The importin-alpha superfamily engages in ethylene signaling by shuttling ETHYLENE INSENSITIVE 2 from the endoplasmic reticulum to the nucleus.

Plant hormones are small molecules that modulate a plethora of growth and developmental pathways. Among these molecules, ethylene is known to modulate several important agronomical traits, including fruit ripening and senescence. However, the mechanisms, pathways, and processes of ethylene signaling from the receptors at the endoplasmic reticulum (ER) membrane to the transcriptional regulators in the nucleus remain to be elucidated. Here, we demonstrate that the importin alpha superfamily of nuclear transport receptors plays a pivotal role by transporting ethylene key regulator ETHYLENE INSENSITIVE 2 (EIN2) from the ER into the nucleus. Our findings show that importin α (impα) single- and triple-mutant seedlings of Arabidopsis thaliana retain a normal ethylene response, as evidenced by the typical triple-response phenotype observed in the presence of ethylene. In vitro and in planta interaction studies demonstrate that EIN2 is recognized as cargo by all nine IMPα isoforms, though with distinct affinities. Specifically, the binding studies reveal that IMPα1/2/3/4/7 are the most relevant isoforms for the nucleocytoplasmic transport of EIN2. Based on computational interaction predictions, we have identified potential binding modes and offer novel mechanistic insights into the interaction between the nuclear localization signal (NLS) motif of EIN2 and the IMPα superfamily. Our results provide novel insights into the mechanism by which the ethylene signal is transmitted from the ER membrane to the nucleus. The data pave the way for a more comprehensive understanding of the ethylene signaling pathway and the central role of EIN2.