Moonlighting Function of Phytochelatin Synthase1 in Extracellular Defense against Fungal Pathogens.

Phytochelatin synthase (PCS) is a key component of heavy metal detoxification in plants. PCS catalyzes both the synthesis of the peptide phytochelatin from glutathione and the degradation of glutathione conjugates via peptidase activity. Here, we describe a role for PCS in disease resistance against plant pathogenic fungi. The pen4 mutant, which is allelic to cadmium insensitive1 (cad1/pcs1) mutants, was recovered from a screen for Arabidopsis mutants with reduced resistance to the nonadapted barley fungal pathogen Blumeria graminis f. sp. hordei PCS1, which is found in the cytoplasm of cells of healthy plants, translocates upon pathogen attack and colocalizes with the PEN2 myrosinase on the surface of immobilized mitochondria. pcs1 and pen2 mutant plants exhibit similar metabolic defects in the accumulation of pathogen-inducible indole glucosinolate-derived compounds, suggesting that PEN2 and PCS1 act in the same metabolic pathway. The function of PCS1 in this pathway is independent of phytochelatin synthesis and deglycination of glutathione conjugates, as catalytic-site mutants of PCS1 are still functional in indole glucosinolate metabolism. In uncovering a peptidase-independent function for PCS1, we reveal this enzyme to be a moonlighting protein important for plant responses to both biotic and abiotic stresses.

Host cell ploidy underlying the fungal feeding site is a determinant of powdery mildew growth and reproduction.

Golovinomyces orontii is an obligate biotrophic powdery mildew (PM) that colonizes Arabidopsis thaliana and agronomic species. It establishes a specialized feeding structure in epidermal cells to fuel its extensive surface hyphal growth and reproduction. Previously, endoreduplication was identified in Arabidopsis mesophyll cells underlying the fungal feeding site, presumably to meet the metabolic demands imposed by the fungus. Furthermore, the cell cycle transcription factor MYB3R4 was shown to regulate this process. Herein, PM-induced endoreduplication is further characterized and three additional factors influencing host ploidy in cells underlying the fungal feeding site are identified. While mutations in PUX2 and PMR6 reduce basal ploidy, mutations in PMR5 (and MYB3R4) abrogate the PM-induced ploidy increase. Moreover, analysis of pmr5 microarray data suggests that PMR5 acts upstream of a MYB3R transcription factor such as MYB3R4 to control PM-induced ploidy. Induced endoreduplication occurs exclusively in mesophyll cells underlying the fungal feeding site at 5 days postinoculation, concomitant with PM reproduction. Gene copy number increases and chromatin remains decondensed, suggesting active, elevated gene expression. Cell ploidy underlying the fungal feeding site is highly correlated with the extent of PM growth and reproduction for these mutants, indicating that (induced) mesophyll cell ploidy is a PM susceptibility determinant.

Loss-of-function mutation of REDUCED WALL ACETYLATION2 in Arabidopsis leads to reduced cell wall acetylation and increased resistance to Botrytis cinerea.

Nearly all polysaccharides in plant cell walls are O-acetylated, including the various pectic polysaccharides and the hemicelluloses xylan, mannan, and xyloglucan. However, the enzymes involved in the polysaccharide acetylation have not been identified. While the role of polysaccharide acetylation in vivo is unclear, it is known to reduce biofuel yield from lignocellulosic biomass by the inhibition of microorganisms used for fermentation. We have analyzed four Arabidopsis (Arabidopsis thaliana) homologs of the protein Cas1p known to be involved in polysaccharide O-acetylation in Cryptococcus neoformans. Loss-of-function mutants in one of the genes, designated REDUCED WALL ACETYLATION2 (RWA2), had decreased levels of acetylated cell wall polymers. Cell wall material isolated from mutant leaves and treated with alkali released about 20% lower amounts of acetic acid when compared with the wild type. The same level of acetate deficiency was found in several pectic polymers and in xyloglucan. Thus, the rwa2 mutations affect different polymers to the same extent. There were no obvious morphological or growth differences observed between the wild type and rwa2 mutants. However, both alleles of rwa2 displayed increased tolerance toward the necrotrophic fungal pathogen Botrytis cinerea.

Host-pathogen warfare at the plant cell wall.

Plants have evolved sensory mechanisms to detect pathogen attack and trigger signalling pathways that induce rapid defence responses. These mechanisms include not only direct detection of pathogen-derived elicitors (e.g. pathogen-associated molecular patterns (PAMPs) and avirulence factors or effectors) but also indirect sensing of pathogens' impact on the host plant. Among the first plant barriers to pathogen ingress are the cell wall and the cuticle. For those pathogens that penetrate the plant cell wall to gain access to water and nutrients of the plant protoplast, small wounds at penetration sites are created by enzymatic or physical disruption of the plant cell wall. Thus, cell wall integrity sensing is one mechanism by which plants may detect pathogen attack. Some plant cell wall fragments, notably oligogalacturonic acids, elicit similar defence responses in plants as the non-specific PAMP elicitors (e.g. production of reactive oxygen species, elevated expression of defence-associated genes), suggesting that PAMP signalling may provide a good model for studying cell wall integrity sensing in plants. However, much remains to be discovered about this sensing mechanism.