**Abstract**

Unlike animals, most of the plants are sessile. This may be a reason why they developed the powerful ability of organ generation throughout their lifetime, which is distinct from the animals, whose generation potential is restricted in a certain period during development. Half part of the plant body, the root system, is hidden under the ground, where there is a competition of resources, for example, water and nutrients or biotic stresses and abiotic stresses surrounding the root system. With its strong regeneration ability, the architecture of the root system is shaped by all of these environmental cues together with the internal developmental signals. In this process, phytohormones work as the regulatory molecules mediating the internal and external developmental signals, thus controlling the morphology and function of the root system architecture. This chapter introduces the development of root system regulated by various phytohormones, like auxin, cytokinin, etc.

**Keywords:** root architecture, auxin, cytokinin, postembryonic organogenesis
