How does auxin influence root and stem growth?

Auxin promotes stem elongation and inhibits root growth in plants.

Auxin, a plant hormone, plays a crucial role in the growth and development of plants. It is primarily produced in the shoot tips and then transported to other parts of the plant. In stems, auxin promotes cell elongation, leading to the growth and stretching of the stem. This is achieved by increasing the plasticity of plant cell walls, allowing them to expand and stretch. The elongation of cells in the stem is a key factor in the plant's ability to grow towards light, a phenomenon known as phototropism.

In contrast to its effect on stems, auxin inhibits the growth of roots. High concentrations of auxin in the root tip suppress cell elongation, resulting in shorter roots. This is a process known as gravitropism, where roots grow downwards in response to gravity. The inhibition of root growth by auxin is thought to be a protective mechanism, preventing roots from overgrowing and potentially damaging the plant.

Furthermore, auxin also influences the formation of lateral roots. It stimulates the cells in the pericycle, a layer of cells just inside the endodermis, to divide and differentiate into lateral root primordia, the precursors of lateral roots. This process is crucial for the establishment of a robust root system, which is essential for the plant's ability to absorb water and nutrients from the soil.

In summary, auxin has a dual role in plant growth, promoting stem elongation while inhibiting root growth. Its differential effects on these two parts of the plant are key to the plant's ability to respond to environmental cues and ensure its survival and growth.