Plant Puberty

MAKING LEMONS and grapes plumper, persuading asparagus to send out more shoots, and waking up a wide variety of seeds from dormancy are all part of gibberellin's job description, but how this plant hormone gets down to business has long been a mystery. Now, two teams of researchers are reporting the first X-ray crystal structures of this important hormone's receptor.

"It's very exciting," says Neil E. Olszewski, a plant biologist at the University of Minnesota. "I've been waiting for the structure. We've known what the gibberellin receptor is, but its structure will provide the framework for understanding how the hormone's signal process works."

The receptor's structure "could help in designing more effective and potentially cheaper gibberellin-like growth regulators for agriculture," Peter Hedden, a plant biochemist at Rothamsted Research, a crop science research institute in England, notes in a published commentary.

Japanese scientists studying rice germination first discovered gibberellin, a diterpenoid carboxylic acid, as a hormone more than 50 years ago. Since then, some 130 different kinds of structurally similar gibberellins have been identified, although not all are biologically active as hormones in plants.

In recent years, researchers figured out that when a gibberellin slips into its receptor, proteins called DELLA, which interfere with gene expression, are targeted for degradation. Destroying DELLA proteins consequently activates genes related to germination, stem elongation, and flowering. But the mechanism for how all this happens was previously unknown, Olszewski says.

The structure of the gibberellin receptor reported in Nature (2008, 456, 459) reveals a deep pocket that can accommodate the hormone. It was solved by Toshio Hakoshima, a crystallographer at the Nara Institute of Science & Technology, in Japan; Tai-ping Sun, a plant biologist from Duke University; and their colleagues.

Gibberellin binding causes an unstructured section of the receptor's N-terminus to collapse into a helical bundle, which then closes over the gibberellin molecule like a lid, Hakoshima explains. The top of the closed lid contains several hydrophobic residues, which then interact with a hydrophobic portion of a DELLA protein. This association with the gibberellin receptor causes DELLA proteins to be chopped up, thereby allowing gene expression.

"Our results reveal a completely different mechanism of plant hormone perception" than that of auxin, the only other plant hormone that's had its receptor's structure solved, Hakoshima says. Although both hormones activate gene expression by targeting transcription blockers for destruction, the two hormones achieve this end differently.

When auxin binds to its receptor, the hormone acts as a "molecular glue" that pulls transcription blockers into the same receptor pocket as the hormone (C&EN, April 9, 2007, page 11). But gibberellin binding initiates a conformational change of the receptor, creating a hydrophobic platform that entices the transcription blocker to bind.

In another paper in the same issue of Nature (2008, 456, 520), a second team of Japanese researchers report the structure of gibberellin and its receptor but without the DELLA transcription blocker. The structure reported by Hiroaki Kato, a structural biologist at Kyoto University, and Makoto Matsuoka, a plant scientist at Nagoya University, reveals a hormone-in-a-box structure similar to that in the first paper.

The second team also mutated sections of the gibberellin-binding pocket, "which provides insight about which functional groups on the gibberellin are important for binding in the receptor pocket and why some gibberellins are biologically inactive," Olszewski says. "In the long term, we may be able to develop new gibberellin mimics or manipulate the plant receptor to recognize synthetic gibberellins."