Abstract
Roots can suffer from hypoxia when soils become waterlogged. Additionally, phytotoxic compounds can accumulate in waterlogged soils. These conditions negatively affect the growth and survival of most plants. The exceptions are wetland species that are well adapted to such conditions. In many wetland species, radial oxygen loss (ROL) is usually greatest near the root tip and declines rapidly with increasing distance from the tip, i.e., in the basal part of roots due to the presence of an ROL barrier. By preventing ROL, the barrier enhances diffusional oxygen transport from the root-shoot junction to the root tip, where it is used for root respiration. However, a positive effect of ROL from the root tip is that is detoxifies reduced phytotoxic-compounds in the rhizosphere. The ROL barrier partly consists of well-suberized cell layers in the outer part of roots. In addition to reducing ROL, the barrier inhibits the entry of phytotoxic compounds from soil. The ROL barrier allows wetland plant to grow under waterlogged conditions. Based on a gene expression analysis, WRKY, AP2, MYB, and NAC transcription factors appear to be involved in regulating suberin biosynthesis during ROL barrier formation. However, so far no mutant defective in an ROL barrier has been found. How the ROL barrier is formed as well as the environmental signals that trigger its formation remain unclear. Further studies are needed to determine the genes, plant hormones, and the signals involved in formation of the ROL barrier in order to understand how wetland plants adapt to waterlogged conditions.
