Plants respond dynamically to environmental stresses in ways that maximize their fitness.  While responses to abiotic stresses and herbivory independently are well known, less is known about interactive effects of multiple stresses.  For example, oxidative stress in plants resulting from tropospheric ozone pollution may influence the interaction of those plants with their herbivores.  Previous work has shown that ozone-stressed trees have markedly lower growth rates than do non-stressed trees in the field, but those trees also suffer less herbivory as a result.  The mechanism for these long-term effects is not well understood.  We conducted a factorial, controlled experiment by manipulating ozone concentration and insect herbivory.  Three genotypes of hybrid poplar trees (Populus deltoides x nigra) were contained within environmental chambers where ozone concentrations could be manipulated.  Ozone concentrations of 80 ppb, added diurnally, were chosen to mimic summer field conditions.  Plants were exposed to ozone until the first symptoms of oxidative stress were observed.  Gypsy moth (Lymantria dispar L.) larvae were then used to generate herbivore damage on half of the plants.  Leaf samples were collected and analyzed for phytohormone concentrations and transcriptional profiling using either a custom array containing approximately 6500 unigenes or a commercially available full genome array.

Results/Conclusions

Ozone exposure dramatically influenced plant responses to herbivores, including the reduction of the entire transcriptome.  Under ambient conditions, approximately 20% of the poplar transcriptome was regulated (either induced or suppressed) by herbivory.  In contrast, only 4% of the transcriptome was regulated by herbivory during ozone fumigation.  Jasmonate synthesis and signaling was one of the critical phytohormone responses altered by ozone.  In particular, the expression of four genes critical in JA synthesis (LOX, AOS, AOC, OPR) was compromised by ozone expression.  This, along with other transcriptome-level changes, was involved in the reduction of herbivore-induced JA concentrations.  Consistently, concentrations of signaling compounds and defense metabolites were significantly reduced despite extraordinary accumulation of fatty acid precursors resulting from ozone-mediated destruction of cell membranes.  The deficient defense signaling system resulted in differential concentrations of defensive metabolites and herbivore performance.  Such short-term responses to realized field levels of ozone may therefore have important consequences for plant demographics and plant-herbivore interactions within changing environments.