The Arabidopsis Nodulin Homeobox Factor AtNDX Interacts with AtRING1A/B and Negatively Regulates Abscisic Acid Signaling

Abstract

The phytohormone abscisic acid (ABA) and the Polycomb group proteins have key roles in regulating plant growth and development; however, their interplay and underlying mechanisms are not fully understood. Here, we identified an Arabidopsis (<i>Arabidopsis thaliana</i>) nodulin homeobox (AtNDX) protein as a negative regulator in the ABA signaling pathway. AtNDX mutants are hypersensitive to ABA, as measured by inhibition of seed germination and root growth, and the expression of <i>AtNDX</i> is downregulated by ABA. AtNDX interacts with the Polycomb Repressive Complex1 (PRC1) core components AtRING1A and AtRING1B in vitro and in vivo, and together, they negatively regulate the expression levels of some ABA-responsive genes. We identified <i>ABA</i>-<i>INSENSITIVE</i> (<i>ABI4</i>) as a direct target of AtNDX. AtNDX directly binds the downstream region of <i>ABI4</i> and deleting this region increases the ABA sensitivity of primary root growth. Furthermore, <i>ABI4</i> mutations rescue the ABA-hypersensitive phenotypes of <i>ndx</i> mutants and <i>ABI4</i>-overexpressing plants are hypersensitive to ABA in primary root growth. Thus, our work reveals the critical functions of AtNDX and PRC1 in some ABA-mediated processes and their regulation of <i>ABI4</i>.