Sensitivity of Rhizoctonia solani Anastomosis Group 2-2 Isolates from Soybean and Sugar Beet to Selected SDHI and QoI Fungicides

Transcriptomics integrated with metabolomics reveal the competitive relationship between co-cultured Trichoderma asperellum HG1 and Bacillus subtilis Tpb55

Microbial co-culture has proven to be an effective way to improve the ability of microorganisms to biocontrol. However, the interactive mechanisms of co-cultural microbes, especially between fungi and bacteria, have rarely been studied. By comparative analysis of morphology, transcriptomics and metabolomics, the interactive mechanisms of a sequential co-culture system of Trichoderma asperellum HG1 and Bacillus subtilis Tpb55 was explored in this study. The results revealed that co- culture has no significant effect on the growth and cell morphology of the two strains, but lead to mycelium wrinkling of HG1. RNA-seq analysis showed that co-culture significantly upregulated the HG1 genes concerning amino acid degradation and metabolism, proteolysis, resisting environmental stress, cell homeostasis, glycolysis, the glyoxylate cycle, and the citric acid (TCA) cycle, while Tpb55 genes related to cell homeostasis, spore formation and membrane fluidization were significantly upregulated, but genes associating to TCA, glycolytic cycles and fatty acid β-oxidation were significantly downregulated. Metabolomic results revealed that some amino acids related to energy metabolism were significantly altered in HG1, whereas palmitic acid, which is related to cell membrane functions, was upregulated in Tpb55. These results indicated that HG1 could interfere with carbon metabolism and cell membrane fluidity, but accelerate spore formation of Tpb55. Biophysical assays further convinced that co-culture could decrease ATP content and inhibit ATPase activity in HG1, and could promote spore formation and reduce the cell membrane fluidity of Tpb55. In addition, co-culture also accelerated the production of intracellular anti-oomycete compound octhilinone. The above results indicate that HG1 and Tpb55 are mainly in a competitive relationship in the co culture system. These findings provide new insights for understanding the interaction mechanism between co cultured microbes.

Anastomosis Groups and Mycovirome of Rhizoctonia Isolates Causing Sugar Beet Root and Crown Rot and Their Sensitivity to Flutolanil, Thifluzamide, and Pencycuron

Anastomosis groups (AGs) or subgroups of 244 Rhizoctonia isolates recovered from sugar beet roots with symptoms of root and crown rot were characterized to be AG-A, AG-K, AG-2-2IIIB, AG-2-2IV, AG-3 PT, AG-4HGI, AG-4HGII, and AG-4HGIII, with AG-4HGI (108 isolates, 44.26%) and AG-2-2IIIB (107 isolates, 43.85%) being predominate. Four unclassified mycoviruses and one hundred and one putative mycoviruses belonging to six families, namely Mitoviridae (60.00%), Narnaviridae (18.10%), Partitiviridae (7.62%), Benyviridae (4.76%), Hypoviridae (3.81%), and Botourmiaviridae (1.90%), were found to be present in these 244 Rhizoctonia isolates, most of which (88.57%) contained positive single-stranded RNA genome. The 244 Rhizoctonia isolates were all sensitive to flutolanil and thifluzamide, with average median effective concentration (EC₅₀) value of 0.3199 ± 0.0149 μg·mL^-1 and 0.1081 ± 0.0044 μg·mL^-1, respectively. Among the 244 isolates, except for 20 Rhizoctonia isolates (seven isolates of AG-A and AG-K, one isolate of AG-4HGI, and 12 isolates of AG-4HGII), 117 isolates of AG-2-2IIIB, AG-2-2IV, AG-3 PT, and AG-4HGIII, 107 isolates of AG-4HGI, and six isolates of AG-4HGII were sensitive to pencycuron, with average EC₅₀ value of 0.0339 ± 0.0012 μg·mL^-1. Correlation index (ρ) of cross-resistance level between flutolanil and thifluzamide, flutolanil and pencycuron, and thifluzamide and pencycuron was 0.398, 0.315, and 0.125, respectively. This is the first detailed study on AG identification, mycovirome analysis, and sensitivity to flutolanil, thifluzamide, and pencycuron of Rhizoctonia isolates associated with sugar beet root and crown rot.

Molecular Mechanisms Associated with the Resistance of Rhizoctonia solani AG-4 Isolates to the Succinate Dehydrogenase Inhibitor Thifluzamide

Thifluzamide, a succinate dehydrogenase (SDH) inhibitor, possesses high activity against Rhizoctonia. In this study, 144 Rhizoctonia solani AG-4 (4HGI, 4HGII, and 4HGIII) isolates, the predominate pathogen associated with sugar beet seedling damping-off, were demonstrated to be sensitive to thifluzamide with a calculated mean median effective concentration of 0.0682 ± 0.0025 μg/ml. Thifluzamide-resistant isolates were generated using fungicide-amended media, resulting in four AG-4HGI isolates and eight AG-4HGII isolates with stable resistance and almost no loss in fitness. Evaluation of cross-resistance of the 12 thifluzamide-resistant isolates and their corresponding parental-sensitive isolates revealed a moderately positive correlation between thifluzamide resistance and the level of resistance to eight other fungicides from three groups, the exception being fludioxonil. An active efflux of fungicide through ATP-binding cassette and major facilitator superfamily transporters was found to be correlated to the resistance of R. solani AG-4HGII isolates to thifluzamide based on RNA-sequencing and quantitative reverse transcription-PCR analyses. Sequence analysis of sdhA, sdhB, sdhC, and sdhD revealed replacement of isoleucine by phenylalanine at position 61 in SDHC in 9 of the 12 generated thifluzamide-resistant isolates. No other mutations were found in any of the other genes. Collectively, the data indicate that the active efflux of fungicide and a point mutation in sdhC may contribute to the resistance of R. solani AG-4HGI and AG-4HGII isolates to thifluzamide in vitro. This is the first characterization of the potential molecular mechanism associated with the resistance of R. solani AG-4 isolates to thifluzamide and provides practical guidance for the use of this fungicide.

Effects of embryonic exposure to fluxapyroxad on zebrafish (Danio rerio) ocular development

Fluxapyroxad (FLU) is a succinate dehydrogenase inhibitor that protects crops from fungal diseases, however, it has been identified as toxicants to aquatic organisms. The objective of this study is to investigate the potential toxicity and underlying mechanisms of FLU on aquatic organisms. Herein, by using zebrafish embryos as a model organism, we demonstrated that FLU can cause microphthalmia in zebrafish embryos. The cell density in ganglion cell layer (GCL) is increased after exposure. Compared with the control, differentiation of the cells in ganglion cell layer, inner nuclear layer (INL), and outer nuclear layer (ONL) were severely disrupted in response to FLU treatment. The data show clear evidence that FLU exhibits development toxicity to zebrafish embryos by inducing retinal cell apoptosis, which causes microphthalmia. Our study provides comprehensive understanding to the underlying mechanism of FLU toxicity.