ABSTRACT
The Arabidopsis heterotrimeric G-protein controls defense responses to necrotrophic and vascular fungi. The agb1 mutant impaired in the Gß subunit displays enhanced susceptibility to these pathogens. Gß/AGB1 forms an obligate dimer with either one of the Arabidopsis Gγ subunits (γ1/AGG1 and γ2/AGG2). Accordingly, we now demonstrate that the agg1 agg2 double mutant is as susceptible as agb1 plants to the necrotrophic fungus Plectosphaerella cucumerina. To elucidate the molecular basis of heterotrimeric G-protein-mediated resistance, we performed a comparative transcriptomic analysis of agb1-1 mutant and wild-type plants upon inoculation with P. cucumerina. This analysis, together with metabolomic studies, demonstrated that G-protein-mediated resistance was independent of defensive pathways required for resistance to necrotrophic fungi, such as the salicylic acid, jasmonic acid, ethylene, abscisic acid, and tryptophan-derived metabolites signaling, as these pathways were not impaired in agb1 and agg1 agg2 mutants. Notably, many mis-regulated genes in agb1 plants were related with cell wall functions, which was also the case in agg1 agg2 mutant. Biochemical analyses and Fourier Transform InfraRed (FTIR) spectroscopy of cell walls from G-protein mutants revealed that the xylose content was lower in agb1 and agg1 agg2 mutants than in wild-type plants, and that mutant walls had similar FTIR spectratypes, which differed from that of wild-type plants. The data presented here suggest a canonical functionality of the Gß and Gγ1/γ2 subunits in the control of Arabidopsis immune responses and the regulation of cell wall composition.
Subject(s)
Arabidopsis Proteins/immunology , Arabidopsis/immunology , Ascomycota/physiology , Cell Wall/immunology , GTP-Binding Protein beta Subunits/immunology , GTP-Binding Protein gamma Subunits/immunology , Plant Diseases/microbiology , Arabidopsis/chemistry , Arabidopsis/genetics , Arabidopsis/microbiology , Arabidopsis Proteins/genetics , Ascomycota/immunology , Cell Wall/chemistry , Cell Wall/genetics , Cell Wall/microbiology , Dimerization , Disease Resistance , GTP-Binding Protein beta Subunits/genetics , GTP-Binding Protein gamma Subunits/genetics , Gene Expression Regulation, Plant , Plant Diseases/immunologyABSTRACT
Some receptor-like kinases (RLK) control plant development while others regulate immunity. The Arabidopsis ERECTA (ER) RLK regulates both biological processes. To discover specific components of ER-mediated immunity, a genetic screen was conducted to identify suppressors of erecta (ser) susceptibility to Plectosphaerella cucumerina fungus. The ser1 and ser2 mutations restored disease resistance to this pathogen to wild-type levels in the er-1 background but failed to suppress er-associated developmental phenotypes. The deposition of callose upon P. cucumerina inoculation, which was impaired in the er-1 plants, was also restored to near wild-type levels in the ser er-1 mutants. Analyses of er cell walls revealed that total neutral sugars were reduced and uronic acids increased relative to those of wild-type walls. Interestingly, in the ser er-1 walls, neutral sugars were elevated and uronic acids were reduced relative to both er-1 and wild-type plants. The cell-wall changes found in er-1 and the ser er-1 mutants are unlikely to contribute to their developmental alterations. However, they may influence disease resistance, as a positive correlation was found between uronic acids content and resistance to P. cucumerina. We propose a specific function for ER in regulating cell wall-mediated disease resistance that is distinct from its role in development.
Subject(s)
Arabidopsis Proteins/physiology , Arabidopsis/microbiology , Cell Wall/physiology , Phyllachorales/physiology , Protein Serine-Threonine Kinases/physiology , Receptors, Cell Surface/physiology , Arabidopsis/genetics , Arabidopsis/metabolism , Arabidopsis Proteins/genetics , Arabidopsis Proteins/metabolism , Cell Wall/metabolism , Glucans/metabolism , Immunity, Innate , Mutation , Phenotype , Plant Leaves/genetics , Plant Leaves/metabolism , Plant Leaves/microbiology , Protein Serine-Threonine Kinases/genetics , Protein Serine-Threonine Kinases/metabolism , Receptors, Cell Surface/genetics , Receptors, Cell Surface/metabolism , Uronic Acids/metabolismABSTRACT
Cellulose is synthesized by cellulose synthases (CESAs) contained in plasma membrane-localized complexes. In Arabidopsis thaliana, three types of CESA subunits (CESA4/IRREGULAR XYLEM5 [IRX5], CESA7/IRX3, and CESA8/IRX1) are required for secondary cell wall formation. We report that mutations in these proteins conferred enhanced resistance to the soil-borne bacterium Ralstonia solanacearum and the necrotrophic fungus Plectosphaerella cucumerina. By contrast, susceptibility to these pathogens was not altered in cell wall mutants of primary wall CESA subunits (CESA1, CESA3/ISOXABEN RESISTANT1 [IXR1], and CESA6/IXR2) or POWDERY MILDEW-RESISTANT5 (PMR5) and PMR6 genes. Double mutants indicated that irx-mediated resistance was independent of salicylic acid, ethylene, and jasmonate signaling. Comparative transcriptomic analyses identified a set of common irx upregulated genes, including a number of abscisic acid (ABA)-responsive, defense-related genes encoding antibiotic peptides and enzymes involved in the synthesis and activation of antimicrobial secondary metabolites. These data as well as the increased susceptibility of ABA mutants (abi1-1, abi2-1, and aba1-6) to R. solanacearum support a direct role of ABA in resistance to this pathogen. Our results also indicate that alteration of secondary cell wall integrity by inhibiting cellulose synthesis leads to specific activation of novel defense pathways that contribute to the generation of an antimicrobial-enriched environment hostile to pathogens.
