Unique Functions of the Ethylene Receptors in Seed Germination

The endogenous phytohormone ethylene regulates many agroeconomically important aspects of plant development, including germination, fruit ripening, leaf and flower senescence, and organ abscission, as well as stress tolerance. In Arabidopsis thaliana, ethylene is perceived by a family of five membrane receptors known as ETHYLENE RESPONSE1 (ETR1), ETR2, ETHYLENE RESPONSE SENSOR1 (ERS1), ERS2, and ETHYLENE INSENSITIVE4 (EIN4). Previous research has shown that these receptors have both overlapping and unique functions in mediating ethylene responses. We have investigated the role of individual ethylene receptors in seed germination during salt stress and following far-red light treatment. Both of these conditions are known to inhibit germination of wild-type seeds by enhancing and reducing production of the phytohormones abscisic acid (ABA) and gibberellic acid (GA) which are known to inhibit and promote germination respectively. We found that ETR1 inhibits while ETR2 promotes seed germination during both salt stress and far-red light treatment. During salt stress, ethylene was found to play only a minor role in the opposing actions of ETR1 and ETR2 on seed germination. Instead, differences in production and/or sensitivity to ABA played the major role in the opposing roles of ETR1 and ETR2 on seed germination during salt stress. Following far-red treatment, ethylene appeared to play a larger role than during salt stress, but ultimately ETR1 likely inhibits germination by affecting ABA and GA synthesis.