Pectin Biosynthesis is Critical for Cell Wall Integrity and Immunity in Arabidopsis 1 thaliana 2 3 RUNNING TITLE : GAE 1 and GAE 6 in Arabidopsis immunity

36 37 Plant cell walls are important barriers against microbial pathogens. Cell walls of 38 Arabidopsis thaliana leaves contain three major types of polysaccharides: cellulose, 39 various hemicelluloses and pectins. UDP-D-galacturonic acid, the key building block of 40 pectins, is produced from the precursor UDP-D-glucuronic acid by the action of 41 glucuronate 4-epimerases (GAEs). Pseudomonas syringae pv. maculicola ES4326 42 (Pma ES4326) repressed expression of GAE1 and GAE6 in Arabidopsis, and immunity 43 to Pma ES4326 was compromised in gae6 and gae1 gae6 mutant plants. These plants 44 had brittle leaves and cell walls of leaves had less galacturonic acid (GalA). Resistance 45 to specific Botrytis cinerea isolates was also compromised in gae1 gae6 double mutant 46 plants. Although oligogalacturonide (OG)-induced immune signaling was unaltered in 47 gae1 gae6 mutant plants, immune signaling induced by a commercial pectinase, 48 macerozyme, was reduced. Macerozyme treatment or infection with B. cinerea released 49 less soluble uronic acid, likely reflecting fewer OGs, from gae1 gae6 cell walls than from 50 wild-type Col-0. Although both OGs and macerozyme induced immunity to B. cinerea in 51 Col-0, only OGs also induced immunity in gae1 gae6. Pectin is thus an important 52 contributor to plant immunity, and this is due at least in part to the induction of immune 53 responses by soluble pectin, likely OGs, that are released during plant-pathogen 54 interactions. 55