Introduction to Fungi, Third Edition

MAGNAPORTHACEAE
383
Fig12.47 (a) Conidiophores of
Magnaporthe grisea emerging from
a stoma on a rice leaf. Detached
conidia are also shown (b).
colonize the plant tissue. These secondary colonizing
hyphae are thinner and straighter than
the bulbous hyphae (Figs. 12.45f,g). Within 5 days
of infection, diamond-shaped lesions of dead
tissue develop, and these emit conidiophores
which often grow out through dead stomata
(Fig. 12.47).
The interaction between M. grisea and rice
is governed by a gene-for-gene relationship
(Valent, 1997; see p. 619). In compatible interactions,
the host response is delayed because the
fungus is not immediately recognized by the
plant. However, even virulent strains of M. grisea
will eventually be exposed to the toxic products
(phytoalexins) of the host’s delayed immune
response, and M. grisea seems to have evolved
mechanisms to deal with phytoalexins by exclusion.
It is becoming clear that ABC transporters
play a crucial role, and we have already come
across them as a resistance mechanism developed
by Candida albicans against clinical drugs
(see p. 278). The report by Urban et al. (1999)
on M. grisea was one of the first to implicate
a phytoalexin-excluding ABC transporter as an
important factor in the colonization of a plant
host by a fungal pathogen. ABC transporters
have now been shown to play a crucial role in the
exclusion of toxic substances as well as fungicides
in several plant-pathogenic fungi (Hayashi
et al., 2002; Stergiopoulos et al., 2002).