The Non-Pathogenic Strain Fusarium oxysporum FO12 Mitigates Simultaneous Verticillium dahliae and Iron Deficiency Stresses in Arabidopsis thaliana.

Verticillium wilt, caused by the soilborne fungus Verticillium dahliae, and iron (Fe) deficiency are major stresses impacting valuable crops in the Mediterranean basin. The plant response to these stresses consists of overlapping pathways that, coupled with the emerging use of endophytic fungi as biological control agents (BCAs) and plant biostimulants, enables the study of their potential to mitigate co-occurring biotic stresses and nutritional deficiencies such as Fe deficiency. To this end, Arabidopsis thaliana plants of the "Columbia" ecotype grown in calcareous soil were inoculated with the non-pathogenic strain Fusarium oxysporum FO12 and/or with the defoliating pathotype of V. dahliae strain V180. Plant growth, plant mineral uptake, and disease progression, along with the molecular quantification of FO12 and V180 fungal biomass and the relative expression of ISR-, SAR-, and Fe deficiency-related genes in shoots, were evaluated. The results revealed that plants inoculated with the FO12 strain increased growth, leaf chlorophyll concentration (SPAD index), and phosphorus (P; 126%) and Fe (9%) contents, partially due to ethylene (ET) and Iron Man peptides (IMA). In addition, FO12 reduced the severity of symptoms by more than 85%. This reduction in disease severity may be attributed to a direct competitive effect on V180 progression and to the induction of several defence-associated genes related to ET and SA synthesis and signaling. This bidirectional crosstalk between Fe deficiency responses and plant defences triggered by FO12 mainly through ET can improve plant resilience while reducing reliance on agrochemicals, offering a sustainable multifaceted tool for crop protection and nutrition management.