Ethylene Inhibited Plasma Membrane H ⁺ ‐ATPase to Decrease Root Na ⁺ Efflux in Maize to Break Na ⁺ /H ⁺ Homeostasis Under Salt Stress

Ethylene plays an indispensable role in regulating plant growth and stress responses. However, the mechanisms underlying the regulation of Na⁺/H⁺ homoeostasis by ethylene and subsequent mediation of maize growth under salt stress remain unclear. ZmACO2, which encodes ethylene biosynthesis enzyme 1-aminocyclopropane-1-carboxylate oxidase2, is induced by salt stress. Thus, ZmACO2-overexpressing (ACO2-OE) and mutant (aco2-cr) plants were used to investigate how ethylene regulates Na⁺/H⁺ homoeostasis in maize under salt stress. The aco2-cr mutants exhibited significantly lower Na⁺ accumulation and Na⁺/K⁺ ratios than the wild-type and ACO2-OE plants. This phenotype was attributed to their higher expression of ZmSOS1 and ZmHKT1, which increased root net Na⁺ efflux by 20.65% and decreased Na⁺ transport from roots to shoots by 42.49% (p < 0.001), respectively. Compared to the other plants, aco2-cr mutants showed higher ZmMHA2 expression and plasma membrane H⁺-ATPase activities, which promoted net root H⁺ efflux to provide a greater H⁺ proton gradient for salt-overly-sensitive 1 (SOS1). Inhibition efficiencies of Na⁺ efflux and H⁺ influx by sodium orthovanadate were lower in aco2-cr mutants than in ACO2-OE and wild-type plants under salt stress; however, ACO2-OE plants showed a salt-sensitive phenotype. Overall, these findings showed that salt-induced ethylene inhibited plasma membrane H⁺-ATPase and SOS1 from disrupting Na⁺/H⁺ homoeostasis, thereby decreasing Na⁺ efflux in maize roots and also provided a strategy to improve salt tolerance by optimising ethylene levels in maize.