Plants can tell the difference between the shade of an inanimate object and the shade of another plant. When a plant detects competition from neighboring plants, it initiates a set of responses, called collectively the shade avoidance syndrome, that alter its growth and physiology. A rapid and transient increase of newly synthesized auxin via a newly discovered auxin synthesis pathway allows plants to elongate and grow toward the sun.
The process has been known for a long time but a study published this week in the Journal Cell explains the molecular details:
"When the major photoreceptor for shade avoidance detects neighbors, it triggers the TAA1 pathway resulting in a rapid increase in free auxin, which is transported to sites in the stem where it can participate in the growth response."
There's a nice report on the work at the ScienceDaily site.