It’s a jungle out there for a plant. If its neighbors grow faster than it does, they will block the sunlight the plant needs to make food and produce seeds. Howard Hughes Medical Institute (HHMI) researchers have now discovered a new molecular mechanism that enables plants to detect when they are in the shade of other plants and adapt by speeding up their growth.
The scientists, led by HHMI Investigator Joanne Chory and postdoctoral researcher Ullas Pedmale of the Salk Institute for Biological Studies, found that light-sensing and DNA-binding proteins team up to switch on a unique set of genes that cause a plant to grow. The scientists publishedexternal link, opens in a new tab their findings on December 24, 2015 in the journal Cell.
“Life is always hard for a plant,” says Chory. Its habitat can be too hot or too cold, too wet or too dry, too bright or too shady. To ensure that they are in sync with their local environment, plants closely monitor and react to factors such as temperature and the amount of light falling on them. Dim light triggers the shade avoidance response, in which the stem and the leaf stalks accelerate their growth, helping the plant escape from the shadow of its rivals.
Researchers have found that plants sense not only light quantity, but also light quality. Vegetation absorbs red and blue light and reflects what’s known as far-red light, which is at the boundary of what our eyes can detect. A surge in the amount of far-red light, relative to the amount of red light, can signal that a competitor is nearby and is casting a shadow on the plant--or is about to.
In 2012, Chory and colleagues uncovered a key step in a plant’s response to the ratio of red to far-red light. They found that a plant molecule that senses red and far-red light turns on the protein PIF7 and, to a lesser extent, its relatives PIF4 and PIF5. These molecules are transcription factors that help control the activity of genes. When a plant absorbs more far-red light, PIF7 attaches to and switches on genes that orchestrate the production of the growth-promoting hormone auxin.
“Shade” is a complex environment whose properties change with seasons, latitude, and whether a plant is growing in the middle or edge of a field. A plant in the shade isn’t just exposed to more far-red light. It might also receive less blue light. For the new study, Chory and Pedmale wanted to determine how decreases in this type of light instigate the shade avoidance response. The team raised mutant seedlings in low-blue-light conditions for four days and then determined whether the loss of particular proteins affected how fast the plants grew. The researchers found that although PIF7 wasn’t involved in the reaction to diminished blue light, PIF4 and PIF5 were.
Chory, Pedmale, and colleagues next wanted to figure out how these proteins work. They discovered that PIF4 and PIF5 bind to CRY1 and CRY2, the proteins that sense blue light. These proteins get together on the promoters in the plants’ DNA, the “on” switches for genes.
When the researchers measured which genes turned on in shaded plants, they got a surprise. Plants don’t activate the same genes in response to low levels of blue light as they do after exposure to increased amounts of far-red light. For example, a plant that senses more far-red light turns on genes that produce auxin, but decreased amounts of blue light cause the plant to flip on genes that stimulate growth in other ways. Depending on which type of light plants are exposed to, “they use the same family of transcription factors to turn on a completely different set of genes,” says Chory.
Plants monitor the overall amount of light that reaches them, as well as the levels of far-red and blue light, and they have different mechanisms for responding to each. Why plants need more than one such mechanism remains unclear, says Chory. The researchers suggest that this redundancy may prevent plants from overreacting to temporary reductions in light, such as when a cloud passes over them.
Chory and her team now want to determine which cells in the plants sense changes in levels of blue light. Answering questions like that could bring practical benefits, she says. Even in agricultural fields, with their evenly spaced rows, plants shade each other, reducing the yield of crops. A better understanding of how shade affects the growth of plants might allow researchers to alter their responses and increase food production. “Ultimately, that will impact yield and our ability to feed 11 billion people” who are projected to live on Earth by the year 2100, Chory says.