The fascinating realm of fungi is governed by intricate systems that connect them to the external environment. Humans, plants, and animals are all affected by the 24-hour cycle of the sun, which influences their biological processes, otherwise known as the circadian rhythm. Similarly, fungi also possess their own internal clock to guide their daily behaviors. This rhythm enables fungi to track time and synchronize their activities with the natural cycle of day and night.
The endogenous (internal) circadian rhythm, also referred to as the internal clock, is an incredible timekeeping system that most living things have. This biological clock helps organisms track the passage of time and regulate physical, mental, and behavioral changes that occur within a 24-hour cycle. The clock plays a significant role in regulating and orchestrating several biochemical, physiological, and behavioral processes, such as hyphal growth, metabolic activity, and spore release times to align with the appropriate time of day. Many fungal species will time their spores’ release to maximize their chances of survival and dispersal. Others may adjust their growth rate to optimize their resource use during specified times of the day (1).
The circadian system is sensitive to external light and darkness, which helps regulate the rest cycles in fungi. Photoreceptors in fungi capture light signals, allowing them to constantly react to these changing conditions. Through the consistent light and dark cycle, the internal clock of fungi can reset every 24 hours to keep their biological processes in sync with the external environment (2).
When fungi are exposed to darkness, certain physiological activities slow down within the fungus, but when they are exposed to light, the internal clock restarts.
Many fungi can keep their own pattern of biochemical and physiological processes without the help of outside signals. Fungi still operate on a 24-hour sequence, yet they do not rely solely on the light-dark cycle of day and night. Even in different period lengths of constant darkness, fungi have been found to carry on with their cycle (3). With this form of circadian rhythmicity, fungi can independently recognize when changes in the environment occur (such as lighting and temperature). However, fungi still have photoreceptors which make them light-responsive. By knowing when external features will change, fungi can anticipate when to move forward with their biological activities according to the time of day. However, there can be significant variability between different fungal species and how their circadian clocks function. Some fungi may more dependent on external cues, while others may rely more on their internal rhythm.
Not all fungi have been studied for having a circadian rhythm, though there’s evidence to suggest that several species have one (4). Not many fungi have yet been researched, though the most notable and well-studied example is Neurospora crassa, a common bread mold used as a default model for fungal research material. Much of the scientific knowledge about the molecular mechanism of circadian rhythms in fungi has been gained from the studying of N. crassa.
The first studies of the fungus were in the late 1950s and 1960s when scientists found that Neurospora continued to regularly create spores despite constant environmental conditions (5). Later, in the 1980s, scientists made a breakthrough discovery when they located a gene in the fungi called frequency (FRQ), responsible for maintaining the circadian clock of Neurospora.
Fungal circadian rhythms are controlled by the circadian oscillator, a network of clock-controlled genes. FRQ interacts with other clock proteins called white collar-1 (wc-1) and white collar-2 (wc-2). When white-collar proteins sense light, they respond by binding to the FRQ gene and activating it. Together they form a self-sustaining feedback loop that activates the transcription and synthesis of more FRQ genes (Dunlap and Loros 2017). This system allows fungi to time various processes in response to circadian cues. Even under constant darkness, this repetitive cycle of the FRQ clock protein matches the fluctuations of the circadian rhythm. The entrainment process ensures that the circadian rhythm is reset and tuned to the time of day so the internal biological clock stays in sync with the external light and dark cycle around it (Liu and Bell-Pedersen 2006).
Not all fungal species have the FRQ protein, but this absence does not necessarily suggest that these fungi do not have a circadian clock. Instead, they may rely on a different type of oscillator, or there is the possibility that the FRQ cannot be found in these fungi through standard techniques (6).
Though N.crassa is the most studied fungi for circadian rhythm, there are several others that scientists have researched to see how the circadian clock impacts them. For instance, many pathogenic fungi have been researched to determine how their circadian rhythmicity impacts their spread and growth.
Botrytis cinerea is a plant pathogen that causes the common gray mold on many common crops. The fungus follows a circadian rhythm that correlates with its pathogenicity. Based on its rhythm, it times the release of its spores, allowing it to infect more plants. As scientists study and understand the cycle of this fungus, they can time fungal treatments during the pathogen’s weakest periods for better disease control (7).
Another dangerous species that scientists are trying to understand is the Candida albicans yeast, which causes severe fungal infections in humans. This fungus is becoming drug-resistant, which is an increasing problem due to the serious nature of its infection. The circadian rhythm of C. Albicans impacts several of its harmful traits, such as its ability to change shape through yeast-to-hyphal transition, which is a key step in establishing infections. This new hyphal form allows the fungus to penetrate tissues and invade host cells which progresses the infection process (8). If scientists can figure out a way to disrupt C. albicans’ internal clock, they can possibly make it less harmful and be able to control or even prevent infections caused by this yeast (9).
Researchers have only skimmed the surface of fungal circadian clocks and how they impact specific fungal species. As we deepen our understanding of this intricate process, science has the potential to revolutionize our approach to pathogen prevention and control. Further research into the circadian rhythms of more fungal species can deepen our knowledge of such an intriguing phenomenon while allowing us to harness the power of the fungal clock to our advantage.