Auxin is a plant growth phytohormone essential for plant development. It plays a major role in controlling their growth. It is involved from the early stages of embryogenesis then controls both the organisation of the apical meristem (phyllotaxis) and the branching of the aerial parts of the plant (apical dominance), as well as the formation of the main root and the initiation of lateral roots and adventitious roots (rhizogenesis). Auxin is also involved in tropisms in response to gravity (gravitropism) or light (phototropism). These multiple effects at plant level result from the control it exerts over cell division, cell elongation and certain differentiation steps.