for a period of time without undergoing nuclear fusion. In fungal cells, maintenance of the dikaryotic state requires an intricate cell division process that often involves the formation of a structure known as the clamp connection as well as the sorting of one of the nuclei to this structure to ensure that each daughter dikaryon inherits a balance of each parental genome. Here, we describe an atypical role of the DNA damage checkpoint kinases Chk1 and Atr1 during pathogenic growth of U. maydis. We found that Chk1 and Atr1 collaborate to control cell cycle arrest during the induction of the virulence program in U. maydis and that Chk1 and Atr1 work together to control the dikaryon formation. These findings uncover a link between a widely conserved signaling cascade and the virulence program in a phytopathogen. We propose a model in which adjustment of the cell cycle by the Atr1-Chk1 axis controls fidelity in dikaryon formation. Therefore, Chk1 and Atr1 emerge as critical cell type regulators in addition to their roles in the DNA damage response.

The transcription factor Rbf1 is the master regulator for b-mating type controlled pathogenic development in Ustilago maydis

In the phytopathogenic basidiomycete Ustilago maydis, sexual and pathogenic development are tightly connected and controlled by the heterodimeric bE/bW transcription factor complex encoded by the b-mating type locus. The formation of the active bE/bW heterodimer leads to the formation of filaments, induces a G2 cell cycle arrest, and triggers pathogenicity. Here, we identify a set of 345 bE/bW responsive genes which show altered expression during these developmental changes; several of these genes are associated with cell cycle coordination, morphogenesis and pathogenicity. 90% of the genes that show altered expression upon bE/bW-activation require the zinc finger transcription factor Rbf1, one of the few factors directly regulated by the bE/bW heterodimer. Rbf1 is a novel master regulator in a multilayered network of transcription factors that facilitates the complex regulatory traits of sexual and pathogenic development.

Activation of the cell wall integrity pathway promotes escape from G2 in the fungus Ustilago maydis

It is widely accepted that MAPK activation in budding and fission yeasts is often associated with negative effects on cell cycle progression, resulting in delay or arrest at a specific stage in the cell cycle, thereby enabling cells to adapt to changing environmental conditions. For instance, activation of the Cell Wall Integrity (CWI) pathway in the budding yeast Saccharomyces cerevisiae signals an increase in CDK inhibitory phosphorylation, which leads cells to remain in the G2 phase. Here we characterized the CWI pathway of Ustilago maydis, a fungus evolutionarily distant from budding and fission yeasts, and show that activation of the CWI pathway forces cells to escape from G2 phase. In spite of these disparate cell cycle responses in S. cerevisiae and U. maydis, the CWI pathway in both organisms appears to respond to the same class cell wall stressors. To understand the basis of such a difference, we studied the mechanism behind the U. maydis response. We found that activation of CWI pathway in U. maydis results in a decrease in CDK inhibitory phosphorylation, which depends on the mitotic phosphatase Cdc25. Moreover, in response to activation of the CWI pathway, Cdc25 accumulates in the nucleus, providing a likely explanation for the increase in the unphosphorylated form of CDK. We also found that the extended N-terminal domain of Cdc25, which is dispensable under normal growth conditions, is required for this G2 escape as well as for resistance to cell wall stressors. We propose that the process of cell cycle adaptation to cell stress evolved differently in these two divergent organisms so that each can move towards a cell cycle phase most appropriate for responding to the environmental signals encountered.

The autophagy genes ATG8 and ATG1 affect morphogenesis and pathogenicity in Ustilago maydis

Autophagy is a complex degradative process in which cytosolic material, including organelles, is randomly sequestered within double-membrane vesicles termed autophagosomes. In Saccharomyces cerevisiae, the autophagy genes ATG1 and ATG8 are crucial for autophagy induction and autophagosome assembly, respectively, and their deletion has an impact on the autophagic potential of the corresponding mutant strains. We were interested in the role of autophagy in the development and virulence of U. maydis. Using a reverse genetic approach, we showed that the U. maydis ATG8 orthologue, atg8, is associated with autophagy-dependent processes. Deletion of atg8 abolished autophagosome accumulation in the vacuoles of carbon-starved cells and drastically reduced the survival of U. maydisDeltaatg8 mutant strains during these conditions. In addition, atg8 deletion had an impact on the budding process during saprobic haploid growth. The infection of maize with compatible Deltaatg8 strains resulted in fewer galled plants, and fungal gall colonization was strongly reduced, as reflected by the very low hyphal density in these tissues. Deltaatg8 infections resulted in the formation of very few teliospores. To corroborate the role of autophagy in U. maydis development, we also deleted the ATG1 orthologue, atg1. Deletion of atg1 yielded phenotypes similar to the Deltaatg8 strains during saprobic growth, but of lower magnitude. The Deltaatg1 strains were only slightly less pathogenic than the wild-type and teliospore production was not affected. Surprisingly, atg1 deletion in the Deltaatg8 background exacerbated those phenotypes already observed in the Deltaatg8 and Deltaatg1 single-mutant strains, strongly suggesting an additive phenotype. In particular, the double mutant was completely suppressed for plant gall induction.

