Ethylene plays a central role in morphological and anatomical acclimations of plants to soil waterlogging. To acclimate to waterlogging, roots of wetland species, and even some crops, form aerenchyma. Aerenchyma is large interconnected gas spaces that connect from shoot to near the tips of roots. Oxygen moves via diffusion in the root aerenchyma. In many wetland species, a barrier to radial oxygen loss (ROL) that greatly reduces oxygen leakage from basal parts, further enhances oxygen diffusion to the apex. Two main types of aerenchyma are recognized: schizogenous and lysigenous aerenchymas. Schizogenous aerenchyma is formed by cell separation without cell death. Lysigenous aerenchyma results from programmed cell death (PCD). Ethylene accumulation in plants during waterlogging induces lysigenous aerenchyma, although many wetland species form some aerenchyma even in drained conditions. The ethylene accumulation may result in the activation of G-proteins and other signal transduction cascades, of which increased cytosolic Ca2+ is a component. Finally, PCD leading to selective cell degradation occurs in the root cortex, with cell wall degrading enzymes, such as cellulase and pectinase, also being activated. On the other hand, ethylene does not appear to induce the barrier to ROL, at least in roots of rice. Thus, induction of the barrier to ROL and formation of aerenchyma appear to be regulated by different signals. Further elucidation of signal transduction pathways initiated by ethylene sensing, as well as non-ethylene dependant responses, and of the genes regulated to control formation of the barrier to ROL and aerenchyma, should be priority areas for future research on plant tolerance of waterlogging. (C) 2008 Elsevier Ireland Ltd. All rights reserved.