No abstract

The plant NADPH oxidases produce reactive oxygen species (ROS) in response to pathogens that have diverse functions in different cellular contexts. Distinct phenotypic outcomes may derive from the interaction of NADPH oxidase-dependent ROS with other signaling components that mediate defense activation. We analyze the interaction between NADPH oxidases AtRbohD and AtRbohF and the Arabidopsis heterotrimeric G protein. The Gβ subunit (AGB1) of the heterotrimeric G protein is required for full disease resistance to different Pseudomonas syringae strains. Genetic studies reveal that, upon P. syringae infection, AGB1 and AtRbohD and AtRbohF can function in the same pathway, as the agb1 null allele is epistatic to the NADPH oxidase null alleles, combinatorial mutants display the agb1 phenotypes, and agb1 suppresses some of the atrbohD atrbohF double mutant phenotypes. In contrast, increased susceptibility to the necrotrophic fungus Plectosphaerella cucumerina displayed by agb1 and atrbohD atrbohF is enhanced in the agb1 atrbohD atrbohF triple mutant, suggesting that NADPH oxidase and heterotrimeric G proteins mediate different response pathways in response to this necrotrophic pathogen. The defense response mediated by AGB1 is independent of pathogen-dependent salicylic acid accumulation and signaling, as the agb1 sid2 (isochorismate synthase 2) double mutant showed enhanced disease susceptibility to P. syringae and Plectosphaerella cucumerina as compared with both single mutants. This study exemplifies the complex interplay between signaling events mediating defense activation, depending on the type of plant-pathogen interaction.

Plant NADPH oxidases, also known as respiratory burst oxidase homologues (RBOHs), produce reactive oxygen species (ROS) that perform a wide range of functions. RbohD and RbohF, two of the 10 Rboh genes present in Arabidopsis, are pleiotropic and mediate diverse physiological processes including the response to pathogens. We hypothesized that the spatio-temporal control of RbohD and RbohF gene expression might be critical in determining their multiplicity of functions. Transgenic Arabidopsis plants with RbohD and RbohF promoter fusions to β-glucuronidase and Luciferase reporter genes were generated. Analysis of these plants revealed a differential expression pattern for RbohD and RbohF throughout plant development and during immune responses. RbohD and RbohF gene expression was differentially modulated by pathogen-associated molecular patterns. Histochemical stains and in vivo expression analysis showed a correlation between the level of RbohD and RbohF promoter activity, H2O2 accumulation and the amount of cell death in response to the pathogenic bacterium Pseudomonas syringae pv. tomato DC3000 and the necrotrophic fungus Plectosphaerella cucumerina. A promoter-swap strategy revealed that the promoter region of RbohD was required to drive production of ROS by this gene in response to pathogens. Moreover, RbohD promoter was activated during Arabidopsis interaction with a non-virulent P. cucumerina isolate, and susceptibility tests with the double mutant rbohD rbohF uncovered a new function for these oxidases in basal resistance. Altogether, our results suggest that differential spatio-temporal expression of the Rboh genes contributes to fine-tune RBOH/NADPH oxidase-dependent ROS production and signaling in Arabidopsis immunity.