The leaf of a normal plant (left), and one from a plant without the brassinosteroid hormone in its epidermis (right).

Sigal Savaldi-Goldstein

How Does Your Plant Garden Grow?

As the first brave flowers start to poke through the cold, wet ground this spring, gardeners might wonder what drives a plant's reach for the sky. They're not the only ones: For over a century, scientists have puzzled over the mechanics of plant growth. Now researchers have found that the key appears to lie in the plant's skin--or epidermis--which tells the rest of the plant when it's time to grow up.

Plant tissue is divided into three main layers: the vascular tissue, which carries water and nutrients; the subepidermis around the core, which is packed with photosynthesizing cells; and the epidermis, a protective layer wrapped around the two other layers. The debate over which layer prompts growth, first raised nearly 150 years ago, pitted the inner layers against the outer one. One theory argued that the inner layers played the active role, pushing against the epidermis like air pumped into a balloon. The other theory held that the epidermis grows first, creating a void that the inner layers are obliged to fill.

To see which is correct, plant geneticist Sigal Savaldi-Goldstein and colleagues at the Salk Institute for Biological Studies in San Diego, California, manipulated a growth-promoting plant hormone known as a brassinosteroid and its receptor in the epidermis and the vasculature tissue of Arabidopsis, a mustard plant. When dwarf strains of the plant, which lack the hormone receptor, were genetically engineered to produce it in their outer tissue, they shot up to normal size. In addition, normal strains engineered to lack the hormone in the epidermis stayed small, the team reports tomorrow in Nature. The researchers suggest that rather than earlier biomechanical theories of layers pushing or pulling, the epidermis chemically signals the inner layers to start or stop growing, and all grow together. "Whether plants decide to grow or not is a matter of life and death," says Savaldi-Goldstein. "Now we know that epidermis plays a key role in this decision."

"It's an important step forward," says plant geneticist Steven Clouse at North Carolina State University in Raleigh. But he notes that the findings don't completely exclude a role for the inner tissues in plant growth. For instance, he says, other studies have shown that when the epidermis is removed, plants still elongate in response to brassinosteroids.

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