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Main conclusionThis article unveiled that ethylene biosynthesis and signaling play a critical role in heat stress response of tomato plants under elevated CO(2.)AbstractPlant responses to elevated CO2 and heat stress are tightly regulated by an intricate network of phytohormones. Plants accumulate ethylene (ET), the smallest hormone, in response to heat stress; however, the role of ET and its signaling in elevated CO2-induced heat stress response remains largely unknown. In this study, we found that transcript levels of multiple genes relating to ET synthesis, signaling, and heat shock proteins (HSPs) were induced by elevated CO2 (800 mu molmol(-1)) compared to ambient CO2 (400 mu molmol(-1)) in tomato leaves under controlled temperature conditions (25 degrees C). Elevated CO2-induced responses to heat stress (42 degrees C) were closely associated with increased ET production and HSP70 expression at both transcript and protein levels. Pretreatment with an antagonist of ET, 1-methylcyclopropene that inhibits ET-dependent responses, abolished elevated CO2-induced stress response without affecting the ET production rate. In addition, silencing of ethylene response factor 1 (ERF1) compromised elevated CO2-induced responses to heat stress, which was associated with the concomitant reduction in the transcript of heat shock factor A2, HSP70 and HSP90, indicating that ERF1 is required for elevated CO2-induced responses to heat. All these results provide convincing evidence on the importance of ET biosynthesis and signaling in elevated CO2-induced heat stress response in tomato plants. Thus, the study advances our understanding of the mechanisms of elevated CO2-induced stress response and may potentially be useful for breeding heat-tolerant tomatoes in the era of climate change.