Calcofluor white hypersensitive proteins contribute to stress tolerance and pathogenicity in entomopathogenic fungus, Metarhizium acridum

The APSES transcription factor CfSwi6 is required for growth, cell wall integrity, and pathogenicity of Ceratocystis fimbriata

Cell wall integrity (CWI) is crucial for the growth, development, and host invasion of pathogenic fungi. The APSES transcription factor Swi6 in fungi plays a role in mediating cell wall integrity through the mitogen-activated protein kinase (MAPK) signaling pathway. Ceratocystis fimbriata is a notorious pathogenic fungus responsible for causing black rot in sweet potatoes. In this study, an orthologous APSES transcription factor Swi6 (CfSwi6) downstream of the CWI regulatory pathway in C. fimbriata was characterized. Deletion of CfSWI6 leads to impaired hyphal development, conidiation, and compromised cell wall integrity, resulting in a significant reduction in virulence. Transcriptome analysis revealed the involvement of CfSWI6 in various pathways, including the MAPK pathway, DNA synthesis and stress response. ChIP-seq data provided predictions of potential target genes regulated by CfSwi6. Through yeast one-hybrid, we confirmed the direct binding of CfSwi6 to the promoter of the chitin synthetase gene. In summary, these findings indicated that CfSwi6 plays an important role in the growth, development, and pathogenicity of C. fimbriata. This study provides new insights into the pathogenic mechanism of C. fimbriata in sweet potato and inspires potential strategies to control sweet potato black rot.

Reduction of histone proteins dosages increases CFW sensitivity and attenuates virulence of Candida albicans

Histone proteins are important components of nucleosomes, which play an important role in regulating the accessibility of DNA and the function of genomes. However, the effect of histone proteins dosages on physiological processes is not clear in the human fungal pathogen Candida albicans. In this study, we found that the deletion of the histone protein H3 coding gene HHT21 and the histone protein H4 coding gene HHF1 resulted in a significant decrease in the expression dosage of the histone proteins H3 and H4, which had a significant impact on the localization of the histone protein H2A and plasmid maintenance. Stress sensitivity experiments showed that the mutants hht21Δ/Δ, hhf1Δ/Δ and hht21Δ/Δhhf1Δ/Δ were more sensitive to cell wall stress induced by Calcofluor White (CFW) than the wild-type strain. Further studies showed that the decrease in the dosage of the histone proteins H3 and H4 led to the change of cell wall components, increased chitin contents, and down-regulated expression of the SAP9, KAR2, and CRH11 genes involved in the cell wall integrity (CWI) pathway. Overexpression of SAP9 could rescue the sensitivity of the mutants to CFW. Moreover, the decrease in the histone protein s dosages affected the FAD-catalyzed oxidation of Ero1 protein, resulting in the obstruction of protein folding in the ER, and thus reduced resistance to CFW. It was also found that CFW induced a large amount of ROS accumulation in the mutants, and the addition of ROS scavengers could restore the growth of the mutants under CFW treatment. In addition, the reduction of the histone proteins dosages greatly weakened systemic infection and kidney fungal burden in mice, and hyphal development was significantly impaired in the mutants under macrophage treatment, indicating that the histone proteins dosages is very important for the virulence of C. albicans. This study revealed that histone proteins dosages play a key role in the cell wall stress response and pathogenicity in C. albicans.

Immunization with a heat-killed prm1 deletion strain protects the host from Cryptococcus neoformans infection

Systemic infection with Cryptococcus neoformans, a dangerous and contagious pathogen found throughout the world, frequently results in lethal cryptococcal pneumonia and meningoencephalitis, and no effective treatments and vaccination of cryptococcosis are available. Here, we describe Prm1, a novel regulator of C. neoformans virulence. C. neoformans prm1Δ cells exhibit extreme sensitivity to various environmental stress conditions. Furthermore, prm1Δ cells show deficiencies in the biosynthesis of chitosan and mannoprotein, which in turn result in impairment of cell wall integrity. Treatment of mice with heat-killed prm1Δ cells was found to facilitate the host immunological defence against infection with wild-type C. neoformans. Further investigation demonstrated that prm1Δ cells strongly promote pulmonary production of interferon-γ, leading to activation of macrophage M1 differentiation and inhibition of M2 polarization. Therefore, our findings suggest that C. neoformans Prm1 may be a viable target for the development of anti-cryptococcosis medications and, cells lacking Prm1 represent a promising candidate for a vaccine.

