Rhythms of differentiation and diacylglycerol in Neurospora

Functional analysis of diacylglycerol O-acyl transferase 2 gene to decipher its role in virulence of Botrytis cinerea

Gray mold disease inflicted by Botrytis cinerea is a serious menace responsible for significant economic loss worldwide. Due to its polyphagous nature, the pathogen has enthused inquisitiveness in researchers to unravel its complexity. Agrobacterium tumefaciens-mediated transformation was used to generate insertional mutants of Botrytis cinerea. A mutant (BCM-55) with disruption in a gene (BcDGAT2) that encodes for diacylglycerol O-acyl transferase 2 (DGAT2), showed enervated virulence on various hosts' tissues. Enzyme DGAT2 is crucial in the final step of synthesis of triacylglycerol (TAG) that plays an important role in homeostasis of membrane and cellular processes. However, the role of DGAT2 has never been reported in a phytopathogenic fungus. In this study, BCM-55 was characterized to ascertain the role of DGAT2 in virulence of B. cinerea. The insertional mutant was defective in spore production and lacked sclerotia formation as a consequence of lower accumulation of TAG. A significant delay in spore germination in BCM-55 was accompanied with a low penetration potential. Hyphae of the mutant formed swollen endings with considerable impairment in penetration. Deletion of BcDGAT2 also led to increased sensitivity towards cell wall and membrane-disturbing agents. Furthermore, BCM-55 was deficient in the production of oxalic acid and showed lower activity of a cell wall-degrading enzyme, polygalacturonase. The role of BcDGAT2 in virulence was further confirmed by targeted deletion and complementation of the gene. The results insinuate a crucial role of BcDGAT2 in penetration and consequently virulence of B. cinerea. The study provides novel insights into plant-pathogen interactions that can be exploited to develop suitable disease management strategies.

Time-Lapse Analysis of the Circadian Rhythms of Conidiation and Growth Rate in Neurospora

The filamentous fungus Neurospora crassa has frequently served as a model organism for the study of circadian rhythms through its ability to form conidial spores on a daily basis. This phenomenon leaves a spatial pattern of conidiation bands along a solid surface of agar after several days of growth. Using time-lapse video, the authors have quantified the rate of conidiation. They have found that conidia do not form at a specified lag time after the growth front is laid down, but rather the band region tends to simultaneously develop over a short time frame. This produces a sharp peak when the conidiation rate is plotted against time. In addition, the authors used time-lapse video to assay growth rate with greater accuracy than previously reported. It is usually assumed that Neurospora’s rate of growth is constant, and this assumption of linear growth has been used extensively to determine period and phase of the conidiation circadian rhythm. The authors have confirmed an earlier report of nonlinear growth rate and have shown that the growth rate varies by a factor of about 2 with each circadian cycle. They have demonstrated that the errors in calculating times of conidiation peaks are maximally 1 to 2 h if linearity is assumed. The conidiation rate and growth rate rhythms are not apparent under conditions (using mutants or high or low temperatures) where the spatial banding rhythm is not observed. In light/dark entraining conditions, the conidiation rate and growth rate rhythms maintain the same phase relationship in different T-cycles. These data are consistent with the hypothesis that the growth rate rhythm is a consequence of the conidiation rate rhythm.

Glycerol-3-phosphate Acyltransferase contributes to triacylglycerol biosynthesis, lipid droplet formation, and host invasion in Metarhizium robertsii

Enzymes involved in the triacylglycerol (TAG) biosynthesis have been well studied in the model organisms of yeasts and animals. Among these, the isoforms of glycerol-3-phosphate acyltransferase (GPAT) redundantly catalyze the first and rate-limiting step in glycerolipid synthesis. Here, we report the functions of mrGAT, a GPAT ortholog, in an insect-pathogenic fungus, Metarhizium robertsii. Unlike in yeasts and animals, a single copy of the mrGAT gene is present in the fungal genome and the gene deletion mutant is viable. Compared to the wild type and the gene-rescued mutant, the ΔmrGAT mutant demonstrated reduced abilities to produce conidia and synthesize TAG, glycerol, and total lipids. More importantly, we found that mrGAT is localized to the endoplasmic reticulum and directly linked to the formation of lipid droplets (LDs) in fungal cells. Insect bioassay results showed that mrGAT is required for full fungal virulence by aiding fungal penetration of host cuticles. Data from this study not only advance our understanding of GPAT functions in fungi but also suggest that filamentous fungi such as M. robertsii can serve as a good model to elucidate the role of the glycerol phosphate pathway in fungal physiology, particularly to determine the mechanistic connection of GPAT to LD formation.

