Effects of elevated CO2 and soil water content on phytohormone transcript induction in Glycine max after Popillia japonica feeding

Plants will experience increased atmospheric CO2 and drought in the future, possibly altering plant- insect dynamics. To investigate the combined effects of these components of global change on plant-insect inter- actions, three major hormone signaling pathways (jasmonic acid (JA), salicylic acid (SA), and ethylene (ET)) and related defenses were examined in undamaged soybean (Glycine max) leaves and after Japanese beetle (Popillia japonica) feeding; plants were grown under elevated CO2 and reduced soil water both independently and simulta- neously. Nutritional quality and Japanese beetle preference for leaf tissue grown under these different conditions also were determined. Elevated CO2 increased the concentra- tion of leaf sugars and dampened JA signaling transcripts but increased the abundance of SA compared with plants grown in ambient CO2. A mild reduction in soil water content had no effect on leaf sugars but stimulated the induction of transcripts related to JA and ET biosynthesis after herbivory. When applied in combination, elevated CO2 and reduced soil water content suppressed the expression of transcripts related to JA and ET gene tran- scription. Exposure to elevated CO2 alone increased sus- ceptibility of soybean to beetle damage. However, exposure to elevated CO2 in combination with reduced soil water content negated the impact of elevated CO2, leaving susceptibility unchanged. Predicting future crop resistance to pests must take into account interactions among indi- vidual components of global climate change.