Revealing How Plants Breathe

Just as plants take in the CO₂ required for photosynthesis, they also release gas on a daily basis. Researchers have long wondered how pores in the leaf epidermis control this gas exchange. Now, a team of researchers at Columbia University has provided a first glimpse of an ion channel, called SLAC1, that helps open and close those pores (Nature 2010, 467, 1074). Its X-ray structure, which boasts a never-before-seen protein fold, may one day be useful for breeding or engineering plants that can better survive in arid environments.

“Every single carbon molecule in our bodies, our food, or our furniture has at one time gone through a plant’s pores—so has a significant portion of the water in our atmosphere,” comments Jaakko Kangasjärvi, a plant physiologist at the University of Helsinki, in Finland, whose group initially identified SLAC1. This ion channel controls passage through those pores, as well as plants’ response to drought conditions, changing CO₂ levels, or even pathogen attacks, Kangasjärvi adds. “This structure will really help the plant biology community,” he says.

SLAC1 is buried in the plant cell membrane, making it hard to crystallize. Instead, researchers led by Wayne A. Hendrickson, a biochemist at Columbia University, crystallized the bacterial equivalent—a so-called homolog—of the plant protein. To show that the structure of the bacterial protein provides legitimate insight into the function of the plant protein, Hendrickson’s team did an extensive battery of biochemical experiments, including mutating essential residues and doing electrophysiology recordings of both the plant and bacteria protein reconstituted into frog egg cells, Kangasjärvi comments.

The X-ray crystal structure reveals that SLAC1 folds into a novel conformation that Hendrickson describes as “helical hairpins in a quasi-five-fold symmetry.” The protein sits in the membrane in groups of three. Each member of the threesome possesses a channel through which negatively charged ions such as chloride and nitrate slip, except when a phenylalanine side chain acts as a stopper. Opening of the channel is likely activated by phosphorylation, which causes conformational changes that “kick the phenylalanine out of the way,” Hendrickson says. When the channels are open, the efflux of negatively charged ions closes the pores in the plant epidermis.

The new structure narrows down which residues may be involved in the phosphorylation that opens the channel in plants, Kangasjärvi says. Pinpointing the precise phosphorylation sites that regulate the pore is just one of many questions that the plant biology community will use the new structure to answer, he adds.