Auxin, synthesized at the shoot tip, promotes cell elongation in response to light. When light is coming from one side, auxin diffuses towards the shady side, stimulating cells to grow longer on the side away from the light, causing the plant to bend towards the light.
How does the hormone auxin contribute to plant growth in response to light, and what is the specific mechanism by which it influences cell elongation?
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Auxin contributes to plant growth in response to light by exhibiting phototropism. In response to unilateral light, auxin accumulates on the shaded side of the stem. This uneven distribution triggers cell elongation on the shaded side, causing the stem to bend towards the light source. The specific mechanism involves auxin promoting cell elongation by acidifying the cell wall through activation of proton pumps. This lowered pH facilitates the activity of enzymes called expansins, which loosen the cell wall structure, allowing for increased water uptake and turgor pressure. The result is directional cell elongation and the bending of the stem towards the light.