Circadian rhythms in fungi

Rhythmidia: A modern tool for circadian period analysis of filamentous fungi

Circadian rhythms are ubiquitous across the kingdoms of life and serve important roles in regulating physiology and behavior at many levels. These rhythms occur in ~24-hour cycles and are driven by a core molecular oscillator. Circadian timekeeping enables organisms to anticipate daily changes by timing their growth and internal processes. Neurospora crassa is a model organism with a long history in circadian biology, having conserved eukaryotic clock properties and observable circadian phenotypes. A core approach for measuring circadian function in Neurospora is to follow daily oscillations in the direction of growth and spore formation along a thin glass tube (race tube). While leveraging robust phenotypic readouts is useful, interpreting the outputs of large-scale race tube experiments by hand can be time-consuming and prone to human error. To provide the field with an efficient tool for analyzing race tubes, we present Rhythmidia, a graphical user interface (GUI) tool written in Python for calculating circadian periods and growth rates of Neurospora. Rhythmidia is open source, has been benchmarked against the current state-of-the-art, and is easily accessible on GitHub.

Plant Defence Mechanisms Are Modulated by the Circadian System

Simple Summary

The circadian clock is an endogenous time keeping mechanism found in living organisms and their respective pathogens. Numerous studies demonstrate that rhythms generated by this internal biological oscillator regulate and modulate most of the physiological, developmental, and biochemical processes of plants. Importantly, plant defence responses have also been shown to be modulated by the host circadian clock and vice versa. In this review we discuss the current understanding of the interactions between plant immunity and the circadian system. We also describe the possibility of pathogens directly or indirectly influencing plants’ circadian rhythms and suggest that these interactions could help us devise better disease management strategies for plants. Our review raises further research questions and we conclude that experimentation should be completed to unravel the complex mechanisms underlying interactions between plant defence and the circadian system.

Abstract

Plant health is an important aspect of food security, with pathogens, pests, and herbivores all contributing to yield losses in crops. Plants’ defence against pathogens is complex and utilises several metabolic processes, including the circadian system, to coordinate their response. In this review, we examine how plants’ circadian rhythms contribute to defence mechanisms, particularly in response to bacterial pathogen attack. Circadian rhythms contribute to many aspects of the plant–pathogen interaction, although significant gaps in our understanding remain to be explored. We conclude that if these relationships are explored further, better disease management strategies could be revealed.

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.

Atmospheric fungal nanoparticle bursts

Aerosol nanoparticles play an important role in the climate system by affecting cloud formation and properties, as well as in human health because of their deep reach into lungs and the circulatory system. Determining nanoparticle sources and composition is a major challenge in assessing their impacts in these areas. The sudden appearance of large numbers of atmospheric nanoparticles is commonly attributed to secondary formation from gas-phase precursors, but in many cases, the evidence for this is equivocal. We report the detection of a mode of fungal fragments with a mobility diameter of roughly 30 nm released in episodic bursts in ambient air over an agricultural area in northern Oklahoma. These events reached concentrations orders of magnitude higher than other reports of biological particles and show similarities to unclarified events reported previously in the Amazon. These particles potentially represent a large source of both cloud-forming ice nuclei and respirable allergens in a variety of ecosystems.

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.

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.

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.

Evolutionary History of the Photolyase/Cryptochrome Superfamily in Eukaryotes

BACKGROUND

Photolyases and cryptochromes are evolutionarily related flavoproteins, which however perform distinct physiological functions. Photolyases (PHR) are evolutionarily ancient enzymes. They are activated by light and repair DNA damage caused by UV radiation. Although cryptochromes share structural similarity with DNA photolyases, they lack DNA repair activity. Cryptochrome (CRY) is one of the key elements of the circadian system in animals. In plants, CRY acts as a blue light receptor to entrain circadian rhythms, and mediates a variety of light responses, such as the regulation of flowering and seedling growth.

RESULTS

We performed a comprehensive evolutionary analysis of the CRY/PHR superfamily. The superfamily consists of 7 major subfamilies: CPD class I and CPD class II photolyases, (6-4) photolyases, CRY-DASH, plant PHR2, plant CRY and animal CRY. Although the whole superfamily evolved primarily under strong purifying selection (average ω = 0.0168), some subfamilies did experience strong episodic positive selection during their evolution. Photolyases were lost in higher animals that suggests natural selection apparently became weaker in the late stage of evolutionary history. The evolutionary time estimates suggested that plant and animal CRYs evolved in the Neoproterozoic Era (~1000-541 Mya), which might be a result of adaptation to the major climate and global light regime changes occurred in that period of the Earth's geological history.

