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

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.