A signal from the cell wall decides that, in the dark, seeds grow into long yellow sprouts, instead of turning green and forming leaves. The signal that switches on the darkness programme in seedling development has not hitherto been identified. Earlier studies had shown that these processes involve photoreceptors inside plant cells. One vital signal outside the cells has now been described by the team of Prof Dr Ute Krämer and Dr Scott Sinclair at Ruhr-Universität Bochum, in collaboration with Prof Dr Dominik Begerow, likewise from Bochum, as well as colleagues from Australia, France, Switzerland, and from the Max Planck Institute of Plant Breeding Research in Cologne, in the journal Current Biology. The article was published online on November 2, 2017.