Identification of Periplasmic Root-Cap Mucilage in Developing Columella Cells of Arabidopsis thaliana

Plant roots secrete various substances with diverse functions against both plants and microbes in the rhizosphere. A major secretory substance is root-cap mucilage, whose functions have been well characterized, albeit mainly in crops. However, little is currently known about the developmental mechanisms of root-cap mucilage. Here, we show the accumulation and extrusion of root-cap mucilage in Arabidopsis. We found propidium iodide (PI) stainable structures between the plasma membrane and cell wall in the sixth layer of columella cells (c6) from the quiescent center. Ruthenium red staining and PI staining with calcium ions suggested that the structure comprises in part pectin polysaccharides. Electron microscopy revealed that the structure had a meshwork of electron-dense filaments that resembled periplasmic mucilage in other plants. In the c6 cells, we also observed many large vesicles with denser meshwork filaments to periplasmic mucilage, which likely mediate the transport of mucilage components. Extruded mucilage was observed outside a partially degraded cell wall in the c7 cells. Moreover, we found that the Class IIB NAC transcription factors BEARSKIN1 (BRN1) and BRN2, which are known to regulate the terminal differentiation of columella cells, were required for the efficient accumulation of root-cap mucilage in Arabidopsis. Taken together, our findings reveal the accumulation of and dynamic changes in periplasmic mucilage during columella cell development in Arabidopsis.