Drought, ozone, ABA and ethylene: new insights from cell to plant to community

Recent reports show ethylene-dependent reductions in stomatal sensitivity to abscisic acid (ABA) under ozone stress. These changes reduce stomatal control of plant water loss in drying soil. Here we review evidence that ABA and ethylene, and interactions between these two stress-induced hormones, control many of the responses of intact plants to drought and ozone stress, with emphasis on effects on stomata and shoot growth. We draw attention to convergent signalling and response pathways induced by ozone and drought that can increase production of hydrogen peroxide (H₂O₂) and nitric oxide (NO). Stomatal responses to a wider range of stresses and developmental cues may also be controlled via the same sets of signalling pathways. Other hormones, or effectors such as xylem/apoplastic pH or changes in plant water status, also play a role in signalling within and between organs. We discuss the implications, for crops, natural ecosystems and water catchment processes, of ethylene's antagonism of the stomatal response to ABA, against a back-drop of predictions for reduced precipitation and increasing ozone pollution, as part of global climate change and increasing urbanization and industrial development.