Question

You are studying a mutant plant that has expansions in the cell walls of the shoots that are not activated by low pH. This is different from the normal response of expansions. Based on this information, how will the plant shoot respond to light from one side of the plant? Be sure the explain the specific steps between the detection of the light in the plant, and the specific growth of the plant.

Answer 1

When light falls on a certain side of a plant, the plant bends in the direction of light. This phenomenon is called as phototropism. When light falls on a plant, the protoreceptors of the plant are stimulated and trigger the release of auxins. Auxin moves to the darker, shaded side of the stem in response to the light exposure. It stimulates the release of hydrogen ions in the cells in the shaded side of the stem, which cause the pH of the cells to decrease. The decrease in pH activates enzymes (called expansins), which cause the cells to swell and lead the stem to bend toward the light.

In a mutant that does not get activated by a low pH, there would be no observable phototropism phenomenon. The light will trigger the release of auxins, which will move to the darker region. However, when these auxins stimulate the hydrogen ions to cause a drop in pH, because the plant is a mutant, it will not show any reaction due to the inability of enzyme expansin to be stimulated. The cell walls of this mutant are already expanded and thus will be bent towards a certain side. The presence of light will not cause the plant to show any photropic movement in response to light.