Cell wall alterations in hypocotyls of bean seedlings protected from Rhizoctonia stem canker by a binucleate Rhizoctonia isolate

The influence exerted by the non-pathogenic binucleate Rhizoctonia (np-BNR) isolate 232-CG in stimulating plant defence reactions in young bean plants inoculated with the root rot fungus Rhizoctonia solani (AG-4) was examined using light and electron microscopy and further investigated by gold cytoc hemistry. Severe necrotic lesions on hypocotyles of diseased beans were obs erved, and the pathogen invaded the cortical tissue causing extensive damag e including cell disorganization and cell wall degradation. In contrast, th ese host reactions were not seen in bean plants inoculated with the non-pat hogenic BNR or in plants that were inoculated with BNR and subsequently cha llenge-inoculated with R. solani. Microscopic examination of hypocotyls ino culated with the nonpathogenic BNR, showed a different host reaction typica l of plant defence reactions. In these samples, epidermal and outer cortica l cells were impregnated with an electron-dense material. Histochemical ass ays of this material confirmed the substantial presence of phenols, pectic substances and suberin. Electron microscope observations clearly showed tha t in non-pathogenic BNR-inoculated plants, fungal cells were confined to th e epidermal layer which was darkly stained. Gold cytochemistry confirmed th e presence of pectic substances in the electron dense material. The possibi lity that pectic oligogalacturonides released after hydrolysis by the nonpa thogenic BNR enzymes may act as elicitors of defence responses is discussed . The present ultrastructural observations corroborate that non-pathogenic BNR isolates may function as potential inducers of plant disease resistance .