untitled

defensible, so long as we recognize that the fungus-like

organisms are not true fungi. The main fungus-like

organisms that can be considered as fungi in a broader

sense (Box 2.1) are:

• The Oomycota, which belong to the Kingdom
  Straminipila (stramenipiles; Dick 2001) and are
  closely related to the golden-brown algae and diatoms.


• The acrasid and dictyostelid slime moulds
  (Acrasiomycotaand Dictyosteliomycota).


• The plasmodial slime moulds (Myxomycota).


• The plasmodiophorids (Plasmodiophoromycota).

The Oomycota

The Oomycota are the most economically important

fungus-like organisms and play extremely significant

roles in many environmental processes. They are

particularly important as plant pathogens, and cause

some of the most devastating plant diseases, such as

potato blight (caused by Phytophthora infestans),

sudden oak death(caused by Phytophthora ramorum),

many seedling and “decline” diseases caused by

Pythiumspp., and several downy mildew diseases.

These diseases are treated in detail in Chapter 14.

Many other aspects of the Oomycota are covered in

Chapters 3 and 10.

One of the most remarkable features of Oomycota

is that they behave exactly like the true fungi. Their

hyphae exhibit apical growth and penetrate plant cells

by producing cell-wall-degrading enzymes. They also

use similar infection strategies to those of fungi. So,

by a remarkable degree of convergent evolution the

Oomycota have developed a lifestyle equivalent to

that of fungi (Latijnhouwers et al. 2003). Nevertheless,

they are wholly unrelated to the true fungi. Apart

from their lack of photosynthetic pigments, they have

many plant-like features, including:

• cell walls composed primarily of glucans, including
  cellulose-like polymers;


• diploid nuclei, in contrast to the haploid nuclei of
  most fungi;


• cell membranes composed of plant sterols, in con-
  trast to ergosterol, the characteristic fungal sterol;


• energy storage compounds similar to those of
  plants, in contrast to the polyols (sugar alcohols) and
  trehalose found in most fungi;


• a range of ultrastructural features more closely
  related to those of plants than of fungi, including
  stacked, plate-like golgi cisternae (the fungi have
  tubular golgi cisternae), and tubular mitochondrial
  cisternae (the fungi have plate-like or disc-like mito-
  chondrial cisternae;


• different sensitivities to a range of antifungal agents.

The characteristic life-cycle features of Oomycota

Figure 2.34 illustrates the major life-cycle features

of Oomycota, represented by the plant-pathogen,

Phytophthora infestans, and water moulds such as

Saprolegniaspp.

Typically, the somatic (vegetative) stages are broad,

fast-growing hyphae with diploid nuclei. The

hyphae lack cross-walls (septa) except when they

produce complete, unperforated septa to isolate the

reproductive structures.

Asexual reproduction involves the production of a

multinucleate sporangium, which is separated by a

septum. The sporangial contents are then cleaved

around the individual nuclei, resulting in the produc-

tion of diploid zoospores, each with an anteriorly

directed tinsel-type flagellum and a trailing whiplash

flagellum. The zoospores are released when the tip of

the sporangium breaks down, but in some species (e.g.

Phytophthora infestans) the whole sporangium can be

released and functions as a dispersal spore. Depending

on environmental conditions (especially temperature)

this can germinate by producing a hypha or it can

undergo cytoplasmic cleavage and release zoospores.

The important downy mildew pathogensof several

crop plants release sporangia that are wind-dispersed

and germinate by a hypha to infect their hosts.

Sexual reproductiontypically involves the produc-

tion of a male sex organ (antheridium) and a female

sex organ (oogonium) that may contain one or sev-

eral eggs. Meiosis occurs within these sex organs, and

fertilization is achieved by the transfer of a single

haploid nucleus through a fertilisation tube to each

haploid egg (Fig. 2.34b). In some Phytophthoraspecies

(e.g. P. cactorum) the antheridia are attached laterally

to the oogonium. But in others (e.g. P. infestans) the

oogonial hypha grows through the antheridium and

then swells to form the oogonium. In any case,

fertilization leads to the development of one or more

thick-walled, diploid oospores. These usually have a

constitutive dormant period before they germinate to

produce either diploid hyphae or a sporangium that

releases diploid zoospores.

There are many variations on these basic themes.

For example, some of the Oomycota are homothallic

(self-fertile) and some heterothallic. Some species (e.g.

Pythium oligandrum) characteristically develop oospores

parthenogenetically, without a sexual process. In some

of the water moulds (Saprolegniales) the zoospores

released from the sporangia encyst immediately and

then germinate to produce a secondary zoospore,

which can swim for many hours. In Pythiumspp. the

sporangia do not release their zoospores “directly” by

dissolution of the tip of the sporangium. Instead, the

sporangium produces a short exit tube and the tip of

this breaks down to form a membrane-bound vesicle

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