A Potential Role for Common Mycorrhizal Networks (CMNs) in Mediating Response Strategies and Signaling Between Different Plant Combinations Under Salt Stress

Soil salinization is one of the main stress factors limiting plant growth and ecosystem restoration in arid regions. Arbuscular mycorrhizal fungi (AMF) can form common mycorrhizal networks (CMNs) that potentially facilitate resource and signal exchange between plants. In this study, we investigated whether such processes associated with AMF connectivity might contribute to salt tolerance in different plant combinations, using Glycyrrhiza inflata and Lycium ruthenicum. However, under salt stress, it remains unclear how different plant combinations (conspecific vs. heterospecific) may differentially benefit from CMN-mediated processes under salt stress, and whether such processes involve coordinated stress signaling and nitrogen transfer. This study used Glycyrrhiza inflata (a leguminous N-fixing plant with a “N-input” strategy) and Lycium ruthenicum (a deep-rooted desert shrub with a “resource-use efficiency” strategy) as materials to construct conspecific and heterospecific plant combinations: G-G (G. inflata-G. inflata), L-L (L. ruthenicum-L. ruthenicum), G-L (G. inflata-L. ruthenicum), and L-G (L. ruthenicum-G. inflata). Four salt stress levels were set (NaCl concentrations of 0, 150, 250, and 350 mmol·L−1), along with AMF inoculation treatments. The study evaluated responses in AMF colonization, nitrogen transfer, biomass, root structure, photosynthetic characteristics, antioxidant capacity, osmotic regulation, and hormone levels. The results show that: (1) AMF colonization rates in all inoculated groups significantly decreased with increasing salt concentration, with the G-L combination showing a smaller decline; (2) The G-G combination maintained strong root activity and photosystem stability under high salt stress, exhibiting higher salt tolerance; (3) In conspecific combinations, the JA-Pro signaling pathway was dominant, whereas in heterospecific combinations, the ABA-SOD pathway prevailed, indicating differences in hormone regulation mechanisms among different combinations; (4) 15N transfer efficiency was significantly higher in conspecific combinations than in heterospecific combinations (p < 0.05), and increasing salt concentrations limited the resource-sharing ability of heterospecific combinations. In summary, our results revealed distinct physiological and hormonal responses in conspecific versus heterospecific plant combinations under salt stress when grown in an AMF-colonized system that permits hyphal connections. These patterns were consistent with a potential role of CMNs in signal coordination and resource sharing, although further experiments with disrupted hyphal connections would be required to confirm this mechanism.