Molecular response of poplar to single and combined ozone and drought.

High concentration in ground-level ozone (O3) and water deficit affect forest ecosystems service. Previously we found intercellular CO2 concentration and isoprene emission were affected by the combination of O3 and drought, but the molecular mechanisms controlling these phenotypes are still open questions. In this study, we investigated the stomatal conductance (gs) and transcriptome changes in an O3-sensitive hybrid poplar exposed to two O3 levels [charcoal-filtered ambient air (CF) and non-filtered ambient air plus 40 ppb (NF40)] and two water conditions [well-watered (W) and moderate drought (D)]. NF40 reduced the gs more under D than W. We identified the differentially expressed genes (DEGs) from pairwise comparisons and found the poplar's molecular response to drought was counteracted by elevated O3. From nine clusters obtained through K-means clustering, 12 core transcription factors were identified. DEGs involved in isoprene biosynthesis and phytohormones signal pathways indicate the molecular response and stomatal closure of poplar under O3 and/or drought might be through MEP/DOXP and ABA-dependent pathways. In addition, 102 Helitrons capturing DEGs were involved in response to O3 and/or drought and related with ABA-dependent pathway. This integrated analysis provides multi-dimensional insights to understand the molecular response to the combination of O3 and drought.