Symbiotic Bacteria System of Locusta migratoria Showed Antifungal Capabilities against Beauveria bassiana

The stability of symbiotic flora is an important indicator of the health of an organism. Symbiotic bacteria have been proven to be closely involved in the immune process of organisms. The pathogenicity of Beauveria bassiana was studied in relation to symbiotic bacteria on the surface and inside of the migratory locust (Locusta migratoria). The results showed that the surface disinfection of test locusts contributed to the pathogenicity of B. bassiana to locusts. Most of the surface bacteria of L. migratoria caused some inhibition of B. bassiana growth, and LM5-4 (Raoultella ornithinolytica), LM5-2 (Enterobacter aerogenes), and LM5-13 (Citrobacter freundii) showed the highest inhibitory effect on the growth of B. bassiana. The inoculation of locusts with additional surface symbiotic bacteria reduced the virulence of B. bassiana to L. migratoria. Infection by different strains of B. bassiana caused similar changes in the symbiotic flora of migratory locusts. The inoculation of locusts with additional intestinal symbiotic bacteria (Enterobacter sp.) reduced the virulence of B. bassiana to L. migratoria. These findings illustrate the effect of bacterial communities on fungal infections in L. migratoria when seen from the perspective of ecology in a microenvironment. The active antifungal substances of such bacteria and their mechanisms of action need further study.

Host–Pathogen Interactions between Metarhizium spp. and Locusts

The progress in research on the interactions between Metarhizium spp. and locusts has improved our understanding of the interactions between fungal infection and host immunity. A general network of immune responses has been constructed, and the pathways regulating fungal pathogenicity have also been explored in depth. However, there have been no systematic surveys of interaction between Metarhizium spp. and locusts. The pathogenesis of Metarhizium comprises conidial attachment, germination, appressorial formation, and colonization in the body cavity of the host locusts. Meanwhile, the locust resists fungal infection through humoral and cellular immunity. Here, we summarize the crucial pathways that regulate the pathogenesis of Metarhizium and host immune defense. Conidial hydrophobicity is mainly affected by the contents of hydrophobins and chitin. Appressorial formation is regulated by the pathways of MAPKs, cAMP/PKA, and Ca²⁺/calmodulin. Lipid droplets degradation and secreted enzymes contributed to fungal penetration. The humoral response of locust is coordinated by the Toll pathway and the ecdysone. The regulatory mechanism of hemocyte differentiation and migration is elusive. In addition, behavioral fever and density-dependent population immunity have an impact on the resistance of hosts against fungal infection. This review depicts a prospect to help us understand host–pathogen interactions and provides a foundation for the engineering of entomopathogenic fungi and the discovery of insecticidal targets to control insect pests.

