Helium governs nitric oxide signaling to improve alfalfa salinity tolerance by reestablishing redox and ion homeostasis.

Helium, previously described as "biologically inert", displays great medicinal potential in a number of respiratory ailments via influencing redox signaling. However, whether or how this gas functions in plant biology is still elusive. Here, hydroponic and pot experiments showed that mimicking the responses achieved by nitric oxide (NO)-releasing compound, exogenous helium supply with helium-enriched solution or its fumigation significantly prevented NaCl-induced growth inhibition of alfalfa seedlings. Lower level of Na+/K+ ratio in both root and shoot parts caused by lower efflux of net K+ and higher efflux of net Na+, as well as higher abundances of ion transport genes (SOS1, SOS2, SOS3, NHX1, HKT1, and AKT1) were simultaneously observed. Consistently, reactive oxygen species (ROS) accumulation and oxidative injury were abolished by helium. These were further supported by stimulating antioxidant machinery and reprogramming gene expression related to antioxidant defence and related transcriptional factors (MYB4, WRKY33, ERF8, and ERF11) against salinity toxicity. Further experiments showed that helium supplementation strengthened endogenous NO production by up-regulating the expression of nitrate reductase (NR2) and its enzymatic activity, and NO-based S-nitrosylation was further influenced. Above responses were remarkably impaired by the removal of endogenous NO when its scavenger was added. Overall, these results revealed that helium control of salinity tolerance is partially mediated by a NO signaling cascade governing redox and ion homeostasis reestablishment, two important defense strategies against salinity stress.