Soil drench with zoospore suspension of P. nicotianae or BABA primes
C. annuum foliar defense responses
Production of autofluorescent compounds was evaluated in plants treated with P. nicotianae at
24 h post foliar inoculation with P. capsici. Confined autofluorescence in epidermal and mesophyll
cells was observed, typically in close proximity to stomatal openings (data not shown).
The hydrogen peroxide indicator stain 3-diaminobenzidine (DAB) was used to visualize
reactive oxygen species production during initial P. capsici infection (Fig 4). At 12 h post inoculation
with P. capsici, encysted zoospores with germination tubes were visible on all leaf surfaces.
For plants treated with P. nicotianae and BABA, DAB staining was visible in single or
multiple plant epidermal cells at or near the encysted zoospore and germ tube while staining in
mock treated plants was indistinct, and often concentrated in the intercellular space between
epidermal cells. At 24 h post inoculation, intense DAB staining was visible on induced plants
within single or multiple epidermal cells near germinated zoospores, while DAB staining in
mock treated plants remained indistinct and P. capsici hyphae could be seen emerging from
leaf tissue. At 72 h post inoculation with P. capsici, induced plants had darkly stained punctate
bodies within DAB stained epidermal cells. By 72 h post inoculation, mock treated plants also
displayed DAB staining in epidermal cells, however, the punctate bodies were not present and
abundant hyphae of P. capsici were present on the leaf surface. In addition, mock treated plants
had a disorganized appearance and intercellular boundaries were not clearly defined. At all
three time points, a limited number of infection sites were visualized in induced plants that
were similar in appearance to mock treated plants and P. capsici hyphae were observed at low
frequency (data not shown). However, no infection sites were visualized in mock treated plants