Genetic characterization of two phototropism mutants of Phycomyces with defects in the genes madI and madJ

A Ras GTPase associated protein is involved in the phototropic and circadian photobiology responses in fungi

Light is an environmental signal perceived by most eukaryotic organisms and that can have major impacts on their growth and development. The MadC protein in the fungus Phycomyces blakesleeanus (Mucoromycotina) has been postulated to form part of the photosensory input for phototropism of the fruiting body sporangiophores, but the madC gene has remained unidentified since the 1960s when madC mutants were first isolated. In this study the madC gene was identified by positional cloning. All madC mutant strains contain loss-of-function point mutations within a gene predicted to encode a GTPase activating protein (GAP) for Ras. The madC gene complements the Saccharomyces cerevisiae Ras-GAP ira1 mutant and the encoded MadC protein interacts with P. blakesleeanus Ras homologs in yeast two-hybrid assays, indicating that MadC is a regulator of Ras signaling. Deletion of the homolog in the filamentous ascomycete Neurospora crassa affects the circadian clock output, yielding a pattern of asexual conidiation similar to a ras-1 mutant that is used in circadian studies in N. crassa. Thus, MadC is unlikely to be a photosensor, yet is a fundamental link in the photoresponses from blue light perceived by the conserved White Collar complex with Ras signaling in two distantly-related filamentous fungal species.

A new genetic linkage map of the zygomycete fungus Phycomyces blakesleeanus

Phycomyces blakesleeanus is a member of the subphylum Mucoromycotina. A genetic map was constructed from 121 progeny of a cross between two wild type isolates of P. blakesleeanus with 134 markers. The markers were mostly PCR-RFLPs. Markers were located on 46 scaffolds of the genome sequence, covering more than 97% of the genome. Analysis of the alleles in the progeny revealed nine or 12 linkage groups, depending on the log of the odds (LOD) score, across 1583.4 cM at LOD 5. The linkage groups were overlaid on previous mapping data from crosses between mutants, aided by new identification of the mutations in primary metabolism mutant strains. The molecular marker map, the phenotype map and the genome sequence are overall congruent, with some exceptions. The new genetic map provides a genome-wide estimate for recombination, with the average of 33.2 kb per cM. This frequency is one piece of evidence for meiosis during zygospore development in Mucoromycotina species. At the same time as meiosis, transmission of non-recombinant chromosomes is also evident in the mating process in Phycomyces. The new map provides scaffold ordering for the genome sequence and a platform upon which to identify the genes in mutants that are affected in traits of interest, such as carotene biosynthesis, phototropism or gravitropism, using positional cloning.

Surface tip-to-base Ca2+ and H+ ionic fluxes are involved in apical growth and graviperception of the Phycomyces stage I sporangiophore

Net fluxes of Ca(2+) and H(+) ions were measured non-invasively close to the surface of Phycomyces blakesleeanus sporangiophores stage I using ion-selective vibrating microelectrodes. The measurements were performed on a wild type (Wt) and a gravitropic mutant A909 kept in either vertical or tilted orientation. Microelectrodes were positioned 4 μm from the surface of sporangiophore, and ion fluxes were recorded from the apical (0-20 μm) and subapical (50-100 μm) regions. The magnitude and direction of ionic fluxes measured were dependent on the distance from the tip along the growing zone of sporangiophore. Vertically oriented sporangiophores displayed characteristic tip-to-base ion fluxes patterns. Ca(2+) and H(+) fluxes recorded from apical region of Wt sporangiophores were inward-directed, while ion fluxes from subapical locations occurred in both directions. In contrast to Wt, mutant A909 showed opposite (outward) direction of Ca(2+) fluxes and reduced H(+) influxes in the apical region. Following gravistimulation, the magnitude and direction of ionic fluxes were altered. Wt sporangiophore exhibited oppositely directed fluxes on the lower (influx) and the upper (efflux) sides of the cell, while mutant A909 did not show such patterns. A variable elongation growth in vertical position and reduced growth rate upon gravistimulation were observed in both strains. The data show that tip-growing sporangiophores exhibit a tip-to-base ion flux pattern which changes characteristically upon gravistimulation in Wt in contrast to the mutant A909 with a strongly reduced gravitropic response.

Photobiology in the Zygomycota: multiple photoreceptor genes for complex responses to light

Light is an environmental signal that modulates many aspects of the biology of zygomycete fungi. Light regulation has been investigated in the zygomycetes Phycomyces blakesleeanus, Mucor circinelloides and Pilobolus crystallinus. Examples of light regulation include the phototropism of the fruiting bodies, the regulation of the development of reproductive structures, and the activation of the biosynthesis of β-carotene. In fungi blue light is perceived by proteins homologous to WC-1, a Neurospora crassa photoreceptor and Zn finger protein that interacts with WC-2 to form a photoresponsive transcription factor complex. Unlike ascomycete and basidiomycete fungi that usually have one wc-1 and one wc-2 gene, several studies have uncovered an unexpected multitude of genes similar to wc-1 and wc-2 in the genomes of several zygomycete fungi. Some of these genes are required for fungal photoresponses, but the function of many of them remains unknown. The presence of multiple wc-1 genes confirms previous suggestions of multiple blue-light photoreceptors in Phycomyces.

