Type One Protein Phosphatase 1 and Its Regulatory Protein Inhibitor 2 Negatively Regulate ABA Signaling

<div><p>The phytohormone abscisic acid (ABA) regulates plant growth, development and responses to biotic and abiotic stresses. The core ABA signaling pathway consists of three major components: ABA receptor (PYR1/PYLs), type 2C Protein Phosphatase (PP2C) and <u>SN</u>F1-<u>r</u>elated protein <u>k</u>inase 2 (SnRK2). Nevertheless, the complexity of ABA signaling remains to be explored. To uncover new components of ABA signal transduction pathways, we performed a yeast two-hybrid screen for SnRK2-interacting proteins. We found that <u>T</u>ype <u>O</u>ne <u>P</u>rotein <u>P</u>hosphatase 1 (TOPP1) and its regulatory protein, <u>At I</u>nhibitor-2 (AtI-2), physically interact with SnRK2s and also with PYLs. TOPP1 inhibited the kinase activity of SnRK2.6, and this inhibition could be enhanced by AtI-2. Transactivation assays showed that TOPP1 and AtI-2 negatively regulated the SnRK2.2/3/6-mediated activation of the ABA responsive reporter gene <i>RD29B</i>, supporting a negative role of TOPP1 and AtI-2 in ABA signaling. Consistent with these findings, <i>topp1</i> and <i>ati-2</i> mutant plants displayed hypersensitivities to ABA and salt treatments, and transcriptome analysis of <i>TOPP1</i> and <i>AtI-2</i> knockout plants revealed an increased expression of multiple ABA-responsive genes in the mutants. Taken together, our results uncover TOPP1 and AtI-2 as negative regulators of ABA signaling.</p>