Analysis of Circadian Rhythms in the Basal Filamentous Ascomycete Pyronema confluens

Identification and expression of functionally conserved circadian clock genes in lichen-forming fungi

Lichen-forming fungi establish stable symbioses with green algae or cyanobacteria. Many species have broad distributions, both in geographic and ecological space, making them ideal subjects to study organism-environment interactions. However, little is known about the specific mechanisms that contribute to environmental adaptation in lichen-forming fungi. The circadian clock provides a well-described mechanism that contributes to regional adaptation across a variety of species, including fungi. Here, we identify the putative circadian clock components in phylogenetically divergent lichen-forming fungi. The core circadian genes (frq, wc-1, wc-2, frh) are present across the Fungi, including 31 lichen-forming species, and their evolutionary trajectories mirror overall fungal evolution. Comparative analyses of the clock genes indicate conserved domain architecture among lichen- and non-lichen-forming taxa. We used RT-qPCR to examine the core circadian loop of two unrelated lichen-forming fungi, Umbilicaria pustulata (Lecanoromycetes) and Dermatocarpon miniatum (Eurotiomycetes), to determine that the putative frq gene is activated in a light-dependent manner similar to the model fungus Neurospora crassa. Together, these results demonstrate that lichen-forming fungi retain functional light-responsive mechanisms, including a functioning circadian clock. Our findings provide a stepping stone into investigating the circadian clock in the lichen symbiosis, e.g. its role in adaptation, and in synchronizing the symbiotic interaction.

Circadian oscillations in Trichoderma atroviride and the role of core clock components in secondary metabolism, development, and mycoparasitism against the phytopathogen Botrytis cinerea

Circadian clocks are important for an individual’s fitness, and recent studies have underlined their role in the outcome of biological interactions. However, the relevance of circadian clocks in fungal–fungal interactions remains largely unexplored. We sought to characterize a functional clock in the biocontrol agent Trichoderma atroviride to assess its importance in the mycoparasitic interaction against the phytopathogen Botrytis cinerea. Thus, we confirmed the existence of circadian rhythms in T. atroviride, which are temperature-compensated and modulated by environmental cues such as light and temperature. Nevertheless, the presence of such molecular rhythms appears to be highly dependent on the nutritional composition of the media. Complementation of a clock null (Δfrq) Neurospora crassa strain with the T. atroviride-negative clock component (tafrq) restored core clock function, with the same period observed in the latter fungus, confirming the role of tafrq as a bona fide core clock component. Confrontation assays between wild-type and clock mutant strains of T. atroviride and B. cinerea, in constant light or darkness, revealed an inhibitory effect of light on T. atroviride’s mycoparasitic capabilities. Interestingly, when confrontation assays were performed under light/dark cycles, T. atroviride’s overgrowth capacity was enhanced when inoculations were at dawn compared to dusk. Deleting the core clock-negative element FRQ in B. cinerea, but not in T. atroviride, was vital for the daily differential phenotype, suggesting that the B. cinerea clock has a more significant influence on the result of this interaction. Additionally, we observed that T. atroviride clock components largely modulate development and secondary metabolism in this fungus, including the rhythmic production of distinct volatile organic compounds (VOCs). Thus, this study provides evidence on how clock components impact diverse aspects of T. atroviride lifestyle and how daily changes modulate fungal interactions and dynamics.

No Evidence That Homologs of Key Circadian Clock Genes Direct Circadian Programs of Development or mRNA Abundance in Verticillium dahliae

Many organisms harbor circadian clocks that promote their adaptation to the rhythmic environment. While a broad knowledge of the molecular mechanism of circadian clocks has been gained through the fungal model Neurospora crassa, little is known about circadian clocks in other fungi. N. crassa belongs to the same class as many important plant pathogens including the vascular wilt fungus Verticillium dahliae. We identified homologs of N. crassa clock proteins in V. dahliae, which showed high conservation in key protein domains. However, no evidence for an endogenous, free-running and entrainable rhythm was observed in the daily formation of conidia and microsclerotia. In N. crassa the frequency (frq) gene encodes a central clock protein expressed rhythmically and in response to light. In contrast, expression of Vdfrq is not light-regulated. Temporal gene expression profiling over 48 h in constant darkness and temperature revealed no circadian expression of key clock genes. Furthermore, RNA-seq over a 24 h time-course revealed no robust oscillations of clock-associated transcripts in constant darkness. Comparison of gene expression between wild-type V. dahliae and a ΔVdfrq mutant showed that genes involved in metabolism, transport and redox processes are mis-regulated in the absence of Vdfrq. In addition, VdΔfrq mutants display growth defects and reduced pathogenicity in a strain dependent manner. Our data indicate that if a circadian clock exists in Verticillium, it is based on alternative mechanisms such as post-transcriptional interactions of VdFRQ and the WC proteins or the components of a FRQ-less oscillator. Alternatively, it could be that whilst the original functions of the clock proteins have been maintained, in this species the interactions that generate robust rhythmicity have been lost or are only triggered when specific environmental conditions are met. The presence of conserved clock genes in genomes should not be taken as definitive evidence of circadian function.