Photobiology of the keystone genus Metarhizium

Metarhizium fungi are soil-inhabiting ascomycetes which are saprotrophs, symbionts of plants, pathogens of insects, and participate in other trophic/ecological interactions, thereby performing multiple essential ecosystem services. Metarhizium species are used to control insect pests of crop plants and insects that act as vectors of human and animal diseases. To fulfil their functions in the environment and as biocontrol agents, these fungi must endure cellular stresses imposed by the environment, one of the most potent of which is solar ultraviolet (UV) radiation. Here, we examine the cellular stress biology of Metarhizium species in context of their photobiology, showing how photobiology facilitates key aspects of their ecology as keystone microbes and as mycoinsectides. The biophysical basis of UV-induced damage to Metarhizium, and mechanistic basis of molecular and cellular responses to effect damage repair, are discussed and interpreted in relation to the solar radiation received on Earth. We analyse the interplay between UV and visible light and how the latter increases cellular tolerance to the former via expression of a photolyase gene. By integrating current knowledge, we propose the mechanism through which Metarhizium species use the visible fraction of (low-UV) early-morning light to mitigate potentially lethal damage from intense UV radiation later in the day. We also show how this mechanism could increase Metarhizium environmental persistence and improve its bioinsecticide performance. We discuss the finding that visible light modulates stress biology in the context of further work needed on Metarhizium ecology in natural and agricultural ecosystems, and as keystone microbes that provide essential services within Earth's biosphere.

The Transient Receptor Potential Channel Yvc1 Deletion Recovers the Growth Defect of Calcineurin Mutant Under Endoplasmic Reticulum Stress in Candida albicans

Transient receptor potential (TRP) channel Yvc1 was related with hyphal growth, oxidative stress response, and pathogenicity. Calcineurin subunit Cnb1 was activated immediately in yeasts when exposed to severe stimulation. However, the relationship between Yvc1 and Cnb1-governed calcium ions and endoplasmic reticulum (ER) stress response remains unrevealed. In this study, we found that the mutant cnb1Δ/Δ was sensitive to TN, which was related with the overexpression of membrane calcium ion channels that could increase the cytosol calcium concentration. However, the growth of the cnb1Δ/Δyvc1Δ/Δ mutant was recovered and its cell vitality was better than the cnb1Δ/Δ strain. Meanwhile, the cellular calcium concentration was decreased and its fluctuation was weakened under ER stress in the cnb1Δ/Δyvc1Δ/Δ strain. To verify the regulation role of Yvc1 in the calcium concentration, we found that the addition of CaCl₂ led to the worse viability, while the growth state was relieved under the treatment of EGTA in the cnb1Δ/Δ strain. In conclusion, the deletion of YVC1 could reduce the cellular calcium and relieve the ER stress sensitivity of the cnb1Δ/Δ strain. Thereby, our findings shed a novel light on the relationship between the Yvc1-governed cellular calcium concentration and ER stress response in C. albicans.

MaAreB, a GATA Transcription Factor, Is Involved in Nitrogen Source Utilization, Stress Tolerances and Virulence in Metarhizium acridum

The nitrogen catabolite repression (NCR) pathway is involved in nitrogen utilization, in which the global GATA transcription factor AreA plays an indispensable role and has been reported in many fungi. However, relatively few studies are focused on AreB, another GATA transcription factor in the NCR pathway and the functions of AreB are largely unknown in entomopathogenic fungi. Here, we characterized MaAreB in the model entomopathogenic fungus Metarhizium acridum. Sequence arrangement found that MaAreB had a conserved GATA zinc finger DNA binding domain and a leucine zipper domain. Disruption of MaAreB affected the nitrogen utilization and led to decelerated conidial germination and hyphal growth, decreased conidial yield, and lower tolerances to UV-B irradiation and heat-shock. Furthermore, the MaAreB mutant (ΔMaAreB) exhibited increased sensitivity to CFW (Calcofluor white), decreased cell wall contents (chitin and β-1,3-glucan) and reduced expression levels of some genes related to cell wall integrity, indicating that disruption of MaAreB affected the cell wall integrity. Bioassays showed that the virulence of the ΔMaAreB strain was decreased in topical inoculation but not in intra-hemocoel injection. Consistently, deletion of MaAreB severely impaired the appressorium formation and reduced the turgor pressure of appressorium. These results revealed that MaAreB regulated fungal nitrogen utilization, cell wall integrity and biological control potential, which would contribute to the functional characterization of AreB homologous proteins in other insect fungal pathogens, and even filamentous fungi.