Growth, Differentiation and Sexuality

252 L.M. Corrochano and P. Galland
in the graviperception of plants, plays a substantial
role also in the gravitropism of Gigaspora rosea,
because lanthanum (a Ca 2+ blocker) and EGTA
(a Ca 2+ chelator) induce hyphal branching, and
atthesametimealsoinhibitionofgravitropism
(Berbara et al. 2002).
The gravisusceptor of these mycorrhizal fungi
are presently unknown. In view of the fact that these
oleaginous fungi contain numerous lipid droplets,
it appears reasonable to assume that the buoyancy
of these droplets mediates graviperception.
We found that vertically growing hyphae of Gigaspora
margarita possess an apical complex of lipid
globules (CLG) that is similar to the one found in
Phycomyces (unpublished data). Even in the CLG of
Gigaspora, the lipid globules are in continual motion.
Lateral hyphae growing horizontally do not
contain apical lipid globules and are agravitropic.
Thus, lipid globules occur only in segments that
grow vertically upward. The close correlation between
gravitropic orientation and the occurrence
of lipid globules clearly indicates their role as gravisusceptors
(Döring, unpublished data).
IV. Conclusions
Light and gravity are environmental stimuli responsible
for the final appearance of fungi. For
many decades, research on fungal photomorphogenesis
and gravitropic responses has concentrated
on the description and characterization of the responses
to light and gravity. Genetics and molecular
biology have been employed only with limited
examples, but nevertheless with great success. The
isolation of blind mutants has allowed the identification
of relevant proteins, like the photoreceptor
WC-1 from Neurospora, confirming the importance
of a detailed genetic analysis in the dissection
of a photoresponse. Additionally, the Neurospora
genome sequence has allowed the identification of
several putative photoreceptors whose functions
are still unknown. The photoreceptor WC-1 binds
DNA promoters of light-inducible genes, establishing
a very simple transduction chain. Fungal homologs
of the WC-1 photoreceptor have been identified
in other ascomycetes and basidiomycetes, but
their function may not be exactly the same as that
of their Neurospora counterparts, based on the absence
of some functional domains and differences
in their pattern of expression. Neurospora photobiology
is now considered a model for other fungi,
but only future research will either confirm this, or
will establish differences in fungal photobiology.
Undoubtedly, the continuing sequencing of fungal
genomes will increase the number of putative elements
in light transduction pathways, and these
willserveasstartingpointsforfutureresearchin
fungal photobiology. The combination of physiology
with genetics and molecular biology should
be employed for a fuller understanding of fungal
photomorphogenesis, from the primary photoreception
to the final developmental response.
The molecular basis of fungal gravitropism,
however, is still rudimentary. Several examples of
gravisusceptors and mechanisms for gravity sensing
have been suggested and presented here. We
hope that the combination of genetics and molecular
biology, with detailed cytological characterization
of model systems, will help to unravel the complexities
of fungal gravitropism. Light and gravity
interact in complex ways to direct sporangiophore
growth in Phycomyces, possibly at the level of the
photoreceptor system itself. It is thus possible that
light and gravity will share some of the elements
of the transduction chain in fungi. Only future research
will confirm the possible relationship between
light and gravity as major signals for fungal
development, growth, and appearance.
References
Adams TH, Wieser JK, Yu JH (1998) Asexual sporulation in
Aspergillus nidulans. Microbiol Mol Biol Rev 62:35–54
Aguirre J, Ríos-Momberg M, Hewitt D, Hansberg W (2005)
Reactive oxygen species and development in microbial
eukaryotes. Trends Miocrobiol 13:111–118
Alvarez MI, Benito EP, Campuzano V, Eslava AP (1992)
Genetic loci of Phycomyces blakesleeanus. In:O’Brien
SJ (ed) Genetic maps. Locus maps of complex genomes,
6th edn. Cold Spring Harbor Laboratory Press, Cold
Spring Harbor, NY, pp 3120–3126
Ambra R, Grimaldi B, Zamboni S, Filetici P, Macino G, Ballario
P (2004) Photomorphogenesis in the hypogeous
fungus Tuber borchii: isolation and characterization of
Tbwc-1, the homologue of the blue-light photoreceptor
of Neurospora crassa. Fungal Genet Biol 41:688–697
Arpaia G, Loros JJ, Dunlap JC, Morelli G, Macino G (1993)
The interplay of light and the circadian clock. Independent
dual regulation of clock-controlled gene ccg-
2(eas). Plant Physiol 102:1299–1305
Arpaia G, Loros JJ, Dunlap JC, Morelli G, Macino G (1995)
Lightinduction ofthe clock-controlledgeneccg-1 is not
transduced through the circadian clock in Neurospora
crassa. Mol Gen Genet 247:157–163
Arpaia G, Cerri F, Baima S, Macino G (1999) Involvement of
protein kinase C in the response of Neurospora crassa
to blue light. Mol Gen Genet 262:314–322