Regulation of ORA59, a key modulator of disease resistance in Arabidopsis

Abstract

Jasmonic acid (JA) is a plant hormone that plays an important role in defense against wounding, insects and microbial pathogens. In defense responses against microbial pathogens JA acts synergistically with the stress hormone ethylene. ORA59, belonging to the plant-specific class of AP2-domain transcription factors, is the main regulator of JA/ethylene-responsive defense gene expression in the model plant species Arabidopsis thaliana. The aim of the research described in this thesis was to study how the activity of ORA59 is regulated by JA. Studies on the promoter of the defense gene PDF1.2 showed that ORA59 regulates gene expression by interaction with GCCGCC motifs. Studies of ORA59 at the protein level indicated that JA controls ORA59 activity by promoting the nuclear localization and stabilization of the protein. Based on the findings it was postulated that there is an F-box protein that targets ORA59 for degradation, and a repressor protein that sequesters ORA59 in the cytoplasm. A protein interacting with ORA59 was identified by yeast two-hybrid screening. Further studies showed that this protein acts as a repressor of ORA59 by retaining it in the cytoplasm, which leads to fine-tuning of basal resistance against pathogens.