Model of lysigenous aerenchyma formation. Waterlogging promotes biosynthesis and accumulation of ethylene, followed by induction of RBOH expression. RBOH activity leads to production and accumulation of O2·- at the apoplast. The O2·- is spontaneously or enzymatically converted to H2O2, which can easily diffuse into the cytosol through the plasma membrane. Under waterlogged conditions, in the cytosol of stelar cells and cells in the outer cell layers, H2O2 and other ROS are scavenged by constitutively-expressed MT. By contrast, in the cortical cells, the decreased MT expression prevents ROS scavenging, thereby leading to greater ROS accumulation, which activates the subsequent processes of programmed cell death and lysis of the cortical cells (i.e., lysigenous aerenchyma formation). Under aerobic conditions, the RBOH gene is expressed at low level and the MT gene is constitutively expressed in the cortical cells. WL, waterlogged conditions; Aer, aerobic conditions; OCL, outer cell layers; ap, apoplast; cs, cytosol.