Sex-specific gene expression during asexual development of Neurospora crassa

The impact of loci that determine sexual identity upon the asexual, dominant stage of fungal life history has been well studied. To investigate their impact, expression differences between strains of different mating type during asexual development were assayed, with RNA sampled from otherwise largely isogenic mat A and mat a strains of Neurospora crassa at early, middle, and late clonal stages of development. We observed significant differences in overall gene expression between mating types across clonal development, especially at late development stages. The expression levels of mating-type genes and pheromone genes were assayed by reverse transcription and quantitative PCR, revealing expression of pheromone and receptor genes in strains of both mating types in all development stages, and revealing that mating type (mat) genes were increasingly expressed over the course of asexual development. Interestingly, among differentially expressed genes, the mat A genotype more frequently exhibited a higher expression level than mat a, and demonstrated greater transcriptional regulatory dynamism. Significant up-regulation of expression was observed for many late light-responsive genes at late asexual development stages. Further investigation of the impact of light and the roles of light response genes in asexual development of both mating types are warranted.

The genetics of circadian rhythms in Neurospora

This chapter describes our current understanding of the genetics of the Neurospora clock and summarizes the important findings in this area in the past decade. Neurospora is the most intensively studied clock system, and the reasons for this are listed. A discussion of the genetic interactions between clock mutants is included, highlighting the utility of dissecting complex mechanisms by genetic means. The molecular details of the Neurospora circadian clock mechanism are described, as well as the mutations that affect the key clock proteins, FRQ, WC-1, and WC-2, with an emphasis on the roles of protein phosphorylation. Studies on additional genes affecting clock properties are described and place these genes into two categories: those that affect the FRQ/WCC oscillator and those that do not. A discussion of temperature compensation and the mutants affecting this property is included. A section is devoted to the observations pertinent to the existence of other oscillators in this organism with respect to their properties, their effects, and their preliminary characterization. The output of the clock and the control of clock-controlled genes are discussed, emphasizing the phasing of these genes and the layers of control. In conclusion, the authors provide an outlook summarizing their suggestions for areas that would be fruitful for further exploration.

Scarification and Environmental Factors that Enhance Carpogenic Germination of Sclerotia of Sclerotinia sclerotiorum

Ascospores of Sclerotinia sclerotiorum are the primary source of inoculum for disease epidemics in many economically important crops. Mass production of ascospores under laboratory conditions is required to prepare inoculum for use in selection of genotypes with resistance against Sclerotinia diseases. A study was undertaken, first, to investigate the effect on carpogenic germination of scarifying sclerotia from two S. sclerotiorum isolates taken from canola (Brassica napus) and, second, to identify environmental factors that enhance carpogenic germination. Seven different environmental treatments were applied to scarified and unscarified sclerotia: (i) sterilized distilled water for 4 months at 15°C, (ii) aerated water for 4 months at 4°C, (iii) constant rinsing with tap water for 8 weeks at 4°C, (iv) cold-conditioning for 4 weeks at 4°C and subsequent transfer into moist unsterilized compost at 15°C, (v) incubation in sterilized river sand at 15°C, (vi) air drying for 2 weeks followed by subsequent transfer into sterilized moist river sand at 15°C, or (vii) placed into 0.5% water agar and incubated at 15°C. Carpogenic germination of scarified sclerotia was significantly greater (P < 0.05) than for unscarified sclerotia. There was significant interaction (P < 0.001) between scarification and the different environmental treatments in relation to the carpogenic germination. Carpogenic germination of scarified sclerotia was enhanced by incubation of sclerotia in compost or in sterilized river sand. Further, overall carpogenic germination of both scarified and unscarified sclerotia occurred to the greatest extent when sclerotia of either of the two isolates were subjected to constant rinsing with tap water. We believe this to be the first report of both the enhanced carpogenic germination by scarification in S. sclerotiorum and the environmental factors we report that enhance carpogenic germination of scarified sclerotia. The progression of carpogenic germination in all the environmental treatments was also monitored as a part of this study across the two consecutive years for the same two isolates. The majority of sclerotia of both isolates germinated between the months of June and September in both years, a period which coincides with the main part of the cropping season when Sclerotinia stem rot is normally observed in rainfed canola in Western Australia. These data suggested the existence of a seasonal rhythm-like pattern in relation to the carpogenic germination of this pathogen.

