Novel halotolerant fungal endophytes from hypersaline coastal ecosystems as sustainable biostimulants for early growth performance of tomato and wheat under saline stress

Abstract

Soil salinization severely restricts seed germination, seedling establishment, and vegetative growth in key crops such as tomato (Solanum lycopersicum L.) and wheat (Triticum aestivum L.). Endophytic fungi from extreme environments are promising biostimulants to alleviate salinity stress, yet systematic assessments of isolates from hypersaline coastal ecosystems remain scarce. Here, we evaluated 28 endophytic fungal isolates from saline coastal ecosystems of the southeastern Iberian Peninsula using a four-stage pipeline: (i) in vitro characterization of salinity and temperature tolerance, (ii) germination bioassays under increasing NaCl gradients, (iii) seedling development under saline stress in controlled-environment pot experiments, and (iv) antioxidant enzyme profiling (POD, CAT, SOD) in both crops. In vitro screening revealed broad phenotypic diversity driven by a significant isolate × temperature × NaCl interaction, resolving two functional superclusters: a thermotolerant group led by Sordaria fimicola (BC05, BC06) with generalist thermal plasticity, and a stress-sensitive mesophilic group including several Trichoderma isolates, with BC47 (Nothophoma gossypiicola) emerging as a halotolerant specialist. Fungal inoculation significantly modulated germination, with responses strongly dependent on crop species, salinity level, and endophyte identity. In tomato, isolates BC06 (S. fimicola), BC17 (Clarireedia sp.), BC47, and BC52 (N. gossypiicola) maintained radicle emergence at 100 mM NaCl, whereas in wheat BC06 was the only isolate capable of overcoming complete germination inhibition at 300 mM NaCl, markedly extending the osmotic tolerance limit of the species. Under saline pot conditions, inoculation enhanced tomato leaf area by up to 60% and root dry weight by up to 340%, while wheat responses were mostly neutral or negative, with BC24 (Trichoderma gamsii) as the sole shoot biomass enhancer (+58.1%) and BC50 (T. saturnisporum) uniquely improving the Dickson Quality Index. Antioxidant enzyme activities displayed contrasting, host- and context-specific patterns between crops. Overall, this study provides the first evidence of S. fimicola, Clarireedia narcissi, and N. gossypiicola as effective fungal biostimulants under saline stress and highlights the biotechnological potential of non-conventional endophytes from extreme coastal environments as a basis for the rational selection of sustainable bioinoculants for salt-affected agricultural systems.