Ethylene receptors signal via a noncanonical pathway to regulate abscisic acid responses ($) (Plant Physiol.)

The two-carbon containing gaseous hydrocarbon, ethylene, is one of the most important phytohormones. It is perceived by a set of receptors (ETR1, ETR2, ERS, ERS2, EIN4). Bakshi et al. found that ETR1 (etr1-6) and ETR2 (etr2-3) mutants have opposite effects on seed germination in the presence of NaCl, KCl, ethanol, CuSO4, and ZnSO4. In the presence of the ABA biosynthesis inhibitor norflurazon (NF), the germination effect is abolished, which suggests that the process is ABA-dependent. Additionally, the authors found that receiver domain of the receptor is critical for the effect on seed germination. Deletion of receiver domains (etr1-6 etr2-3 ein4-4) causes wild-type like seed germination. This receiver domain-dependent seed germination process is independent of the immediate downstream signaling molecule, CTR1. Interestingly, ABA alters the transcript levels of ETR1, ETR2 and EIN4 but not ERS1 and ERS2. By using the receiver domain for a protein-protein interaction study, the authors identified three proteins (LEB, RD21A, DI19). Mutants of these three proteins have reduced ABA responsiveness and faster seed germination like the etr1-6 mutant. (Summary by Arif Ashraf) Plant Physiol. 10.1104/pp.17.01321