A role for the DNA-damage checkpoint kinase Chk1 in the virulence program of the fungus Ustilago maydis

During induction of the virulence program in the phytopathogenic fungus Ustilago maydis, the cell cycle is arrested on the plant surface and it is not resumed until the fungus enters the plant. The mechanism of this cell cycle arrest is unknown, but it is thought that it is necessary for the correct implementation of the virulence program. Here, we show that this arrest takes place in the G2 phase, as a result of an increase in the inhibitory phosphorylation of the catalytic subunit of the mitotic cyclin-dependent kinase Cdk1. Sequestration in the cytoplasm of the Cdc25 phosphatase seems to be one of the reasons for the increase in inhibitory phosphorylation. Strikingly, we also report the DNA-damage checkpoint kinase Chk1 appears to be involved in this process. Our results support the emerging idea that checkpoint kinases have roles other than in the DNA-damage response, by virtue of their ability to interact with the cell cycle machinery.

DNA-damage response in the basidiomycete fungus Ustilago maydis relies in a sole Chk1-like kinase

Chk1 is a protein kinase that acts as a key signal transducer within the complex network responsible of the cellular response to different DNA damages. It is a conserved element along the eukaryotic kingdom, together with a second checkpoint kinase, called Chk2/Rad53. In fact, all organisms studied so far carried at least one copy of each kind of checkpoint kinase. Since the relative contribution to the DNA-damage response of each type of kinase varies from one organism to other, the current view about the roles of Chk1 and Chk2/Rad53 during DNA-damage response is one of mutual complementation and intimate cooperation. However, in this work it is reported that Ustilago maydis - a phytopathogenic fungus exhibiting extreme resistance to UV and ionizing radiation - have a single kinase belonging to the Chk1 family but strikingly no kinases related to Chk2/Rad53 family are apparent. The U. maydis Chk1 kinase is able to respond to different classes of DNA damages and its activity is required for the cellular adaptation to such damages. As other described components of the Chk1 family of kinases, U. maydis Chk1 is phosphorylated and translocated to nucleus in response to DNA-damage signals. Interestingly subtle differences in this response depending on the kind of DNA damage are apparent, suggesting that in U. maydis the sole Chk1 kinase recapitulates the roles that in other organisms are shared by Chk1 and the Chk2/Rad53 family of protein kinases.

Spa2 is required for morphogenesis but it is dispensable for pathogenicity in the phytopathogenic fungus Ustilago maydis

The increasing evidence linking regulation of polar growth and pathogenicity in fungi has elicited a significant effort devoted to produce a better understanding of mechanisms determining polarization in pathogenic fungi. Here we characterize in the phytopathogenic basidiomycete Ustilago maydis, the Spa2 protein, a well-known component of polarisome, firstly described in Saccharomyces cerevisiae. U. maydis display a dimorphic switch between budding growth of hapoid cells and filamentous growth of the dikaryon. During yeast growth, a GFP-tagged Spa2 protein localized to distinct growth sites in a cell cycle-specific manner, while during hyphal growth is persistently located to hyphal tips. Deletion of spa2 gene produces rounder budding cells and thicker filaments than wild-type cells, suggesting a role of Spa2 for the determination of the growth area in U. maydis. We also address the connections between Spa2 and the actin- and microtubule-cytoskeleton. We found that the absence of Spa2 does not affect cytoskeleton organization and strikingly, interference with actin filament or microtubule formation does not affect the polar localization of Spa2. In contrast, defects in the small GTPase Rac1 seems to affect the ability of Spa2 to locate to precise sites at the tip cell. Finally, to our surprise, we found that cells defectives in Spa2 function were as pathogenic as wild-type cells.

