NO more ABA activity
Abscisic acid (ABA) is a critical plant hormone, controlling developmental processes and immune responses. Long-term regulation of ABA signaling involves changes in gene expression that reduce ABA synthesis and enhance ABA metabolism. Castillo et al. found that various members of the ABA receptor PYR/PYL/RCAR family were modified posttranslationally by tyrosine nitration or S-nitrosylation at cysteine residues, two covalent modifications that can result from increased nitric oxide (NO). These NO-mediated modifications and polyubiquitylation, which target proteins for degradation, occurred in a complex, potentially interconnected, and receptor-specific pattern in plants overexpressing individual receptors. Tyrosine nitration, but not S-nitrosylation, inhibited ABA-induced activity in vitro, suggesting that tyrosine nitration may be a mechanism to rapidly tune the cellular responsiveness to ABA.
Abstract
Abscisic acid (ABA) is a phytohormone that inhibits growth and enhances adaptation to stress in plants. ABA perception and signaling rely on its binding to receptors of the pyrabactin resistance1/PYR1-like/regulatory components of ABA receptors (PYR/PYL/RCAR) family, the subsequent inhibition of clade A type 2C protein phosphatases (PP2Cs), and the phosphorylation of ion channels and transcription factors by protein kinases of the SnRK2 family. Nitric oxide (NO) may inhibit ABA signaling because NO-deficient plants are hypersensitive to ABA. Regulation by NO often involves posttranslational modification of proteins. Mass spectrometry analysis of ABA receptors expressed in plants and recombinant receptors modified in vitro revealed that the receptors were nitrated at tyrosine residues and S-nitrosylated at cysteine residues. In an in vitro ABA-induced, PP2C inhibition assay, tyrosine nitration reduced receptor activity, whereas S-nitrosylated receptors were fully capable of ABA-induced inhibition of the phosphatase. PYR/PYL/RCAR proteins with nitrated tyrosine, which is an irreversible covalent modification, were polyubiquitylated and underwent proteasome-mediated degradation. We propose that tyrosine nitration, which requires NO and superoxide anions, is a rapid mechanism by which NO limits ABA signaling under conditions in which NO and reactive oxygen species are both produced.
