The overlooked salt: Impact of dark septate endophytes on alfalfa at varying sodium sulfate levels

Sodium sulfate (Na₂SO₄) is one sodium salt extensively found in saline soils; in certain regions, it is the dominant salt present. Dark septate endophytes (DSE) are competent in enhancing plants’ resistance to stressed environments. Nevertheless, little is known about the role of DSE in enhancing plant tolerance to Na₂SO₄. This study examined DSE growth and its impacts on alfalfa plants exposed to varying Na₂SO₄ concentrations (0%, 0.15%, 0.3%, and 0.45% (w/w)). Our findings revealed that DSE can thrive even in salt-stress environments. On the 8^th day of cultivation, their biomass reached the highest level under 0.45% salt concentration. Moreover, DSE successfully colonized alfalfa roots and significantly enhanced plant growth and development across the various salt gradients. Notably, DSE made the highest contribution 68% to the total biomass of alfalfa at 0.45% salt concentration. Meanwhile, DSE significantly decreased the presence of root’s Na⁺ across varying salt gradients. Additionally, DSE significantly increased catalase (CAT) activity at salt concentrations of 0.3% and 0.45%. Our study also revealed strong positive correlations of plant biomass with the root index, root’s K⁺ content, and K⁺/Na⁺ ratio, and strong negative correlations of plant biomass with root’s Na⁺ content and soil’s Na⁺ and SO₄²⁻ contents. Structural equation modeling (SEM) demonstrated that DSE indirectly enhanced plant’s shoot biomass under various salt stresses via increasing root length, decreasing root’s Na⁺ content, and raising CAT activity, while salt indirectly reduced plant’s shoot weight via reducing root length or increasing root’s Na⁺ content or exerted a direct negative effect on plant shoot biomass. Thus, DSE are instrumental in bolstering the salt tolerance of plants, which holds strategic importance for the management of saline-alkali soils.