A Root-Colonizing Fusarium Isolate from a Salt Marsh Improves the Salinity Tolerance of a Commercial Cultivar of Festuca rubra via Enhanced Root K⁺ Homeostasis

Salinity poses a major threat to sustainable agriculture and coastal ecosystems resulting in a substantial loss of plant productivity and biodiversity. Although some coastal grass species exhibit natural adaptation to saline conditions, the physiological mechanisms underlying salt tolerance remain incompletely understood, particularly regarding the role of plant-associated microbial symbionts. In a previous study, a commercial cultivar of red fescue (Festuca rubra ssp. rubra cv. Rafael) was evaluated as salt sensitive grown hydroponically, whereas wild populations of F. rubra commonly occur in coastal salt marshes (possibly ssp. litoralis). We hypothesized that this difference in salt tolerance is partly associated with fungal plant-microbe interactions. To test this, we investigated whether inoculation with a root-colonizing fungal isolate (identified as Fusarium sp. 1), isolated from F. rubra growing on a salt marsh along the Dutch Wadden Sea coast, could improve the salinity tolerance of a commercial cultivar. The results showed that inoculation with Fusarium sp. 1 alleviated the salt-induced growth inhibition. At 100 mM NaCl, the shoot and root biomass of inoculated plants were partially restored compared with non inoculated controls, accompanied by a significant increase in the shoot-to-root ratio. To elucidate the physiological basis of this response, we applied the microelectrode ion flux estimation (MIFE) technique to quantify Na⁺ -induced K⁺ efflux in roots. Inoculated plants exhibited improved K⁺ homeostasis, characterized by a reduced instantaneous Na⁺-induced K⁺ efflux and a faster recovery of root fluxes. Moreover, inoculated plants grown at 50 and 100 mM NaCl showed an increases in root K⁺ influx of 333 and 397%, respectively, compared with non-inoculated controls. Our results indicated that inoculation with a root-colonizing fungal isolate can improve salinity tolerance of F. rubra, likely through enhanced root K⁺ retention. These findings suggest that commercial F. rubra cultivars remain responsive to microbial associations and highlight the potential of exploring plant- microbe interactions from natural environments to improve salinity resilience in grasses and potentially other crops.