Structure and function of wall appositions. 2. Callose and the resistance of oversize papillae to penetration by Erysiphe graminis f. sp. hordei

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Abstract
Callose, a β-1,3- or β-1,3-1,4-glucan, is one of the most commonly reported constituents of papillae. Its putative roles include conferring resistance to fungal penetration by sequestering fungitoxic compounds or decreasing molecular communication between pathogen and suscept. Oversize papillae induced in partially dissected coleoptiles of barley are known to be resistant to penetration by a challenge inoculum of a compatible race of Erysiphe graminis D.C. f. sp. hordei Em. Marchal. When such coleoptiles were treated with laminarinase, predominantly a β-1,3-glucanase, callose was removed from the papillae as evidenced by the loss of aniline blue induced fluorescence. Upon subsequent challenge inoculation, there was no decrease in the resistance of the papillae: areas without papillae were penetrated at 43% of the 538 sites examined, whereas none of the 26 laminarinase-digested, oversize papillae was penetrated. Because the enzyme treatment removed not only callose but also autofluorescent materials, we conclude that these components were unnecessary for the resistance of oversize papillae. Some other component, not identified in this study, confers this resistance.

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