Fungal Cell Cycle: A Unicellular versus Multicellular Comparison

Gamma-irradiated Aspergillus conidia show a growth curve with a reproductive death phase

In this study, we evaluated the effects of gamma irradiation on the germination of Aspergillus conidia and mycelial growth using microscopy and predictive microbiological modeling methods. A dose of 0.4 kGy reduced the germination rate by 20% compared to the untreated control, indicating interphase death due to the high radiation dose. The number of colonies formed (5.5%) was lower than the germination rate (69%), suggesting that most colonies died after germination. Microscopic observations revealed that mycelial elongation ceased completely in the middle of the growth phase, indicating reproductive death. The growth curves of irradiated conidia exhibited a delayed change in the growth pattern, and a decrease in slope during the early stages of germination and growth at low densities. A modified logistic model, which is a general purpose growth model that allows for the evaluation of subpopulations, was used to fit the experimental growth curves. Dose-dependent waveform changes may reflect the dynamics of the subpopulations during germination and growth. These methods revealed the occurrence of two cell death populations resulting from gamma irradiation of fungal conidia and contribute to the understanding of irradiation-induced cell death in fungi.

Aspergillus SUMOylation mutants exhibit chromosome segregation defects including chromatin bridges

Functions of protein SUMOylation remain incompletely understood in different cell types. Via forward genetics, here we identified ubaBQ247*, a loss-of-function mutation in a SUMO activation enzyme UbaB in the filamentous fungus Aspergillus nidulans. The ubaBQ247*, ΔubaB, and ΔsumO mutants all produce abnormal chromatin bridges, indicating the importance of SUMOylation in the completion of chromosome segregation. The bridges are enclosed by nuclear membrane containing peripheral nuclear pore complex proteins that normally get dispersed during mitosis, and the bridges are also surrounded by cytoplasmic microtubules typical of interphase cells. Time-lapse sequences further indicate that most bridges persist through interphase prior to the next mitosis, and anaphase chromosome segregation can produce new bridges that persist into the next interphase. When the first mitosis happens at a higher temperature of 42°C, SUMOylation deficiency produces not only chromatin bridges but also many abnormally shaped single nuclei that fail to divide. UbaB-GFP localizes to interphase nuclei just like the previously studied SumO-GFP, but the nuclear signals disappear during mitosis when the nuclear pores are partially open, and the signals reappear after mitosis. The nuclear localization is consistent with many SUMO targets being nuclear proteins. Finally, although the budding yeast SUMOylation machinery interacts with LIS1, a protein critical for dynein activation, loss of SUMOylation does not cause any obvious defect in dynein-mediated transport of nuclei and early endosomes, indicating that SUMOylation is unnecessary for dynein activation in A. nidulans.

The biological relevance of the FspTF transcription factor, homologous of Bqt4, in Fusarium sp. associated with the ambrosia beetle Xylosandrus morigerus

Transcription factors in phytopathogenic fungi are key players due to their gene expression regulation leading to fungal growth and pathogenicity. The KilA-N family encompasses transcription factors unique to fungi, and the Bqt4 subfamily is included in it and is poorly understood in filamentous fungi. In this study, we evaluated the role in growth and pathogenesis of the homologous of Bqt4, FspTF, in Fusarium sp. isolated from the ambrosia beetle Xylosandrus morigerus through the characterization of a CRISPR/Cas9 edited strain in Fsptf. The phenotypic analysis revealed that TF65-6, the edited strain, modified its mycelia growth and conidia production, exhibited affectation in mycelia and culture pigmentation, and in the response to certain stress conditions. In addition, the plant infection process was compromised. Untargeted metabolomic and transcriptomic analysis, clearly showed that FspTF may regulate secondary metabolism, transmembrane transport, virulence, and diverse metabolic pathways such as lipid metabolism, and signal transduction. These data highlight for the first time the biological relevance of an orthologue of Bqt4 in Fusarium sp. associated with an ambrosia beetle.

Aspergillus SUMOylation mutants have normal dynein function but exhibit chromatin bridges

Functions of protein SUMOylation remain incompletely understood in different cell types. The budding yeast SUMOylation machinery interacts with LIS1, a protein critical for dynein activation, but dynein-pathway components were not identified as SUMO-targets in the filamentous fungus Aspergillus nidulans. Via A. nidulans forward genetics, here we identified ubaBQ247*, a loss-of-function mutation in a SUMO-activation enzyme UbaB. Colonies of the ubaBQ247*, ΔubaB and ΔsumO mutants looked similar and less healthy than the wild-type colony. In these mutants, about 10% of nuclei are connected by abnormal chromatin bridges, indicating the importance of SUMOylation in the completion of chromosome segregation. Nuclei connected by chromatin bridges are mostly in interphase, suggesting that these bridges do not prevent cell-cycle progression. UbaB-GFP localizes to interphase nuclei just like the previously studied SumO-GFP, but the nuclear signals disappear during mitosis when the nuclear pores are partially open, and the signals reappear after mitosis. The nuclear localization is consistent with many SUMO-targets being nuclear proteins, for example, topoisomerase II whose SUMOylation defect gives rise to chromatin bridges in mammalian cells. Unlike in mammalian cells, however, loss of SUMOylation in A. nidulans does not apparently affect the metaphase-to-anaphase transition, further highlighting differences in the requirements of SUMOylation in different cell types. Finally, loss of UbaB or SumO does not affect dynein- and LIS1-mediated early-endosome transport, indicating that SUMOylation is unnecessary for dynein or LIS1 function in A. nidulans.

