untitled

be studied using capillaries filled with potential attract-

ants. Often the zoospores of plant pathogens show

chemotaxis to several individual sugars or amino

acids, or to volatile compounds such as ethanol and

aldehydes, which are likely to be released as fermen-

tation products of roots in moist soil conditions.

However, the strongest responses are usually seen with

mixtures of compounds, such as seed and root exudates.

Most Pythiumand Phytophthoraspecies show taxis

to the roots of both host and nonhost plants, but a

few interesting examples of host-specific taxis have been

reported. For example, the host-specific pathogen

Phytophthora sojaeshows chemotaxis in vitroto the

flavonoids daidzein and genistein, which are known to

be present in the soybean host. Similarly, Aphanomyces

cochlioidesshows strong chemotaxis to the flavonoid

“cochliophilin A” from spinach plants. It should not

be assumed that these compounds are the only factors

involved in attraction to the host roots, but the

findings are notable because they parallel the behavior

of Rhizobiumspp., which show in vitrochemotaxis to

the specific flavonoids of their hosts.

The next phase of the sequence seems to involve

recognition of a host surface component, because

the zoospores move over the host surface, with their

flagella in contact with this. Some zoosporic fungi

might have specific host surface requirements, but

often they respond to pectin and other polyuronates

(e.g. alginate) in vitro. For example, Fig. 10.20 shows

the massing and encystment of Pythiumzoospores on

the root tip mucilage secreted by root cap cells of

wheat. Similar massing and encystment is seen in the

elongation zone of roots that are coated with calcium

alginate gel – a treatment that masks any specific

receptors on the root surface.

Recognition of a host surface component (perhaps

coupled with a high concentration of root-derived

nutrients) leads to orientated encystment, with the

200 CHAPTER 10

Fig. 10.19The homing and docking sequence of zoospores.

Fig. 10.20Accumulation and encyst-
ment of Pythiumzoospores on wheat
roots. (a) Zoospores (z) accumulating
in large numbers on the surface of a
ball of root tip mucilage; some root cap
cells are indicated (rc). (b) Zoospores
(z) accumulating and encysting in the
elongation zone of wheat roots. In this
case the root had been coated with a
double layer of calcium alginate gel to
block any specific receptor compounds
(a) (b) on the root surface.