Saline stress affects the pH-dependent regulation of the transcription factor PacC in the dermatophyte Trichophyton interdigitale

Transcription factors SltA and CrzA reversely regulate calcium homeostasis under calcium-limited conditions

IMPORTANCE

Calcium ions are ubiquitous intracellular signaling molecules for many signaling pathways regulating the fungal response to stress and antifungal drugs. The concentration of intracellular calcium is tightly regulated in its storage, release, and distribution. CrzA is the best-studied transcription factor that regulates this process under sufficient calcium or other external signals. However, CrzA was excluded from nuclei and then lost transcriptional activation under calcium-limited conditions. The regulators in the Ca2+ signaling pathway under calcium-limited conditions remain unclear. Here, we identified SltA as a key regulator in the Ca2+ signaling pathway under calcium-limited conditions, and the underlying mechanisms were further explored in Aspergillus fumigatus. These findings reveal a transcriptional control pathway that precisely regulates calcium homeostasis under calcium-limited conditions.

Phenotypic and Genomic Difference among Four Botryosphaeria Pathogens in Chinese Hickory Trunk Canker

Botryosphaeria species are amongst the most widespread and important canker and dieback pathogens of trees worldwide, with B. dothidea as one of the most common Botryosphaeria species. However, the information related to the widespread incidence and aggressiveness of B. dothidea among various Botryosphaeria species causing trunk cankers is still poorly investigated. In this study, the metabolic phenotypic diversity and genomic differences of four Chinese hickory canker-related Botryosphaeria pathogens, including B. dothidea, B. qingyuanensis, B. fabicerciana, and B. corticis, were systematically studied to address the competitive fitness of B. dothidea. Large-scale screening of physiologic traits using a phenotypic MicroArray/OmniLog system (PMs) found B. dothidea has a broader spectrum of nitrogen source and greater tolerance toward osmotic pressure (sodium benzoate) and alkali stress among Botryosphaeria species. Moreover, the annotation of B. dothidea species-specific genomic information via a comparative genomics analysis found 143 B. dothidea species-specific genes that not only provides crucial cues in the prediction of B. dothidea species-specific function but also give a basis for the development of a B. dothidea molecular identification method. A species-specific primer set Bd_11F/Bd_11R has been designed based on the sequence of B. dothidea species-specific gene jg11 for the accurate identification of B. dothidea in disease diagnoses. Overall, this study deepens the understanding in the widespread incidence and aggressiveness of B. dothidea among various Botryosphaeria species, providing valuable clues to assist in trunk cankers management.

A Dual-Plasmid-Based CRISPR/Cas9-Mediated Strategy Enables Targeted Editing of pH Regulatory Gene pacC in a Clinical Isolate of Trichophyton rubrum

Trichophyton rubrum is the most prevalent causative agent responsible for 80-90% of all known superficial fungal infections in humans, worldwide. Limited available methods for genetic manipulations have been one of the major bottlenecks in understanding relevant molecular mechanisms of disease pathogenesis in T. rubrum. Here, a dual-plasmid-based CRISPR/Cas9 strategy to edit pH regulatory transcription factor, pacC, of a clinical isolate of T. rubrum by non-homologous end joining (NHEJ) repair is presented. A cas9-eGFP fusion that aids pre-screening of primary transformants through detection of GFP fluorescence is expressed from one plasmid while target-specific sgRNA from the other brings about mutagenesis of pacC with an overall efficiency of 33.8-37.3%. The mutants had reduced transcript levels of pacC at both acidic and alkaline pH with several morphological abnormalities. We believe this dual-plasmid-based CRISPR/Cas9 strategy will aid functional genomics studies, especially in non-lab-adapted clinical strains of T. rubrum.

The bZIP Ap1 transcription factor is a negative regulator of virulence attributes of the anthropophilic dermatophyte Trichophyton rubrum

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The ap1 gene negatively regulates virulence traits in Trichophyton rubrum.

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Ap1 regulates T. rubrum growth in keratin, playing a vital role in nail infection.

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Ap1 may contribute to the chronicity of onychomycosis.

Trichophyton rubrum is a fungus that causes chronic skin and nail infections in healthy individuals and immunocompromised patients. During infection, T. rubrum invades host cutaneous tissues by adapting to the acidic pH and the innate immune response of the host. Several genes are upregulated during the growth of T. rubrum in substrates found in human tissue, including the ap1 gene, which codes for the transcription factor Ap1. Here, we generated a null mutant strain by deleting the T. rubrum ap1 gene and performed a functional analysis of this gene. Our results showed that the Δap1mutant increased its growth in nail fragments and co-cultures with keratinocytes compared to the wild type. Furthermore, the mutant displayed hyperpigmentation, thickening of the conidia cell wall, increased conidia susceptibility to calcofluor-white compared to the wild type, and loss of control of the keratinolytic activity. Although the ap1 gene was upregulated during exposure to the antifungal drugs amphotericin B, nystatin, and terbinafine, its deletion did not alter the fungal susceptibility to these drugs, revealing the role of the ap1 gene in the physiological response to the stress caused by these drugs, but not in their resistance. Moreover, ap1 was also involved in the oxidative stress response caused by menadione, but not paraquat or hydrogen peroxide. These findings indicate that the ap1 gene plays a role in the negative control of virulence-related attributes and may contribute to the chronicity of nail infection caused by T. rubrum.

Organic acid, a virulence factor for pathogenic fungi, causing postharvest decay in fruits

Decay due to fungal infection is a major cause of postharvest losses in fruits. Acidic fungi may enhance their virulence by locally reducing the pH of the host. Several devastating postharvest fungi, such as Penicillium spp., Botrytis cinerea, and Sclerotinia sclerotiorum, can secrete gluconic acid, oxalic acid, or citric acid. Emerging evidence suggests that organic acids secreted by acidic fungi are important virulence factors. In this review, we summarized the research progress on the biosynthesis of organic acids, the role of the pH signalling transcription factor PacC in regulating organic acid, and the action mechanism of the main organic acid secreted via postharvest pathogenic fungi during infection of host tissues. This paper systematically demonstrates the relationships between tissue acidification and postharvest fungal pathogenicity, which will motivate the study of host-pathogen interactions and provide a better understanding of virulence mechanisms of the pathogens so as to design new technical strategies to prevent postharvest diseases.

State-of-the-Art Dermatophyte Infections: Epidemiology Aspects, Pathophysiology, and Resistance Mechanisms

The burden of fungal infections is not widely appreciated. Although these infections are responsible for over one million deaths annually, it is estimated that one billion people are affected by severe fungal diseases. Mycoses of nails and skin, primarily caused by fungi known as dermatophytes, are the most common fungal infections. Trichophyton rubrum appears to be the most common causative agent of dermatophytosis, followed by Trichophyton interdigitale. An estimated 25% of the world’s population suffers from dermatomycosis. Although these infections are not lethal, they compromise the quality of life of infected patients. The outcome of antidermatophytic treatments is impaired by various conditions, such as resistance and tolerance of certain dermatophyte strains. The adage “know your enemy” must be the focus of fungal research. There is an urgent need to increase awareness about the significance of these infections with precise epidemiological data and to improve knowledge regarding fungal biology and pathogenesis, with an emphasis on adaptive mechanisms to tackle adverse conditions from host counteractions. This review outlines the current knowledge about dermatophyte infections, with a focus on signaling pathways required for fungal infection establishment and a broad perspective on cellular and molecular factors involved in antifungal resistance and tolerance.