Cellular lignification as a factor in the hypersensitive resistance of wheat to stem rust

Fluorescence microscopy, histochemistry and autoradiography all indicated the cellular accumulation of phenolic compounds during hypersensitive necrosis induced by Puccinia graminis f. sp. tritici in resistant wheat cultivars. Hypersensitivity and phenolic accumulation were observed in near-isogenic lines carrying the Sr 5 or Sr 6 (at 19°C) alleles for resistance, but not in the susceptible reactions of the parent cultivar Marquis ( sr 5, sr 6), or the Sr 6 line when tested at 26°C. Toluidine blue staining and fluorescence microscopy indicated the accumulation of a phenolic material in hypersensitively necrotic cells. Removal of wall-bound phenolic acids with hot alkali caused little loss in fluorescent intensity, but subsequent delignification with chlorine dioxide reduced fluorescence to negligible levels. Autoradiography, using the lignin precursors [ 3 H]-phenylalanine and [ 14 C]-cinnamic acid, showed incorporation of label into cytoplasmic regions of necrotic and degenerating host cells, corresponding to the sites of autofluorescence and histochemical staining. [ 3 H]-Phenylalanine incorporated into healthy cells and fungal structures was lost from both resistant and susceptible tissue on alkali treatment, but some label was retained in necrotic cells. Necrotic regions from infected resistant cultivars were resistant to cell wall degrading enzymes, but extraction of lignin allowed subsequent tissue maceration. The accumulation of a phenolic material which is alkali-insoluble, enzyme resistant and chlorine dioxide extractable indicates that lignification occurs in necrotic cells of the hypersensitive response. A role for cellular lignification as an induced barrier is postulated and its direct or indirect effect on fungal growth in resistant tissue is discussed.