An Arabidopsis Homeodomain Transcription Factor, <i>OVEREXPRESSOR OF CATIONIC PEROXIDASE 3</i>, Mediates Resistance to Infection by Necrotrophic Pathogens

The mechanisms controlling plant resistance to necrotrophic fungal pathogens are poorly understood. We previously reported on <i>Ep5C</i>, a gene shown to be induced by the H₂O₂ generated during a plant–pathogen interaction. To identify novel plant components operating in pathogen-induced signaling cascades, we initiated a large-scale screen using <i>Arabidopsis thaliana</i> plants carrying the ß-glucuronidase reporter gene under control of the H₂O₂-responsive <i>Ep5C</i> promoter. Here, we report the identification and characterization of a mutant, <i>ocp3</i> (for <i>overexpressor of cationic peroxidase 3</i>), in which the reporter construct is constitutively expressed. Healthy <i>ocp3</i> plants show increased accumulation of H₂O₂ and express constitutively the <i>Glutathione S-transferase1</i> and <i>Plant Defensine 1.2</i> marker genes, but not the salicylic acid (SA)–dependent pathogenesis-related <i>PR-1</i> gene. Strikingly, the <i>ocp3</i> mutant shows enhanced resistance to the necrotrophic pathogens <i>Botrytis cinerea</i> and <i>Plectosphaerella cucumerina</i>. Conversely, resistance to virulent forms of the biotrophic oomycete <i>Hyaloperonospora parasitica</i> and the bacterial pathogen <i>Pseudomonas syringae</i> pv <i>tomato</i> DC3000 remains unaffected in <i>ocp3</i> plants when compared with wild-type plants. Consistently with this, <i>ocp3</i> plants are not affected in SA perception and express normal levels of <i>PR</i> genes after pathogen attack. To analyze signal transduction pathways where <i>ocp3</i> operates, epistasis analyses between <i>ocp3</i> and <i>pad4</i>, <i>nahG</i>, <i>npr1</i>, <i>ein2</i>, <i>jin1</i>, or <i>coi1</i> were performed. These studies revealed that the resistance signaling to necrotrophic infection in <i>ocp3</i> is fully dependent on appropriate perception of jasmonic acid through COI1 and does not require SA or ethylene perception through NPR1 or EIN2, respectively. The <i>OCP3</i> gene encodes a homeodomain transcription factor that is constitutively expressed in healthy plants but repressed in response to infection by necrotrophic fungi. Together, these results suggest that OCP3 is an important factor for the COI1-dependent resistance of plants to infection by necrotrophic pathogens.