Sustained cell polarity and virulence in the phytopathogenic fungus Ustilago maydis depends on an essential cyclin-dependent kinase from the Cdk5/Pho85 family

Cyclin-dependent kinases from the Cdk5/Pho85 family are thought to play important roles in morphogenesis in organisms as diverse as yeast and humans. Here we used the corn smut fungus Ustilago maydis to address the role of Cdk5/Pho85 kinases in the morphogenesis and virulence of dimorphic phytopathogens. We found that Cdk5 is essential for growth in U. maydis. A temperature-sensitive cdk5 mutant caused cell wall and morphology defects at the restrictive temperature. Actin patches labeled with a fimbrin-GFP fusion protein were delocalized and a GFP-Myo5 fusion was directed towards the growing cell pole and rapidly dissociated from the tip. These defects were found to be due to an impairment in the maintenance of cell polarity. Our results indicated that Cdk5 is required for the activity of Rac1, probably at the level of the localization of its GEF, Cdc24. Cdk5 was required for full virulence, probably because mutant cells are unable to sustain the dramatic polar growth required for the formation of the infective structures. These results support a major role for morphogenesis in the virulence program of dimorphic fungi.

Genetics of morphogenesis and pathogenic development of Ustilago maydis

Ustilago maydis has emerged as an important model system for the study of fungi. Like many fungi, U. maydis undergoes remarkable morphological transitions throughout its life cycle. Fusion of compatible, budding, haploid cells leads to the production of a filamentous dikaryon that penetrates and colonizes the plant, culminating in the production of diploid teliospores within fungal-induced plant galls or tumors. These dramatic morphological transitions are controlled by components of various signaling pathways, including the pheromone-responsive MAP kinase and cAMP/PKA (cyclic AMP/protein kinase A) pathways, which coregulate the dimorphic switch and sexual development of U. maydis. These signaling pathways must somehow cooperate with the regulation of the cytoskeletal and cell cycle machinery. In this chapter, we provide an overview of these processes from pheromone perception and mating to gall production and sporulation in planta. Emphasis is placed on the genetic determinants of morphogenesis and pathogenic development of U. maydis and on the fungus-host interaction. Additionally, we review advances in the development of tools to study U. maydis, including the recently available genome sequence. We conclude with a brief assessment of current challenges and future directions for the genetic study of U. maydis.

Pathocycles: Ustilago maydis as a model to study the relationships between cell cycle and virulence in pathogenic fungi

Activation of virulence in pathogenic fungi often involves differentiation processes that need the reset of the cell cycle and induction of a new morphogenetic program. Therefore, the fungal capability to modify its cell cycle constitutes an important determinant in carrying out a successful infection. The dimorphic fungus Ustilago maydis is the causative agent of corn smut disease and has lately become a highly attractive model in addressing fundamental questions about development in pathogenic fungi. The different morphological and genetic changes of U. maydis cells during the pathogenic process advocate an accurate control of the cell cycle in these transitions. This is why this model pathogen deserves attention as a powerful tool in analyzing the relationships between cell cycle, morphogenesis, and pathogenicity. The aim of this review is to summarize recent advances in the unveiling of cell cycle regulation in U. maydis. We also discuss the connection between cell cycle and virulence and how cell cycle control is an important downstream target in the fungus-plant interaction.

The cdc25 phosphatase is essential for the G2/M phase transition in the basidiomycete yeast Ustilago maydis

Cdc25-related phosphatases reverse the inhibitory phosphorylation of mitotic Cyclin-dependent kinases mediated by Wee1-related kinases, thereby promoting entry into mitosis. In the fission yeast, Schizosaccharomyces pombe, Cdc25 is required for entry into mitosis, while in the budding yeast Saccharomyces cerevisiae, Mih1 (the homologue of Cdc25) is not required for entry into mitosis or for viability. As these differences were linked to the different cell division and growth mechanism of these species, we sought to analyse the roles of Cdc25 in Ustilago maydis, which as S. cerevisiae divides by budding, but relies in a polar growth. This basidiomycete yeast is perfectly suited to analyse the relationships between cell cycle and morphogenesis. We show that U. maydis contains a single Cdc25-related protein, which is essential for growth. Loss of Cdc25 function results in a specific G2 arrest that correlated with high level of Tyr15 phosphorylation of Cdk1. Moreover, we show genetic interactions of cdc25 with wee1 and clb2 that support the notion that in U. maydis Cdc25 counteracts the Wee1-mediated inhibitory phosphorylation of Cdk1-Clb2 complex. Our results supports a model in which inhibitory phosphorylation of Cdk1 is a primary mechanism operating at G2/M transition in this fungus.