Arabidopsis thaliana seedlings show positive phototropism: they bend and grow toward the source of (blue) light. However, phototropism is not observed if the seedlings are treated with an auxin efflux inhibitor called NPA. Knowing that auxin stimulates elongation of the cells in the growing shoot, these observations are consistent with a model in which, in the presence of a lateral light, ...

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In the presence of a lateral light, auxin efflux transporter proteins become primarily localized to the side of the cell that is away from the light source. This redirects the auxin flux mainly to the darker part of the growing shoot, where it stimulates the elongation of the cells (in contrast to its inhibitory effect on the elongation of root epidermal cells seen in gravitotropism), bending the shoot towards the light source.

Arabidopsis Thaliana Seedlings Show Positive Phototropism: They Bend and Grow