The Clock Keeps on Ticking: Emerging Roles for Circadian Regulation in the Control of Fungal Physiology and Pathogenesis

Tic-tac, tic-tac, the sound of time is familiar to us, yet, it also silently shapes daily biological processes conferring 24-hour rhythms in, among others, cellular and systemic signaling, gene expression, and metabolism. Indeed, circadian clocks are molecular machines that permit temporal control of a variety of processes in individuals, with a close to 24-hour period, optimizing cellular dynamics in synchrony with daily environmental cycles. For over three decades, the molecular bases of these clocks have been extensively described in the filamentous fungus Neurospora crassa, yet, there have been few molecular studies in fungi other than Neurospora, despite evidence of rhythmic phenomena in many fungal species, including pathogenic ones. This chapter will revise the mechanisms underlying clock regulation in the model fungus N. crassa, as well as recent findings obtained in several fungi. In particular, this chapter will review the effect of circadian regulation of virulence and organismal interactions, focusing on the phytopathogen Botrytis cinerea, as well as several entomopathogenic fungi, including the behavior-manipulating species Ophiocordyceps kimflemingiae and Entomophthora muscae. Finally, this review will comment current efforts in the study of mammalian pathogenic fungi, while highlighting recent circadian lessons from parasites such as Trypanosoma and Plasmodium. The clock keeps on ticking, whether we can hear it or not.

Daily rhythms and enrichment patterns in the transcriptome of the behavior-manipulating parasite Ophiocordyceps kimflemingiae

Various parasite-host interactions that involve adaptive manipulation of host behavior display time-of-day synchronization of certain events. One example is the manipulated biting behavior observed in Carpenter ants infected with Ophiocordyceps unilateralis sensu lato. We hypothesized that biological clocks play an important role in this and other parasite-host interactions. In order to identify candidate molecular clock components, we used two general strategies: bioinformatics and transcriptional profiling. The bioinformatics approach was used to identify putative homologs of known clock genes. For transcriptional profiling, RNA-Seq was performed on 48 h time courses of Ophiocordyceps kimflemingiae (a recently named species of the O. unilateralis complex), whose genome has recently been sequenced. Fungal blastospores were entrained in liquid media under 24 h light-dark (LD) cycles and were harvested at 4 h intervals either under LD or continuous darkness. Of all O. kimflemingiae genes, 5.3% had rhythmic mRNAs under these conditions (JTK Cycle, ≤ 0.057 statistical cutoff). Our data further indicates that a significant number of transcription factors have a peaked activity during the light phase (day time). The expression levels of a significant number of secreted enzymes, proteases, toxins and small bioactive compounds peaked during the dark phase or subjective night. These findings support a model whereby this fungal parasite uses its biological clock for phase-specific activity. We further suggest that this may be a general mechanism involved in parasite-host interactions.

Spontaneous circadian rhythms in a cold-adapted natural isolate of Aureobasidium pullulans

Circadian systems enable organisms to synchronize their physiology to daily and seasonal environmental changes relying on endogenous pacemakers that oscillate with a period close to 24 h even in the absence of external timing cues. The oscillations are achieved by intracellular transcriptional/translational feedback loops thoroughly characterized for many organisms, but still little is known about the presence and characteristics of circadian clocks in fungi other than Neurospora crassa. We sought to characterize the circadian system of a natural isolate of Aureobasidium pullulans, a cold-adapted yeast bearing great biotechnological potential. A. pullulans formed daily concentric rings that were synchronized by light/dark cycles and were also formed in constant darkness with a period of 24.5 h. Moreover, these rhythms were temperature compensated, as evidenced by experiments conducted at temperatures as low as 10 °C. Finally, the expression of clock-essential genes, frequency, white collar-1, white collar-2 and vivid was confirmed. In summary, our results indicate the existence of a functional circadian clock in A. pullulans, capable of sustaining rhythms at very low temperatures and, based on the presence of conserved clock-gene homologues, suggest a molecular and functional relationship to well-described circadian systems.

