An Arabidopsis berberine-bridge enzyme-like protein specifically oxidizes cellulose oligomers and plays a role in immunity

The plant cell wall is the barrier that pathogens must overcome to cause a disease and to this purpose they secrete degrading enzymes of the various cell wall components. Due to the complexity of these components, several types of oligosaccharide fragments may be released during pathogenesis and some of these can act as Damage-Associated Molecular Pattern (DAMPs). Well-known DAMPs are the oligogalacturonides (OGs) released upon degradation of homogalacturonan and the products of the cellulose breakdown, i.e. the cellodextrins (CDs). We have previously reported that four Arabidopsis Berberine Bridge Enzyme-like (BBE-like) proteins (OGOX1-4) oxidize OGs and impair their elicitor activity. We show here that another Arabidopsis BBE-like protein, which is expressed coordinately with OGOX1 during immunity, specifically oxidizes CDs with a preference for cellotriose (CD3) and longer fragments (CD4-6). Oxidized CDs show a negligible elicitor activity and are less utilizable by the fungus Botrytis cinerea as a carbon source. The enzyme, named CELLOX (CELLODEXTRIN OXIDASE), is encoded by the gene At4g20860. Plants overexpressing CELLOX display an enhanced resistance to B. cinerea likely because oxidized CDs are a less valuable carbon source. Thus, the capacity of oxidizing and impairing the biological activity of the cell wall-derived oligosaccharides seems to be a general trait of the family of BBE-like proteins, which may serve for the homeostatic control of the level of DAMPs to prevent their hyper-accumulation.