Impacts of CO2 pollution on plant-insect interactions: A transcriptional and biochemical analysis of soybean defense signaling

Anthropogenic CO₂ levels are rising in the atmosphere. Growth in elevated CO₂ generally reduces the nutritional quality of plant tissues for insects. Insects may increase food intake to compensate for reduced leaf nitrogen content, although this is not always the case. Incomplete understanding of the mechanisms governing plant-insect interactions under elevated CO₂ makes it currently difficult to predict alterations in future ecosystems. Based on previous studies examining global transcriptional profiles in soybean, elevated CO₂ exposure is hypothesized to modulate plant resistance through alterations in hormone signaling and defenses against herbivorous pests. To determine the impact of elevated CO₂ exposure on plant-insect interactions in soybean, magnitude and timing of two major hormone signaling pathways (jasmonic acid [JA] and salicylic acid [SA]) and related defenses were examined in open field environments under elevated CO₂. Elevated CO₂ exposure altered kinetics of JA and SA related transcripts and metabolites (JA, polyphenol oxidase; PPO, protease inhibitors; PIs and SA, phenolics). Accumulation of JA related transcripts and metabolites initially increased to a higher level after Japanese beetle attack in elevated CO₂ compared to ambient grown plants, but over time decreased to levels lower then ambient grown plants. In addition, elevated CO₂ increased the accumulation of SA related transcripts and metabolites in soybeans; in ambient grown plants the relationship between these two signaling pathways appears antagonistic. The modulation of JA signaling transcripts and metabolites resulted in lowered plant chemical defense and can explain increases in insect damage observed in previous soybean studies that were not explained by changes in plant nutrition.