Scientists Discover How Plants Escape Shade

Story by:

Salk Institute for Biological Studies

First posted on 04-17-2012

Mild mannered though they seem, plants are extremely competitive, especially when it comes to getting their fair share of sunlight. Whether a forest or a farm, where plants grow, a battle wages for the sun’s rays.

A plant’s primary weapon in this fight is the ability to grow towards the light, getting just the amount it needs and shadowing its competition. Now, scientists at the Salk Institute for Biological Studies have determined precisely how leaves tell stems to grow when a plant is caught in a shady place.

The researchers report that a protein—known as phytochrome interacting factor 7 (PIF7)—serves as the key messenger between a plant’s cellular light sensors and the production of auxins, which are hormones that stimulate stem growth.

“We knew how leaves sensed light and that auxins drove growth, but we didn’t understand the pathway that connected these two fundamental systems,” says Joanne Chory, professor and director of the Salk’s Plant Biology Laboratory.

Plants gather intelligence about their light situation—including whether they are surrounded by light-thieving plants—through photosensitive molecules in their leaves. These sensors determine whether a plant is in full sunlight, or in the shade of other plants based on the wavelength of red light striking the leaves.

If a sun-loving plant finds itself in a shady place, the sensors will tell cells in the stem to elongate, causing the plant to grow upwards towards sunlight.

When a plant remains in the shade for a prolonged period, however, it may flower early and produce fewer seeds in a last ditch effort to help its offspring spread to sunnier real estate. This response, known as shade avoidance syndrome, results in loss of agricultural crop yield due to closely planted rows of plants that block each other’s light.

Chory and her team showed that when a particular plant is placed in shade, a cascade of molecular changes occurs in the cells of the leaves: photoreceptors cause chemical changes in PIF7, which then activates genes that direct the cell to produce auxin.

“We already knew that auxin is made in the leaves and travels to the stem to stimulate growth,” says Chory. “Now we know how shade stimulates the leaves to produce auxin, and it turns out that it’s a remarkably simple pathway to control such an important function.”

She added that the findings may offer new avenues for developing crops with stem architectures better suited to tightly planted field rows, making them less prone to shade avoidance syndrome. If successful, such crops would produce higher yields of foods and biofuels than existing strains.