Auxin in Phototropism

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Phototropism, or a plant’s ability to manipulate their growth in order to grow in the direction of a light source, is primarily observed in plants, but also in organisms such as fungi. Phototropism is one of the many “tropisms” or movements in which plants react from external stimuli. Growth towards a light source is known as “positive phototropism” while growth away from such a source is called “negative phototropism. Plant shoots tend to utilize positive phototropism and roots often exhibit negative phototropism. Vine shoot tips are abnormal in their tendencies to utilize negative tropism with which they are able to grow towards darker, secure objects and climb them. This occurs due to a reaction of auxin in the plant. Auxins, found in the plant cells that are farthest from the light, are plant hormones with a variety of functions including embryonic development, leaf formation, apical dominance and root development. Relative to the process of phototropism, auxins transport protons by activating proton pumps which consequently decreases the pH levels in the cells on the darker side of the plant. This region becomes more acidic and activates enzymes known as “expansins”, which break bonds within the cell wall structure. The cell wall therefore becomes less rigid in this area. This flaccidity directly changes the shape of the plant as it curves towards the light source. Separately, the acidic environment causes a breakup of the hydrogen bonds in the cellulose that makes up the cell wall. As the cell wall loses strength, the cells swell, which provides the mechanical pressure needed for phototropic movement.
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