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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).