How Do Plants Respond to Stimuli? Tropisms and Auxin
Learn how plant shoots and roots respond to light and gravity, how IAA controls cell elongation, and why auxin has different effects in shoots and roots.
Plants respond to stimuli by changing the direction of their growth. These growth responses are called tropisms. A shoot usually grows towards light, while a root usually grows downwards in response to gravity. The plant growth factor auxin, known as IAA, helps control both responses.
How does a tropism work?
A stimulus is a change in the environment, such as light coming from one direction or the pull of gravity. Growth towards a stimulus is a positive tropism; growth away from it is a negative tropism. Growing towards light is positive phototropism. Growing in the direction of gravity is positive gravitropism.
When light reaches a shoot from one side, more IAA accumulates on the shaded side. In shoots, IAA stimulates cell elongation. Cells on the shaded side therefore grow longer than those on the lit side, bending the shoot towards the light.
Gravity also leads to an uneven distribution of IAA. In a horizontal root, more IAA accumulates on its lower side. Unlike in shoots, a higher concentration of IAA inhibits cell elongation in roots. Cells on the upper side elongate more, so the root bends downwards. In a horizontal shoot, IAA on the lower side stimulates elongation, so the shoot bends upwards.
For example, imagine a seedling shoot growing beside a window. Light reaches one side, so the shaded side elongates faster and the shoot curves towards the window. This positions its leaves to receive more light for photosynthesis. If the seedling is turned on its side, its shoot curves upwards while its root curves downwards.
Exam mistake to avoid
Do not write that auxin always speeds up growth. Its effect depends on the tissue: a higher IAA concentration stimulates elongation in shoots but inhibits elongation in roots. To explain the direction of bending, identify which side elongates more.
