The nutritional control of root development

Root development is remarkably sensitive to variations in the supply and di stribution of inorganic nutrients in the soil. Here we review examples of t he ways in which nutrients such as N, P, K and Fe can affect developmental processes such as root branching, root hair production, root diameter, root growth angle, nodulation and proteoid root formation. The nutrient supply can affect root development either directly, as a result of changes in the external concentration of the nutrient, or indirectly through changes in th e internal nutrient status of the plant. The direct pathway results in deve lopmental responses that are localized to the part of the root exposed to t he nutrient supply; the indirect pathway produces systemic responses and se ems to depend on long-distance signals arising in the shoot. We propose the term `trophomorphogenesis' to describe the changes in plant morphology tha t arise from variations in the availability or distribution of nutrients in the environment. We discuss what is currently known about the mechanisms o f external and internal nutrient sensing, the possible nature of the long-d istance signals and the role of hormones in the trophomorphogenic response.