Introduction to Fungi, Third Edition

5
Straminipila: Oomycota
5.1 Introduction
The phylum Oomycota, alternatively called
Peronosporomycetes (Dick, 2001a), currently comprises
some 800 1000 species (Kirk et al., 2001).
The Oomycota as a whole have been resolved
as a monophyletic group within the kingdom
Straminipila in recent phylogenetic studies (e.g.
Riethmüller et al., 1999; Hudspeth et al., 2000;
see Fig. 4.2), although considerable rearrangements
are still being performed at the level of
orders and families. A scholarly treatment of the
Oomycota has been published by Dick (2001a)
and will remain the reference work for many
years to come. Because of the outstanding significance
of Oomycota, especially in plant pathology,
we give an extended treatment of this group.
5.1.1 The vegetative hypha
Although some members of the Oomycota grow
as sac-like or branched thalli, most of them
produce hyphae forming a mycelium. Oomycota
are now known to be the result of convergent
evolution with the true fungi (Eumycota), and
their hyphae differ in certain details. However,
the overall functional similarities are so great
that they provide a persuasive argument for
the fundamental importance of the hypha in
the lifestyle of fungi (Barr, 1992; Carlile, 1995;
Bartnicki-Garcia, 1996). Much physiological work
has been carried out on hyphae of Oomycota (see
Chapter 1), and the results have a direct bearing
on our understanding of the biology of the
Eumycota. Like them, the hyphae of Oomycota
display apical growth and enzyme secretion,
ramify throughout the substratum by branching
to form a mycelium, and can show morphogenetic
plasticity by differentiation into specialized
structures such as appressoria or haustoria.
The hyphae of Oomycota are coenocytic, i.e.
they generally do not form cross-walls (septa)
except in old compartments or at the base of
reproductive structures. The cytoplasm is generally
coarsely granular and contains vacuoles,
Golgi stacks, mitochondria and diploid nuclei.
The apex is devoid of organelles other than
numerous secretory vesicles. These are not, as
in the Eumycota, arranged into a Spitzenkörper
because the microvesicles which contain chitin
synthase and make up the Spitzenkörper core are
lacking. This is in line with the general absence,
with a few exceptions, of chitin from the walls of
Oomycota; instead, cellulose, a crystalline b-(1,4)-
glucan, contributes the main fibrous component.
As in the Eumycota, these structural fibres
are cross-linked by branched b-(1,3)- and b-(1,6)-
glucans, although the biochemical properties of
the glucan synthases seem to differ fundamentally
between those of Eumycota on the one hand
and those of Oomycota and plants on the other
(Antelo et al., 1998). Other biochemical differences
include the lysine synthetic pathway (DAP
in plants and Oomycota; AAA in true Fungi;
see p. 67) and details of sterol metabolism (Nes,
1990; Dick, 2001a).
The mitochondria of Oomycota are indistinguishable
by light microscopy from those of the
Eumycota, but when viewed with the transmission
electron microscope they have tubular