Building a morphogen gradient by simple diffusion in a growing plant leaf

Spatial gradient of signaling molecules is critical for establishing developmental patterns of multicellular organisms. Around half a century ago, a seminal work proposed a theory that pure diffusion of signaling molecules from a restricted source may explain the establishment of such tissue-scale gradients (Crick, 1970). Despite the prominence of this diffusion-based model in development, quantitative studies, largely performed in animals, have not yet demonstrated this simple mechanism in multicellular tissues.

In an article published in Biophysical Journal, Dr. Kawade's team at OIBB/NIBB, in collaboration with scientists in the Institut Jacques Monod (France), Rikkyo University (Japan) and University of Tokyo (Japan), solved a long-standing argument of the diffusion-based model for morphogen gradient formation. They measured molecular transport through plasmodesmata, a unique cellular channel in plants directly connecting neighboring cells, by trans-scale FRAP (Fluorescence Recovery After Photobleaching) assays. This analyses revealed biophysical properties of diffusive molecular transport through plasmodesmata. Based on this diffusion-based framework, they succeeded in demonstrating that the AN3 gradient could be achieved solely by pure diffusion process through plasmodesmata in developing leaf primordia. Because the AN3 signaling gradient corresponds to the cell-proliferation domain, this study can explain how spatial and temporal dynamics of cell proliferation, and therefore tissue growth, is regulated during leaf development.

These discoveries provide a significant step forward in our understanding of how the simple biophysical phenomena 'diffusion' governs developmental patterning in multicellular organisms.