VIEW ARTICLE

Cytology and Histology

Physical Entrapment of Pseudomonads in Bean Leaves by Films Formed at Air-Water Interfaces. D. C. Hildebrand, Associate research plant pathologist, University of California, Berkeley 94720; M. C. Alosi(2), and M. N. Schroth(3). (2)(3)Staff research associate, and professor of plant pathology, University of California, Berkeley 94720. Phytopathology 70:98-109. Accepted for publication 1 August 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-98.

Water infiltrated into the intercellular spaces of bean leaves appears to dissolve materials from the cell wall surfaces. Subsequent transpiration causes intercellular fluid to recede until films, consisting of amorphous and/or fibrillar material condensing from the dissolved material, form at air-water interfaces. Bacteria introduced during water infiltration become trapped by films on cell wall surfaces or in corners between juxtaposed mesophyll cells. After infiltration, no difference in entrapment was observed at 3 hr between homologous or heterologous strains of Pseudomonas syringae, and the heterologous pathogens, P. marginalis, P. pisi, and P. tomato, or the saprophyte P. fluorescens. By 12 hr, bacterial multiplication was occurring with both the homologous P. syringae strain and heterologous P. tomato and colonies thereof, bounded by films, were expanding. Cells of pathogens or saprophytes were not attached to the cell wall and remained suspended in the intercellular fluid if the leaves were kept watersoaked for 12 hr. Under these conditions both pathogens and saprophytes appeared to multiply. Films observed in the water-soaked leaves were associated with gas pockets found in crevices between plant cells and not with bacterial cells. Leaf age greatly affected the amount of material that condensed in the films and the deposition of materials behind them. More films were formed and a greater amount of material was deposited in younger leaves. Vacuum infiltration of the electron microscopy fixative into tissue affected the integrity of the films. It is postulated that films may play a role in pathogenesis by assisting in the retention of moisture in the microenvironment surrounding the pathogen, thus enabling it to multiply.