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FUNGAL GROWTH 75

example, towards a nutrient source or away from a

potential inhibitor. Despite the fact that all fungi

require nutrients, and therefore might be expected

to orientate towards nutrient-rich zones, there is no

evidence that the normal somatic hyphae of true

fungi do this. Only the Oomycota show this nutrient-

seeking behavior, and it can be strain-specific – some

strains of Saprolegnia orAchlyaspp. orientate towards

mixtures of amino acids, whereas other strains show

no response. Where it occurs, the response can be very

striking. On a nutrient-poor medium the hyphae will

turn through 180 degrees to an agar disk containing

casein hydrolysate or other amino acid mixtures

(Fig. 4.10) and the hyphae also branch from the side

Fig. 4.9Behavior of spores (S) of Idriella bolleyion wheat

root hairs in aseptic conditions. (a) Spores germinate to-

wards a dead root hair, and then envelop and penetrate

the root hair. (b) Spores germinate away from a living root

hair (rh). (From Allan et al. 1992.)

(a)

(b)

cathode. In a more recent study (Lever et al. 1994) the

galvanotropic responses of somatic hyphae were found

to be pH- and calcium-dependent. Neurosporahyphae

even changed from being strongly cathodotropic at

pH 4.0 to strongly anodotropic at pH 7. Given the range

of different responses to electrical fields it is difficult to

summarize this topic, except to say that fungal hyphae

can be responsive to electrical/ionic fields. Gow (2004)

recently reviewed this topic.

Hyphal tropisms

In all the examples above we have used the term trop-

ism loosely to describe the position where a hyphal tip

emerges. But, strictly, a tropism is a bending response

that orientates a hypha in a particular direction – for

Fig. 4.10(a) Orientation of hyphal tips and orientated

emergence of hyphal branches of Achlyaand Saprolegnia

spp. towards an agar disk (black circle) containing a

mixture of amino acids. (b) Reorientation of hyphae from

germinating spores of Pythium aphanidermatumtowards

a mixture of amino acids (Mitchell & Deacon 1986).

(a)

(b)