Chasing the Sun: Auxins in Plants Explained

If a plant needs to grow taller or bend toward a stimulus, then it must lengthen its existing cells. If auxin is the hormone that triggers this stretching, then its primary function is promoting cell elongation.

If a plant stem curves in the direction of a sunlit window, then it is responding to light. If "photo" means light and "tropism" means turning, then this behavior is correctly called phototropism.

If the growth of a plant starts from the highest point of the stem, then that tissue is the apical meristem. If this is the site of hormone synthesis, then auxin is produced in the apical meristem.

If light hits one side of a stem, then the auxin molecules are sensitive and migrate away from the brightness. If they move away from light, then they accumulate on the shaded side.

If auxin allows cells to expand, it must weaken the rigid cell wall. If it promotes root branching and stem curvature, then those are its known effects; however, it does not reduce chlorophyll or stop water intake.

If auxin promotes elongation and is more concentrated on the shaded side, then the shaded cells will stretch more than the sunny cells. If one side stretches more, then the stem must bend toward the light.

If high levels of auxin at the tip of the plant prevent side buds from growing, then the top "dominates" the growth. If the tip is the apex, then this is called apical dominance.

If different tissues have different sensitivities, then they react differently to the same hormone. If shoots need lots of auxin to grow but roots are stunted by that same amount, then auxin stimulates shoots but inhibits roots.

If synthetic auxins can cause "overgrowth" that kills weeds or trigger root development on a cut branch, then they are useful tools; however, they do not help plants grow without light or change soil salt.

If gravity causes auxin to settle on the lower side of a horizontal root, and if high auxin inhibits root growth, then the top of the root will grow faster. If the top grows faster, then the root bends downward.

If auxin triggers the movement of protons (H+) into the cell wall, then the pH drops and enzymes called expansins activate. If these enzymes loosen the fibers, then the cell can stretch under water pressure.

If the tip is the factory for auxin, then cutting it off stops the flow of the hormone downward. If the inhibitor is gone, then the side branches are free to grow, making the bush wider.

If light and gravity determine where auxin migrates, they are primary factors. If the plant's age and hydration affect hormone levels and transport speed, then they also matter; flower color, however, does not control auxin flow.

If the plant needs to coordinate growth from the top down, then the hormone must follow a specific path. If auxin primarily travels from the shoot tip toward the roots using carrier proteins, then the transport is polar (one-way).

If the settling of auxin on the lower side of a stem requires a gravitational pull to orient the molecules, then in zero gravity, there is no "bottom." If there is no pull, then the auxin distribution remains even.

If scientists have identified the specific chemical structure of the main plant growth hormone, then they have named it. If that name is Indole-3-acetic acid, then IAA is the abbreviation for natural auxin.

If an overdose of auxin forces a plant to elongate its cells without stop, then the plant's metabolic systems will collapse. If the plant grows itself to death, then the auxin-based herbicide has worked.

If the bending of a plant in response to light, gravity, or a physical support (like a vine on a pole) requires uneven cell growth, then auxin is the driver for all three of these responses.

If auxin is a signaling molecule that tells cells how to behave rather than being a fuel source, then it is a hormone, not a nutrient or protein food.

If a steering wheel changes the direction of a car, then a mechanism that changes the direction of a stem does the same. If auxin causes one side to grow longer than the other, then it effectively "steers" the plant's shape.