The wild Arabidopsis<\/em>, 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<\/em>, which produces a protein that may help waterproof the cell wall to ensure that the plant\u2019s air shafts are watertight.<\/p>\n

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. \u201cWe can see that these plants are genetically normal,\u201d Legris said. \u201cThe only things they\u2019re missing are these air channels.\u201d<\/p>\n

The researchers deduced that the plant orients itself to light through a mechanism based on the phenomenon of refraction \u2014 the tendency of light to change direction as it passes through different media. Because of refraction, Legris explained, light passing through a normal Arabidopsis<\/em> 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\u2019s direction and then grow toward it.<\/p>\n

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.<\/p>\n