Plant cells react to localized stress by forming wall appositions outside their protoplasts on the inner surface of their cellulose walls. For many years it has been inferred that appositions-elicited by encroaching fungi, termed "papillae," may subsequently also deter them and thus represent a disease-resistance mechanism. Recently, it has been shown that preformed, oversized papillae, experimentally produced in coleoptile cells of compatible barley, Hordeum vulgare, can completely prevent direct entry of Erysiphe praminis 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 papillaV, It is suggested that some wall appositions could serve in disease resistance as viscoelastic barriers to mechanical forces exerted by the special penetration structures of advancing pathogenic fungi. Living plant cells rapidly react to localized stress by directly apposing substances onto the inner surface of their cellulose walls. Because such wall appositions (1) could, by healing, forestall death of injured cells (2), their timely formation could also contribute to the survival of whole plants when injury from pathogen penetration and subsequent infection are imminent. It has even been suggested that wall appositions elicited by fungi, termed "papillae" (3), may represent a disease-resistance mechanism because in some cases they seem to intercept invading pathogens (for review, see ref.2).In the past, papilla function has usually been judged from after-the-fact correlations, often taken from spent or preserved parasite-host systems, which have thus been equivocal (2). Recently, however, some workers have turned to evaluations of living systems, and only very lately has the process been examined experimentally.In the most recent report (4), part of an extended series of studies done in our laboratories (5-14), we showed that oversized, preformed papillae experimentally produced in coleoptile cells of compatible barley (Hordeum vulgare) Acoustic microscopy represents a new capability for visualizing certain physical properties, not dependent upon refractive index, of cells and tissues (15). Point-by-point properties of structures can be evaluated nondestructively as they react in situ to periodic stress waves. With acoustic microscopy one may view structures within living samples and gain detailed information about their elasticity, density, or viscosity. This information is unobtainable by any other technique. Thus, the acoustic microscope seemed uniquely suited for estimating the "strength" of papillae, as they repel or yield to attacking parasitic fungi.The goals of this study were simple an...
