In our research, we aim to reveal the subcellular mechanisms of growth and morphogenesis in plants.  In contrast to animal cells, which can migrate inside tissues, plant cells are immobilized by their cell walls and morphogenesis occurs only through cell division and cell expansion. The increase in cell size requires the expansion of the cell walls. In the conventional view, plant growth is fueled by turgor and canalized by the mechanical compliance of the cell wall. Our research has recently identified an alternative fuel for growth, the demethylation-induced swelling of the pectic polysaccharide homogalacturonan, which in principle can sustain growth independently from the turgor pressure. This intrinsic cell wall expansion mechanism shows that the cell wall is not only undergoing turgor-driven stress relaxation but also can generate the energy to expand by itself. Using 3D dSTORM nanoscopy we observed that, at least in certain cell walls, homogalacturonan, instead of forming an amorphous gel, can form nanofilaments that can expand radially upon demethylation. The orientation of those nanofilaments perpendicularly to the growth axis can control the direction of cell wall expansion. It also can contribute to the formation of lobes, like those of leaf epidermal cells, by the preferential demethylation and expanding on one side of a common cell wall (See Illustration 1)  Next step after the discovery of this new fuel for growth, our goal is to dissect the molecular mechanism that controls this growth motor and how it interfaces with turgor-driven cell wall remodeling.


Figure 1. Morphogenesis through expansion of oriented nanofilaments. A) Classical fluorescence image of lobed epidermal pavement cells on the Arabidopsis Thaliana cotyledon. B) A super-resolved image of the lobed region highlighted in (A) with a red rectangle showing axially arranged HG nanofilaments. In (A) and (B) HG0 - methylated (neutral) pectin homogalacturonan, HG-- - unmethylated (polyanionic) pectin homogalacturonan C) A cartoon representation of the 'intrinsic cell wall expansion' through a pectin demethylation-driven nanofilament swelling. Pectin HG demethylation causes a radial swelling of the nanofilament and consequently cell wall expansion. D) Cartoon representation of the lobe formation through asymmetric HG demethylation along and across the cell wall.