Plants exhibit various mechanisms to adapt to different nutrient supply conditions. Among these mechanisms, the plasticity of root development is vital. Plant root systems continuously branch and form lateral roots. Lateral roots develop from founder cells in the pericycle, the outermost layer of the vascular cylinder (stele) of the root. Auxin plays a dominant role in the specification of founder cells that give rise to lateral root initiation and the later stages of development, and differential distribution of auxin is required for LR organogenesis.
Ammonium (NH4+) is the preferred form of N over NO3– in rice (Oryza sativa), due to its waterlogged growth environment. Although the predominant form of mineral N in bulk soil for paddy rice fields is likely to be NH4+, rice roots are exposed to partial NO3− nutrition by nitrification in the rice rhizosphere. The practice of intermittent flooding during rice cultivation, which causes an uneven distribution of NH4+ and NO3– within the soil horizon under field conditions, is being adopted by increasing numbers of Chinese farmers. In rice, the growth of lateral roots (LRs) is generally enhanced by partial nitrate nutrition. The roles of auxin distribution and polar transport in lateral root formation in response to localized nitrate availability are not known.
A recent paper in Annals of Botany examines two rice cultivars with differing responsiveness to nitrate and finds that initiation of lateral roots is only enhanced in the one with higher responsiveness. In this cultivar, auxin accumulation in the LR zone is greater in response to NO3– compared with NH4+, and a greater number of auxin transporter genes show increased expression in the LR zone under PNN than in the less-responsive cultivar. The authors conclude that partial nitrate nutrition enhances auxin polar transport from the shoot to root and induces greater auxin flux into the lateral root zone, enhancing lateral root proliferation in the NO3–-responsive ‘Nanguang’ cultivar.
