The dual role of ethylene in plant salt tolerance: mechanisms and species specificity

Soil salinity threatens over a third of irrigated land, and ethylene is a central but species-dependent regulator of plant responses. It is broadly protective in many eudicots, whereas stronger signaling is associated with salt sensitivity in several rice studies. This review separates demonstrated mechanisms from unresolved hypotheses. Salt-responsive ACS/ACO expression, ACC content and ethylene emission represent distinct regulatory levels, and matched cross-clade emission kinetics are lacking. The Yang cycle supports continued biosynthesis but does not exclude precursor limitation. The strongest salt-specific contrast occurs at EIN3/EIL target selection. In rice, OsEIL1/OsEIL2 directly activate OsHKT2;1, OsERF2 and OsBURP16, and OsERF2 activates OsRbohH, promoting Na+ uptake, ROS accumulation or salt sensitivity. In Arabidopsis, AtEIN3 directly activates protective stress- and ROS-response genes. Receptor-specific ABA and MAPK–ethylene connections provide additional comparison points, but rice receptor–ABA coupling and matched kinase–substrate analyses remain unresolved. Effects on osmotic adjustment, ion homeostasis and ROS homeostasis vary with organ, stage, dose and duration. ACC deaminase-producing microorganisms can lower plant ACC pools and often reduce stress-induced ethylene, although selective suppression of a later ethylene phase remains unproven. Thus, EIL target selection is the clearest eudicot–rice divergence; other effects remain context dependent.