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

Publisher Website
Google Scholar
Abstract
Leaves of susceptible soybean cultivar Clark were spray-inoculated with either of two virulent strains of Xanthomonas campestris pv. glycines, the causative agent of bacterial pustule disease of soybean [Glycine max]. After 7 days, leaf tissue bearing pustules was prepared for light microscopy and transmission or scanning electron microscopy. Pustules usually protruded from the abaxial surface of the leaf and were composed of enlarged, closely spaced parenchyma cells. Affected palisade parenchyma cells were longer and larger than control palisade cells. The increased number of cells and the presence of cross walls in some palisade cells indicated that cell division, as well as cell enlargement, was a mechanism of pustule development. Cells at the apices of developing pustules or throughout mature pustules contained scant cytoplasm with few organelles. In pustule cells closer to the midplane of the leaf, active wall synthesis was indicated by the presence of many dictyosomes and Golgi vesicles filled with fibrillar substance. The existing wall structure appeared to be loosened, with microfibrils fraying at the outer wall surfaces. Softening of the cell wall and increased wall synthesis may be involved in the enlargement of cells composing the pustules.