Acclimation to soil flooding - sensing and signal-transduction

Flooding results in major changes in the soil environment. The slow diffusion rate of gases in water limits the oxygen supply, which affects aerobic root respiration as well as many (bio)geochemical processes in the soil. Plants from habitats subject to flooding have developed several ways to acclimate to these growth-inhibiting conditions, ranging from pathways that enable anaerobic metabolism to specific morphological and anatomical structures that prevent oxygen shortage. In order to acclimate in a timely manner, it is crucial that a flooding event is accurately sensed by the plant. Sensing may largely occur in two ways: by the decrease of oxygen concentration, and by an increase in ethylene. Although ethylene sensing is now well understood, progress in unraveling the sensing of oxygen has been made only recently. With respect to the signal-transduction pathways, two types of acclimation have received most attention. Aerenchyma formation, to promote gas diffusion through the roots, seems largely under control of ethylene, whereas adventitious root development appears to be induced by an interaction between ethylene and auxin. Parts of these pathways have been described for a range of species, but a complete overview is not yet available. The use of molecular-genetic approaches may fill the gaps in our knowledge, but a lack of suitable model species may hamper further progress.