Cytological studies of the interactions between the cowpea rust fungus and silicon-depleted French bean plants

Abstract

To investigate the role of silica-rich wall deposits in resistance to the cowpea rust fungus, French bean plants were grown hydroponically in nutrient solutions supplemented with or depleted in silicon. Primary leaves supplied with adequate silicon responded to fungal infection by the autofluorescence of guard cell walls, the limited autofluorescence of mesophyll cell walls (both visualized in cleared tissue), and the deposition in and on the latter of silica. If infected, silicon-depleted plants, light microscopy, electron microscopy, and energy dispersive X-ray analysis indicated that silica deposits were absent. The incidence of autofluorescence of guard cells was comparable to that in the silicon-supplemented plants, but the incidence and extent of mesophyll wall autofluorescence was greatly enhanced. The autofluorescence of mesophyll cells, but not guard cells, corresponded to areas of the wall that gave a colour reaction with toluidine blue indicative of phenolic compounds. Callose (aniline blue positive material), in the form of papillae, was also extremely common at infection sites in silicon-depleted leaves. Infection hyphae rarely formed haustoria in either silicon-depleted plants or those given adequate silicon, although these hyphae grew as well, and appeared equally healthy, in both types of plants. In silicon-depleted plants, pre-inoculation heat treatments or injection of intercellular fluids from bean rust-infected bean leaves, increased the incidence of haustorium formation and decreased the incidence of all observed plant responses. The results suggest that either silica deposition is not the primary barrier to haustorium formation in normal plants, or that a second barrier, such as the impregnation of the plant wall with phenolic materials, comes into play if silica deposition is prevented.