Galactofuranose (Galf)-containing sugar chain contributes to the hyphal growth, conidiation and virulence of F. oxysporum f.sp. cucumerinum

The ascomycete fungus Fusarium oxysporum f.sp. cucumerinum causes vascular wilt diseases in cucumber. However, few genes related to morphogenesis and pathogenicity of this fungal pathogen have been functionally characterized. BLASTp searches of the Aspergillus fumigatus UgmA and galatofuranosyltransferases (Galf-transferases) sequences in the F. oxysporum genome identified two genes encoding putative UDP-galactopyranose mutase (UGM), ugmA and ugmB, and six genes encoding putative Galf-transferase homologs. In this study, the single and double mutants of the ugmA, ugmB and gfsB were obtained. The roles of UGMs and GfsB were investigated by analyzing the phenotypes of the mutants. Our results showed that deletion of the ugmA gene led to a reduced production of galactofuranose-containing sugar chains, reduced growth and impaired conidiation of F. oxysporum f.sp. cucumerinum. Most importantly, the ugmA deletion mutant lost the pathogenicity in cucumber plantlets. Although deletion of the ugmB gene did not cause any visible phenotype, deletion of both ugmA and ugmB genes caused more severe phenotypes as compared with the ΔugmA, suggesting that UgmA and UgmB are redundant and they can both contribute to synthesis of UDP-Galf. Furthermore, the ΔgfsB exhibited an attenuated virulence although no other phenotype was observed. Our results demonstrate that the galactofuranose (Galf) synthesis contributes to the cell wall integrity, germination, hyphal growth, conidiation and virulence in Fusarium oxysporum f.sp. cucumerinum and an ideal target for the development of new anti-Fusarium agents.

Identification of Two Mannosyltransferases Contributing to Biosynthesis of the Fungal-type Galactomannan α-Core-Mannan Structure in Aspergillus fumigatus

Fungal-type galactomannan (FTGM) is a polysaccharide composed of α-(1 → 2)-/α-(1 → 6)-mannosyl and β-(1 → 5)-/β-(1 → 6)-galactofuranosyl residues located at the outer cell wall of the human pathogenic fungus Aspergillus fumigatus. FTGM contains a linear α-mannan structure called core-mannan composed of 9 or 10 α-(1 → 2)-mannotetraose units jointed by α-(1 → 6)-linkages. However, the enzymes involved in core-mannan biosynthesis remain unknown. We speculated that two putative α-1,2-mannosyltransferase genes in A. fumigatus, Afu5g02740/AFUB_051270 (here termed core-mannan synthase A [CmsA]) and Afu5g12160/AFUB_059750 (CmsB) are involved in FTGM core-mannan biosynthesis. We constructed recombinant proteins for CmsA and detected robust mannosyltransferase activity using the chemically synthesized substrate p-nitrophenyl α-d-mannopyranoside as an acceptor. Analyses of CmsA enzymatic product revealed that CmsA possesses the capacity to transfer a mannopyranoside to the C-2 position of α-mannose. CmsA could also transfer a mannose residue to α-(1 → 2)-mannobiose and α-(1 → 6)-mannobiose and showed a 31-fold higher specific activity toward α-(1 → 6)-mannobiose than toward α-(1 → 2)-mannobiose. Proton nuclear magnetic resonance (¹H-NMR) spectroscopy and gel filtration chromatography of isolated FTGM revealed that core-mannan structures were drastically altered and shortened in disruptant A. fumigatus strains ∆cmsA, ∆cmsB, and ∆cmsA∆cmsB. Disruption of cmsA or cmsB resulted in severely repressed hyphal extension, abnormal branching hyphae, formation of a balloon structure in hyphae, and decreased conidia formation. The normal wild type core-mannan structure and developmental phenotype were restored by the complementation of cmsA and cmsB in the corresponding disruptant strains. These findings indicate that both CmsA, an α-1,2-mannosyltransferase, and CmsB, a putative mannosyltransferase, are involved in FTGM biosynthesis.