Modulation of ABA signaling in vivo by an engineered receptor-insensitive PP2C allele

Abstract

The plant hormone abscisic acid (ABA) plays a crucial role in the control of the stress response and the regulation of plant growth and development. ABA binding to PYR/PYL/RCAR intracellular receptors leads to inhibition of key negative regulators of ABA signaling, i.e. clade A protein phosphatases type 2C (PP2Cs) such as ABI1 and HAB1, causing the activation of the ABA signaling pathway. In order to gain further understanding on the mechanism of hormone perception, PP2C inhibition and its implications for ABA signaling, we have performed a structural and functional analysis of the PYR1-ABA-HAB1 complex. Based on structural data, we generated a gain-offunction mutation in a critical residue of the phosphatase, hab1W385A, which abolished ABA-dependent receptor-mediated PP2C inhibition without impairing basal PP2C activity. As a result, hab1W385A caused constitutive inactivation of the protein kinase OST1 even in the presence of ABA and PYR/PYL proteins, in contrast to the receptorsensitive HAB1, and therefore hab1W385A qualifies as a hypermorphic mutation. Expression of hab1W385A in Arabidopsis thaliana plants leads to a strong, dominant ABA-insensitivity, which demonstrates that this conserved Trp residue can be targeted for the generation of dominant clade A PP2C alleles. Moreover, our data highlight the critical role of molecular interactions mediated by Trp385 equivalent residues for clade A PP2C function in vivo and the mechanism of ABA perception and signaling.