Impacts of climate change on herbivore induced plant signaling and defenses

The accumulation of CO2 and O3 in the troposphere alters phytochemistry which in turn influences the interactions between plants and insects. To examine the effect of elevated atmospheric CO2 and O3 concentrations on plant-insect interaction, I measured changes in transcription using microarray analysis of field-grown soybean (Glycine max). I found that the number of transcripts in the leaves affected by Japanese beetle (JB; Popillia japonica) herbivory was greater when plants were grown under elevated CO2, O3 and the combination of both compared to ambient atmosphere. The effect of herbivory on transcription diminished strongly with time, and elevated CO2 interacted more strongly with herbivory than did elevated O3 in my study. Constitutive levels and the induction by herbivory of key transcripts associated with defense and hormone signaling were down-regulated under elevated CO2, suggesting susceptibility may be altered.;To examine the impact of elevated CO2 exposure on susceptibility to herbivory in soybeans in more detail, the magnitude and timing of transcripts related to three major hormone signaling pathways (jasmonic acid [JA], salicylic acid [SA], ethylene [ET]) and related defenses were examined in field environments under elevated CO2 after JB feeding. In addition, JB preference between elevated and ambient-grown tissue was determined. Elevated CO 2 decreased the induction of JA and ET related transcripts ( lox7, aos, hpl and acc1), resulting in decreased accumulation of defenses (polyphenol oxidase, protease inhibitors) over time compared to ambient-grown plants. Elevated CO2-grown tissue was preferred by JB in choice experiments. Elevated CO2 also increased the accumulation of SA in soybeans. SA and JA are known to have an antagonistic relationship in other plants, and this antagonism may explain the reduction in JA related transcripts. These results suggest elevated CO2 exposure could cause increases in insect damage and reduction in diseases caused by pathogen sensitive to the SA defense pathway in the future.;In addition to elevated CO2, models predict that plants will also experience increased drought in the future, possibly altering plant-insect dynamics in unanticipated ways. To investigate the combined effects of drought and elevated CO2 on plant-insect interactions, components of susceptibility and palatability in soybeans were examined under field conditions. The effect of elevated CO2 exposure on phytohormone signaling was consistent with previous studies. Exposure to mild drought stimulated the induction of the JA/ET signaling pathways but had no impact on nutritional components. However, elevated CO2 exposure in combination with drought amplified the induction of JA and ET signaling transcripts and the accumulation of related defenses in soybeans after beetle herbivory. Overall, in combination with drought exposure, increased susceptibility of soybean to herbivores resulting from elevated CO2 exposure was removed. This study suggests soybean in areas experiencing elevated drought will have an advantage over well-watered plants grown under elevated CO2.;It is not known if the impact of elevated CO2 on phytohormones and induced defenses is a generalized response in soybean or if it varies across plant species. In an attempt to address these questions, phytohormone signaling was examined under ambient and elevated CO2 concentrations across six soybean cultivars and six different plant species using a common protocol across all experiments to aid in comparison. Elevated CO2 reduced constitutive levels of JA in some but not all soybean cultivars; there was extensive variation in the response. Unexpectedly, constitutive and induced ET signaling increased in some soybean cultivars. In contrast to the variation seen in JA and ET, constitutive levels of SA were elevated universally across soybean cultivars grown under elevated CO2 with little variation in the response. Across species examined, elevated CO2 had a similar impact as with soybean cultivars, generally reducing constitutive JA signaling transcripts in most species examined. However, in contrast to soybean there was no impact of elevated CO2 on levels of SA across species. This study suggests some pathways may experience generalized changes for a species with little variation (e.g. SA in soybean), while others may not (JA/ET) and interactions with herbivores can change the response (constitutive versus induced). Thus, the modulation of hormone signaling by elevated CO2 may cause increases in chewing insect damage and reduction in pathogen infections sensitive to the SA defense pathway in the future. However, the interactions between different aspects of global change with elevated CO2 may alter the response, playing a more important role in determining plant-insect interactions than previously hypothesized. In conclusion, the impact of elevated CO2 on phytohormone signaling appears complex, dependent on interactions, individual signaling pathways, cultivars and species examined.