Visualization of A. citrulli in pericarp- and pistil-inoculated
seeds. Sections from pistil- and pericarp-inoculated watermelon
seeds that were labeled with anti-rabbit IgG conjugated with
Alexa-fluor 488 alone did not yield fluorescent signals. This
indicated nonspecific binding of the secondary antibody to watermelon
seed tissues did not occur (data not shown). Additionally,
fluorescence was not observed in sections of negative control
seeds from PBS-treated blossoms when labeled with the anti-A.
citrulli primary antibody. Toluidine blue-stained seed sections
(Fig. 3A and B) showed the PE envelope enclosing the cotyledons.
In sections of pistil-inoculated seeds, A. citrulli cells were
observed not only in the inner cells of the PE envelope, but also in
the intercellular spaces of parenchyma tissue of the embryo (Fig.
3C). In pericarp-inoculated seeds, A. citrulli was observed only in
the outer layer of PE envelope (Fig. 3D). A. citrulli cells were not
observed in the embryo of pericarp-inoculated seeds.