Innovative fungal bioagents: producing siderophores, IAA, and HCN to support plants under salinity stress and combat microbial plant pathogens.

Salinity stress is a major environmental problem affecting agricultural productivity worldwide. Bioagents such as plant growth-promoting fungi (PGPF) are gained increasing attention to improve plant growth and resilience to this problem. This study addresses the isolation and screening of endophytic fungal isolates from Atriplex nummularia as well as soil fungi for salinity tolerance. Screening revealed two fungal isolates AS1 and B4, exhibiting exceptional salt tolerance at different concentrations of NaCl from 2 to 10%. Morphological and molecular identification confirmed AS1 was identified as Alternaria sp. and B4 as Aspergillus terreus. Results revealed that, both fungal strains are plant growth promoters under normal and saline conditions in vitro. In normal conditions, endophytic Alternaria sp. AS1 produced indole acetic acid (IAA) and solubilized phosphate with quantities 39.0 and 58.438 µg/ml; and A. terreus B4 with quantities 52.90 and 63.07 µg/ml respectively. In saline conditions, IAA production by both fungal strains was decreased gradually with increasing salt concentration. On the other hand, phosphates solubilization was increased with increasing salt concentration up to 8% where the quantity was 81.917 and 85.677 in the case of endophytic Alternaria sp. AS1 and A. terreus B4, respectively. Furthermore, both fungi produced siderophores and hydrogen cyanide, with A. terreus exhibiting high production under both normal and saline conditions compared to the endophytic Alternaria sp. AS1. Antagonistic assays revealed that both AS1 and B4 effectively inhibited the growth of fungal plant pathogens Alternaria alternata and Fusarium oxysporum using dual culture technique. Antimicrobial assay demonstrated significant efficacy of ethyl acetate extracts of both fungi against A. alternata, F. oxysporum and Ralstonia solanacearum using the agar well diffusion method. Furthermore, seed treatment with both fungal strains and their consortia alleviated the harmful effect of salinity stress and improved seedling growth parameters compared to untreated wheat seeds. Our findings suggest that endophytic Alternaria sp. and soil fungus Aspergillus terreus have potential as bio-inoculants to improve plant growth and its resilience in saline environments.