The idea of being able to precisely control plant height might not sound that exciting at first, but ... what if your lawn just couldn't grow into a scraggly forest of grass, staying short and neat without ever needing mowing? Or what if crops could be made to grow larger, producing more food per acre?
As plant scientist and Howard Hughes Medical Institute investigator Joanne Chory explains, these aren't fantasies. She and her team at the Salk Institute have clarified the signaling pathway of a class of plant hormones called brassinosteroids that play a central role in regulating plant stature. These steroids are so important in telling plants to grow that disrupting the pathway turns plants into tiny dwarves.
"Every cell is smaller than it should be, and though it looks exactly like the big plant, it's just like a tiny little bonsai version," says Chory.
Applying this to plants of interest could lead to grass that stays perpetually short, more compact varieties of plants for cities, or any number of other horticultural applications. On the flip side, increasing the brassinosteroid signal makes plants grow larger, which Chory thinks could one day be important for increasing the yields of major food crops such as rice. "I am very interested in how we could increase yield and better feed the world," Chory says.
As Chory explains, when plant cells receive the steroid signal it triggers a cascade of events that turns on hundreds of genes and leads to plant growth. Removing the brassinosteroids miniaturizes plants, but also has secondary effects on plant aging and reproduction. Chory explains that plants age differently from animals, one leaf at a time. Brassinosteroids must promote this aging program because as she says, "When we don't have brassinosteroids in the plant, the leaves stay green, they don't turn yellow, and they also don't fall off." Knocking out the pathway also makes plants male sterile, which means they shouldn't be able to make pollen. Chory says they see these same effects in every plant in which they block the pathway.
In their most recent report in the journal Nature, Chory and her colleague Grégory Vert explained how the nuclear portion of the steroid signaling actually works in plants. Even though steroid signaling is highly conserved, preceding the split between animals and plants that occurred over a billion years ago, this portion of the pathway is drastically different in plants and was unknown till now.
Chory and her team identify the functions of genes in the brassinosteroid pathway by observing changes in plant stature. She explains that dwarfing the plants intentionally can be done in multiple ways, such as by replacing critical genes with non-functioning mutant versions, or by enhancing the gene for an enzyme that inactivates the steroid. Chory says that this is a fairly easy thing to do in most plants.
She says it should also prove easy to do in grass. "You could probably make grass any height you wanted. So you could make it really, really short, you could make it intermediate, you could even make it longer if you put in a different gene," she says.
Such dwarf grass would use less water, less fertilizer, stay green longer, and never need mowing. Chory says that as with any genetically modified organism, there is always the concern that the modified genes could leak into the environment, a problem known as gene flow. Even though such dwarf grass shouldn't be able to make pollen, which should keep it from spreading, companies interested in making it into a product will still have to weed through a complex approval process.
As for Chory, she's most satisfied with the new knowledge that her research has contributed to explaining the basic mysteries of plant functioning. "It's a very simple question – how do plants grow?" says Chory, "I'm excited that we've made a contribution to a very fundamental area in plant sciences about which we know so little."
