Scientists discover key cog in receptor that governs ripening

Digging deep into the protein molecules that govern ripening and aging in plants, scientists have found an ion of copper — and a genetic link to some of the oldest life forms on the planet.

The discovery reveals another key component of the ethylene receptor, a protein sensor that helps plants detect the slightest whiffs of ethylene, a gaseous hormone that prompts fruit to ripen and flowers to form.

Writing in the Friday, Feb. 12 edition of the journal Science, a team directed by UW–Madison botany professor Anthony B. Bleecker reports that a copper ion is a key mediator in the process by which plants sense minuscule concentrations of ethylene. The discovery confirms a long-standing hypothesis that protein receptors somehow get the help of a transition metal like copper to sniff out important but barely detectable environmental and developmental cues.

“It does solve the mystery of how a biological sensor can sense such a small molecule,” said Bleecker, who three years ago led a team that discovered the gene that governs the receptor for ripening and aging in plants. That discovery underpins the promise of one day being able to precisely control the process of aging and ripening, to make fruits ripen on schedule and cut flowers last many more days in their vases.

The new finding, according to Bleecker, takes that discovery a step further by revealing the key that permits ethylene — a hormone produced by plants themselves and that serves as a chemical cue to tell plants to ripen or age — to gain access to cells in plants and their fruit.

“It’s a kind of tinkering, like figuring out how a clock or a watch works,” Bleecker said. The finding “tells us how the system works at a chemical level.”

Because plants produce the hormone at such low levels, there has been a great deal of curiosity to know just how plants are able to detect it. “It’s tricky to sense low concentrations of small gaseous molecules like ethylene. The theory is the protein that senses gets help somehow.” One idea was a transition metal, metals like copper, silver and gold, which mediate reactions in organic chemistry, for example.

The new discovery, says Bleecker, confirms that theory, but also has an important evolutionary twist. The focus of Bleecker’s work has been in a plant known as Arabidopsis, a member of the mustard family and a relative of broccoli that serves plant molecular biologists like the fruit fly serves those who study genes in animals.

Combing plant genome databases, Bleecker’s team discovered a similar gene exists in a strain of blue-green algae, Synechocystis, a member of a class of organisms that are among the oldest known. Fossil evidence for such primitive photosynthetic bacteria places them on Earth as long as 3.5 billion years ago. That, said Bleecker, suggests “a very ancient origin for this protein function,” one that predates the evolution of land plants altogether.

Moreover, similar genes are known to exist in other plants, the tomato, for example, which is only a distant relative of Arabidopsis. What that all suggests, Bleecker said, is an intriguing economy of nature where genes are invented just a few times early on, but are modified many times over the course of evolution to fit the needs of different host organisms. The new mystery, he said, is precisely what role this gene plays in blue-green alga, which neither makes nor responds to ethylene.

“Although we don’t know, this new finding suggests where the plant receptor came from,” Bleecker said. “It is much more ancient than plants. It shows that parts of proteins are recruited in evolution and mixed and matched” to suit new needs.

Other members of Bleecker’s team include Fernando I. Rodriguez, Jeffrey J. Esch, Anne E. Hall, Brad M. Binder, all of UW–Madison; and G. Eric Schaller, now at the University of New Hampshire.