We have all seen it on a slice of old bread or a piece of cheese lying too long in the fridge. An almost perfectly circular carpet of thin fungal material (mycelia), radiating out from the point of origin. It is easy to get the impression of a blind organism at work, shooting filaments of mycelium (hyphae) in all directions in an attempt to cover as much substrate as possible. However, the fungal mycelium is by no means blind.
In a recent PNAS paper by Carol A Kumamoto, a study on the pathogenic fungus Candida albicans has shown previously known virulence proteins to be produced at high rate throughout the organism upon first contact with the fungus' human host. (A contact-activated kinase signals Candida albicans invasive growth and biofilm development. PNAS, 102:15, 5576-5581. 2005). This stresses the importance of fungi's highly sophisticated, but often overlooked, internal communication systems.
A fungal spore landing on a suitable surface will soon germinate and begin a filamentous growth. The initial even spread in all directions is only true on a perfectly homogenous substrate, which is rarely the case in nature. In nature, for example in soil, the radial hyphal spread will go on until any part of the mycelia encounters some carbon source, like a piece of wood. The whole mycelia will then begin to relocate its resources to the hyphae in contact with the source, prioritizing this growth for as long as there is easily available carbon left. Thus, the entire mycelium of one individual, originating from the same spore, is highly “aware” of what is going on at different ends of the mycelium. In the same manner, if part of the mycelium reaches particulary nutrient-poor or even toxic environment, the rest of the mycelia will stay clear of that area, directing its growth in other directions instead.
This system is based on complicated protein signals. Very specific receptors located in the hyphal tips respond to different kinds of conditions outside the hyphal cell membrane. If concentrations of available carbon or other nutrients reach high enough levels, the receptor is activated and releases signalling proteins into the hyphal cytosol, triggering a chain reaction throughout the entire mycelium, resulting in more resources being transferred through the hyphae to the area in question. Conversely, low enough levels of nutrients or extreme pH levels trigger other receptors and other pathways, causing the mycelium to terminate growth and transferring resources away from the area instead.
Generally, it is becoming clear that not only the growing hyphal tips of a mycelium are critical for the fungal individual. The existing mycelia, sometimes falsely assumed to be just a skeleton of past expansion, is equally crucial for advanced signalling and transportation of water and nutrients throughout the fungal body.