Transcription Activator Swi6 Interacts with Mbp1 in MluI Cell Cycle Box-Binding Complex and Regulates Hyphal Differentiation and Virulence in Beauveria bassiana

Mbp1 protein acts as a DNA-binding protein in MluI cell cycle box-binding complex (MBF) and plays an essential role in filamentous myco-pathogen Beauveria bassiana.In the current study, BbSwi6 (a homologue of yeast Swi6) was functionally characterized in B.bassiana. Both BbSwi6 and BbMbp1 localize in the nucleus and display a direct interaction relationship which is indicated by a yeast two-hybrid assay. BbSwi6 significantly contributes to hyphal growth, asexual sporulation and virulence. On the aerial surface, ΔBbSwi6 grew slower on various nutrients and displayed abnormal conidia-producing structures, which hardly produced conidia. In liquid media, BbSwi6 loss led to 90% reduction in blastospore yield. Finally, the virulence of the ΔBbSwi6 mutant was modestly weakened with a reduction of 20% in median lethal time. Comparative transcriptomics revealed that BbSwi6 mediated different transcriptomes during fungal development into conidia and blastospores. Notably, under the indicated condition, the BbSwi6-mediated transcriptome significantly differed to that mediated by BbMbp1. Our results demonstrate that, in addition to their roles as the interactive components in MBF, BbSwi6 and BbMbp1 mediate divergent genetic pathways during morphological transitions in B. bassiana.

Aspergillus fumigatus, One Uninucleate Species with Disparate Offspring

Establishment of a fungal infection due to Aspergillus fumigatus relies on the efficient germination of the airborne conidia once they penetrate the respiratory tract. However, the features of conidial germination have been poorly explored and understood in this fungal species as well as in other species of filamentous fungi. We show here that the germination of A. fumigatus is asynchronous. If the nutritional environment and extensive gene deletions can modify the germination parameters for A. fumigatus, the asynchrony is maintained in all germinative conditions tested. Even though the causes for this asynchrony of conidial germination remain unknown, asynchrony is essential for the completion of the biological cycle of this filamentous fungus.

Fungal oxylipins direct programmed developmental switches in filamentous fungi

Filamentous fungi differentiate along complex developmental programs directed by abiotic and biotic signals. Currently, intrinsic signals that govern fungal development remain largely unknown. Here we show that an endogenously produced and secreted fungal oxylipin, 5,8-diHODE, induces fungal cellular differentiation, including lateral branching in pathogenic Aspergillus fumigatus and Aspergillus flavus, and appressorium formation in the rice blast pathogen Magnaporthe grisea. The Aspergillus branching response is specific to a subset of oxylipins and is signaled through G-protein coupled receptors. RNA-Seq profiling shows differential expression of many transcription factors in response to 5,8-diHODE. Screening of null mutants of 33 of those transcription factors identifies three transcriptional regulators that appear to mediate the Aspergillus branching response; one of the mutants is locked in a hypo-branching phenotype, while the other two mutants display a hyper-branching phenotype. Our work reveals an endogenous signal that triggers crucial developmental processes in filamentous fungi, and opens new avenues for research on the morphogenesis of filamentous fungi.

Fungi produce oxygenated fatty acids, or oxylipins, of unclear function. Here, Niu et al. show that an Aspergillus oxylipin induces various developmental processes in several fungi, including lateral branching in human pathogenic Aspergillus species, and appressorium formation in the plant pathogen Magnaporthe grisea.

Molecular basis of resistance to the microtubule-depolymerizing antitumor compound plocabulin

Plocabulin (PM060184) is a microtubule depolymerizing agent with potent antiproliferative activity undergoing phase II clinical trials for the treatment of solid tumors. Plocabulin shows antifungal activity virtually abolishing growth of the filamentous fungus Aspergillus nidulans. A. nidulans hyphae depend both on mitotic and interphase microtubules, as human cells. Here, we exploited the A. nidulans genetic amenability to gain insight into the mechanism of action of plocabulin. By combining mutations in the two A. nidulans β-tubulin isotypes we obtained a plocabulin-insensitive strain, showing that β-tubulin is the only molecular target of plocabulin in fungal cells. From a genetic screen, we recovered five mutants that show plocabulin resistance but do not carry mutations in β-tubulin. Resistance mutations resulted in amino acid substitutions in (1) two subunits of the eukaryotic translation initiation factor eIF2B activating the General Amino Acid Control, (2) TIM44, an essential component of the inner mitochondrial membrane translocase, (3) two transcription factors of the binuclear zinc cluster family potentially interfering with the uptake or efflux of plocabulin. Given the conservation of some of the identified proteins and their respective cellular functions in the tumor environment, our results pinpoint candidates to be tested as potential biomarkers for determination of drug efficiency.