Cross talk between phytohormone signals is thought to regulate the specificity of defense responses to biotic attackers; however, which these signals mediate induced plant volatile emission during pathogen infection is unclear. We have previously reported that a complex volatile blend including ocimene, methyl salicylate (MeSA), and numerous sesquiterpenes were released by tobacco plants infected with the avirulent/incompatible strain of Pseudomonas syringae pv. tomato (Pst). In contrast, a volatile blend consisting of only MeSA and two sesquiterpenes was released by plants infected with P. syringae pv. tabaci (Pstb) in a virulent/compatible interaction. In this study, we demonstrate that in the compatible interaction, Pstb trigger tobacco plants to produce an extremely large ethylene burst; while in the incompatible interaction there was no such induction. To examine the interaction of multiple pathogen stresses, we inoculated one leaf with Pst and another leaf from the same plant with Pstb. Combined infection resulted in significantly reduced emission of ocimene and MeSA compared to plants infected with Pst alone, but with no significant changes in total sesquiterpene emissions. We hypothesized that Pstb induced ethylene production may play an important regulatory role in altering the typical volatile emission in response to Pst infection. To clarify the role of ethylene, we dynamically applied ethylene to the headspace of tobacco plants following infection with Pst. Ethylene addition significantly reduced both ocimene and MeSA emissions, and selectively altered the ratios and amounts of induced sesquiterpenes emissions. Our findings strongly suggest that ethylene regulates the magnitude and blend of induced volatile emissions during pathogen infection.
Keywords: volatile emission
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