Auxin is a plant growth hormone that is essential forplant development. It plays a major role in controlling plant growth. It is involved from the earliest stages of embryogenesis and controls the organization oftheapicalmeristem(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 and adventitious roots (rhizogenesis). Auxin is also involved intropismsin response to gravity (gravitropism) or light (phototropism). These multiple effects at the plant level result from its control overcell division, cell elongation, and certain stages of differentiation.
Source: Auxin – Wikipedia
