Fig. 2.

Schematic overview on proposed signal mobility and specificity in cell–cell communication. Some signals can freely diffuse (O₂), while others migrate through plasmodesmata, e.g., transcription factors and small RNAs. Also, small RNAs might be transported in vesicles between cells. Proteins and ligands are secreted by unconventional and conventional secretion into the plant apoplast. There they can interact, for example, with receptor-like kinases or might be taken up by other cells. All these modes of mobility can result in signal gradient formation along tissues. Mobility can be limited by size exclusion of plasmodesmata or capture in certain tissues. Specificity of signals can be achieved by expression of response molecules in certain cell layers