Transcription factor HAT1 is a substrate of SnRK2.3 kinase and negatively regulates ABA synthesis and signaling in Arabidopsis responding to drought

<div><p>Drought is a major threat to plant growth and crop productivity. The phytohormone abscisic acid (ABA) plays a critical role in plant response to drought stress. Although ABA signaling-mediated drought tolerance has been widely investigated in Arabidopsis thaliana, the feedback mechanism and components negatively regulating this pathway are less well understood. Here we identified a member of Arabidopsis HD-ZIP transcription factors HAT1 which can interacts with and be phosphorylated by SnRK2s. <i>hat1hat3</i>, loss-of-function mutant of HAT1 and its homolog HAT3, was hypersensitive to ABA in primary root inhibition, ABA-responsive genes expression, and displayed enhanced drought tolerance, whereas <i>HAT1</i> overexpressing lines were hyposensitive to ABA and less tolerant to drought stress, suggesting that HAT1 functions as a negative regulator in ABA signaling-mediated drought response. Furthermore, expression levels of ABA biosynthesis genes <i>ABA3</i> and <i>NCED3</i> were repressed by HAT1 directly binding to their promoters, resulting in the ABA level was increased in <i>hat1hat3</i> and reduced in <i>HAT1OX</i> lines. Further evidence showed that both protein stability and binding activity of HAT1 was repressed by SnRK2.3 phosphorylation. Overexpressing <i>SnRK2</i>.<i>3</i> in <i>HAT1OX</i> transgenic plant made a reduced HAT1 protein level and suppressed the <i>HAT1OX</i> phenotypes in ABA and drought response. Our results thus establish a new negative regulation mechanism of HAT1 which helps plants fine-tune their drought responses.</p>