Watermelon seeds can become infested by Acidovorax citrulli, the
causal agent of bacterial fruit blotch (BFB) of cucurbits via penetration of
the ovary pericarp or by invasion of the pistil. This study investigated the
effect of these invasion pathways on A. citrulli localization in seeds. Seed
samples (n = 20 or 50 seeds/lot) from pistil- and pericarp-inoculated lots
were dissected into testa, perisperm-endosperm (PE) layer, and embryo
tissues and tested for A. citrulli by species-specific polymerase chain
reaction (PCR) and by plating on semiselective media. Less than 8% of
the testa samples were A. citrulli-positive regardless of the method of
seed inoculation. Additionally, the difference in percentages of contaminated
testae between the two seed lot types was not significant (P = 0.64).
The percentage of A. citrulli-positive PE layer samples as determined by
real-time PCR assay was significantly greater for seeds from pistilinoculated
lots (97%) than for seeds from pericarp-inoculated lots
(80.3%). The mean percentage of A. citrulli-positive embryo samples was
significantly greater for seeds from pistil-inoculated lots (94%) than for
seeds from pericarp-inoculated lots (≈8.8%) (P = 0.0001). Removal of PE
layers and testae resulted in a significant reduction in BFB seed-toseedling
transmission percentage for seeds from pericarp-inoculated lots
(14.8%) relative to those from pistil-inoculated lots (72%). Additionally,
using immunofluorescence microscopy, A. citrulli cells were observed in
the PE layers and the cotyledons of pistil-inoculated seeds but only in the
PE layers of pericarp-inoculated seeds. These results suggest that pericarp
invasion results in superficial contamination of the testae and PE layers
while pistil invasion results in the deposition of A. citrulli in seed
embryos.