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What Really Triggers This Plant to Grow?

Is it a shoot's outer waxy layer? Its inner layer studded with chloroplasts? Or the vascular system that moves nutrients and water? The answer could have great implications for modern agriculture, which desires a modern magical bean or two.

Researchers in the Plant Biology Laboratory at the Salk Institute have unlocked the mystery after successfully making tiny plants big and big plants tiny by controlling growth signals emanating from the plants' outer layer, their epidermis.

The findings, reported in Nature, could eventually be used by agronomists to manipulate plant growth pathways to maximize crop yield, says the study's lead author Joanne Chory, professor and director of the Plant Biology Laboratory and investigator with the Howard Hughes Medical Institute.

Chory and her team have spent years helping to define how a plant "knows" when to grow and when to stop – which is a "big question in developmental biology," she says. For their experiments, they rely on the model system Arabidopsis thaliana, a small plant related to cabbage and mustard, whose genome has been decoded.

The researchers have built up a whole tool kit over the years, learning how to add and subtract genes in order to determine form and function. Among their discoveries is a class of dwarf plants whose size is about one-tenth the size of a single leaf of the full-sized plant.

Over the past decade, Chory's laboratory and others have shown that these dwarf plants are defective in making or responding to a steroid hormone called brassinolide. Among the genes identified was the plant steroid receptor BRI1 ("bry-one"). The dwarfed Arabidopsis doesn't express BRI1 at all, unlike normal Arabidopsis, which expresses BRI1 on both the outer waxy epidermis, and the inner sub-epidermal layer, which contains the chloroplasts that conduct photosynthesis. In the current study, first author Sigal Savaldi-Goldstein, a postdoctoral researcher in the Plant Biology Laboratory, and Charles Peto, an electron microscopy specialist in the Laboratory of Neuronal Structure and Function, conducted a series of experiments that address an old question: Which leaf tissue controls growth?

They found that when they drive the expression of the BRI1 receptor in the epidermis of a dwarf Arabidopsis while leaving the sub-epidermal layer alone (without BRI1 receptors), the tiny plant morphed into a full-sized plant. In the second set of experiments, they used an enzyme to break down the steroid hormones in the epidermis, and found that a normal sized plant shrunk into a dwarf. "These are simple experiments, but it took 10 years of work in order for us to be able to ask this question," Chory says.