Siderophore-producing Aspergillus species as multifunctional bioinoculants for sustainable cultivation of cowpea (Vigna unguiculata (L.) Walp.).

The excessive reliance on synthetic fertilizers and pesticides has undeniably enhanced crop productivity; however, it has also led to long-term environmental degradation, highlighting the urgent need for sustainable alternatives. Bioinoculants, particularly siderophore-producing fungi, represent a potential eco-friendly approach to improving nutrient acquisition, plant health, and resistance to pathogens. In the present study, 23 fungal species were isolated from agricultural and non-agricultural soils, among which four exhibited plant growth-promoting fungal (PGPF) activity. Of these, Aspergillus awamori (SA2) and Aspergillus terreus (SA3), identified through ITS gene sequencing, demonstrated the highest PGPF potential. Both strains synthesized hydroxamate-type siderophores, as confirmed by Chrome Azurol S (CAS) assay, HPLC, FTIR, and NMR analyses. In addition, they exhibited multiple plant growth-promoting (PGP) traits, including indole-3-acetic acid and ammonia production, as well as phosphate solubilizing activity. Treatment of Vigna unguiculata (L.) Walp. seeds with the filtrate of siderophore- producing isolates significantly enhanced germination rate, biomass accumulation, chlorophyll content, and levels of secondary metabolites (phenolics and flavonoids), while also displaying antagonistic activity against Fusarium oxysporum and Rhizoctonia solani under controlled conditions. These findings suggest that Aspergillus awamori and Aspergillus terreus have potential as plant growth-promoting and biocontrol agents; however, further validation under field conditions is required.