Galacturonic acid oxidation: a radical way to stick together

The middle lamellae (ML) of plant cells, enriched in Homogalacturonan (HG) is considered to function as a crucial glue responsible for cell-cell cohesion (for review see 1). Mutants with defective HG content exhibit cell adhesion defects2,3. Despite advances, the mechanisms governing cell adhesion during plant development remain elusive. We previously hypothesized that cell-cell cohesion relies on cell wall integrity signaling, yet the specifics remain undefined4. OligoGalacturonans (OG), degradation products of HG, are prime candidates for informing cells about ML status and thereby influencing cell adhesion. This integrity signal is crucial for adhesion homeostasis 4. OGs serve as signaling molecules, recognized by membrane-bound cell wall receptors. Notably, restoring adhesion in qua2-1 mutants by modulating pectin response gene expression in esmd/qua2 mutants underscores potential OGs importance4-6. Deciphering the diversity and role of endogenous OGs is imperative for understanding cell adhesion modulation. Our study aims to identify compounds in this signalling pathway that regulate cell adhesion. We focused on characterizing HG degradation products in dark-grown hypocotyls. Our findings highlight various oligomers, along with two key monomers: galacturonic acid and its oxidized form, galactaric acid. These monomers appear to play a pivotal role in controlling cell adhesion by indirectly enhancing the crosslinking of extensin, a cell wall structural protein. This crosslinking leads to the densification of extensin-based cell wall networks, ultimately restoring cell adhesion in defective mutants. Our research sheds light on the intricate interplay between HG degradation product monomer and cell adhesion mechanisms.