| The phytohormone abscisic acid (ABA) regulates growth, development, and response to abiotic stress in plants. The core ABA signaling has three key components: ABA receptors PYRABACTIN RESISTANCE 1 and PYR1-Like protein (PYR1/PYLs), type 2C protein phosphatases (PP2Cs) and SNF1- related protein Kinase 2s (SnRK2s). In the presence of ABA, PYR1/PYLs bind to and inhibit PP2Cs, leading to the release of SnRK2s from suppression, to activate downstream events such as ABA-responsive gene expression. To identify new components in the ABA signaling pathway, we performed a yeast two-hybrid screen for PYLs and SnRK2s interactors. We identified two proteins that interact with PYLs and SnRK2s: Ty pe One Protein Phosphatase 1 (TOPP1) and its regulatory protein At Inhibitor 2 (AtI-2). We show that TOPP1 and AtI-2 inhibit SnRK2.6 activity in vitro and that the inhibition is reversed by PYLs in the presence of ABA. Mutants topp1-1topp4-1 and ati-2 are hypersensitive to ABA during seed germination. ABA-responsive genes are highly up regulated in topp1-1topp4-1 and ati-2 mutants. Taken together, our results suggest that the TOPP1 and AtI-2 form a complex that negatively modulates ABA signaling by interacting with PYLs and SnRK2s. |
