• Highlights
• The upper hypocotyl perceives the phototropic stimulus in Arabidopsis seedlings
• Cotyledons and the hypocotyl apex are not necessary for phototropism in Arabidopsis
• Local activation of phototropin 1 leads to a rapid global response
• Spatial aspects of signaling during phototropism differ between grasses and dicots

Summary

Phototropism is an adaptive response allowing plants to optimize photosynthetic light capture [1,2,3,4,5,6,7]. This is achieved by asymmetric growth between the shaded and lit sides of the stimulated organ [8,9]. In grass seedlings, the site of phototropin-mediated light perception is distinct from the site of bending [10,11,12]; however, in dicotyledonous plants (e.g., Arabidopsis), spatial aspects of perception remain debatable. We use morphological studies and genetics to show that phototropism can occur in the absence of the root, lower hypocotyl, hypocotyl apex, and cotyledons. Tissue-specific expression of the phototropin1 (phot1) photoreceptor [13] demonstrates that light sensing occurs in the upper hypocotyl and that expression of phot1 in the hypocotyl elongation zone is sufficient to enable a normal phototropic response. Moreover, we show that efficient phototropism occurs when phot1 is expressed from endodermal, cortical, or epidermal cells and that its local activation rapidly leads to a global response throughout the seedling. We propose that spatial aspects in the steps leading from light perception to growth reorientation during phototropism differ between grasses and dicots. These results are important to properly interpret genetic experiments and establish a model connecting light perception to the growth response, including cellular and morphological aspects.