Detrimental Effects of Multiple Mutations in Position 240 of Fusarium graminearum β2-Tubulin

Genetic Diversity, Mycotoxin Profiles, and Population Structure of Fusarium spp. Associated with Fusarium Head Blight in Georgia, United States

Fusarium head blight (FHB) has become a limiting factor in soft red winter wheat production in the southeast United States. Recent epidemics have occurred in Georgia, but genetic information on the Fusarium species responsible for FHB is unknown. This study aimed to assess pathogen population structure and genetic diversity, trichothecene profiles, and representative pathogenicity of 196 Fusarium isolates collected from 44 wheat (n = 85) and 53 corn (n = 111) fields in Georgia. Phylogenetic analysis using the translation elongation factor 1-alpha (635 bp) and RNA polymerase second largest subunit (930 bp) sequence data resolved isolates into 185 haplotypes, which represented 12 Fusarium species grouped under five species complexes. F. graminearum with 15-acetyl-deoxynivalenol (15ADON) chemotype (75.6%) and F. incarnatum (57.7%) predominated in wheat and corn, respectively, with a surprisingly higher frequency of nivalenol (NIV) F. graminearum (21.8%). Using nine variable numbers of tandem repeat markers, 82 multilocus genotypes out of 86 F. graminearum isolates were identified and grouped into two genetic clusters, pop1fg (n = 29) and pop2fg (n = 32), as part of the North American populations (NA1 and NA2) but with no chemotype differentiation. F. graminearum populations in Georgia are mostly clonal and might have evolved through at least two introductions from the northeast United States and Canada and local adaptation to maintain high genetic diversity. Pathogenicity of F. graminearum isolates from wheat and corn had high FHB severity (>60%) in wheat, which depicted the risk they can pose towards future FHB outbreaks. Overall, this baseline study provided important information on Fusarium species diversity including F. graminearum associated with FHB in Georgia that will be useful to formulate integrated disease management incorporating improved host resistance and fungicide spray programs.

Functional Roles of Two β-Tubulin Isotypes in Regulation of Sensitivity of Colletotrichum fructicola to Carbendazim

Colletotrichum fructicola is the major pathogen of anthracnose in tea-oil trees in China. Control of anthracnose in tea-oil trees mainly depends on the application of chemical fungicides such as carbendazim. However, the current sensitivity of C. fructicola isolates in tea-oil trees to carbendazim has not been reported. Here, we tested the sensitivity of 121 C. fructicola isolates collected from Guangdong, Guangxi, Guizhou, Hainan, Hunan, Jiangsu, and Jiangxi provinces in China to carbendazim. One hundred and ten isolates were sensitive to carbendazim, and 11 isolates were highly resistant to carbendazim. The growth rates, morphology, and pathogenicity of three resistant isolates were identical to those of three sensitive isolates, which indicates that these resistant isolates could form a resistant population under carbendazim application. These results suggest that carbendazim should not be the sole fungicide in control of anthracnose in tea-oil trees; other fungicides with different mechanisms of action or mixtures of fungicides could be considered. In addition, bioinformatics analysis identified two β-tubulin isotypes in C. fructicola: Cfβ1tub and Cfβ2tub. E198A mutation was discovered in the Cfβ2tub of three carbendazim-resistant isolates. We also investigated the functional roles of two β-tubulin isotypes. CfΔβ1tub exhibited slightly increased sensitivity to carbendazim and normal phenotypes. Surprisingly, CfΔβ2tub was highly resistant to carbendazim and showed a seriously decreased growth rate, conidial production, pathogenicity, and abnormal hyphae morphology. Promoter replacement mutant CfΔβ2-2×β1 showed partly restored phenotypes, but it was still highly resistant to carbendazim, which suggests that Cfβ1tub and Cfβ2tub are functionally interchangeable to a certain degree.

Functional roles of α1-, α2-, β1-, β2-tubulin in vegetative growth, microtubule assembly and sexual reproduction of Fusarium graminearum

The plant pathogen Fusarium graminearum contains two α-tubulin (α₁ and α₂) isotypes and two β-tubulin isotypes (β₁ and β₂). The functional roles of these tubulins in microtubule assembly are not clear. Previous studies showed that α₁- and β₂-tubulin deletion mutants showed severe growth defects and hypersensitivity to carbendazim, which have not been well explained. Here, we investigated the interaction between α- and β-tubulin of F. graminearum. Co-localization experiments demonstrated that β₁- and β₂-tubulin are co-localized. Co-immunoprecipitation experiment suggested that β₁-tubulin binds to both α₁- and α₂-tubulin and β₂-tubulin can also bind to α₁- or α₂-tubulin. Interestingly, deletion of α₁-tubulin increased the interaction between β₂-tubulin and α₂-tubulin. Microtubule observation assays showed that deletion of α₁-tubulin completely disrupted β₁-tubulin-containing microtubules and significantly decreased β₂-tubulin-containing microtubules. Deletion of α₂-, β₁- or β₂-tubulin respectively had no obvious effect on the microtubule cytoskeleton. However, microtubules in α₁- and β₂-tubulin deletion mutants were easily depolymerized in the presence of carbendazim. The sexual reproduction assay indicates that α₁- and β₁-tubulin deletion mutants could not produce asci and ascospores. These results implied that α₁-tubulin may be essential for the microtubule cytoskeleton. However, our Δα₁-2×α₂ mutant (α₁-tubulin deletion mutant containing two copies of α₂-tubulin) exhibited a normal microtubule network, growth and sexual reproduction. Interestingly, the Δα₁-2×α₂ mutant was still hypersensitive to carbendazim. In addition, both β₁-tubulin and β₂-tubulin were found to bind the mitochondrial outer membrane voltage-dependent anion channel (VDAC), indicating they could regulate the function of VDAC. Importance: In this study, we found that F. graminearum contains four different α-/β-tubulin heterodimers (α₁-β₁, α₁-β₂, α₂-β₁ and α₂-β₂) and they assemble together into a single microtubule. Moreover, α₁-, α₂-tubulins are functionally interchangeable in microtubule assembly, vegetative growth and sexual reproduction. These results provide more insights into functional roles of different tubulins of F. graminearum which could be helpful for purification of tubulin heterodimers and developing new tubulin-binding agents.