Nitrate assimilation pathway (NAP): role of structural (nit) and transporter (ntr1) genes in Fusarium oxysporum f.sp. lycopersici growth and pathogenicity

MaNrtB, a Putative Nitrate Transporter, Contributes to Stress Tolerance and Virulence in the Entomopathogenic Fungus Metarhizium acridum

Nitrogen is an essential nutrient that frequently determines the growth rate of fungi. Nitrate transporter proteins (Nrts) play a crucial role in the cellular absorption of nitrate from the environment. Entomopathogenic fungi (EPF) have shown their potential in the biological control of pests. Thus, comprehending the mechanisms that govern the pathogenicity and stress tolerance of EPF is helpful in improving the effectiveness and practical application of these fungal biocontrol agents. In this study, we utilized homologous recombination to create MaNrtB deletion mutants and complementation strains. We systematically investigated the biological functions of the nitrate transporter protein gene MaNrtB in M. acridum. Our findings revealed that the disruption of MaNrtB resulted in delayed conidial germination without affecting conidial production. Stress tolerance assays demonstrated that the MaNrtB disruption strain was more vulnerable to UV-B irradiation, hyperosmotic stress, and cell wall disturbing agents, yet it exhibited increased heat resistance compared to the wild-type strain. Bioassays on the locust Locusta migratoria manilensis showed that the disruption of MaNrtB impaired the fungal virulence owing to the reduced appressorium formation on the insect cuticle and the attenuated growth in the locust hemolymph. These findings provide new perspectives for understanding the pathogenesis of EPF.

Comparative genomics of Metarhizium brunneum strains V275 and ARSEF 4556: unraveling intraspecies diversity

Entomopathogenic fungi belonging to the Order Hypocreales are renowned for their ability to infect and kill insect hosts, while their endophytic mode of life and the beneficial rhizosphere effects on plant hosts have only been recently recognized. Understanding the molecular mechanisms underlying their different lifestyles could optimize their potential as both biocontrol and biofertilizer agents, as well as the wider appreciation of niche plasticity in fungal ecology. This study describes the comprehensive whole genome sequencing and analysis of one of the most effective entomopathogenic and endophytic EPF strains, Metarhizium brunneum V275 (commercially known as Lalguard Met52), achieved through Nanopore and Illumina reads. Comparative genomics for exploring intraspecies variability and analyses of key gene sets were conducted with a second effective EPF strain, M. brunneum ARSEF 4556. The search for strain- or species-specific genes was extended to M. brunneum strain ARSEF 3297 and other species of genus Metarhizium, to identify molecular mechanisms and putative key genome adaptations associated with mode of life differences. Genome size differed significantly, with M. brunneum V275 having the largest genome amongst M. brunneum strains sequenced to date. Genome analyses revealed an abundance of plant-degrading enzymes, plant colonization-associated genes, and intriguing intraspecies variations regarding their predicted secondary metabolic compounds and the number and localization of Transposable Elements. The potential significance of the differences found between closely related endophytic and entomopathogenic fungi, regarding plant growth-promoting and entomopathogenic abilities, are discussed, enhancing our understanding of their diverse functionalities and putative applications in agriculture and ecology.

The Ubiquitous Wilt-Inducing Pathogen Fusarium oxysporum-A Review of Genes Studied with Mutant Analysis

Fusarium oxysporum is one of the most economically important plant fungal pathogens, causing devastating Fusarium wilt diseases on a diverse range of hosts, including many key crop plants. Consequently, F. oxysporum has been the subject of extensive research to help develop and improve crop protection strategies. The sequencing of the F. oxysporum genome 14 years ago has greatly accelerated the discovery and characterization of key genes contributing to F. oxysporum biology and virulence. In this review, we summarize important findings on the molecular mechanisms of F. oxysporum growth, reproduction, and virulence. In particular, we focus on genes studied through mutant analysis, covering genes involved in diverse processes such as metabolism, stress tolerance, sporulation, and pathogenicity, as well as the signaling pathways that regulate them. In doing so, we hope to present a comprehensive review of the molecular understanding of F. oxysporum that will aid the future study of this and related species.

Temperature-Responded Biological Fitness of Carbendazim-Resistance Fusarium graminearum Mutants Conferring the F167Y, E198K, and E198L Substitutions

Understanding the effects of temperature on Fusarium graminearum infection can provide theoretical guidance for chemical control of Fusarium head blight (FHB). Here, we evaluated the effects of various temperatures on biological fitness development of wild-type sensitive strain 2021 and carbendazim-resistance mutants conferring β₂-tubulin substitutions F167Y, E198K, and E198L. The results showed that mycelial growth and conidiation of four strains increased with the increase in temperature between 10 and 25°C. Conidia of F167Y displayed strong adaptability to low temperature. The virulence of the four strains was largely similar at the same temperature, showing an upward trend between 10 and 25°C. At 10°C, the hyphal growth of all strains was significantly inhibited, metabolism was slowed down, and the accumulation of secondary metabolites decreased. Subsequently, the production of deoxynivalenol (DON) and its intermediates pyruvate and aurofusarin decreased at low temperature, and the expression of DON biosynthesis-related genes Tri5, FgPK, and AUR decreased accordingly. At the same temperature, the aurofusarin production of the strains F167Y and E198K was higher than that of strains 2021 and E198L. The contents of DON and pyruvic acid in carbendazim-resistance mutants were higher than those in the wild-type strain 2021. The sensitivity of four strains to different fungicides changed at various temperatures. The sensitivity to most fungicides increased with decreasing temperature. The carbendazim-resistance mutants showed positive cross-resistance with other benzimidazoles. However, there was no cross-resistance to pyraclostrobin and azoles. These results would direct us to use fungicides preventing the infection of F. graminearum with changeable atmospheric temperature at the wheat flower stage.

