Air channels create a directional light signal to regulate hypocotyl phototropism

In light-limiting conditions, aerial organs of most plants reorient their growth towards the light to improve photosynthesis, through a process known as phototropism1-3. The blue light receptors phototropin control phototropic responses through light-induced protein kinase activity4. Current models posit that asymmetric activation of these sensory receptors across a unilaterally illuminated organ leads to asymmetric distribution of the growth hormone auxin ultimately leading to growth re-orientation4,5. However, the tissue properties required to generate a light gradient across the stem triggering phototropism remain unclear1. Here we show that inter-cellular air channels6,7 are required for an efficient phototropic response. These channels enhance light scattering (refraction and reflection) in Arabidopsis hypocotyls thereby enhancing the light gradient across the photo-stimulated organ. We identify an embryonically expressed ABC transporter that is required to keep air in inter-cellular spaces in seedlings and for efficient phototropism. Our work suggests that this transporter shapes cell wall properties to maintain air between cells. Moreover, we establish the functional importance of inter-cellular air channels in the hypocotyl for phototropism.