Assessment of Circadian and Light-Entrainable Parameters in Mice Using Wheel-Running Activity

In most organisms, physiological variables are regulated by an internal clock. This endogenous circadian (∼24-hr) clock enables organisms to anticipate daily environmental changes and modify behavioral and physiological functions appropriately. Processes regulated by the circadian clock include sleep-wake and locomotor activity, core body temperature, metabolism, water/food intake, and available hormone levels. At the core of the mammalian circadian system are molecular oscillations within the hypothalamic suprachiasmatic nucleus. These oscillations are modifiable by signals from the environment (so called zeitgebers or time-givers) and, once integrated within the suprachiasmatic nucleus, are conveyed to remote neural circuits where output rhythms are regulated. Disrupting any of a number of neural processes can affect how rhythms are generated and relayed to the periphery and disturbances in circadian/entrainment parameters are associated with numerous human conditions. These non-invasive protocols can be used to determine whether circadian/entrainment parameters are affected in mouse mutants or treatment groups. Curr. Protoc. Mouse Biol. 1:369-381 © 2011 by John Wiley & Sons, Inc.

Circadian rhythms in Neurospora crassa: Downstream effectors

The circadian rhythm in Neurospora crassa is exhibited as alternating areas of conidiating and non-conidiating mycelia growth. A significant role in this circadian rhythm is played by the frq (frequency) and wc (white-collar) genes, comprising the "FWC" oscillator. Strains lacking the FWC can be restored to rhythmicity, which has been attributed to a second oscillator, called the FLO (frq-less oscillator). This study reports additional conditions that allow this rhythmicity to occur. Rhythmicity was restored to mutants lacking either the frq, or wc-1, or wc-2 genes in D/D (constant darkness) or L/L (constant light) by the addition of low levels of menadione, a known stimulator of ROS (reactive oxygen species). Additional studies are reported on the rhythm effects from caffeine, a known cAMP phospho-diesterase inhibitor as well as the effects from mutations in the csp-1 gene, the rco-1 gene, and other genes. A theme ties all of these "downstream effects" together, i.e., they affect either components thought to be part of the conidiation process itself, or the RAS-cAMP-protein kinase pathway. Since mutations in these components unexpectedly had rhythm effects, this suggests that these components may be good candidates for some part of the frq-less oscillator.

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.

Cellular signalling and the complexity of biological timing: insights from the ultradian clock of Schizosaccharomyces pombe

The molecular bases of circadian clocks are complex and cannot be sufficiently explained by the relatively simple feedback loops, based on transcription and translation, of current models. The existence of additional oscillators has been demonstrated experimentally, but their mechanism(s) have so far resisted elucidation and any universally conserved clock components have yet to be identified. The fission yeast, Schizosaccharomyces pombe, as a simple and well-characterized eukaryote, is a useful model organism in the investigation of many aspects of cell regulation. In fast-growing cells of the yeast an ultradian clock operates, which can serve as a model system to analyse clock complexity. This clock shares strict period homeostasis and efficient entrainment with circadian clocks but, because of its short period of 30 min, mechanisms other than a transcription/translation-based feedback loop must be working. An initial systematic screen involving over 200 deletion mutants has shown that major cellular signalling pathways (calcium/phosphoinositide, mitogen-activated protein kinase and cAMP/protein kinase A) are crucial for the normal functioning of this ultradian clock. A comparative examination of the role of cellular signalling pathways in the S.pombe ultradian clock and in the circadian timekeeping of different eukaryotes may indicate common principles in biological timing processes that are universally conserved amongst eukaryotes.

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.