6.15B - Auxin and tropisms

6.15B - Hormones coordinate plant growth

A plant hormone is a chemical messenger made by plant tissues. In small amounts, plant hormones coordinate growth, such as cell division and cell elongation, and development, such as germination, flowering, fruit formation and ripening. Coordination matters because cells in different regions can respond by different amounts, or even respond differently to the same hormone.

Auxin is especially important in the growing regions near the tips of shoots and roots. A stimulus can change how auxin is distributed. Cells on opposite sides then elongate by different amounts, making one side longer than the other and causing the organ to curve. The plant has not chosen a direction and the whole plant has not moved from place to place: its direction changes through unequal growth.

Light produces a phototropism, while gravity produces a gravitropism. To explain either response, track the same three links: where there is more auxin, how that auxin changes cell elongation in that tissue, and how the unequal growth changes direction.

6.15B - Phototropism in shoots

Consider a young shoot receiving light from one side.

1. Auxin made near the shoot tip is redistributed so that more auxin is present on the shaded side.
2. In shoot tissue, auxin stimulates cell elongation. Cells on the shaded side therefore elongate more than cells on the light-facing side.
3. The shaded side becomes longer, so it forms the outside of the curve and the tip bends towards the light.
4. This is positive phototropism. The resulting position exposes the shoot and its leaves to more light for photosynthesis.

If light arrives evenly from above, the auxin effect is more even on the two sides, so the shoot grows more nearly straight. The key phrase is unequal cell elongation: saying only that "the stem elongates" does not explain a bend.