PMR5, 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 PMR5 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 PMR5 orthologs are necessary for PMR5 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, RWA2 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.

Etiolated Seedling Development Requires Repression of Photomorphogenesis by a Small Cell-Wall-Derived Dark Signal

Etiolated growth in darkness or the irreversible transition to photomorphogenesis in the light engages alternative developmental programs operating across all organs of a plant seedling. Dark-grown Arabidopsis de-etiolated by zinc (dez) mutants exhibit morphological, cellular, metabolic, and transcriptional characteristics of light-grown seedlings. We identify the causal mutation in TRICHOME BIREFRINGENCE encoding a putative acyl transferase. Pectin acetylation is decreased in dez, as previously found in the reduced wall acetylation2-3 mutant, shown here to phenocopy dez. Moreover, pectin of dez is excessively methylesterified. The addition of very short fragments of homogalacturonan, tri-galacturonate, and tetra-galacturonate, restores skotomorphogenesis in dark-grown dez and similar mutants, suggesting that the mutants are unable to generate these de-methylesterified pectin fragments. In combination with genetic data, we propose a model of spatiotemporally separated photoreceptive and signal-responsive cell types, which contain overlapping subsets of the regulatory network of light-dependent seedling development and communicate via a pectin-derived dark signal.