Membrane transporters mediating root adaptive repsonses to flooding

The lack of oxygen in flooded soils blocks aerobic respiration and ATP synthesis in mitochondria, with major implications to root metabolism and nutrient acquisition. Excess water also causes a sharp decrease in soil redox potential, resulting in very significant changes to the soil elemental profile and causing elemental toxicities. In addition to inorganic phytotoxins such as Fe2+, Mn2+, or H2S, a significant accumulation of organic substances (e.g. ethanol, acetaldehyde, and various short-chain fatty acids and phenolics), also occurs in waterlogged soils. Each of these substances may significantly affect plant physiological performance, both at the cellular and the whole-plant levels. Finally, hypoxic conditions favour generation of reactive oxygen species (ROS). These ROS can each damage to plant cells and tissues by causing lipid peroxidation in membranes, DNA damage, protein denaturation, carbohydrate oxidation, pigment breakdown and an impairment of enzymatic activity. In this talk, I discuss the ionic and molecular mechanisms underlying plant adaptive responses to above factors. I highlight the importance of maintenance of membrane potential and discuss the role of H+-ATPase and H+-PPase in this process. I show that reduced oxygen availability and associated reduction in the rate of H+-ATPase pumping causes a substantial membrane depolarization, making passive K+ uptake thermodynamically impossible and causing significant K+ loss through depolarization-activated outward-rectifying K+ channels. I also show that increase ROS production after prolonged hypoxia, or upon re-oxygenation, directly control activity of Ca2+ and K+-permeable plasma membrane ion channels. Both depolarization- and ROS-mediated K+ loss from the cytosol causes significant perturbations in intracellular ionic homeostasis and may result in a programmed cell death. The latter process may be of adaptive significance, contributing to aerenchyma formation by a mechanism unrelated to lysogeny. Implications of these findings for plant breeding for waterlogging stress tolerance in crops are discussed.