The Arabidopsis NPR1 protein is a receptor for the plant defense hormone salicylic acid

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Highlights? NPR1 is a salicylic acid (SA) receptor, binding specifically to SA via Cys^521/529 ? NPR1 binds copper in vivo via Cys^521/529, and metals are required for SA binding ? SA directly regulates the conformation of NPR1 by deoligomerizing NPR1 into a dimer ? The NPR1 BTB/POZ domain autoinhibits the function of the NPR1 transactivation domain     

The NPR1 family of transcription cofactors in papaya: insights into its structure, phylogeny and expression

The NPR1 (non-expressor of pathogenesis related gene 1) gene was initially identified in Arabidopsis as a master regulator of the systemic acquired resistance (SAR). Five additional NPR1 homologues have been identified in Arabidopsis whose function range from regulators of SAR to plant development. In the present study, we characterized the structure, phylogeny and expression of the NPR1 family in papaya (Carica papaya L.), one of the most important tropical fruit crops. We identified four NPR1 homologues in the papaya genome sequence (CpNPR1 to CpNPR4). Overall, the four papaya predicted NPR1 proteins showed the characteristic BTB/POZ and ankyrin domains of the Arabidopsis NPR1 family. Twelve additional open reading frames showing homology to retrotransposon elements or genes involved in different physiological processes were found in close proximity to the papaya NPR1 homologues. The phylogenetic analysis revealed that the papaya NPR1 sequences resolved in three clades, each clade containing two Arabidopsis NPR1 homologues involved either in the positive regulation of SAR (clade I), negative regulation of SAR (clade II) or plant development (clade III), suggesting a similar function for the corresponding papaya NPR1 homologues. Furthermore, the expression of the four papaya NPR1 homologues was detected in both vegetative and reproductive tissues. The present study has provided the first comparative analysis of the NPR1 family in a tropical fruit crop and expanded our knowledge on this type of genes in dicotyledoneous plants. The identification of the full set of papaya NPR1 homologues will pave the way for their systematic functional analysis and new opportunities for engineering disease resistance in this crop.     

Negative regulation of defense responses in Arabidopsis by two NPR1 paralogs

NPR1 is required for systemic acquired resistance, and there are five NPR1 paralogs in Arabidopsis. Here we report knockout analysis of two of these, NPR3 and NPR4. npr3 single mutants have elevated basal PR-1 expression and the npr3 npr4 double mutant shows even higher expression. The double mutant plants also display enhanced resistance against virulent bacterial and oomycete pathogens. This enhanced disease resistance is partially dependent on NPR1, can be in part complemented by either wild-type NPR3 or NPR4, and is not associated with an elevated level of salicylic acid. NPR3 and NPR4 interact with TGA2, TGA3, TGA5 and TGA6 in yeast two-hybrid assays. Using bimolecular fluorescence complementation analysis, we show that NPR3 interacts with TGA2 in the nucleus of onion epidermal cells and Arabidopsis mesophyll protoplasts. Combined with our previous finding that basal PR-1 levels are also elevated in the tga2 tga5 tga6 triple mutant, we propose that NPR3 and NPR4 negatively regulate PR gene expression and pathogen resistance through their association with TGA2 and its paralogs.