Phycomyces MADB interacts with MADA to form the primary photoreceptor complex for fungal phototropism

The fungus Phycomyces blakesleeanus reacts to environmental signals, including light, gravity, touch, and the presence of nearby objects, by changing the speed and direction of growth of its fruiting body (sporangiophore). Phototropism, growth toward light, shares many features in fungi and plants but the molecular mechanisms remain to be fully elucidated. Phycomyces mutants with altered phototropism were isolated approximately 40 years ago and found to have mutations in the mad genes. All of the responses to light in Phycomyces require the products of the madA and madB genes. We showed that madA encodes a protein similar to the Neurospora blue-light photoreceptor, zinc-finger protein WC-1. We show here that madB encodes a protein similar to the Neurospora zinc-finger protein WC-2. MADA and MADB interact to form a complex in yeast 2-hybrid assays and when coexpressed in E. coli, providing evidence that phototropism and other responses to light are mediated by a photoresponsive transcription factor complex. The Phycomyces genome contains 3 genes similar to wc-1, and 4 genes similar to wc-2, many of which are regulated by light in a madA or madB dependent manner. We did not detect any interactions between additional WC proteins in yeast 2-hybrid assays, which suggest that MADA and MADB form the major photoreceptor complex in Phycomyces. However, the presence of multiple wc genes in Phycomyces may enable perception across a broad range of light intensities, and may provide specialized photoreceptors for distinct photoresponses.

Topology of an intracellular transduction chain (phototropism of Phycomyces): 1. Joint review of functional, temporal, and spatial aspects

Two light-induced growth reactions in a unicellular cylindrical sporangiophore of Phycomyces blakesleeanus-vertical growth acceleration under symmetrical irradiation (photomecism) and directional growth under unilateral irradiation (phototropism)-share common input light perception as well as common output growth mechanism but have strongly divergent dynamics and other distinctive features. This divergence culminates in the phototropic paradoxes the main of which states that photomecism shows total adaptation, while phototropism does not adapt. The basis for this contradiction is that the phototropic transduction chain, unlike that of photomecism, faces a spatially non-uniform stimulus and processes a series of spatial patterns (light and absorption profiles, adaptation profile, etc.). The only way to resolve the paradoxes and correlate features of both responses within a single transduction chain is to assume non-local signal transduction, e.g. a cross-talk between different azimuthal locations within the cylindrical cell. On the other hand, to establish the presence of an appropriate cross-talk is equivalent of gaining insight into the topology of the transduction chain. This series of two papers contains a review reconsidering the entire field from this viewpoint (Paper 1) and a mathematical model of pattern transduction which unifies features of phototropism and resolves the paradoxes (Paper 2). At the same time, this is the first "proof of concept" for the "activity/pooling (a/p) networks"-a specific mathematical apparatus designed to analyse systemic properties and control in metabolic pathways.

Blue-light receptor requirement for gravitropism, autochemotropism and ethylene response in Phycomyces

Light, gravity and ethylene represent for plants and fungi important environment cues for spatial orientation and growth regulation. Coordination of the frequently conflicting stimuli requires signal-integration sites, which, however, remain largely unidentified. The genetic and physiological basis for signal integration was investigated with a set of phototropism mutants (genotype mad) of the UV- and blue-light-sensitive fungus Phycomyces blakesleeanus, which responds also to gravity, ethylene and nearby obstacles (autochemotropism or avoidance response). Both, class 1 and class 2 mutants display a reduced sensitivity to visible light. Class 1 mutants with defects in genes madA, B, C, I have preserved their sensitivity to gravity and ethylene, whereas class 2 mutants with defects in genes madD,E,F,G,J have lost it. We found that the phototropic sensitivity of class 1 mutants is affected roughly to the same extent in far UV and blue light. In contrast, the sensitivity loss of class 2 mutants is restricted mainly to the near-UV and the blue-light region, whereas the sensitivity to far UV is only mildly affected. This behavior of the class 2 mutants indicates that different photoreceptors mediate phototropism in far-UV and in near-UV/ blue light. The photogravitropic action spectra for two class 2 mutants with defects in genes madF and madJ display distortions between 342 and 530 nm and a bathochromic shift relative to the action spectrum of the wild type. These features indicate that the madF and madJ mutants are affected at the level of the blue-light photoreceptor system. As an implication we infer that an intact near-UV/blue-light photoreceptor system is required even in darkness for negative gravitropism, the ethylene response and autochemotropism. In Phycomyces, signal integration occurs, at least in part, at the level of the near-UV/blue-light photoreceptor system.