We recently characterized a highly dynamic
fungal disease outbreak in native populations of Nicotiana attenuata in the southwestern United States. Here, we explore how
phytohormone signalling contributes to the observed disease dynamics. Single inoculation with three native Fusarium and Alternaria fungal pathogens, isolated from diseased plants growing in native populations, resulted in disease symptoms characteristic for each pathogen species. While Alternaria sp.-infected plants displayed fewer symptoms and recovered, Fusarium spp.-infected plants became chlorotic and frequently spontaneously wilted.
Jasmonic acid (JA) and
salicylic acid (SA) levels were differentially induced after Fusarium or Alternaria
infection. Transgenic N. attenuata lines silenced in JA production or JA conjugation to
isoleucine (
JA-Ile), but not in JA perception, were highly susceptible to
infection by F. brachygibbosum Utah 4, indicating that products derived from the
JA-Ile biosynthetic pathway, but not their perception, is associated with increased Fusarium resistance.
Infection assays using ov-nahG plants which were silenced in pathogen-induced SA accumulations revealed that SA may increase N. attenuata's resistance to
Fusarium infection but not to Alternaria. Taken together, we propose that the dynamics of
fungal disease symptoms among plants in native populations may be explained by a complex interplay of
phytohormone responses to attack by multiple pathogens.