The garden at Joe Noel's San Diego home features coastal sage, California lilacs, and other chaparral plants that once dominated the local landscape.

While he appreciates his garden's vibrant colors and water efficiency, like the biochemist he is, Noel sees beyond all that: “There's a tremendous amount of chemistry happening, most evident from the very interesting smells released from the plants throughout the day.”

An HHMI investigator at the Salk Institute for Biological Studies, Noel is one of a growing group of scientists fascinated by the extraordinary diversity of volatile and non-volatile compounds found in plants. Plants use these small molecules, which diffuse easily through the membranes of the cells that produce them, to communicate and interact with the outside world.

Often aromatic and almost always highly specialized for a particular ecological niche, volatiles in particular help each plant flourish in its environment, acting as an odiferous language among plants and between plants and other organisms. The chemicals attract pollinators, summon natural predators of pests, or provide protection through their antimicrobial properties.

Scientists have so far identified more than 1,000 plant volatiles. Even closely related plants produce their own unique sets of volatiles—evidence, says Noel, that the pathways giving rise to these chemicals and their biosynthetic precursors (known as secondary metabolism, because their products are not essential for a plant's growth or reproduction) are subject to rapid evolution.

“By evolving new compounds or new ways of making existing compounds, the enzymes in these biosynthetic pathways that produce volatiles and related compounds provide adaptive advantages to organisms,” Noel says. “And they're evolving so rapidly that you can begin to piece together a historical record of how these systems originated, what changes they've undergone, and how they're utilized in the present. Ultimately,” he adds, “I think we can also learn what changes we are in store for well into the future.”

Illustration: Christoper Silas Neal

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