Mutants of Phycomyces with abnormal phototropism induced by ICR-170

Grafting as a Tool for Study of Phototropism in Phycomyces

Physiological responses, such as phototropism and carotenogenesis, are usually exhibited through a specific cascade composed of several gene functions. The number of such gene functions can be determined by means of complementation analysis. For this purpose, a procedure is needed to produce heterokaryons easily and with a high success rate. Here, we present a method of grafting sporangiophores from different mutants to obtain heterokaryotic regenerates at the graft union, based on the large size of the sporangiophore and its remarkable regeneration capability.

The sporangiophore of Phycomyces blakesleeanus: a tool to investigate fungal gravireception and graviresponses

The giant sporangiophore of the single-celled fungus, Phycomyces blakesleeanus, utilises light, gravity and gases (water and ethylene) as environmental cues for spatial orientation. Even though gravitropism is ubiquitous in fungi (Naturwissenschaftliche Rundschau, 1996, 49, 174), the underlying mechanisms of gravireception are far less understood than those operating in plants. The amenability of Phycomyces to classical genetics and the availability of its genome sequence makes it essential to fill this knowledge gap and serve as a paradigm for fungal gravireception. The physiological phenomena describing the gravitropism of plants, foremost adherence to the so-called sine law, hold even for Phycomyces. Additional phenomena pertaining to gravireception, specifically adherence to the novel exponential law and non-adherence to the classical resultant law of gravitropism, were for the first time investigated for Phycomyces. Sporangiophores possess a novel type of gravisusceptor, i.e. lipid globules that act by buoyancy rather than sedimentation and that are associated with a network of actin cables (Plant Biology, 2013). Gravitropic bending is associated with ion currents generated by directed Ca(2+) and H(+) transport in the growing zone (Annals of the New York Academy of Sciences, 2005, 1048, 487; Planta, 2012, 236, 1817). A set of behavioural mutants with specific defects in gravi- and/or photoreception allowed dissection of the respective transduction chains. The complex phenotypes of these mutants led to abandoning the concept of simple linear transduction chains in favour of interacting networks with molecular modules of physically interacting proteins.

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.

The Phycomyces madA gene encodes a blue-light photoreceptor for phototropism and other light responses

Phycomyces blakesleeanus is a filamentous zygomycete fungus that produces striking elongated single cells that extend up to 10 cm into the air, with each such sporangiophore supporting a sphere containing the spores for dispersal. This organism has served as a model for the detection of environmental signals as diverse as light, chemicals, touch, wind, gravity, and adjacent objects. In particular, sporangiophore growth is regulated by light, and it exhibits phototropism by bending toward near-UV and blue wavelengths and away from far-UV wavelengths in a manner that is physiologically similar to plant phototropic responses. The Phycomyces madA mutants were first isolated more than 40 years ago, and they exhibit reduced sensitivity to light. Here, we identify two (duplicated) homologs in the White Collar 1 family of blue-light photoreceptors in Phycomyces. We describe that the madA mutant strains contain point mutations in one of these genes and that these mutations cosegregate with a defect in phototropism after genetic crosses. Thus, the phototropic responses of fungi through madA and plants through phototropin rely on diverse proteins; however, these proteins share a conserved flavin-binding domain for photon detection.

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

Two Phycomyces genes, madI and madJ, which are involved in phototropism, were characterized by recombination and complementation analyses. The madI gene was located on linkage group IV of the genetic map of Phycomyces, 27 map units away from the gene carA. Complementation and recombination studies involving the genes madD, madE, madF, and madG, in combination with previous genetic studies, show that the recently isolated mad-407 mutation defines a novel behavioural gene, madJ, of Phycomyces. A regulatory role of the madJ gene product in the light-sensory transduction pathway is suggested.

Isolation and characterization of phototropism mutants of Phycomyces insensitive to ultraviolet light

Phototropism mutants of the zygomycete fungus Phycomyces blakesleeanus were isolated on the basis of their loss of responsivity to UV light. Four of these mutants had retained a partial sensitivity to near-UV and to blue light. Gravitropism and the avoidance response were unaffected in these mutants. One mutant, A909, had lost most of its sensitivity to near-UV and blue light while the sensitivity to far-UV light was only slightly affected. Additionally, the gravitropic and the avoidance responses were significantly reduced in A909. A complementation analysis of the five strains of Phycomyces with known phototropism mutants indicated that strains A896, A897, and A898 were defective in the madA gene, and that A905 was affected in the madC gene. In strain A909 the input, as well as the output, of the transduction chain is affected.

A new gene (madI) involved in the phototropic response of Phycomyces

Only eight genes are known to be involved in the phototropic response of Phycomyces (madA-H). Mutants affected in these genes have played a major role in the analysis of photosensory transduction processes in this system. A set of new mutants isolated by Alvarez et al. (1989) that are unable to bend towards dim unilateral blue light were studied by complementation and recombination. Two of these mutants have mutations in madE, one has a mutation in madF and one is a double madE madF mutant. The three remaining mutants tested did not complement each other and showed positive complementation with strains carrying mutations in the genes madA, madB, and madC, indicating that they carried mutations in a new gene designated madI. Recombination analysis showed that madI is unlinked to madA, madB and madC.