What is the role of the nitrate reductase (euknr) gene in fungi that live in nitrate-free environments? A targeted gene knock-out study in Ampelomyces mycoparasites

Mycoparasitic fungi can be utilized as biocontrol agents (BCAs) of many plant pathogens. Deciphering the molecular mechanisms of mycoparasitism may improve biocontrol efficiency. This work reports the first functional genetic studies in Ampelomyces, widespread mycoparasites and BCAs of powdery mildew fungi, and a molecular genetic toolbox for future works. The nitrate reductase (euknr) gene was targeted to reveal the biological function of nitrate assimilation in Ampelomyces. These mycoparasites live in an apparently nitrate-free environment, i.e. inside the hyphae of powdery mildew fungi that lack any nitrate uptake and assimilation system. Homologous recombination-based gene knock-out (KO) was applied to eliminate the euknr gene using Agrobacterium tumefaciens-mediated transformation. Efficient KO of euknr was confirmed by PCR, and visible phenotype caused by loss of euknr was detected on media with different nitrogen sources. Mycoparasitic ability was not affected by knocking out euknr as a tested transformant readily parasitized Blumeria graminis and Podosphaera xanthii colonies on barley and cucumber, respectively, and the rate of mycoparasitism did not differ from the wild type. These results indicate that euknr is not involved in mycoparasitism. Dissimilatory processes, involvement in nitric oxide metabolism, or other, yet undiscovered processes may explain why a functional euknr is maintained in Ampelomyces.

Role for nitrate assimilatory genes in virulence of Ustilago maydis

Ustilago maydis can utilize nitrate as a sole source of nitrogen. This process is initiated by transporting nitrate from the extracellular environment into the cell by a nitrate transporter and followed by a two-step reduction of nitrate to ammonium via nitrate reductase and nitrite reductase enzymes, respectively. Here, we characterize the genes encoding nitrate transporter, um03849 and nitrite reductase, um03848 in U. maydis based on their roles in mating and virulence. The deletion mutants for um03848, um03849 or both genes were constructed in mating compatible haploid strains 1/2 and 2/9. In addition, CRISPR-Cas9 gene editing technique was used for um03849 gene to create INDEL mutations in U. maydis mating strains. For all the mutants, phenotypes such as growth ability, mating efficiency and pathogenesis were examined. The growth of all the mutants was diminished when grown in a medium with nitrate as the source of nitrogen. Although no clear effects on haploid filamentation or mating were observed for either single mutant, double Δum03848 Δum03849 mutants showed reduction in mating, but increased filamentation on low ammonium, particularly in the 1/2 background. With respect to pathogenesis on the host, all the mutants showed reduced degrees of disease symptoms. Further, when the deletion mutants were paired with wild type of opposite mating-type, reduced virulence was observed, in a manner specific to the genetic background of the mutant's progenitor. This background specific reduction of plant pathogenicity was correlated with differential expression of genes for the mating program in U. maydis.

Mechanism insights into polyhydroxyalkanoate-regulated denitrification from the perspective of pericytoplasmic nitrate reductase expression

For enhanced biological nutrient removal (BNR) process, the polyhydroxyalkanoate (PHA) can be used as an eco-friendly internal as well as external substrate for regulating the growth of heterotrophic denitrifiers and promoting the denitrification process for deep nitrogen removal from wastewater. However, the exact mechanisms by which PHA impacts bacterial metabolism and affects the electron transfer of denitrification remain unknown. In this study, the in-depth mechanism investigation for PHA-mediated denitrification based on the jointly applied transcriptomic, proteomic and Western Blotting techniques was performed on a model denitrifier, Pseudomonas stutzeri. Results showed that PHA dramatically fostered the growth of Pseudomonas stutzeri, resulting in improved nitrate removal efficiency from 32.8% to 45.8%. Comparison of protein expression profiles indicated that PHA promoted the expression of enzyme NapB and NapA by approximately 10.34 and 20.01 times, respectively, which were both in charge of reduction from nitrate to nitrite. Based on transcriptional sequencing and Tandem Mass Tags, the correlation results also showed that differential proteins and genes with the same expression trend were positively correlated (R² = 0.427, p-value<0.033). Western Blotting approach was further developed to confirm the up-regulated expression of target protein with the higher proportion of PHA in carbon source of the medium, which proved the reliability of proteomics results. All the findings presented here are believed to deepen the understanding of microbial mechanism about PHA-enhanced denitrification from the novel perspective of associated electron-transfer enzymatic proteins.