The plant cell wall is a polysaccharide-based extracellular matrix that surrounds and protects all plant cells. Since plants are constantly growing and developing within the confines of their cell walls, plant cells must be in constant communication with their cell walls. Furthermore, cell walls are a critical line of defense between plant cells and their environment; changes to the cell wall are often early warning signs of pathogen attack or abiotic stress, and plants fortify their cell walls in response to these stresses. This ongoing communication between the plant cells and their cell walls is collectively called “cell wall signaling”. Attempts to modify plant cell walls for improved materials or biofuels have exposed a critical gap in our understanding: inadvertent activation of cell wall signaling typically cause yield penalties that render these cell wall “improvements” agriculturally/economically unviable. We have been using hemicellulose (xyloglucan) synthesis in Arabidopsis as a model system to understand what types of modifications the plant cell wall can tolerate without triggering yield losses. We have shown that disrupting xyloglucan D-galactose sidechain synthesis via several genetic approaches results in intracellular aggregations of cell wall material that disrupt subcellular trafficking and organelle morphology. Altering xyloglucan synthesis via addition of a D-galacturonic acid side chain prevents these phenotypes, suggesting that the presence, but not the composition, of XyG sidechains is essential, likely by ensuring XyG solubility. Our results suggest a common model in which cell wall polysaccharides are synthesized in a highly substituted form for efficient secretion, and then later modified by cell-wall-localized enzymes to fine-tune cell wall mechanical properties.
