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76 CHAPTER 4

closest to the amino acid source. Manavathu &
Thomas (1985) investigated this for a strain of Achlya
ambisexualisand found that, of all the single amino acids
tested, only the sulfur-containing amino acid methio-
nine could elicit hyphal tropism. However, gradients
of many other single amino acids would elicit tropism
if the medium contained a uniform background of
cysteine. As an explanation of this it was suggested
that cysteine, when taken up by cells, can donate one
of its sulphydryl (-SH) groups to other amino acids and
thereby generate methionine. In bacteria the attraction
of cells to several types of compound is mediated by
chemoreceptor complexes, and this involves a role for
methionyl derivatives which donate methyl groups
to the interior domains of the receptor complexes
(Armitage & Lackie 1990).
A similar system might be involved in chem-
otropism by the Oomycota, because Manavathu &
Thomas found that several methyl-donor compounds
could elicit a tropic response. In work with a strain of
Achlya bisexualis, Schreurs et al. (1989) found that the
hyphal tips orientate towards the tips of micropipettes
containing either methionine or phenylalanine. Also,
when micropipettes containing attractant amino acids
were placed behind the hyphal tips the branches
emerged from the hyphae and grew towards the
attractants. Thus it seems that the initiation of
branching and the tropism of hyphal tips are closely
related responses to environmental signals, and in
some Oomycota these responses might be mediated by
plasma membrane receptors for specific amino acids.

The hyphae of many fungi show tropic responses to

non-nutrient factorsof potential ecological relevance. For

example, germ-tubes arising from spores of the arbus-

cular mycorrhizal fungi (Glomeromycota; Chapter 2)

can grow towards volatile metabolites (perhaps alde-

hydes) from roots; some wood-rotting fungi (e.g.

Chaetomium globosum, Ascomycota) orientate towards

volatile compounds from freshly cut wood blocks, and

hyphae of the seedling pathogen Athelia (Sclerotium) rolf-

sii(Basidiomycota) orientate towards methanol and

other short-chain alcohols from freshly decomposing

organic matter (Chapter 14). Sexual pheromones also

elicit orientation responses (discussed in Chapter 5).

The yeast cell cycle

In contrast to fungal hyphae, which grow continuously

from a hyphal tip, yeasts typically grow by a repeated

budding process to produce colonies of single cells.

As shown for Saccharomyces cerevisiae in Fig. 4.11,

in each “turn” of the yeast cell cycle a young bud

emerges from a predictable point on the mother cell.

The bud grows apically by channeling of wall com-

ponents to the bud tip, but at a later stage the mode

of growth switches and wall components are inserted

uniformly over the cell, so that the bud becomes more

swollen. Meanwhile, the cell has undergone mitosis,

and one of the two daughter nuclei enters the bud. The

final stage of cytokinesis (cell separation) occurs by

the development of a primary septum, composed of

Fig. 4.11Events in the cell cycle of

Saccharomyces cerevisiae. (Based on a draw-

ing by Hartwell 1974.)