Ultrastructural observations of penetration sites of the cowpea rust fungus in untreated and silicon-depleted French bean cells

Abstract

The region where the cowpda rust fungus initiates haustorium formation in plant mesophyll cells was compared ultrastructurally in susceptible cowpea plants, nonhost French bean plants given access to silicon (−Si plants), and French bean plants depleted in silicon (−Si plants). In +Si plants, fungal development apparently ceased while the penetration peg was traversing the haustorial mother cell (HMC) wall, often before the peg reached the adjacent silicified wall of the plant. In −Si plants, where silica deposits were absent, HMCs at three out of 10 infection sites had formed a haustorium. At the majority of the remaining sites, fungal growth appeared to have ceased before the initiation of a visible penetration peg and during a stage of development, also seen in susceptible cowpea, where the HMC and the plant wall were bridged by electron-opaque material. It is suggested that this stage represents the incipient degradation of the fungal, and possibly the plant, wall prior to the appearance of a distinguishable penetration peg. The fact that most penetration pegs stopped their development earlier in −Si than in +Si plants supports the previous suggestion that the higher levels of wall-associated phenolic compounds in the former results in a more rapid inhibition of fungal enzymes involved in the formation of the penetration peg. These and other ultrastructural observations suggest that in +Si plants the silicified plant walls may (1) reduce the interchange of materials between plant and fungus so that lesser amounts of phenolic materials are produced, (2) restrict the flow of materials to the HMC that normally prevent its premature senescence, and/or (3) act as a physical barrier if the penetration peg reaches the plant wall.