The diversity and evolution of circadian clock proteins in fungi

Circadian rhythms are endogenous cellular patterns that associate multiple physiological and molecular functions with time. The Neurospora circadian system contains at least three oscillators: the FRQ/WC-dependent circadian oscillator (FWO), whose core components are the FRQ, WC-1, WC-2, FRH, and FWD-1 proteins; the WC-dependent circadian oscillator (WC-FLO); and one or more FRQ/ WC-independent oscillators (FLO). Little is known about the distribution of homologs of the Neurospora clock proteins or about the molecular foundations of circadian rhythms across fungi. Here, we examined 64 diverse fungal proteomes for homologs of all five Neurospora clock proteins and retraced their evolutionary history. The FRH and FWD-1 proteins were likely present in the fungal ancestor. WC-1 and WC-2 homologs are absent from the early diverging chytrids and Microsporidia but are present in all other major clades. In contrast to the deep origins of these four clock proteins FRQ homologs are taxonomically restricted within Sordariomycetes, Leotiomycetes and Dothideomycetes. The large number of FRH and FWD-1 homologs identified and their lack of concordance with the fungal species phylogeny indicate that they likely underwent multiple rounds of duplications and losses. In contrast, the FRQ, WC-1 and WC-2 proteins exhibit relatively few duplications and losses. A notable exception is the 10 FRQ-like proteins in Fusarium oxysporum, which resulted from nine duplication events. Our results suggest that the machinery required for FWO oscillator function is taxonomically restricted within Ascomycetes. Although the WC proteins are widely distributed, the functional diversity of the few non-Neurospora circadian oscillators suggests that a WC-FLO oscillator is unlikely to fully explain the observed rhythms. The contrast between the diversity of circadian oscillators and the conservation of most of their machinery is likely best explained by considering the centrality of noncircadian functions in which RNA helicase (FRH), F-box (FWD-1), WC-1 and WC-2 (light-sensing) proteins participate in fungi and eukaryotes.

Circadian rhythms in Neurospora crassa: dynamics of the clock component frequency visualized using a fluorescent reporter

The frequency (frq) gene of Neurospora crassa has long been considered essential to the function of this organism's circadian rhythm. Increasingly, deciphering the coupling of core oscillator genes such as frq to the output pathways of the circadian rhythm has become a major focus of circadian research. To address this coupling it is critical to have a reporter of circadian activity that can deliver high resolution spatial and temporal information about the dynamics of core oscillatory proteins such as FRQ. However, due to the difficulty of studying the expression of circadian rhythm genes in aerobic N. crassa cultures, little is known about the dynamics of this gene under physiologically realistic conditions. To address these issues we report a fluorescent fusion to the frq gene using a codon optimized version of the mCherry gene. To trace the expression and accumulation of FRQ-mCherryNC (FRQ-mCh) during the circadian rhythm, growing vegetative hyphae were scanned every hour under confocal microscopy (100x). Fluorescence of FRQ-mCh was detected only at the growing edge of the colony, and located in the cytoplasm and nuclei of vegetative hyphae for a distance of approximately 150-200microm from the apices of leading hyphae. When driven by the frq promoter, apparently there was also a second FRQ entrance into the nucleus during the circadian cycle; however the second entrance had a lower accumulation level than the first entrance. Thus this fluorescent fusion protein has proven useful in tracking the spatial dynamics of the frq protein and has indicated that the dynamics of the FRQ protein's nuclear trafficking may be more complex than previously realized.

Impact of Diurnal Periodicity, Temperature, and Light on Sporulation of Bremia lactucae

ABSTRACT We evaluated direct and interactive effects of light quality and intensity, temperature and light, diurnal rhythms, and timing of high relative humidity during long day lengths on sporulation of Bremia lactucae, the causal agent of lettuce downy mildew, using inoculated lettuce seedlings and detached cotyledons. Suppression of sporulation by light was strongly dependent upon temperature and there was little suppression at </=10 degrees C. The most suppressive waveband was in the range from 400 to 450 nm, although a lesser effect of wavebands from 450 to 500 and 500 to 550 nm could be detected. At 15 degrees C, near the lower threshold for suppression of sporulation by light, a clear diurnal pattern of sporulation was observed independent of light and darkness. This diurnal rhythm potentially could interact with light and temperature to confound the results of controlled environment studies, and may be the controlling factor in timing of sporulation at low temperatures. Forecasting models that currently use sunrise and sunset to delimit periods conducive to sporulation can be adapted to short nights and extended twilight conditions by incorporating the effects reported herein. Additionally, models of sporulation could be adapted to better reflect a decrease or absence of the suppressive effect of light at <15 degrees C.

