EIN3 and SOS2 synergistically modulate plant salt tolerance

Ethylene biosynthesis and the ethylene signaling pathway regulate plant salt tolerance by activating the expression of downstream target genes such as those related to ROS and Na(+)/K(+) homeostasis. The Salt Overly Sensitive (SOS) pathway regulates Na(+)/K(+) homeostasis in Arabidopsis under salt stress. However, the connection between these two pathways is unclear. Through genetic screening, we identified two sos2 alleles as salt sensitive mutants in the ein3-1 background. Neither Ethylene-Insensitive 2 (EIN2) nor EIN3 changed the expression patterns of SOS genes including SOS1, SOS2, SOS3 and SOS3-like Calcium Binding Protein 8 (SCaBP8), but SOS2 activated the expression of one target gene of EIN3, Ethylene and Salt-inducible ERF 1 (ESE1). Moreover, Ser/Thr protein kinase SOS2 phosphorylated EIN3 in vitro mainly at the S325 site and weakly at the S35, T42 and S606 sites. EIN3 S325A mutation reduced its transcriptional activating activity on ESE1 promoter:GUS in a transient GUS assay, and impaired its ability to rescue ein3-1 salt hypersensitivity. Furthermore, SOS2 activated salt-responsive ESE1 target gene expression under salt stress. Therefore, EIN3-SOS2 might link the ethylene signaling pathway and the SOS pathway in Arabidopsis salt responses.