Plants synthesize and emit volatile molecules as signals for pollinating insects to come hither, for nearby plants to ready defenses against hungry herbivores, and for many other purposes. Now scientists have discovered that plants actively release these volatile organic compounds (VOCs) with the help of protein transporters in their cell membranes (Science 2017, DOI: 10.1126/science.aan0826).
“It’s pretty huge,” says Jonathan Gershenzon of the Max Planck Institute for Chemical Ecology, who was not involved in the work. “For years, people thought that plant volatiles just passed through membranes and that there was no active mechanism for getting them out of cells.” This discovery, he says, suggests plants have invested a lot of resources in emitting VOCs, in part to control the timing and rates of their release for certain functions.
Researchers at Purdue University, led by Natalia Dudareva and including Joshua Widhalm and John Morgan, started to question the diffusion mechanism a few years ago. They calculated that for plants to release VOCs at the rates scientists had observed, cells would have to accumulate high millimolar concentrations of the molecules in their lipid membranes. Such concentrations would be toxic to the cells, Widhalm says.
So in the new study, the team examined gene expression patterns in petunia plants (Petunia hybrida) to find proteins that might actively release VOCs. The researchers looked for genes with high expression levels two days after flower opening—the peak VOC emission time—and low expression levels at the budding stage, when there is little VOC release.
One gene that fits that description is PhABCG1, which encodes for an adenosine triphosphate-binding cassette transporter. When the researchers used RNA interference to lower expression of PhABCG1, the petunias’ VOC emission rates dropped and the volatile molecules started accumulating inside the plants’ cells, indicating that the transporter protein controlled the release of the compounds.
Gershenzon says the work raises many new questions about VOC transport, including how cells move VOCs from where they are synthesized to transporters and how the compounds get through the hydrophilic cell wall.