Bacterial Pustule Disease of Soybean: Electron Microscopy of Pustule Development in a Susceptible Cultivar

Abstract

Leaves of the susceptible soybean cv. Clark were inoculated with virulent strains of Xanthomonas campestris pv. glycines, causative agent of bacterial pustule disease in soybean. After 7 d, leaf tissue bearing pustules was prepared for transmission and scanning electron microscopy. Pustules developed in the spongy parenchyma. Vascular-associated cells, including bundle sheath extensions, were not affected. The heaviest concentration of bacteria was in the intercellular spaces in or near pustules and bacterial extracellular polysaccharide was present. Bacteria were widely dispersed among the palisade cells, but the palisade parenchyma was not affected. There was no envelopment of bacteria at host cell wall surfaces and no bacteria were observed in the xylem or phloem elements of the minor leaf veins. Pustules were comprised of large mesophyll cells with uniform shape, unlike control spongy parenchyma cells which were irregular in shape. Affected cells had more endoplasmic reticulum and more ribosomes, both free and membrane-bound, than the control, and had few chloroplasts. Deep, focal invaginations of plasma membrane and tonoplast appeared to be involved in endocytic transport of membrane lamellae, small membrane vesicles, and non-organellar cytoplasmic constituents to the central vacuole.KeywordsCentral VacuoleXanthomonas CampestrisPlant Pathogenic BacteriumPalisade ParenchymaPalisade CellThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.