An Abscisic Acid Receptor ABAR/CHLH Mediated ABA Signaling Pathway In Arabidopsis

The phytohormone abscisic acid (ABA) plays a vital role in plant development and response to environmental challenges, but the complex networks of ABA signaling pathways are poorly understood. We previously reported that a chloroplast protein, the magnesium-protoporphyrin IX chelatase H subunit (CHLH/ABAR), functions as a receptor for ABA in Arabidopsis thaliana, which spans chloroplast envelope. Here we report that the cytosolic C-terminus of ABAR interacts with a group of WRKY transcription factors (AD1A/WRKY40, AD1B/WRKY18 and AD1C/WRKY60) that function as negative regulators of ABA signaling in seed germination and postgermination growth. AD1A, a central negative regulator, inhibits expression of ABA-responsive genes such as ABI5. The interaction between ABAR and AD1A is affected by ABA. In response to a high level of ABA signal that recruits AD1A from the nucleus to the cytosol and promotes ABAR-AD1A interaction, ABAR relieves ABI5 gene of inhibition by repressing AD1A expression. In Barley, the homologous gene of ABAR is XanF, which also interacts with the AD1A/AD1B/AD1C. These findings describe a unique ABA signaling pathway from the early signaling events to downstream gene expression.