The rhythms of life: circadian output pathways in Neurospora

Research in Neurospora crassa pioneered the isolation of clock-controlled genes (ccgs), and more than 180 ccgs have been identified that function in various aspects of the fungal life cycle. Many clock-controlled genes are associated with damage repair, stress responses, intermediary metabolism, protein synthesis, and development. The expression of most of these genes peaks just before dawn and appears to prepare the cells for the desiccation, mutagenesis, and stress caused by sunlight. Progress on characterization of the output signaling pathways from the circadian oscillator mechanism to the ccgs is discussed. The authors also review evidence suggesting that, similar to other clock model organisms, a connection exists between the redox state of the cell and the Neurospora clock. The authors speculate that the clock system may sense not only light but also the redox potential of the cell through one of the PAS domains of the core clock components WC-1 or WC-2.

Circadian rhythms in Neurospora crassa: clock gene homologues in fungi

Computer-based analysis of a total of 17 filamentous fungal and yeasts genomes has shown: (1) homologues of frq, wc-1, wc-2, and vvd, key gene components of the Neurospora crassa clock, are present in Magnaporthe grisea, Gibberella zeae, and Podospora anserina, suggesting an frq-based oscillator in these organisms; (2) some fungal species that are more distantly related to Neurospora, such as Rhizopus oryzae do not appear to have frq homologues; (3) many fungal species that do not appear to contain frq, such as Aspergillus nidulans, do contain wc homologues; (4) of 11 well-described genes classified as clock-controlled genes (ccgs), in Neurospora, all of them were found to have homologues in other fungi; (5) the ccg-8 gene of N. crassa has homologies to opi1p, a transcriptional regulatory gene in Saccharomyces cerevisiae involved in inositol regulation. This suggests the possibilities of rhythmic inositol regulation, and/or a cascade of rhythmic activation of other genes in N. crassa.

Circadian rhythms in microorganisms: new complexities

Recent advances in understanding circadian (daily) rhythms in the genera Neurospora, Gonyaulax, and Synechococcus are reviewed and new complexities in their circadian systems are described. The previous model, consisting of a unidirectional flow of information from input to oscillator to output, has now expanded to include multiple input pathways, multiple oscillators, multiple outputs; and feedback from oscillator to input and output to oscillator. New posttranscriptional features of the frq/white-collar oscillator (FWC) of Neurospora are described, including protein phosphorylation and degradation, dimerization, and complex formation. Experimental evidence is presented for frq-less oscillator(s) (FLO) downstream of the FWC. Mathematical models of the Neurospora system are also discussed.

The Neurospora crassa pheromone precursor genes are regulated by the mating type locus and the circadian clock

Pheromones play important roles in female and male behaviour in the filamentous ascomycete fungi. To begin to explore the role of pheromones in mating, we have identified the genes encoding the sex pheromones of the heterothallic species Neurospora crassa. One gene, expressed exclusively in mat A strains, encodes a polypeptide containing multiple repeats of a putative pheromone sequence bordered by Kex2 processing sites. Strains of the opposite mating type, mat a, express a pheromone precursor gene whose polypeptide contains a C-terminal CAAX motif predicted to produce a mature pheromone with a C-terminal carboxy-methyl isoprenylated cysteine. The predicted sequences of the pheromones are remarkably similar to those encoded by other filamentous ascomycetes. The expression of the pheromone precursor genes is mating type specific and is under the control of the mating type locus. Furthermore, the genes are highly expressed in conidia and under conditions that favour sexual development. Both pheromone precursor genes are also regulated by the endogenous circadian clock in a time-of-day-specific fashion, supporting a role for the clock in mating.

The Neurospora circadian clock: simple or complex?

