ETHYLENE INSENSITIVE2-like protein mediates submergence and drought responses in Physcomitrium patens

ETHYLENE INSENSITIVE 2 (EIN2) is an Nramp family transmembrane protein recognized as an essential component of ethylene signaling in angiosperms. However, its functions in other plant systems are not fully understood. Here, we demonstrate that ppein2ab mutants of the moss Physcomitrium patens, in which both EIN2-like genes have been disrupted, do not show a typical ethylene-mediated escape response following submergence. Interestingly, ppein2ab mutants showed reduced sensitivity to abscisic acid (ABA), a phytohormone that mediates drought stress responses. The ppein2ab plants were sensitive to hyperosmosis and freezing stress and exhibited reduced late embryogenesis abundant protein accumulation. Furthermore, ppein2ab mutants showed reduced activation of both SNF1-related protein kinase2 (SnRK2), the central activator of ABA and osmostress signaling, and the B3-RAF kinase ARK/PpCTR1L, a positive regulator of SnRK2. These results indicate that EIN2 is a dual function signaling component mediating both submergence and drought signaling in bryophytes. The diminished ABA responses in ppein2ab were restored by introduction of Arabidopsis EIN2 and the EIN2 orthologs of the Charophyceaen alga Chara braunii, suggesting functional conservation of EIN2 orthologs in Phragmoplastophyta.