Partial characterization of the electron-opaque deposits formed in the non-host plant, French bean, after cowpea rust infection

Abstract

French bean cultivars respond to cowpea rust infection by the production of naturally electron-opaque deposits on the surrounding mesophyll cell walls. Deposition of this material was inhibited by prior treatment of the leaves with actinomycin D, cycloheximide and blasticidin S, and was elicited by washings from fungal infection structures, or less reliably, by injection of water. Energy dispersive X-ray (EDX) analysis of rust-induced deposits revealed silicon as the only element (with an atomic number of 11 or higher) present in significant amounts. Concentrations were roughly proportional to the amount of electronopaque material present and no silicon was detected in walls lacking such deposits. Cytochemical tests at the light microscope level suggested that deposits only rarely contained high levels of phenolic compounds. Similarly, EDX analysis of FeCl 3-treated tissue revealed only low levels of bound iron (possibly indicative of phenolic compounds) in the deposits while higher levels were found in deposit-lacking walls of cells in the immediate vicinity of the fungus. These results suggest that silicon, rather than phenolic compounds, is the primary electron-opaque component of the wall deposits formed in this non-host interaction and that, contrary to suggestions for silicon deposition in some other situations, formation of these deposits is controlled by metabolic activity of the protoplast.