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Induction of Transfer Cell Formation in a Plant Infected by a Biotrophic Fungal Parasite

In this study a combination of scanning (SEM) and transmission electron microscopy (TEM) was used to examine leaf cells of the common weed Indian strawberry (Duchesnea indica) infected by the biotrophic rust fungus Frommeëla mexicana var. indicae. For SEM, free-hand sections of infected leaves cut with a razor blade were placed in a 5% (wt/vol) aqueous solutions of Ariel (Proctor and Gamble, Cincinnati, OH), a washing powder containing a bacterial protease. This treatment removed exposed host cell cytoplasm so as to reveal details of fungal haustoria and host cell wall responses to infection (Figs. 1, 2). Samples then were processed for study according to procedures described previously. For TEM, samples were prepared for study using high pressure freezing followed by freeze substitution. This fixation procedure has been used with excellent results to study hostpathogen interactions in rust-infected plants. Frommeëla mexicana var. indicae produced specialized hyphal branches known as haustoria that extended through the walls of infected cells. Each haustorium possessed a long, slender neck region (Fig. 3) and an expanded body (Figs. 1, 4) that was separated from the host cell cytoplasm by the invaginated host cell plasma membrane. At maturity the neck region was surrounded by a extensive labyrinth cell wall in-growth or elaboration (Figs. 1-3) that appeared to be identical to wall elaborations that characterize plant transfer cells. Such cells are thought to be involved in intensive solute transfer over short distances. While transfer cells have been reported within cells of the haustoria of various plant parasitic higher plants and in plant giant cells induced by certain plant parasitic nematodes, this appears to be the first report of their formation in response to infection by a plant pathogenic fungus. Not only did these elaborations form in the immediate vicinity of haustoria, they also arose apart from haustoria in infected cells. However, such elaborations were not found in cells of uninfected leaves.