An important role of a BAHD acyl transferase-like protein in plant innate immunity

Salicylic acid (SA) is an important regulator of plant resistance to biotrophic and hemi-biotrophic pathogens. The enhanced pseudomonas susceptibility 1 ( eps1 ) mutant in Arabidopsis thaliana is hypersusceptible to both virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae . Through positional cloning, the EPS1 gene was isolated and found to encode a novel member of the BAHD acyltransferase superfamily. Pathogen-induced accumulation of SA and expression of pathogenesis-related ( PR ) genes were compromised in the eps1 mutant. SA could induce PR1 gene expression and restore disease resistance in the eps1 mutant. These results suggest that EPS1 functions upstream of SA and may be involved directly in synthesis of a precursor or a regulatory molecule for SA biosynthesis. Mutations of EPS1 or other genes important for SA accumulation or signaling conferred enhanced resistance to the necrotrophic fungal pathogens Botrytis cinerea and Alternaria brassicicola in the Nossen-0 background but had little effect in the Columbia-0 background. These results suggest that there is natural variation among Arabidopsis ecotypes with respect to the antagonistic cross-talk between defense signaling pathways against various types of microbial pathogens.     

Evidence for an Important Role of WRKY DNA Binding Proteins in the Regulation of NPR1 Gene Expression

The Arabidopsis NPR1 gene is a positive regulator of inducible plant disease resistance. Expression of NPR1 is induced by pathogen infection or treatment with defense-inducing compounds such as salicylic acid (SA). Transgenic plants overexpressing NPR1 exhibit enhanced resistance to a broad spectrum of microbial pathogens, whereas plants underexpressing the gene are more susceptible to pathogen infection. These results suggest that regulation of NPR1 gene expression is important for the activation of plant defense responses. In the present study, we report the identification of W-box sequences in the promoter region of the NPR1 gene that are recognized specifically by SA-induced WRKY DNA binding proteins from Arabidopsis. Mutations in these W-box sequences abolished their recognition by WRKY DNA binding proteins, rendered the promoter unable to activate a downstream reporter gene, and compromised the ability of NPR1 to complement npr1 mutants for SA-induced defense gene expression and disease resistance. These results provide strong evidence that certain WRKY genes act upstream of NPR1 and positively regulate its expression during the activation of plant defense responses. Consistent with this model, we found that SA-induced expression of a number of WRKY genes was independent of NPR1.     

The pepper extracellular peroxidase CaPO2 is required for salt, drought and oxidative stress tolerance as well as resistance to fungal pathogens

In plants, biotic and abiotic stresses regulate the expression and activity of various peroxidase isoforms. Capsicum annuum EXTRACELLULAR PEROXIDASE 2 (CaPO2) was previously shown to play a role in local and systemic reactive oxygen species bursts and disease resistance during bacterial pathogen infection. Here, we report CaPO2 expression patterns and functions during conditions of biotic and abiotic stress. In pepper plants, CaPO2 expression was strongly induced by abscisic acid, but not by defense-related plant hormones such as salicylic acid, ethylene and jasmonic acid. CaPO2 was also strongly induced by abiotic and biotic stress treatments, including drought, cold, high salinity and infection by the hemibiotrophic fungal pathogen Colletotrichum coccodes. Loss-of-function of CaPO2 in virus-induced gene silenced pepper plants led to increased susceptibility to salt- and osmotic-induced stress. In contrast, CaPO2 overexpression in transgenic Arabidopsis thaliana plants conferred enhanced tolerance to high salt, drought, and oxidative stress, while also enhancing resistance to infection by the necrotrophic fungal pathogen Alternaria brassicicola. Taken together, these results provide evidence for the involvement of pepper extracellular peroxidase CaPO2 in plant defense responses to various abiotic stresses and plant fungal pathogens.