Plants turn toward light - that much has been known for a long time. But the mechanism by which they do so has only recently been uncovered by an international team of biologists.
Phototropism is the process describing how plants point themselves toward the Sun, in order to gather as much light as possible to fuel photosynthesis. Auxin, a plant hormone, has been found to be the driving force behind this movement. The hormone is also used by plants to direct its growth in a form suitable to its environment.
Young plants exhibit phototropism to an even greater degree than older plants. When seeds sprout under the soil, they have a small amount of starch and lipids from which to feed. The seedling pops up through the ground by sensing which way gravity is pulling it, and heading in the opposite direction. Light-sensing proteins seek out the light, and direct the plant toward the source.
"Even mature plants bend toward the strongest light. They do this by elongating the cells of the stem on the side that is farthest from the light," Claus Schwechheimer, Plant Systems Biology Chairman at the Technische Universität München (TUM), said.
The elongation of these cells that cause phototropism is directed by the phytohormone auxin, according to Schwechheimer. Created in cells located at the tip of the shoot, auxin is transported from cell to cell by import and export proteins before finally reaching the areas which are its intended target. Modified export proteins called PIN's direct auxin on its journey, but not on their own.
"They require the signal of the D6PK protein kinase. The kinase enzyme modifies the PINs through the transfer of phosphate groups - thus activating them as auxin transporters," Schwechheimer said.
The research team investigating the role that auxin transport plays in helping plants turn toward light removed the kinase proteins from plants, which then became unresponsive to light. The plants did, however, continue to grow upward against gravity.
Charles Darwin made the first in-depth study of this behavior of plants to turn toward the light in his book The power of movement in plants. Auxin was first proposed as the hormone responsible for phototropism in 1937 by Frits Went, but this new research was able to prove that theory for the first time, and uncover the role played by D6PK.
