Infection of Kikuyu grass (Pennisetum clandestinum) by the rust fungus Phakopsora apoda.

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 significance). 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 elaborations. 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.