TotM: Hex-1 and hyphal compartimentalization in Pezizomycotina


We live in a universe that is very hostile to life as we know, and human life is particularly fragile. We can survive sheltered in a small range of pressure of a mix of gases with a particular composition, ensuring a proper balance between molecular oxygen and chemically inert gases such as molecular nitrogen. We need to take care also to keep ourselves within certain temperatures, not too cold, not too hot. Radiation is highly destructive for living beings and gamma rays soak almost every corner of the cosmos. Yet, as a species we have always dreamed to go further and further, until our modest planet became small. In order to explore outer space we had to protect ourselves from vacuum, extreme temperatures and cosmic rays; encasing our bodies in highly controlled environments.


While in practice we have been able to do a tiny walk in our own planet satellite, the dream has inspired endless works of fiction. And in such, loaded with action and danger, it is rather common that something threatens to break the insignificant barriers that keeps us away from a certain death. Quite often the damage is even self induced. Just think in the archetypical solution of launching the big baddie to outer space, letting the vacuum to suck your problems away. But in order to avoid losing all the precious air in the ship, we are made to know that the structure of the whole thing can be sealed in case something bad happens, limiting the damage to just some areas.


320px-Neurospora_crassahyphae.jpgSeparated from the rest of the universe by a tiny layer of lipids and proteins, cells must face similar dangers and have found similar solutions. The intracellular space is highly controlled and cannot afford to be dramatically affected by the external world. If the membrane breaks the cell might die. In a multicellular organism, facing the death of one or two of their cells usually has little relevance. However, filamentous fungi form cylindrical mycelia that are syncytial, a set of many nuclei sharing cytoplasm and thus acting both as a multicellular and a unicellular being. And while fungi are protected by a chitinous armor, it is not impossible to break. The fungal mycelium as a whole cannot be compromised by a single injury.


One of the most common mechanisms to avoid such fate is the development of septa that define compartments within the mycelium. Regularly spaced septa in vegetative mycelium is found in Kicxellomycotina, Agaricomycotina, Pezizomycotina and the genus Neolecta (Taphrinomycotina). In Pezizomycotina, that comprises most of filamentous Ascomycota, septa present holes that can be plugged with a peroxisome-derived organelle called Woronin body. In case of injury, the Woronin body is able to block sections of the cytoplasm, thus containing all possible damage.



The Woronin body forms a crystal structure in its core, formed by the protein Hex. As we can observe in our tree of the month, Hex is highly conserved across Pezizomycotina. Hex and the Woronin bodies are required for many other functions in different filamentous fungi, including resistance to chemical damage, hyphal morphological differentiation or formation of specialized structures during parasitism. Woronin body formation and regulation are also attractive targets for antifungal drugs, given the key roles these cellular structures play in fungal biology and its exclusivity to Pezizomycotina.




Photogram from Cowboy Bebop, episode 11: Toys in the attic (1998)

Sepatetd hyphae from Neurospora crassa. Photography by Roland Gromes. CC license.

Transmission electron micrography of Woronin bodies in Neofusicoccum parvum (White arrow). Extracted from Dalmas et al (2013)