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.

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Buckwheat is one of the most popular cereal crops in Japan, and is consumed as noodles. In 2014, 59,000 ha was used for buckwheat cultivation, but the average yield was only 520 kg ha^-1. One of the reasons for the low yield is lodging. Therefore, improvement of lodging resistance is an important issue in breeding programs. To resolve this issue, NARO Tohoku Agricultural Research Center released a lodging-resistant variety, ‘Nijiyutaka,’ in 2011. However, root characteristics related to lodging resistance were not well studied. We cultivated the new variety ‘Nijiyutaka’ and the control variety ‘Hashikami-wase’ at different seeding densities in three fields and analyzed root characteristics at different growth stages. We found that ‘Nijiyutaka’ had a larger number of lateral roots, a wider extension of hard lateral roots in the horizontal direction, a larger hard lateral root spreading angle and a larger hard lateral root length than ‘Hashikami-wase.’ These root characteristics supported stability of shoot and prevented lodging. Further, we found that decreasing the seeding density enhanced these root characteristics in both varieties. Our results are useful for the future development of new varieties and cultivation techniques.

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