Abstract
Similar to mycorrhizal mutualists, the rhizospheric and endophytic fungi are also
considered to act as active regulators of host fitness (e.g., nutrition and stress tolerance).
Despite considerable work in selected model systems, it is generally poorly understood
how plant-associated fungi are structured in habitats with extreme conditions and to
what extent they contribute to improved plant performance. Here, we investigate the
community composition of root and seed-associated fungi from six halophytes growing
in saline areas of China, and found that the pleosporalean taxa (Ascomycota) were most
frequently isolated across samples. A total of twenty-seven representative isolates were
selected for construction of the phylogeny based on the multi-locus data (partial 18S
rDNA, 28S rDNA, and transcription elongation factor 1-a), which classified them into
seven families, one clade potentially representing a novel lineage. Fungal isolates were
subjected to growth response assays by imposing temperature, pH, ionic and osmotic
conditions. The fungi had a wide pH tolerance, while most isolates showed a variable
degree of sensitivity to increasing concentration of either salt or sorbitol. Subsequent
plant–fungal co-culture assays indicated that most isolates had only neutral or even
adverse effects on plant growth in the presence of inorganic nitrogen. Interestingly,
when provided with organic nitrogen sources the majority of the isolates enhanced
plant growth especially aboveground biomass. Most of the fungi preferred organic
nitrogen over its inorganic counterpart, suggesting that these fungi can readily mineralize
organic nitrogen into inorganic nitrogen. Microscopy revealed that several isolates
can successfully colonize roots and form melanized hyphae and/or microsclerotia-like
structures within cortical cells suggesting a phylogenetic assignment as dark septate
endophytes. This work provides a better understanding of the symbiotic relationship
between plants and pleosporalean fungi, and initial evidence for the use of this fungal
group in benefiting plant production.