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Summary  The biosynthesis of cell surface‐located galactofuranose (Gal f )‐containing glycostructures such as galactomannan,  N ‐glycans and  O ‐glycans in filamentous fungi is important to secure the integrity of the cell wall. UgmA encodes an UDP‐galactopyranose mutase, which is essential for the formation of Gal f . Consequently, the  ΔugmA  mutant lacks Gal f ‐containing molecules. Our previous work in  Aspergillus niger  work suggested that loss of function of  ugmA  results in activation of the cell wall integrity (CWI) pathway which is characterized by increased expression of the  agsA  gene, encoding an α‐glucan synthase. In this study, the transcriptional response of the  ΔugmA  mutant was further linked to the CWI pathway by showing the induced and constitutive phosphorylation of the CWI‐MAP kinase in the  ΔugmA  mutant. To identify genes involved in cell wall remodelling in response to the absence of galactofuranose biosynthesis, a genome‐wide expression analysis was performed using RNAseq. Over 400 genes were higher expressed in the  ΔugmA  mutant compared to the wild‐type. These include genes that encode enzymes involved in chitin ( gfaB ,  gnsA ,  chsA ) and α‐glucan synthesis ( agsA ), and in β‐glucan remodelling ( bgxA ,  gelF  and  dfgC ), and also include several  g lycosyl p hosphatidyl i nositol (GPI)‐anchored cell wall protein‐encoding genes.  In silico  analysis of the 1‐kb promoter regions of the up‐regulated genes in the  ΔugmA  mutant indicated overrepresentation of genes with RlmA, MsnA, PacC and SteA‐binding sites. The importance of these transcription factors for survival of the  ΔugmA  mutant was analysed by constructing the respective double mutants. The  ΔugmA / ΔrlmA  and  ΔugmA/ΔmsnA  double mutants showed strong synthetic growth defects, indicating the importance of these transcription factors to maintain cell wall integrity in the absence of Gal f  biosynthesis.

Transcriptomic and molecular genetic analysis of the cell wall salvage response of Aspergillus niger to the absence of galactofuranose synthesis