The fungus Neurospora crassa is being used by a number of research groups as a model organism to investigate circadian (daily) rhythmicity. In this review we concentrate on recent work relating to the complexity of the circadian system in this organism. We discuss: the advantages of Neurospora as a model system for clock studies; the frequency (frq), white collar-1 and white collar-2 genes and their roles in rhythmicity; the phenomenon of rhythmicity in null frq mutants and its implications for clock mechanisms; the study of output pathways using clock-controlled genes; other rhythms in fungi; mathematical modelling of the Neurospora circadian system; and the application of new technologies to the study of Neurospora rhythmicity. We conclude that there may be many gene products involved in the clock mechanism, there may be multiple interacting oscillators comprising the clock mechanism, there may be feedback from output pathways onto the oscillator(s) and from the oscillator(s) onto input pathways, and there may be several independent clocks coexisting in one organism. Thus even a relatively simple lower eukaryote can be used to address questions about a complex, networked circadian system.

Understanding circadian rhythmicity in Neurospora crassa: from behavior to genes and back again

Circadian clocks have been described in organisms ranging in complexity from unicells to mammals, in which they function to control daily rhythms in cellular activities and behavior. The significance of a detailed understanding of the clock can be appreciated by its ubiquity and its established involvement in human physiology, including endocrine function, sleep/wake cycles, psychiatric illness, and drug tolerances and effectiveness. Because the clock in all organisms is assembled within the cell and clock mechanisms are evolutionarily conserved, simple eukaryotes provide appropriate experimental systems for dissecting the clock. Significant progress has been made in deciphering the circadian system in Neurospora crassa using both genetic and molecular approaches, and Neurospora has contributed greatly to our understanding of (1) the feedback cycle that comprises a circadian oscillator, (2) the mechanisms by which the clock is kept in synchrony with the environment, and (3) the genes that reside in rhythmic output pathways. Importantly, the lessons learned in Neurospora are relevant to our understanding of clocks in higher eukaryotes.

Phosphorylation of the Neurospora clock protein FREQUENCY determines its degradation rate and strongly influences the period length of the circadian clock

Under free running conditions, FREQUENCY (FRQ) protein, a central component of the Neurospora circadian clock, is progressively phosphorylated, becoming highly phosphorylated before its degradation late in the circadian day. To understand the biological function of FRQ phosphorylation, kinase inhibitors were used to block FRQ phosphorylation in vivo and the effects on FRQ and the clock observed. 6-dimethylaminopurine (a general kinase inhibitor) is able to block FRQ phosphorylation in vivo, reducing the rate of phosphorylation and the degradation of FRQ and lengthening the period of the clock in a dose-dependent manner. To confirm the role of FRQ phosphorylation in this clock effect, phosphorylation sites in FRQ were identified by systematic mutagenesis of the FRQ ORF. The mutation of one phosphorylation site at Ser-513 leads to a dramatic reduction of the rate of FRQ degradation and a very long period (>30 hr) of the clock. Taken together, these data strongly suggest that FRQ phosphorylation triggers its degradation, and the degradation rate of FRQ is a major determining factor for the period length of the Neurospora circadian clock.

Spore Release of Bremia lactucae on Lettuce Is Affected by Timing of Light Initiation and Decrease in Relative Humidity

ABSTRACT A suction-impaction mini-spore trap was developed to study the effect of light initiation and decreasing relative humidity (RH) on spore release of Bremia lactucae in a controlled environment. Three light periods (from 0400 to 1600, 0600 to 1800, and 0800 to 2000 h, circadian time) at a constant RH of 99 to 100% were used for studying the effect of light initiation on spore release. Few spores were released during the dark periods. Spore release increased sharply after the initiation of the three light periods, reached a maximum 1 to 2 h after light initiation, and then declined until only a few spores could be detected. The effect of reduction in RH on spore release was studied by comparing decreases in RH 2 h before and 2 h after light initiation at 0800 h. When RH decreased from 100 to 94% 2 h before light initiation, spore release increased within 1 h, followed by a second increase after light initiation. When RH decreased 2 h after light initiation, spore release continued to increase after the initial increase after light initiation, reached a maximum 1 h after the reduction in RH, and then declined. The results suggest that both light initiation and reduction in RH can trigger spore release and that these factors have separate effects on spore release of Bremia lactucae.

