Deciphering the uniqueness of Mucoromycotina cell walls by combining biochemical and phylogenomic approaches

Abstract

Most fungi from the Mucoromycotina lineage occur in ecosystems as saprobes, although some species are phytopathogens or may induce human mycosis. Mucoromycotina represent early diverging models that are most valuable for understanding fungal evolution. Here we reveal the uniqueness of the cell wall structure of the Mucoromycotina Rhizopus oryzae and Phycomyces blakesleeanus compared with the better characterized cell wall of the ascomycete Neurospora crassa. We have analysed the corresponding polysaccharide biosynthetic and modifying pathways, and highlight their evolutionary features and higher complexity in terms of gene copy numbers compared with species from other lineages. This work uncovers the presence in Mucoromycotina of abundant fucose-based polysaccharides similar to algal fucoidans. These unexpected polymers are associated with unusually low amounts of glucans and a higher proportion of chitin compared with N. crassa. The specific structural features are supported by the identification of genes potentially involved in the corresponding metabolic pathways. Phylogenomic analyses of genes encoding carbohydrate synthases, polysaccharide modifying enzymes and enzymes involved in nucleotide-sugar formation provide evidence for duplication events during evolution of cell wall metabolism in fungi. Altogether, the data highlight the specificity of Mucoromycotina cell walls and pave the way for a finer understanding of their metabolism.

Mechanisms of Bacterial (Serratia marcescens) Attachment to, Migration along, and Killing of Fungal Hyphae

“…sized that S. marcescens utilizes secreted chitinases to specifically target and kill zygomycete molds, because they contain unusually high levels of chitin (in the form of chitosan) in their cell walls (12). Surprisingly, we found that an S. marcescens triple-deletion mutant (⌬chiA ⌬chiB ⌬chiC) with no detectable secreted chitinase activity retained wild-type fungal killing abilities.…”

“…To measure the rate of bacterial spreading over fungal mycelia, fresh spores of R. oryzae 3465 were spread over an SOC plate at a concentration of 2.5 ϫ 10 4 spores/cm 2 , allowed to germinate for 12 h at 37°C, and then inoculated at the center of the plate by pipetting 5 l of bacterial suspension grown at 30°C in SOC to exponential phase (OD ϭ 0.5) on the agar surface. After incubation at 30°C, the plates were sampled by the insertion of a 96-pin metal replicator (of which 66 pins fit into the plate) into the agar and replication onto fresh SOC plates at 4,8,12, and 24 h postinoculation. Replicated plates were incubated for 24 h at 37°C, when bacterial colonies were clearly visible.…”

“…Mucormycosis, an infection caused by fungi of the order Mucorales, is an uncommon yet often deadly infection in immunocompromised patients (11). It is noteworthy that zygomycetes are the only members of the fungal kingdom whose cell wall is composed primarily of chitin (12), which is susceptible to enzymatic breakdown by chitinases secreted by S. marcescens (10).…”

Genome-wide analyses of chitin synthases identify horizontal gene transfers towards bacteria and allow a robust and unifying classification into fungi

“…In fungal cell walls, there is a common fibrillar core composed of branched beta-(1,3)-glucan to which chitin and other polysaccharides are covalently bound. Chitin accounts for 1-2% of the cell wall mass in yeasts and up to 30% in molds [9, 10]. Elongation of the chitin polymer is catalyzed by a highly conserved enzyme called chitin synthase, CHS (UDP-N-acetyl-D-glucosamine: chitin 4-beta-N-acetylglucosaminyl-transferase; EC 2.4.1.16).…”

Nicotiana alata Defensin Chimeras Reveal Differences in the Mechanism of Fungal and Tumor Cell Killing and an Enhanced Antifungal Variant

“…However, the activity of L1B against yeasts and A. niger was similar to that of NaD1. The observation that the enhancement in antifungal activity varied widely between fungal strains points to the potential involvement of the fungal cell wall, which varies between fungal species (33), and/or to differences in the ability of the fungi to respond to cell wall, osmotic, or oxidative stress (14,21). Variations in cell wall composition have been proposed as an explanation for the different effects that the defensin MtDef4 displays against Neurospora crassa and F. graminearum (34).…”