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of asexual sporing structures or sexual reproductive
structures in several (but not all) fungi. For example,
the toadstools and similar fruitbodies of many
Basidiomycota are formed in response to light, but
often with an additional requirement for a low level
of CO 2. Often these photoresponses are elicited by
NUV or blue light (about 450 nm), implicating a
flavin-type photoreceptor. But there is considerable
variation in the photoresponses of different fungi,
almost certainly related to habitat requirements. For
example, Alternariaspp. are induced to sporulate by UV
irradiation (280 – 290 nm), and in Botrytis cinereathe
triggering by NUV is reversed by subsequent exposure
to blue light (Chapter 5). The sporulation of some other
fungi is regulated by exposure to red/far red light, but
this is less common than the blue light responses. Light
also has other effects on fungal reproductive structures,
notably in eliciting phototropism of the sporangiophores
of some Zygomycota and of the ascus tips of some
Ascomycota (Chapter 10).


Genetic dissection of blue light
perception in Neurospora crassa

Neurospora crassahas always been an important model
eukaryote, owing to its relatively small genome (about
40 megabases), its rapid growth and manipulability,
its ease of genetic manipulation by random and stable
integration of foreign DNA, and an abundance of


well-characterized mutants. It is also a preferred model
organism for investigating light perception, for two
reasons. First, it perceives light only in the blue/UV
range, and relatively few genes (which are well-
characterized) seem to be involved in this response.
Second, it shows a pronounced circadian rhythm –
a molecular clock that has an innate period length
close to 24 hours and that is compensated against
temperature and nutrition, but can be reset by envir-
onmental (light) cues. In fact, there is a strong
interaction between the circadian clock of Neurospora
and the perception of light and subsequent signal
transduction pathways. The recent publication of a high-
quality draft genome sequence of N. crassa(Galagan
et al. 2003) should provide further insights into light
perception.
Turning specifically to the light response, a recent
series of papers have characterized the first fungal blue
light photoreceptor. It is a regulatory protein termed
White Collar 1 (WC-1) linked to a chromophore – the
yellow-pigmented flavin adenine dinucleotide (FAD).
The WC-1 protein interacts with DNA to initiate gene
transcription. Another protein (WC-2) also acts as a
transcription factor and forms a complex with WC-1.
This WC-1/WC-2 complex is localized in the nucleus
and targets the light signal to the promoters of the
blue-light-regulated genes. The wild-type genes for
both of these proteins had been known for some time,
and strains carrying mutant WC genes were known
to be “blind” – they were unable to induce carotenoid

156 CHAPTER 8

Fig. 8.13Examples of darkly pigmented
spores of fungi that commonly grow on
senescing leaves and stems of plants,
but also grow on bathroom and kitchen
walls. These fungi are often termed
dematiaceous hyphomycetes (with
darkly pigmentedhyphae and spores).
(a) Several spores of Alternaria spp.
(30– 40μm) viewed by phase-contrast
or bright-field microscopy. (b) Spores
of Cladosporiumspp. and related fungi
(a) (b) (about 10–15mm).

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