Cell wall appositions and plant disease resistance: Acoustic microscopy of papillae that block fungal ingress

Abstract

Plant cells react to localized stress by forming wall appositions, papillae, outside their protoplasts on the inner surface of their cellulose walls. Preformed, oversized papillae, experimentally produced in coleoptile cells of compatible barley, Hordeum vulgare, can completely prevent direct entry of Erysiphe graminis f. sp. hordei that ordinarily penetrates and causes disease. To discover how these papillae may function, acoustic microscopy was used to contrast their in vivo elastic properties with those of ineffective normal papillae and contiguous cell wall. Raster and line scans showed intense acoustic activity at sites of preformed papillae; scans in selected focal planes identified this activity with the papillae, not with subtending cell wall. Minimal acoustic activity was found in normal papillae. Some wall appositions could probably serve in disease resistance as viscoelastic barriers to mechanical forces exerted by the special penetration structures of advancing pathogenic fungi.