Natural Variation of the Circadian Clock in Neurospora

Most living organisms on earth experience daily and expected changes from the rotation of the earth. For an organism, the ability to predict and prepare for incoming stresses or resources is a very important skill for survival. This cellular process of measuring daily time of the day is collectively called the circadian clock. Because of its fundamental role in survival in nature, there is a great interest in studying the natural variation of the circadian clock. However, characterizing the genetic and molecular mechanisms underlying natural variation of circadian clocks remains a challenging task. In this chapter, we will summarize the progress in studying natural variation of the circadian clock in the successful eukaryotic model Neurospora, which led to discovering many design principles of the molecular mechanisms of the eukaryotic circadian clock. Despite the success of the system in revealing the molecular mechanisms of the circadian clock, Neurospora has not been utilized to extensively study natural variation. We will review the challenges that hindered the natural variation studies in Neurospora, and how they were overcome. We will also review the advantages of Neurospora for natural variation studies. Since Neurospora is the model fungal species for circadian study, it represents over 5 million species of fungi on earth. These fungi play important roles in ecosystems on earth, and as such Neurospora could serve as an important model for understanding the ecological role of natural variation in fungal circadian clocks.

Making Time: Conservation of Biological Clocks from Fungi to Animals

The capacity for biological timekeeping arose at least three times through evolution, in prokaryotic cyanobacteria, in cells that evolved into higher plants, and within the group of organisms that eventually became the fungi and the animals. Neurospora is a tractable model system for understanding the molecular bases of circadian rhythms in the last of these groups, and is perhaps the most intensively studied circadian cell type. Rhythmic processes described in fungi include growth rate, stress responses, developmental capacity, and sporulation, as well as much of metabolism; fungi use clocks to anticipate daily environmental changes. A negative feedback loop comprises the core of the circadian system in fungi and animals. In Neurospora, the best studied fungal model, it is driven by two transcription factors, WC-1 and WC-2, that form the White Collar Complex (WCC). WCC elicits expression of the frq gene. FRQ complexes with other proteins, physically interacts with the WCC, and reduces its activity; the kinetics of these processes is strongly influenced by progressive phosphorylation of FRQ. When FRQ becomes sufficiently phosphorylated that it loses the ability to influence WCC activity, the circadian cycle starts again. Environmental cycles of light and temperature influence frq and FRQ expression and thereby reset the internal circadian clocks. The molecular basis of circadian output is also becoming understood. Taken together, molecular explanations are emerging for all the canonical circadian properties, providing a molecular and regulatory framework that may be extended to many members of the fungal and animal kingdoms, including humans.

Circadian Oscillators: Around the Transcription-Translation Feedback Loop and on to Output

From cyanobacteria to mammals, organisms have evolved timing mechanisms to adapt to environmental changes in order to optimize survival and improve fitness. To anticipate these regular daily cycles, many organisms manifest ∼24h cell-autonomous oscillations that are sustained by transcription-translation-based or post-transcriptional negative-feedback loops that control a wide range of biological processes. With an eye to identifying emerging common themes among cyanobacterial, fungal, and animal clocks, some major recent developments in the understanding of the mechanisms that regulate these oscillators and their output are discussed. These include roles for antisense transcription, intrinsically disordered proteins, codon bias in clock genes, and a more focused discussion of post-transcriptional and translational regulation as a part of both the oscillator and output.

The circadian system as an organizer of metabolism

The regulation of metabolism by circadian systems is believed to be a key reason for the extensive representation of circadian rhythms within the tree of life. Despite this, surprisingly little work has focused on the link between metabolism and the clock in Neurospora, a key model system in circadian research. The analysis that has been performed has focused on the unidirectional control from the clock to metabolism and largely ignored the feedback from metabolism on the clock. Recent efforts to understand these links have broken new ground, revealing bidirectional control from the clock to metabolism and vise-versa, showing just how strongly interconnected these two cellular systems can be in fungi. This review describes both well understood and emerging links between the clock and metabolic output of fungi as well as the role that metabolism plays in influencing the rhythm set by the clock.