[Science] Pectin homogalacturonan nanofilament expansion drives morphogenesis in plant epidermal cells

• The 3D-dSTORM nanoscopy and cryo-SEM reveal HG nanofilaments.
• HG methylation affects cell wall filament diameter
• Demethylation-mediated nanofilament inflation underlies anisotropic growth
• HG methylation asymmetry affects lobe formation
  
• HG demethylation alone leads to tissue expansion

Wavy walls built by nanofilaments
In the model plant Arabidopsis, pavement cells fit together with the lobes and curves of jigsaw puzzle pieces. Such complex cell shapes, in plants, were generally thought to be driven by turgor pressure. Haas et al. now show that the extracellular cell wall can actively shape the cell it contains without relying on turgor pressure. Nanofilaments of pectin homogalacturonan in the cell wall shift between crystalline and anisotropic phases according to whether they are methylated. The shift in form drives changes in cell wall shape that stand independent of turgor pressure.

Abstract

The process by which plant cells expand and gain shape has presented a challenge for researchers. Current models propose that these processes are driven by turgor pressure acting on the cell wall. Using nanoimaging, we show that the cell wall contains pectin nanofilaments that possess an intrinsic expansion capacity. Additionally, we use growth models containing such structures to show that a complex plant cell shape can derive from chemically induced local and polarized expansion of the pectin nanofilaments without turgor-driven growth. Thus, the plant cell wall, outside of the cell itself, is an active participant in shaping plant cells. Extracellular matrix function may similarly guide cell shape in other kingdoms, including Animalia.