Salk scientists discover how plants grow to escape shade
Mild mannered though they seem, plants are extremely competitive, especially when it comes to getting their fair share of sunlight. A plant's primary weapon in this contest is the ability to grow toward light, getting the amount it needs and shadowing its competition.
Scientists knew that a pigment found in leaves of thale cress plants (Arabidopsis thaliana), phytochrome B (PHYB), is excited by both the red wavelengths of light that drive photosynthesis, as well as the near infrared light that is enriched in shady spots. But no one had found a direct link between this response to light and the hormone-driven growth response to shade.
A team in the lab of Joanne Chory recently determined precisely how leaves tell stems to grow when a plant is caught in a shady place. In a paper published in Genes and Development, the researchers reported 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, hormones that stimulate stem growth.
If a sun-loving plant finds itself in a shady place, sensors in its leaves will tell cells in the stem to elongate, causing the plant to grow upward toward 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. In agriculture, this response, known as shade avoidance syndrome, results in loss of crop yield due to closely planted rows of plants that block each other's light.
In their study, Chory and her colleagues used biochemical and genomic analyses to identify PIF7 as the key molecular link between a plant's light sensors and production of auxins.
"We already knew that auxin is made in the leaves and travels to the stem to stimulate growth," she says. "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."
These 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 and capable of producing higher food and biofuel yields than existing strains.