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130 Cha pte r F i v e

Fungal growth is localized at the cell tip, eventually creating a
tubular hypha. The hyphae of many fungi (e.g., asexual stages of
Ascomycetes) including A. nidulans are internally subdivided by
cross walls called septa into cell-like compartments (Fig. 5.1b). Septa
contain additional wall layers as well as a central core that appears
(using transmission electron microscopy) to have the same composition
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as the lateral walls. In vegetative hyphae, septa have a pore that
maintains cytoplasmic and pressure continuity between compartments;
septum-associated organelles called Woronin bodies protect hyphae from
excessive loss of cytoplasm following damage. 20
Using conditional septation-defective strains of A. nidulans, septa
have been shown to be important for sporulation. 21,22 Isolation of a
portion of hyphal cytoplasm is an early stage in spore formation.
Notably, the conidiophore foot cell is isolated from the hyphal
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cytoplasm by an additional wall layer, which is consistent with
work in other systems showing that the inner cell wall has important
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regulatory functions. In fungi such as Rhizopus, which have aseptate
vegetative hyphae, there is a cross wall that delimits the sporangium
from its supporting cell, the sporangiophore. In the oomycete
Saprolegnia, which also has aseptate vegetative hyphae, septum
formation is required for commitment to asexual spore formation. 25
Both of these specialized septa lack a pore, making them substantially
similar to the secondary wall of the Aspergillus conidiophore foot
cell. Taken together, septa are important for both vegetative growth
and for sporulation, and may have additional roles that are less well
understood. However, because septal walls likely comprise less than
5 percent of the total fungal wall material, studies characterizing their
composition have been limited. 26

5.3 Vibrational Spectroscopy
As this book is entirely devoted to applications of vibrational
spectroscopy in biosciences, a complete introduction to the topic, in
every chapter, is clearly superfluous. Here we present the basics, with
some general tissue examples and then focus on aspects that we have
found to be relevant to analyses of fungi.
Vibrational spectroscopy (IR and Raman) is a long-established
technique used to identify molecular structures and specific molecular
functional groups. Energies are typically reported as inverse
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−1
wavelength, or wavenumber, hence the unit is cm . In FTIR,
molecules absorb energy (photons in the mid-infrared region of the
electromagnetic spectrum) and begin to vibrate: bonds stretch;
segments of the molecule twist, rock, or bend. The specific frequencies
of light absorbed depend on the number and type of molecular
bonds present in the sample.
For any given molecule, there will be a total of 3N-6 possible
vibrational modes, where N is the number of atoms in the molecule.