|dc.description.abstract||A rust fungus on kikuyu grass (Pennisetum clandestinum Hochst. ex Chiov.) was
identified as Phakopsora apoda (Har. & Pat.) Mains., and was reported for the first
time in South Africa.
An investigation was conducted into the direct penetration mechanisms employed
by the urediospore germlings, using light, scanning and transmission electron
microscopy. The infection processes of P. apoda were found to parallel closely
those of Phakopsora pachyrhizi.
Infection structures of P. apoda produced on artificial membranes are similar to
those observed in the host leaf. The binucleate urediospore germinates to form a
typically short germ tube, which differentiates an appressorium, delimited by a
septum. The two nuclei migrate into the appressorium, where mitosis occurs,
resulting in four nuclei in the mature appressorium.
Appressoria appear to form preferentially at epidermal cell junctions. When
germinated on artificial membranes, germlings differentiate appressoria against
microfabricated ridges and on smooth surfaces, possibly with a marginal preference
for ridge-associated differentiation (significant at the 5% but not the 1% level of
A penetration pore develops in the basal wall of the mature appressorium, over the
infection site. A cone-like structure develops around the pore. The basal wall of the
appressorium is thinned and electron dense in the vicinity of the cone, and the cone
appears to be attached to the appressorial wall by means of a collar of similar,
electron-dense material. The membrane-bound cone is elaborately branched, and
numerous, electron-dense glycogen-like particles are associated with these cone
A penetration peg, its walls continuous with an inner wall layer of the cone,
penetrates the epidermal cell wall and expands into an intracellular penetration
hypha within the host cell.
The penetration hole formed by the peg has smooth edges, and there is little
deformation of the epidermal cell wall fibrils, indicating a predominantly enzymatic
mode of penetration.
The penetration hypha traverses the epidermal cell, and emerges into an
intercellular space of the mesophyll, narrowing to form a penetration neck at the exit
site. Both the penetration peg and the neck, contain multivesicular bodies
associated with parallel arrays of microtubules. The intercellular penetration hypha
contains an elaborate endomembrane system, and is virtually devoid of glycogen-like
particles. Lomasomes often occur in the vicinity of the penetration neck.
Signs of host disruption and the formation of papilla-like structures are apparent in
mesophyll cells adjacent to the infection site 12 hours after inoculation.
A septum delimits the penetration hypha from the primary hypha, which extends
further into the mesophyll and forms secondary branches.||en