The wild Arabidopsis, like most plants, has air channels between its cells. These structures are like ventilation shafts woven around the sealed cellular compartments, and they are known to play important roles both in photosynthesis and in oxygenating cells. But the air channels of the mutant plant were flooded with water. The team tracked the mutation to the gene abcg5, which produces a protein that may help waterproof the cell wall to ensure that the plant’s air shafts are watertight.

Intrigued, the researchers tried an experiment. They filled the intercellular air shafts of non-mutant plants with water to see whether this affected their growth. Like the mutants, these plants had a difficult time determining where the light was coming from. “We can see that these plants are genetically normal,” Legris said. “The only things they’re missing are these air channels.”

The researchers deduced that the plant orients itself to light through a mechanism based on the phenomenon of refraction — the tendency of light to change direction as it passes through different media. Because of refraction, Legris explained, light passing through a normal Arabidopsis will scatter under the surface of the stem: Every time it moves through a plant cell, which is mostly water, and then through an air channel, it changes direction. Since some of the light is redirected in the process, the air channels establish a steep light gradient across different cells, which the plant can use to assess the light’s direction and then grow toward it.

In contrast, when these air channels are filled with water, the scattering of light is reduced. Plant cells refract light in a similar way as a flooded channel, since they both contain water. Instead of scattering, the light passes almost straight through the cells and the flooded channels to deeper within the tissue, decreasing the light gradient and depriving the seedling of differences in light intensity.

Seeing the Light

The research suggests that these air channels play a critical role in helping young plants track light. Roger Hangarter, a plant biologist at Indiana University Bloomington, who was not involved in the new study, hailed it for finding a clever solution to a long-standing problem. Fankhauser, Legris and their colleagues “pretty well put the nail in the coffin on the importance of these air spaces,” he said.

The idea has come up before, Hangarter noted. In 1984, a team of researchers at the University of York suggested that air channels between plant cells might help establish the necessary light gradient. But since the team didn’t have the funding to carry out expensive experiments, their suggestion went untested.

“It was always baffling to us how these little, tiny — almost transparent — [embryonic plants] could detect a gradient,” Hangarter said. “We never really gave much credence to the air-space thing because we were distracted looking for molecules that were involved. You get on a certain research path, and you get blinders on.”