92 CHAPTER 5
the substomatal cavity and produces a substomatal
vesicle. Then an infection hypha develops from this to
produce a haustorium mother cell on one of the leaf
parenchyma cells. The infection process is completed
when the fungus penetrates the host cell to form a
haustorium, a specialized nutrient-absorbing struc-
ture (Chapter 14).
Studies of this sequence forU. appendiculatushave
shown that all the events up to, and including, the
development of the infection hypha are induced when
germ-tubes locate the “stomata” on nail varnish replicas
of leaf surfaces. These events can also be induced by
scratches on other artificial surfaces. But the forma-
tion of the haustorial mother cell usually depends on
chemical recognition of a leaf cell wall. Thus, most of
this developmental process is pre-programmed and it
requires only an initial topographical signal. To invest-
igate this further, Hoch and his co-workers exploited
the techniques of microelectronics to make silicon
wafers with precisely etched ridges and grooves of
different heights and spacings. The wafers were then
used as templates to produce transparent polystyrene
replicas on which rust spores would germinate and
the responses to surface topography could be studied.
Initial studies with U. appendiculatus showed that
appressoria were induced in response to single ridges
or grooves of precise height (or depth), about 0.5μm,
but little or no differentiation occurred in response
to ridges or grooves lower or higher than this (Hoch
et al. 1987). The inductive height corresponds to the
height of the lip on the guard cells of bean stomata,
which probably is the inductive signal in vivo.
In a further study, a total of 27 rust species were tested
on ridges of different heights (Allen et al. 1991), and
this enabled the species to be categorized into four
groups. Group 1included U. appendiculatusand seven
other species, which produced appressoria in response
to a single ridge or groove of quite precisely defined
height; ridges or grooves higher or lower than this had
little effect (Fig. 5.8a). Group 2included Puccinia men-
thae(mint rust) and three other species, which needed
a minimum ridge height (0.4μm for P. menthae) for
production of appressoria but also responded to all ridge
heights above this, up to at least 2.25μm (Fig. 5.8b).
Group 3was represented by a single species, Phakopsora
pachyrhizi(soybean rust), which could form appressoriaFig. 5.7Infection structures of the bean rust fungus,
Uromyces appendiculatus, penetrating a stomatal opening
from a germinating uredospore. A =appressorium; GC =
stomatal guard cell; GT =germ-tube; H =haustorium;
HMC =haustorial mother cell; IH =infection hypha; IP =
infection peg; SSC =substomatal cavity; U =uredospore;
V =substomatal vesicle. (Based on a drawing by H.C. Hoch
& R.C. Staples; see Hoch et al. 1987.)Fig. 5.8Production of appressoria by two rust fungi in response to single ridges of different heights on polystyrene
replicas of microfabricated silicon wafers. (a) Uromyces appendiculatusresponded only to ridges of narrowly defined
height. (b) Puccinia menthaeresponded to all ridges above a minimum height. (Based on Allen et al. 1991.)