AtHSPR Plays a Positive Role in Arabidopsis Resistance Against Pseudomonas syringae pv. tomato DC3000 by Interacting with TOP1

The Arabidopsis thaliana Heat Shock Protein-Related (AtHSPR) gene participates in plant growth and abiotic stress tolerance, while its role in biotic stress resistance remains unclear. Here, we report that the athspr mutant is sensitive to Pseudomonas syringae pv. tomato (Pst) DC3000, whereas over-expression of AtHSPR enhances the defense of Arabidopsis against the pathogen. AtHSPR expression was induced by treatment with Pst DC3000, flg22, or salicylic acid (SA). Transcriptome analysis showed that mutation of AtHSPR changed the expression patterns of genes associated with defense response, oxidation–reduction, and SA responses, as well as transcription factors. The biochemical evidence revealed that AtHSPR interacted with Thimet Oligopeptidase 1 (TOP1), which modulated the SA-mediated immune response. Co-expression of AtHSPR and TOP1 showed that the TOP1 protein, normally located in the chloroplasts, gathered around the nucleus in response to a pathogen. After pathogen treatment, dynamic tubular projections (stromules) were present, extending from the chloroplasts toward the nucleus, and TOP1 was observed in the nucleus, together with AtHSPR. The top1athspr double mutant had lower SA levels and was more sensitive to pathogens than the top1 and athspr single mutants. Taken together, our results demonstrated that the interaction between AtHSPR and TOP1 plays a positive role in SA-mediated plant resistance against Pst DC3000.