untitled

accumulate at the root tips in response to root tip nutri-

ents and then lyse within a few minutes (Fig. 10.17).

The use of surfactants such as rhamnolipids, or even

crude extracts of saponin-containing tissues such as oat

roots, could provide disease control in hydroponic

glasshouse-cropping systems where zoosporic fungi can

cause serious diseases. This is now being investigated

in several laboratories, to find environmentally safe

alternatives to the use of fungicides.

Zoospore motility

In appropriate conditions zoospores of Oomycota

can swim for 10 hours or more, at rates of at least

100 μms−^1 fuelled by endogenous nutrient reserves. So

they could, in theory, swim as far as 3– 4 meters for

dispersal to new environments. However, the zoospores

make frequent random turns (Fig. 10.18), and because

of this the rate of dispersion by Phytophthorazoospores

in still water has been found to be little more than the

rate of diffusion of a small molecule such as HCl.

Clearly, the swimming activity of zoospores must

serve other roles. One of these roles is that swimming

zoospores can remain in suspension and be carried

in moving water, whereas nonmotile spores tend to

settle out. This has been demonstrated both in artificial

soils and in field soils, where zoospores of Oomycota

can escape entrapment in narrow, water-filled soil

pores, so they can remain suspended and spread in

surface run-off water, whereas zoospore cysts are easily

trapped in soil. But the main role of zoospores is that

their swimming is linked to sensory perception: they

can swim towards attractants such as nutrients or

oxygen (positive chemotaxis) or avoid unsuitable

chemical environments (negative chemotaxis). Zoo-

spores also can respond to pH gradients, to electrical

or ionic fields (electrotaxis), and they can accumulate

by autoaggregation. The extreme responsiveness of

zoospores enables them to settle and encyst in envir-

onments that are most appropriate for subsequent

development. For example, zoospores often accumulate

in large numbers near root tips, at plant wound sites,

or around individual stomata on a leaf surface. This

“homing and docking” sequence of zoospores, dis-

cussed below, is as sophisticated and rapid as any that

have been described in the biological world.

The “homing and docking” sequence of

zoospores

The events in the “homing and docking” sequence of

zoospores (Fig. 10.19) have been studied in detail for

Pythiumand Phytophthoraspecies. The sequence begins

when a zoospore detects a gradient of chemoattractant,

which causes a partial suppression of random turns so

that the spore tends to move up the attractant gradient.

This phase of zoospore taxis, or zoospore kinesis, can

FUNGAL SPORES, SPORE DORMANCY, AND SPORE DISPERSAL 199

(c)
i ii iii iv

vii vi v

(a) (b)

Fig. 10.17(a) Oat root tips with natural blue auto-

fluorescence caused by the presence of avenacin. (b) A

Pythiumzoospore undergoing disorganization and lysis

in the presence of avenacin. (c) Responses of wall-less

zoospores of Oomycota to saponins such as avenacin or

b-aescin: (i) motile zoospore; (ii) immobilization and

rounding-up; (iii) development of phase-dark granules;

(iv) localization of granules and development of vacuoles;

(v) lysis; (vi,vii) ballooning followed by lysis. Disruption of

the zoospores usually occurs within 5–10 minutes. (From

Deacon & Mitchell 1985.)

Fig. 10.18Zoospore tracks of Oomycota, captured as neg-

atives on photographic film during a 5-second exposure

and showing frequent random turns in the absence of an

attractant.