Identification and characterization of signal components in early ABA signaling in Arabidopsis

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 SnRK2