Cytological responses induced by five phytopathogenic fungi in a nonhost plant, Phaseolus vulgaris

Abstract

Responses of the French bean plant (Phaseolus vulgaris L. cv. Pinto) to the nonpathogenic fungi Helminthosporium maydis Nisikado and Miyake (perfect stage Cochliobolus heterostrophus (Drechsler) Drechsler), Stemphylium sarcinaeforme (Cav.) Wiltish., S. botryosum Wallr., Cladosporium fulvum Cooke, and Uromyces vignae Barcl. were examined by cytological and histochemical techniques. All fungi penetrated bean leaves through stomata, and elicited similar modifications in guard and mesophyll cells. Responses involving cell contents were collapse, browning, autofluorescence, and a toluidine blue reaction indicative of the presence of phenolic compounds. Responses associated with cell walls were yellowing, autofluorescence, staining for phenolic compounds, and deposition of callose and of noncallosic, refractive material. In some of the interactions, decolorization and clearing of the tissue for light microscopy induced changes in the frequencies of autofluorescence of cell contents and walls when compared with those observed in fresh tissue. Despite the general similarity of responses elicited by each fungus, striking variations were found among fungi in the percentage of infection sites showing a particular response. For some responses, this variation seemed to be related to the type of interaction each of the fungi has with its susceptible host. It is suggested that even in nonhost plants, the nature and activities of the fungus determine the frequency and extent of the plant's responses. Also, it seems unlikely that all of these responses are involved in the cessation of fungal growth.