Argon-stimulated nitric oxide production and its function in alfalfa cadmium tolerance

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.

Alfalfa MsbHLH115 confers tolerance to cadmium stress through activating the iron deficiency response in Arabidopsis thaliana

Cadmium (Cd) pollution severely affects plant growth and development, posing risks to human health throughout the food chain. Improved iron (Fe) nutrients could mitigate Cd toxicity in plants, but the regulatory network involving Cd and Fe interplay remains unresolved. Here, a transcription factor gene of alfalfa, MsbHLH115 was verified to respond to iron deficiency and Cd stress. Overexpression of MsbHLH115 enhanced tolerance to Cd stress, showing better growth and less ROS accumulation in Arabidopsis thaliana. Overexpression of MsbHLH115 significantly enhanced Fe and Zn accumulation and did not affect Cd, Mn, and Cu concentration in Arabidopsis. Further investigations revealed that MsbHLH115 up-regulated iron homeostasis regulation genes, ROS-related genes, and metal chelation and detoxification genes, contributing to attenuating Cd toxicity. Y1H, EMSA, and LUC assays confirmed the physical interaction between MsbHLH115 and E-box, which is present in the promoter regions of most of the above-mentioned iron homeostasis regulatory genes. The transient expression experiment showed that MsbHLH115 interacted with MsbHLH121pro. The results suggest that MsbHLH115 may directly regulate the iron-deficiency response system and indirectly regulate the metal detoxification response mechanism, thereby enhancing plant Cd tolerance. In summary, enhancing iron accumulation through transcription factor regulation holds promise for improving plant tolerance to Cd toxicity, and MsbHLH115 is a potential candidate for addressing Cd toxicity issues.