Hex1, the Major Component of Woronin Bodies, Is Required for Normal Development, Pathogenicity, and Stress Response in the Plant Pathogenic Fungus Verticillium dahliae

Role of cell membrane homeostasis in the pathogenicity of pathogenic filamentous fungi

The cell membrane forms a fundamental part of all living cells and participates in a variety of physiological processes, such as material exchange, stress response, cell recognition, signal transduction, cellular immunity, apoptosis, and pathogenicity. Here, we review the mechanisms and functions of the membrane structure (lipid components of the membrane and the biosynthesis of unsaturated fatty acids), membrane proteins (transmembrane proteins and proteins contributing to membrane curvature), transcriptional regulation, and cell wall components that influence the virulence and pathogenicity of filamentous fungi.

Components of TOR and MAP kinase signaling control chemotropism and pathogenicity in the fungal pathogen Verticillium dahliae

Filamentous fungi can sense useful resources and hazards in their environment and direct growth of their hyphae accordingly. Chemotropism ensures access to nutrients, contact with other individuals (e.g., for mating), and interaction with hosts in the case of pathogens. Previous studies have revealed a complex chemotropic sensing landscape during host-pathogen interactions, but the underlying molecular machinery remains poorly characterized. Here we studied mechanisms controlling directed hyphal growth of the important plant-pathogenic fungus Verticillium dahliae towards different chemoattractants. We found that the homologs of the Rag GTPase Gtr1 and the GTPase-activating protein Tsc2, an activator and a repressor of the TOR kinase respectively, play important roles in hyphal chemotropism towards nutrients, plant-derived signals, and heterologous α-pheromone of Fusarium oxysporum. Furthermore, important roles of these regulators were identified in fungal development and pathogenicity. We also found that the mitogen-activated protein kinase (MAPK) Fus3 is required for chemotropism towards nutrients, while the G protein-coupled receptor (GPCR) Ste2 and the MAPK Slt2 control chemosensing of plant-derived signals and α-pheromone. Our study establishes V. dahliae as a suitable model system for the analysis of fungal chemotropism and discovers new components of chemotropic signaling during growth and host-pathogen interactions of V. dahliae.

The NADPH Oxidase A of Verticillium dahliae Is Essential for Pathogenicity, Normal Development, and Stress Tolerance, and It Interacts with Yap1 to Regulate Redox Homeostasis

Maintenance of redox homeostasis is vital for aerobic organisms and particularly relevant to plant pathogens. A balance is required between their endogenous ROS production, which is important for their development and pathogenicity, and host-derived oxidative stress. Endogenous ROS in fungi are generated by membrane-bound NADPH oxidase (NOX) complexes and the mitochondrial respiratory chain, while transcription factor Yap1 is a major regulator of the antioxidant response. Here, we investigated the roles of NoxA and Yap1 in fundamental biological processes of the important plant pathogen Verticillium dahliae. Deletion of noxA impaired growth and morphogenesis, compromised formation of hyphopodia, diminished penetration ability and pathogenicity, increased sensitivity against antifungal agents, and dysregulated expression of antioxidant genes. On the other hand, deletion of yap1 resulted in defects in conidial and microsclerotia formation, increased sensitivity against oxidative stress, and down-regulated antioxidant genes. Localized accumulation of ROS was observed before conidial fusion and during the heterokaryon incompatibility reaction upon nonself fusion. The frequency of inviable fusions was not affected by the deletion of Yap1. Analysis of a double knockout mutant revealed an epistatic relationship between noxA and yap1. Our results collectively reveal instrumental roles of NoxA and ROS homeostasis in the biology of V. dahliae.

Starvation-induced cell fusion and heterokaryosis frequently escape imperfect allorecognition systems in an asexual fungal pathogen

Background

Asexual fungi include important pathogens of plants and other organisms, and their effective management requires understanding of their evolutionary dynamics. Genetic recombination is critical for adaptability and could be achieved via heterokaryosis — the co-existence of genetically different nuclei in a cell resulting from fusion of non-self spores or hyphae — and the parasexual cycle in the absence of sexual reproduction. Fusion between different strains and establishment of viable heterokaryons are believed to be rare due to non-self recognition systems. Here, we investigate the extent and mechanisms of cell fusion and heterokaryosis in the important asexual plant pathogen Verticillium dahliae.

Results

We used live-cell imaging and genetic complementation assays of tagged V. dahliae strains to analyze the extent of non-self vegetative fusion, heterokaryotic cell fate, and nuclear behavior. An efficient CRISPR/Cas9-mediated system was developed to investigate the involvement of autophagy in heterokaryosis. Under starvation, non-self fusion of germinating spores occurs frequently regardless of the previously assessed vegetative compatibility of the partners. Supposedly “incompatible” fusions often establish viable heterokaryotic cells and mosaic mycelia, where nuclei can engage in fusion or transfer of genetic material. The molecular machinery of autophagy has a protective function against the destruction of “incompatible” heterokaryons.

Conclusions

We demonstrate an imperfect function of somatic incompatibility systems in V. dahliae. These systems frequently tolerate the establishment of heterokaryons and potentially the initiation of the parasexual cycle even between strains that were previously regarded as “incompatible.”

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01101-5.

Establishment of conidial fusion in the asexual fungus Verticillium dahliae as a useful system for the study of non-sexual genetic interactions

Cell-to-cell fusion is a fundamental biological process across the tree of life. In filamentous fungi, somatic fusion (or anastomosis) is required for the normal development of their syncytial hyphal networks, and it can initiate non-sexual genetic exchange processes, such as horizontal genetic transfer and the parasexual cycle. Although these could be important drivers of the evolution of asexual fungi, this remains a largely unexplored possibility due to the lack of suitable resources for their study in these puzzling organisms. We thus aimed at the characterization of cell fusion in the important asexual fungus Verticillium dahliae via Conidial Anastomosis Tubes (CATs), which can be useful for the analysis of parasexuality. We optimized appropriate procedures for their highly reproducible quantification and live-cell imaging, which were used to characterize their physiology and cell biology, and to start elucidating their underlying genetic machinery. Formation of CATs was shown to depend on growth conditions and require functional Fus3 and Slt2 MAP kinases, as well as the NADPH oxidase NoxA, whereas the GPCR Ste2 and the mating-type protein MAT1-2-1 were dispensable. We show that nuclei and other organelles can migrate through CATs, which often leads to the formation of transient dikaryons. Their nuclei have possible windows of opportunity for genetic interaction before degradation of one by a presumably homeostatic mechanism. We establish here CAT-mediated fusion in V. dahliae as an experimentally convenient system for the cytological analysis of fungal non-sexual genetic interactions. We expect that it will facilitate the dissection of sexual alternatives in asexual fungi.