Cytoplasmic H2O2 prevents translocation of NPR1 to the nucleus and inhibits the induction of PR genes in Arabidopsis.

Plants activate a number of defense reactions in response to pathogen attack. One of the major pathways involves biosynthesis of Salicylic Acid (SA), which acts as a signaling molecule that regulates local defense reaction at the infection site and in induction of systemic acquired resistance (SAR). SA is sensed and transduced by NPR1 protein, which is a redox sensitive protein that acts as a central transcription activator of many pathogenesis related and defense related genes. In its uninduced state NPR1 exists as an oligomer in the cytoplasm. Following pathogen attack and SAR induction, cells undergo a biphasic change in cellular redox, resulting in reduction of NPR1 to a monomeric form,which moves to the nucleus. Recently, it was shown that pathogen attack or SA treatment cause S-nitrosylation of NPR1, promoting NPR1 oligomerization and restricting it in the cytoplasm. We used A. thaliana mutants in cytosolic ASCORBATE PEROXIDASE, apx1, and plants expressing antisense CATALASE gene, as well as the CATALASE inhibitor 3-amino-1,2,4-triazole, to examine the effect of H2O2 on the pathogen-triggered translocation of the NPR1 to the nucleus. Our results show that the pathogen-triggered or SA-induced nuclear translocation is prevented by accumulation of H2O2 in the cytosol. Moreover, we show that increased accumulation of cytoplasmic ROS in apx1 mutants reduced the NPR1-dependent gene expression. We suggest that H2O2 has a signaling role in pathogenesis, acting as a negative regulator of NPR1 translocation to the nucleus, limiting the NPR1-dependent gene expression.