PMR 5, an acetylation protein at the intersection of pectin biosynthesis and defense against fungal pathogens

Powdery mildew ( Golovinomyces cichoracearum ), one of the most prolific obligate biotrophic fungal pathogens worldwide, infects its host by penetrating the plant cell wall without activating the plant's innate immune system. The Arabidopsis mutant powdery mildew resistant 5 ( pmr5 ) carries a mutation in a putative pectin acetyltransferase gene that confers enhanced resistance to powdery mildew. Here, we show that heterologously expressed PMR 5 protein transfers acetyl groups from [ 14 C]‐acetyl‐CoA to oligogalacturonides. Through site‐directed mutagenesis, we show that three amino acids within a highly conserved esterase domain in putative PMR 5 orthologs are necessary for PMR 5 function. A suppressor screen of mutagenized pmr5 seed selecting for increased powdery mildew susceptibility identified two previously characterized genes affecting the acetylation of plant cell wall polysaccharides, RWA 2 and TBR . The rwa2 and tbr mutants also suppress powdery mildew disease resistance in pmr6 , a mutant defective in a putative pectate lyase gene. Cell wall analysis of pmr5 and pmr6 , and their rwa2 and tbr suppressor mutants, demonstrates minor shifts in cellulose and pectin composition. In direct contrast to their increased powdery mildew resistance, both pmr5 and pmr6 plants are highly susceptibile to multiple strains of the generalist necrotroph Botrytis cinerea , and have decreased camalexin production upon infection with B. cinerea . These results illustrate that cell wall composition is intimately connected to fungal disease resistance and outline a potential route for engineering powdery mildew resistance into susceptible crop species. The PMR 5 gene was discovered nearly two decades ago through a forward‐genetics screen for powdery mildew resistance, although its biochemical function has remained unknown. Here, we present a detailed cell wall biochemistry analysis of the pmr5 mutant and two disease suppressors, demonstrate purified pmr5 activity, identify three amino acids in the PMR 5 protein essential for function, and show that the pmr5 mutant has increased susceptibility to Botrytis cinerea .