Phytohormones mediate volatile emissions during the interaction of compatible and incompatible pathogens: the role of ethylene in Pseudomonas syringae infected tobacco.

Interactions between the phytohormones ethylene, salicylic acid (SA), and jasmonic acid (JA) are thought to regulate the specificity of induced plant defenses against microbial pathogens and herbivores. However, the nature of these interactions leading to induced plant volatile emissions during pathogen infection is unclear. We previously demonstrated that a complex volatile blend including (E)-beta-ocimene, methyl salicylate (MeSA), and numerous sesquiterpenes was released by tobacco plants, Nicotiana tabacum K326, infected with an avirulent/incompatible strain of Pseudomonas syringae pv. tomato (Pst DC3000). In contrast, a volatile blend, mainly consisting of MeSA and two unidentified sesquiterpenes, was released by plants infected with P. syringae pv. tabaci (Pstb) in a virulent/compatible interaction. In this study, we examined the interaction of multiple pathogen stresses, phytohormone signaling, and induced volatile emissions in tobacco. Combined pathogen infection involved the inoculation of one leaf with Pst DC 3000 and of a second leaf, from the same plant, with Pstb. Combined infection reduced emissions of ocimene and MeSA compared to plants infected with Pst DC 3000 alone, but with no significant changes in total sesquiterpene emissions. In the compatible interaction, Pstb elicited a large ethylene burst with a peak emission occurring 3 days after inoculation. In contrast, the incompatible interaction involving Pst DC3000 displayed no such ethylene induction. Pstb-induced ethylene production was not significantly altered by Pst DC3000 in the combined infection. We postulated that Pstb-induced ethylene production may play a regulatory role in altering the typical volatile emission in tobacco in response to Pst DC3000 infection. To clarify the role of ethylene, we dynamically applied ethylene to the headspace of tobacco plants following infection with Pst DC3000. Consistent with Pstb-induced ethylene, exogenous ethylene reduced both ocimene and MeSA emissions, and selectively altered the ratios and amounts of induced sesquiterpene emissions. Our findings suggest that ethylene can regulate the magnitude and blend of induced volatile emissions during pathogen infection.