ABA Signaling Crucial Regulator SOAR1 Regulates Development And Stress Responses In Arabidopsis

The phytohormone abscisic acid(ABA) plays vital roles in regulating many aspects of plant growth and development, as well as plant responses to various stresses. The pentatricopeptide repeat(PPR) superfamily proteins, which are encoded by one of the largest gene families in plants, play important roles in plant developmental processes and responses to environmental stresses. However, no nucleus- or cytosol-localized PPR protein has been found to regulate plant ABA signaling and abiotic stress responses.In this dissertation, we find a cytosol-nucleus dual-localized pentatricopeptide repeat(PPR) protein SOAR1. A dominant suppressor of ABAR overexpressor, soar1-1D, was screened from ABAR overexpression lines, and SOAR1 gene was cloned in Arabidopsis thaliana. Down-regulation of SOAR1 strongly enhances, but up-regulation of SOAR1 almost completely impairs ABA responses in ABA-induced seed germination inhibition and post- germination growth arrest, indicating that changing the expression level of SOAR1 sharply regulates ABA tolerance in Arabidopsis. Changes in SOAR1 expression alter dramatically expression of a subset of key, nuclear ABA-responsive genes. These results indicate that SOAR1 is a crucial regulator of ABA signaling. We also put forward an eco-friendly weed control strategy with ABA as selective herbicide for SOAR1-overexpression transgenic plants.Besides, SOAR1 regulates plant development and floral transition in Arabidopsis. SOAR1 knockdown mutants display the phenotypes of delayed germination rate, enlongated hypocotyl length and earl y flowering; while SOAR1-overexpression lines show opposite phenotypes such as accelerated germination rate, shortened hypocotyl length and late flowering.Additionally, SOAR1 plays important roles in regulating main abiotic stress responses. SOAR1 knockdown mutants display impaired drought, salt, low temperature and osmotic stress responses. Nevertheless, SOAR1-overexpression lines are characterized by significantly enhanced tolerance to these stresses, and even germinated in salt treatment of artificial sea water’s Na Cl content. These results reveal that SOAR1 is critical in regulating stress responses.Furthermore, genetic analysis supports that SOAR1 acts downstream of ABAR and upstream of a nuclear ABA-responsive b ZIP transcription factor ABI5. SOAR1 negatively regulates ABI5 expression. Moreover, RNA Immunoprecipitation(RIP) result indicates that SOAR1 binds ABI5 mRNA in vivo. These results indicate that in the CHLH/ABAR- mediated ABA signaling pathway, SOAR1 may influence the RNA expression level by binding the target RNA(such as ABI5), thereby regulates ABA signaling network.In conclusion, this dissertation demonstrates the crucial function of SOAR1 in regulating ABA signaling, development and stress responses in plant. These findings provide important information to elucidate the functional mechanism of PPR proteins and ABA signalling network.