Fungicide Resistance Evolving in Ramularia collo-cygni Population in Estonia

Fungicide Sensitivity Profile of Pyrenophora teres f. teres in Field Population

Pyrenophora teres f. teres (Ptt) is a severe pathogen to spring barley in Northern Europe. Ptt with relevant mutations in fungicide target proteins, sterol 14α-demethylase (CYP51A), cytochrome b (Cyt b), and succinate dehydrogenase (SDH) would put efficient disease control at risk. In the growing seasons of 2021 and 2022, 193 Ptt isolates from Estonia were analysed. In this study, mutation detection and in vitro fungicide sensitivity assays of single-spore isolates were carried out. Reduced sensitivity phenotype to mefentrifluconazole was evident in Ptt isolates with a F489L mutation in CYP51A or with 129 bp insert in the Cyp51A gene-promoter region. However, sensitivity to a prothioconazole-desthio remained high regardless of these molecular changes. The Ptt population was mostly sensitive to bixafen, fluxapyroxad, pyraclostrobin, and azoxystrobin. The sensitivity of fluxapyroxad and bixafen has been affected by two mutations, C-S135R and D-H134R, found in SDH subunits. The F129L mutation in Cyt b influenced azoxystrobin but not pyraclostrobin sensitivity. In total, 30 isolates from five fields had relevant mutations in three target protein genes simultaneously. Most of these isolates had a reduced sensitivity phenotype to mefentrifluconazole, fluxapyroxad, and azoxystrobin, while sensitivity to other tested fungicides remained high. Furthermore, possible sexual reproduction may enhance the pathogen's fitness and help it adapt to fungicides.

The sphinganine C4-hydroxylase FgSur2 regulates sensitivity to azole antifungal agents and virulence of Fusarium graminearum

Lipid rafts consisting of ergosterol and sphingolipids in the lipid membrane of cells play important roles in various cellular processes. However, the functions of sphingolipids and their synthetic genes in phytopathogenic fungi have not been well understood yet. In this study, we conducted genome-wide searches and carried out systematic gene deletion analysis of the sphingolipid synthesis pathway in Fusarium graminearum, a causal agent of Fusarium head blight of wheat and other cereal crops worldwide. Mycelial growth assays showed that deletion of FgBAR1, FgLAC1, FgSUR2 or FgSCS7 resulted in markedly reduced hyphal growth. Fungicide sensitivity tests showed that the sphinganine C4-hydroxylase gene FgSUR2 deletion mutant (ΔFgSUR2) exhibited significantly increased susceptibility to azole fungicides. In addition, this mutant displayed a remarkable increase in cell membrane permeability. Importantly, ΔFgSUR2 was defective in deoxynivalenol (DON) toxisome formation, leading to dramatically decreased DON biosynthesis. Moreover, the deletion of FgSUR2 resulted in dramatically decreased virulence of the pathogen on host plants. Taken together, these results indicate that FgSUR2 plays an important role in regulating the susceptibility to azoles and virulence of F. graminearum.

Biosynthesis of Rubellins in Ramularia collo-cygni-Genetic Basis and Pathway Proposition

The important disease Ramularia leaf spot of barley is caused by the fungus Ramularia collo-cygni. The disease causes yield and quality losses as a result of a decrease in photosynthesis efficiency due to the appearance of necrotic spots on the leaf surface. The development of these typical Ramularia leaf spot symptoms is thought to be linked with the release of phytotoxic secondary metabolites called rubellins in the host. However, to date, neither the biosynthetic pathways leading to the production of these metabolites nor their exact role in disease development are known. Using a combined in silico genetic and biochemistry approach, we interrogated the genome of R. collo-cygni to identify a putative rubellin biosynthetic gene cluster. Here we report the identification of a gene cluster containing homologues of genes involved in the biosynthesis of related anthraquinone metabolites in closely related fungi. A putative pathway to rubellin biosynthesis involving the genes located on the candidate cluster is also proposed.