The Mg-chelatase H subunit is an abscisic acid receptor

Abstract

Abscisic acid (ABA) is a vital phytohormone that regulates mainly stomatal aperture and seed development, but ABA receptors involved in these processes have yet to be determined. We previously identified from broad bean an ABA-binding protein (ABAR) potentially involved in stomatal signalling, the gene for which encodes the H subunit of Mg-chelatase (CHLH), which is a key component in both chlorophyll biosynthesis and plastid-to-nucleus signalling. Here we show that Arabidopsis ABAR/CHLH specifically binds ABA, and mediates ABA signalling as a positive regulator in seed germination, post-germination growth and stomatal movement, showing that ABAR/CHLH is an ABA receptor. We show also that ABAR/CHLH is a ubiquitous protein expressed in both green and non-green tissues, indicating that it might be able to perceive the ABA signal at the whole-plant level.

Overexpression of AtMYB44 Enhances Stomatal Closure to Confer Abiotic Stress Tolerance in Transgenic Arabidopsis

“…Enhanced expression of genes encoding subunits of magnesium-protoporphyrin-IX chelatase (Mg-chelatase), including CHLH, was also notable. CHLH specifically binds ABA and thereby mediates plastid-to-nucleus signaling as a positive regulator in seed germination, postgermination growth, and stomatal movement (Nott et al, 2006;Shen et al, 2006).…”

Structural insights into PYR/PYL/RCAR ABA receptors and PP2Cs

“…Glu94Lys, Glu141Lys, Lys59Gln, abolish or reduce PYR1 function. In this context, it is interesting to mention that coupling of ABA through its carboxylic group to the amino group of a 10-atom spacer arm of a Sepharose resin has been used as a tool to identify ABA binding proteins, which resulted in the identification of the Mg-chelatase H subunit as an ABA receptor [6,25]. Such approach is likely to interfere with the binding of the carboxylate group to the receptor, so the mechanism of ABA binding by Mg-chelatase has still to be explained.…”

“…Functional redundancy or pleiotropic effects including embryo or gamete lethality looked as sound arguments to justify this failure. Indeed, the first approaches to identify ABA receptors, marked by retracted data in one case [5], used biochemical techniques to identify ABA-binding proteins (CHLH/ABAR/GUN5) [6] or followed pharmacological evidence suggesting the involvement of Gprotein coupled signaling in the ABA pathway (GTG1/GTG2) [7]. Finally, a chemical genetic approach using a synthetic selective ABA agonist, pyrabactin, made it possible to identify a family of soluble ABA receptors, named PYR/PYL/RCAR for PYRABACTIN RESISTANCE1/PYR1-LIKE/ REGULATORY COMPONENTS OF ABA RECEPTORS, respectively [8,9].…”

The multifaceted role of ABA in disease resistance

“…One of these, a G-protein-coupled receptor, is located in the plasma membrane [61], although subsequent genetic characterization failed to confirm its role in the ABA response [62]. Another putative ABA receptor was identified as a chloroplast-located magnesium-chelatase that is involved in chlorophyll synthesis [63]. It is currently unknown how these ABA-binding proteins fit into the complex signaling network of the ABA response [64].…”