Positive regulation of adenylyl cyclase activity by a galphai homolog in Neurospora crassa

GNA-1 and GNA-2 are two G protein alpha subunits from the filamentous fungus Neurospora crassa. Loss of gna-1 leads to multiple phenotypes, while Deltagna-2 strains do not exhibit visible defects. However, Deltagna-1Deltagna-2 mutants are more affected in Deltagna-1 phenotypes. Here we report a biochemical investigation of the roles of GNA-1 and GNA-2 in cAMP metabolism. Assays of Mg2+ ATP-dependent adenylyl cyclase activity (+/-GppNHp) in extracts from submerged cultures indicated that Deltagna-2 strains were normal, whereas Deltagna-1 and Deltagna-1Deltagna-2 strains had only 10-15% the activity of the wild-type control. Levels of the Gbeta protein, GNB-1, were normal in Deltagna-1 strains, excluding altered GNB-1 production as a factor in loss of adenylyl cyclase activity. Steady-state cAMP levels in Deltagna-1 and Deltagna-1Deltagna-2 mutants were reduced relative to wild-type under conditions that result in morphological abnormalities (solid medium), while levels in submerged culture were normal. cAMP phosphodiesterase activities in submerged cultures of Deltagna-1 and/or Deltagna-2 strains were lower than in wild-type; the individual deletions were additive in decreasing activity. These results suggest that in submerged culture, N. crassa, like mammalian systems, possesses compensatory mechanisms that maintain cAMP at relatively constant levels. Furthermore, the finding that Mg2+ATP-dependent adenylyl cyclase activity in wild-type cell extracts could be inhibited using anti-GNA-1 IgG suggests that GNA-1 directly interacts with adenylyl cyclase in N. crassa.

Time at the end of the millennium: the Neurospora clock

In the past two years we have entered the log phase for unraveling the molecular clockworks. Rapid progress in understanding the Neurospora clock has been complemented by a flood of information from diverse systems including cyanobacteria, insects and mice. There are broadly conserved features in transcription/translation based feedback loops. Conservation is also found at the sequence level, from fungi to mammals, in the PAS domains of the heterodimeric partners of the transcription factors that act as the positive components of the feedback cycle. Pivotal PAS proteins from Neurospora, the WCs, provide an evolutionary link connecting the clock in insects and mammals to the fungi and to light-harvesting proteins from bacteria.

How temperature changes reset a circadian oscillator

Circadian rhythms control many physiological activities. The environmental entrainment of rhythms involves the immediate responses of clock components. Levels of the clock protein FRQ were measured in Neurospora at various temperatures; at higher temperatures, the amount of FRQ oscillated around higher levels. Absolute FRQ amounts thus identified different times at different temperatures, so temperature shifts corresponded to shifts in clock time without immediate synthesis or turnover of components. Moderate temperature changes could dominate light-to-dark shifts in the influence of circadian timing. Temperature regulation of clock components could explain temperature resetting of rhythms and how single transitions can initiate rhythmicity from characteristic circadian phases.

Behavior of SCN-lesioned chipmunks in natural habitat: a pilot study

The necessity of a circadian pacemaker, the suprachiasmatic nuclei (SCN), for survival was evaluated in a population of approximately 65 wild eastern chipmunks, Tamias striatus. The research involved over 3000 h of field-work between May 1995 and October 1997 on a study site at Mountain Lake Biological Station, Virginia. The 28 chipmunks randomly designated as project animals included 10 SCN-lesioned chipmunks, 5 surgical controls (sham-lesioned), and 13 intact controls. Visual observation, live trapping, and radio telemetric tracking were used to assess 6 aspects of survival and reproduction. Upon release after surgery, every animal returned to its den site and was able to maintain its home territory. In warm months from May through October, all chipmunks were active above ground with a strictly day-active pattern. During the remaining cold months, they were normothermic but relatively torpid in their underground dens for extended periods of time. Short-term mortality for the initial 3 months included only a single intact control chipmunk; loss for the extended period from August 1995 to October 1996 was 40% for the SCN-lesioned animals, 0% for surgical controls, and 15.4% for the intact controls. Survival differences were not significant between surgical control and intact control groups but were significant (alpha = .10) between SCN-lesioned and pooled control groups. Annual body weight patterns were similar for both groups. Most individuals in both SCN-lesioned and control groups were reproductively active in the spring and fall breeding periods. Lack of major differences may be attributable to the exceptionally favorable conditions for survival such as a very abundant fall acorn crop, medium population size, and absence of heavy predation.